IB Biology HL Continuity and Change Concepts

DNA replication produces exact copies of DNA with identical base sequences, apart from rare copying errors.

Accurate copies preserve genetic information when cells or organisms reproduce. In multicellular organisms, replication supplies genomes for cell division during growth and replacement of damaged or worn tissues.

Original DNA sequence → replication → two matching DNA molecules → genetic continuity in reproduction, growth and tissue replacement.

Before a skin cell divides to replace lost tissue, its DNA is copied so both daughter cells can inherit the same base sequence.

Replication copies DNA; transcription makes RNA and translation makes polypeptide. ‘Identical’ describes base-sequence information, not two newly synthesized strands without templates.

Semi-conservative replication produces DNA molecules in which each double helix contains one parental strand and one newly synthesized strand.

When the original strands separate, each acts as a template. Complementary base pairing preserves information while retaining one physical strand from the original molecule in each product.

Identify a product by checking: one old strand; one new strand; complementary pairing between them.

After one round, heavy parental DNA in a density experiment is replaced by two intermediate molecules, each containing one old and one new strand.

Semi-conservative does not mean half the bases are copied randomly; the strand pattern is the key prediction.

Helicase separates the two DNA strands, while DNA polymerase builds complementary DNA strands from the exposed templates.

Helicase unwinds the double helix and breaks hydrogen bonds between complementary bases. DNA polymerase selects complementary DNA nucleotides and joins them into a growing strand.

Helicase: unwind and break inter-strand hydrogen bonds. DNA polymerase: use each original strand as a template and join complementary nucleotides.

At a replication fork, helicase exposes template bases; polymerase then places A opposite T and C opposite G while extending the new DNA.

Helicase does not synthesize DNA, and polymerase does not separate the original strands. Detailed 5′/3′ directionality belongs to the later HL objective.

PCR amplifies a selected DNA region; gel electrophoresis then separates DNA fragments mainly by length.

Primers define the target ends. Each thermal cycle uses high temperature to separate strands, lower temperature for primer binding, and a suitable extension temperature for heat-stable Taq DNA polymerase to synthesize new DNA.

Load amplified fragments into wells. Negatively charged DNA moves toward the positive electrode through the gel; shorter fragments move farther than longer fragments in the same time.

Primers amplify a variable DNA locus; electrophoresis separates the resulting fragments, and a size marker allows their approximate lengths to be compared.

PCR increases the amount of target DNA; electrophoresis separates fragments. Primer specificity, contamination controls and the size marker affect interpretation.

PCR and gel electrophoresis support DNA profiling by amplifying and comparing variable DNA markers.

In a paternity investigation, a child's marker alleles must be explainable by the biological parents. In a forensic investigation, a crime-scene DNA pattern can be compared with reference samples.

Use several independent markers: each additional matching marker reduces the probability that an unrelated person shares the full pattern by chance. Include positive/negative controls and guard against contamination.

A child's allele not supplied by the known parent must match the candidate parent's allele at each tested marker; one matching marker is weak, whereas a consistent multi-marker pattern is stronger evidence.

A profile supports or excludes a biological relationship/source; it does not alone prove when or how DNA reached a location. More markers reduce false-match probability but do not make laboratory error impossible.

DNA strands have chemically different 5′ and 3′ ends, and DNA polymerase synthesizes only in the 5′→3′ direction.

The 5′ end is associated with the phosphate on the sugar's 5′ carbon, while the 3′ end has a free hydroxyl on the 3′ carbon. Polymerase attaches the 5′ phosphate of an incoming DNA nucleotide to the free 3′ end of the growing strand.

New nucleotide's 5′ end + growing strand's 3′ end → phosphodiester bond. Therefore extension occurs only at 3′ and the new strand lengthens 5′→3′.

A polymerase moving along a template 3′→5′ builds the complementary strand 5′→3′, adding every next nucleotide at the new strand's 3′ end.

5′→3′ describes the direction of new-strand synthesis. Polymerase cannot add to the growing strand's 5′ end.

At each replication fork, the leading strand is synthesized continuously and the lagging strand discontinuously as Okazaki fragments.

DNA polymerase can extend only 5′→3′ while the templates are antiparallel. The leading template supports synthesis toward the fork; the lagging template requires repeated synthesis away from the fork as more DNA is exposed.

Leading strand: one RNA primer, then continuous synthesis. Lagging strand: repeated RNA primers, discontinuous Okazaki-fragment synthesis, followed by primer replacement and fragment joining.

As helicase advances, one polymerase follows the fork continuously; on the other template, each newly exposed section receives another primer and becomes a separate Okazaki fragment.

Both new strands are synthesized 5′→3′. Leading/lagging refer to synthesis pattern relative to fork movement, not gene content or biological importance.

In prokaryotic DNA replication, primase, DNA polymerase III, DNA polymerase I and DNA ligase perform distinct sequential jobs.

On the lagging strand, primase repeatedly starts fragments, polymerase III extends them, polymerase I replaces each primer, and ligase seals the final backbone gaps.

This named-enzyme sequence is limited to the prokaryotic system. Polymerase I replaces primers; ligase does not synthesize the missing DNA nucleotides.

DNA polymerase III proofreads a newly added base at the growing strand's 3′ terminal and corrects a mismatch before replication continues.

A non-complementary base pair distorts the new DNA. Polymerase III removes the mismatched terminal nucleotide, exposes the 3′ end again and inserts a nucleotide complementary to the template.

Mismatch at 3′ terminal → polymerase III detects it → incorrect nucleotide removed → correct complementary nucleotide added → 5′→3′ extension resumes.

If an incorrect nucleotide is added opposite a template G, proofreading removes it from the 3′ end and replaces it with C before the strand is extended further.

Proofreading greatly improves accuracy but does not eliminate every mutation. This objective is specifically polymerase III correction of a mismatched 3′ terminal.

Transcription is the synthesis of an RNA strand using one DNA strand as the template.

RNA polymerase binds the selected DNA region, separates a short section, matches RNA nucleotides to exposed template bases and joins the nucleotides into RNA.

DNA template exposed → complementary RNA nucleotides pair → RNA polymerase forms the RNA backbone → RNA separates and DNA re-forms its double helix.

For a protein-coding gene, RNA polymerase produces an mRNA transcript whose base sequence can later be translated.

Transcription synthesizes RNA, not DNA or polypeptide. Promoter/transcription-factor detail belongs to the later HL initiation objective.

Temporary hydrogen bonds between complementary bases allow an RNA sequence to be specified from a DNA template during transcription.

DNA hydrogen bonds separate locally while the covalent sugar–phosphate backbones remain intact. RNA nucleotides form complementary hydrogen bonds to exposed template bases before RNA polymerase joins them.

DNA template A pairs with RNA U; DNA template T pairs with RNA A; DNA C pairs with RNA G; DNA G pairs with RNA C.

A DNA template segment 3′-TACG-5′ specifies RNA 5′-AUGC-3′: template adenine is represented by uracil in RNA, not thymine.

Complementary does not mean identical. Hydrogen bonds guide base choice; phosphodiester bonds form the stable RNA backbone.

A single DNA strand can act repeatedly as a transcription template without its base sequence changing.

RNA polymerase reads the template and joins separate RNA nucleotides; it does not consume or replace the DNA bases. After the transcription bubble passes, the DNA strands pair again.

DNA opens locally → RNA copy is synthesized → RNA leaves → original DNA sequence remains conserved and available for later transcription.

A non-dividing neuron may transcribe the same essential gene many times while conserving that gene's DNA sequence throughout the life of the cell.

Stable does not mean inaccessible: local opening permits transcription. The RNA transcript can change or degrade while the DNA template sequence remains unchanged.

Transcription is the first stage of gene expression and a key point at which expression can be switched on or off.

Cells contain many genes but do not express all of them at the same time. If a gene is not transcribed, its mRNA is unavailable for translation; starting transcription makes expression possible.

Regulatory state → transcription on/off → mRNA absent/present → translation possible/not possible. Different cell types express different subsets of the same genome.

A cell can switch on transcription of an enzyme gene when the enzyme is needed while other genes remain untranscribed.

Transcriptional control is a key switch, not the only possible control. More mRNA does not guarantee proportionally more protein if later steps are limiting.

Translation uses a ribosome to read mRNA codons and join amino acids in the encoded order.

tRNA molecules pair anticodons with codons and deliver specific amino acids. The ribosome forms peptide bonds as it moves from start toward stop.

Trace: start codon; tRNA matching; peptide bond; codon movement; stop codon.

A ribosome reading three codons adds three specified amino acids before a stop signal ends the chain.

Translation reads RNA; it does not copy mRNA back into DNA. The ribosome assembles the chain; later folding and modification determine whether the polypeptide becomes functional.

During translation, mRNA supplies codon order, tRNAs deliver specific amino acids and the ribosome positions the molecules and catalyses peptide-bond formation.

mRNA first binds to the small ribosomal subunit. The large subunit forms the catalytic complex and can hold two tRNAs simultaneously so the amino acid on one can be linked to the growing chain on the other.

mRNA: ordered codons. tRNA: anticodon plus specific amino acid. Small subunit: binds/positions mRNA. Large subunit: binds two tRNAs and supports peptide-bond formation.

One tRNA holds the growing chain while a second matching tRNA places the next amino acid beside it; a peptide bond extends the polypeptide.

No component determines the protein alone: mRNA sets sequence, tRNAs match codons, and the ribosome coordinates assembly.

A three-base tRNA anticodon forms complementary base pairs with a three-base mRNA codon during translation.

Correct codon–anticodon pairing positions the tRNA carrying the amino acid specified by that codon, transferring mRNA sequence information into polypeptide sequence.

Read the mRNA codon 5′→3′; write the antiparallel complementary anticodon; pair A–U and C–G; identify the amino acid from the mRNA codon, not the anticodon.

mRNA codon 5′-AUG-3′ pairs with tRNA anticodon 3′-UAC-5′, positioning the tRNA carrying methionine.

Codon and anticodon are complementary and antiparallel, not identical. DNA–RNA pairing during transcription is a different Objective.

The genetic code is a triplet, degenerate and nearly universal mapping from mRNA codons to amino acids or stop signals.

Four bases taken two at a time give only 16 combinations, fewer than the 20 amino acids; triplets give 4³ = 64 codons, enough for all amino acids plus stop signals.

Degenerate means more than one codon can specify the same amino acid. Universal means the same codon usually specifies the same amino acid across organisms, with limited exceptions.

UUU and UUC both code for phenylalanine, showing degeneracy; UUU has one defined meaning, so degeneracy is not ambiguity.

Read codons in a fixed triplet frame. Universality is a broad biological pattern, not a claim that no exceptions exist.

A genetic-code table is used by reading mRNA codons 5' to 3' and matching each to its amino acid.

The first, second and third bases select positions in the table. Reading-frame errors shift every later codon, so strand labels and grouping matter.

Use: locate start; split triplets; read each codon; stop at a stop codon; report amino-acid order.

For 5'-AUG-GCU-UAA-3', AUG starts, GCU adds alanine and UAA terminates. If one base is omitted before AUG, every later triplet may be regrouped, so the table must be used from the correct start frame.

Do not use the DNA template directly as if it were mRNA; transcribe first.

During elongation, a matching tRNA enters, a peptide bond forms and the ribosome moves to the next codon.

Repeating this cycle extends the chain in a fixed direction while mRNA advances through the ribosome. Energy from charged tRNAs and factors supports the process.

Track each cycle: codon recognition; chain transfer; translocation; empty tRNA exit.

After one cycle the chain has one additional amino acid and the next codon is exposed.

Elongation is not random amino-acid addition; codon recognition controls the order. The cycle stops at a termination signal rather than continuing until the physical end of the mRNA.

A point mutation can change one mRNA codon, replace one amino acid and alter how a polypeptide folds or functions.

The effect depends on code degeneracy, mutation position and the chemical properties of the original and replacement amino acids. Some substitutions are synonymous; others alter interactions stabilizing protein structure.

In sickle-cell disease, a point substitution changes a β-globin codon from GAG to GUG in mRNA, replacing glutamic acid with valine. The hydrophobic replacement promotes abnormal haemoglobin association and changes red-cell shape under low oxygen.

DNA base substitution → changed mRNA codon → possible amino-acid replacement → altered side-chain interactions → altered protein structure/function.

A mutation is not automatically harmful or structure-changing: degeneracy can make it synonymous, and some amino-acid replacements have little effect.

Transcription and translation both proceed with respect to the mRNA 5′→3′ direction.

RNA polymerase reads the DNA template 3′→5′ and adds RNA nucleotides to the transcript's 3′ end, so RNA is synthesized 5′→3′.

The ribosome moves along mRNA from its 5′ end toward its 3′ end, reading successive codons in that direction while the polypeptide elongates.

Label both ends before interpreting a diagram: a template shown 3′→5′ supports a transcript drawn antiparallel 5′→3′, and the ribosome follows the transcript toward 3′.

‘Left to right’ is meaningless without strand labels. Transcription direction refers to new RNA synthesis; translation direction refers to ribosome movement along mRNA.

A promoter is a DNA region where transcription machinery binds to position RNA polymerase and define where transcription begins.

Promoter sequence and associated factors determine strand choice, start site and transcription level. Opening and early RNA synthesis proceed from that positioned complex.

Name: promoter; binding factor; start site; direction of RNA synthesis.

A promoter mutation can reduce mRNA production without changing the protein-coding sequence. RNA polymerase begins at the promoter and then moves along the template, so promoter position fixes which downstream sequence is copied.

A promoter is regulatory DNA, not the entire gene or translated region.

Non-coding DNA does not code for polypeptide sequence, but it can have regulatory, processing, structural or functional-RNA roles.

A tRNA gene is transcribed to a functional tRNA molecule; its RNA product participates in translation but is not itself translated.

Non-coding does not mean non-functional or never transcribed. Limit examples here to the four syllabus categories.

Eukaryotic pre-mRNA is processed by adding a 5' cap and poly-A tail and removing introns so mature mRNA can be exported and translated.

Processing protects RNA, supports export and helps ribosomes recognize the message. Exons are joined in the order defining the coding sequence.

Trace: pre-mRNA; cap and tail; intron removal; exon joining; mature mRNA export.

Removing an intron prevents its sequence from being read as part of the final translated message.

Processing changes RNA, not the original DNA sequence; different processing choices can alter the final protein.

Alternative splicing joins different combinations of exons from one pre-mRNA, allowing one gene to code for different polypeptides.

After introns are removed, selected exons can be retained or omitted in different mature mRNAs. Each exon combination creates a different codon sequence for translation.

One DNA gene → one pre-mRNA containing exons/introns → introns removed → different exon combinations joined → different mature mRNAs → polypeptide variants.

If mature mRNA A contains exons 1–2–3 and mature mRNA B contains exons 1–3, translation can produce two polypeptides from the same gene.

Alternative splicing changes RNA processing, not the DNA gene. Specific named protein examples or detailed splicing-factor mechanisms are not required.

Translation initiation assembles the ribosome at the mRNA start codon and establishes the reading frame; A, P and E sites then organize tRNA movement during elongation.

The small ribosomal subunit attaches near the mRNA 5′ terminal and moves to the start codon. An initiator tRNA pairs with that codon; the large subunit attaches, and the next matching tRNA can enter.

After peptide-bond formation and translocation, tRNAs shift through the sites: the growing-chain tRNA moves to P, the empty tRNA moves to E, and A becomes available for the next tRNA.

The start codon fixes the triplet reading frame. A/P/E describe tRNA positions, not three separate ribosomes.

Many translated polypeptides must be folded, cleaved or chemically modified before becoming functional proteins.

Insulin provides a required two-stage example: removal of the signal peptide converts pre-proinsulin to proinsulin; later removal of the connecting C-peptide converts proinsulin to mature insulin, whose A and B chains remain linked by disulfide bonds.

Pre-proinsulin → signal peptide removed → proinsulin folds/disulfide bonds form → C-peptide removed → functional insulin.

Cleavage changes one precursor polypeptide into the mature hormone structure able to bind its receptor appropriately.

Translation alone does not guarantee a functional protein. The insulin example is processing after translation, not alternative splicing of insulin exons.

Proteasomes break selected proteins into peptides, allowing amino acids to be recycled for new protein synthesis or metabolism.

Damaged or short-lived proteins are tagged and fed into a proteasome, where peptide bonds are hydrolysed. Released amino acids return to the cellular pool.

Trace: protein tag; proteasome entry; peptide breakdown; amino-acid reuse or catabolism.

A misfolded protein can be removed and its amino acids reused to build a different protein.

Proteasomal degradation is controlled recycling, not random digestion of every protein. Proteasomes do not recycle every protein immediately; tagging and recognition determine which substrates are selected.

A gene mutation is a structural change in the base sequence of DNA within a gene.

Changing 5′-ACT-3′ to 5′-AGT-3′ is a substitution; changing it to 5′-ACCT-3′ is an insertion.

A mutation is the DNA sequence change itself. Its effect on a codon, protein or phenotype is a possible consequence, not part of the definition.

A single-nucleotide polymorphism (SNP) results from a base substitution, but the substitution may or may not change one amino acid in a polypeptide.

An mRNA codon change from GAA to GAG is silent because both specify glutamate; a change to a stop codon can shorten the polypeptide.

A substitution does not automatically change protein function. First identify the new codon and its amino-acid or stop outcome.

Insertions and deletions are likely to stop a polypeptide functioning when they shift its reading frame or change a large section of its sequence.

Ribosomes read mRNA in triplets. Adding or removing a number of bases that is not a multiple of three regroups every downstream codon, often changing many amino acids and creating an early stop codon.

Not a multiple of three → frameshift and changed downstream codons. Multiple of three → no frameshift, but amino acids are added or removed. A major insertion or deletion can still disrupt structure and function even without a frameshift.

Deleting one base near the start of a coding sequence shifts the triplet grouping for most of the remaining mRNA and is therefore likely to produce a non-functional polypeptide.

A three-base insertion or deletion avoids a frameshift, but it is not automatically harmless because the added or missing amino acid may be important.

Gene mutations can result from errors in DNA replication or repair and from DNA damage caused by mutagens.

Damage is not yet a permanent mutation if accurate repair restores the original sequence. It becomes a mutation when the altered sequence remains and is copied.

A mutagen increases mutation probability; it does not produce the same mutation in every exposed cell.

Mutations occur randomly with respect to an organism's need: no known natural mechanism deliberately changes a particular base in order to create a useful trait.

A mutation can occur anywhere in the genome before its consequence is tested by the environment. Natural selection later changes variant frequencies because some carriers reproduce more successfully.

Random relative to need does not mean uniform probability. Base identity and sequence context, DNA repair, gene activity and mutagen exposure can make some sites or cells more likely to mutate than others.

Antibiotic exposure does not instruct bacteria to make a resistance mutation. A resistant variant may already exist, then increase in frequency when susceptible cells die.

Mutation bias can make some changes more frequent without making them purposeful or directed toward advantage.

The consequence of a mutation depends on whether it occurs in the germ line or in a somatic cell lineage.

A mutation in a sperm cell may be inherited by a child, whereas a mutation acquired in one skin cell can spread through a local clone but is not normally passed to offspring.

Somatic does not mean harmless: a non-inherited mutation can still cause cancer or other serious effects in the individual.

Gene mutation is the original source of new alleles and therefore of all genetic variation.

Most mutations are neutral or harmful to an individual, but a population needs heritable variants for natural selection to act on. Selection changes allele frequencies; it does not create the initial DNA differences.

Mutation creates a new allele → inheritance can place it in a population → environmental conditions affect reproductive success → natural selection can change its frequency over generations.

A new allele that improves drought survival may spread when carriers leave more offspring in dry conditions, while the same allele may provide no advantage in another environment.

Mutation alone is not adaptation. The variant must be heritable and influence reproductive success in the relevant environment.

Gene knockout investigates gene function by changing a chosen gene so that it becomes inoperative.

Researchers compare the knockout phenotype with an appropriate control. A consistent difference suggests that the disabled gene contributes to the affected process, while rescue or other controls strengthen the inference.

Libraries of knockout organisms are available for some research species, allowing scientists to study many genes systematically. Common model groups include mice, fruit flies, zebrafish and Arabidopsis plants.

If p53-knockout mice develop tumours and are then used to test anti-cancer treatments, the knockout model connects loss of the tumour-suppressor gene with the cancer phenotype.

Students do not need the technical steps used to create a knockout. A missing phenotype also does not prove that the gene has no function, because another gene may compensate.

CRISPR-Cas9 gene editing uses a guide RNA to direct the Cas9 enzyme to a complementary DNA target, where Cas9 cuts the DNA so the sequence can be changed.

Changing the guide RNA changes the target sequence. After cutting, gene editing can delete DNA, insert or replace a sequence, or disrupt the gene so that it no longer functions.

Choose a target → design a complementary guide RNA → guide RNA directs Cas9 → Cas9 cuts near the target → the DNA is edited → verify the sequence and biological result.

Successful-use example from an official IB paper: CRISPR-Cas9 repair of a mutated DMD gene restored expression of dystrophin. Depending on the mutation, editing can replace a changed codon, add DNA missing after a deletion, or remove DNA added by an insertion.

Restoring dystrophin expression demonstrates a successful molecular result; it does not by itself prove complete, safe clinical treatment. Delivery, unintended edits and regulation must still be considered.

Conserved gene sequences remain identical or similar across species; highly conserved sequences remain similar across long evolutionary periods.

If the same coding region is very similar in distantly related species, researchers can hypothesize that its amino-acid sequence is strongly constrained by the protein's function.

Conservation supports hypotheses; it does not by itself identify the exact function or distinguish functional constraint from a lower mutation rate. Further evidence is required.

In every living organism, cell division generates new cells when one parent cell divides to produce two daughter cells.

Producing daughter cells allows an organism to grow, replace worn cells, repair damaged tissue or reproduce asexually. The genetic material must be distributed before the cytoplasm separates so both daughters can function.

A skin cell can divide into two daughter cells that replace cells lost from the surface, while a single-celled organism can divide to produce two organisms.

The parent cell is sometimes called a mother cell, but this does not imply sex or fertilization. Cell division means generation of daughter cells, not growth of one cell alone.

Cytokinesis splits the cytoplasm of a parent cell between its daughter cells, but animal and plant cells achieve the split differently.

An animal cell pinches from its outer edge toward the centre, whereas a plant cell builds the separating plate from the centre outward.

Cytokinesis divides cytoplasm; mitosis or meiosis divides the nucleus. The events can overlap in time but are not the same process.

Cytokinesis is usually equal, but unequal cytokinesis can give daughter cells very different amounts of cytoplasm.

Whatever their final sizes, both daughters must receive at least one mitochondrion and any other organelle that can arise only by growth and division of a pre-existing organelle.

Unequal cytoplasm does not mean unequal nuclear DNA: chromosomes can still be segregated correctly before asymmetric cytokinesis.

Eukaryotic cells use mitosis to maintain chromosome number and genome, whereas meiosis halves chromosome number and generates genetic diversity.

Nuclear division must occur before cell division so each daughter receives a nucleus rather than becoming an anucleate cell.

Meiosis is not simply mitosis twice: homologous chromosomes pair and separate in its first division, creating the reduction in chromosome number.

DNA replication occurs before nuclear division so each daughter nucleus can receive a complete chromosome set.

During S phase, each chromosome becomes two sister chromatids joined at a centromere. Division then separates chromatids or homologues according to the process.

Trace: replication; duplicated chromosome; spindle attachment; chromosome separation; daughter nuclei.

If a cell entered mitosis without replicating DNA, one daughter could receive too little genetic material.

Replication doubles DNA amount, not chromosome number in the usual chromosome-counting convention.

Mitosis and meiosis both condense chromosomes and move them accurately between new nuclei.

Histones organize DNA into nucleosomes, and further supercoiling condenses the long chromatin fibres into compact chromosomes that can be moved without tangling.

Spindle microtubules attach to chromosomes, and microtubule motors plus microtubule shortening or growth generate directed movement. Alignment and separation differ between divisions, but the same general machinery organizes chromosome distribution.

In both mitosis and meiosis II, sister chromatids move toward opposite poles; in meiosis I, homologous chromosomes move apart while sister chromatids remain together.

Shared condensation and movement mechanisms do not make the outcomes identical: the chromosome partners separated and the number of divisions differ.

Mitosis proceeds through prophase, metaphase, anaphase and telophase, each solving a different chromosome-distribution problem.

Chromosomes condense and attach to spindle fibers, align at the equator, separate sister chromatids and re-form nuclei at opposite poles. Cytokinesis follows.

Use the sequence: condense; align; separate chromatids; rebuild nuclei.

A metaphase cell has chromosomes aligned at the equator; anaphase begins when sister chromatids move apart.

Interphase is not a mitosis stage, although it prepares the cell by growing and replicating DNA.

Identify a mitosis phase from the position and appearance of chromosomes, using several features rather than cell shape alone.

Two groups of V-shaped chromatids moving away from the equator indicate anaphase, whether seen in a diagram, a prepared root-tip cell or a micrograph.

A dark stain or rounded cell outline is not enough to identify a phase. Confirm chromosome condensation, alignment or separation and nuclear-envelope state.

Meiosis is a reduction division: one diploid nucleus undergoes two nuclear divisions after one DNA replication to produce four haploid nuclei.

Diploid nuclei contain two homologous chromosome sets; haploid nuclei contain one. Homologous chromosomes separate in meiosis I, halving the number of sets, and sister chromatids separate in meiosis II without another round of replication.

Diploid nucleus → DNA replication → homologous pairs segregate in meiosis I → two haploid nuclei with duplicated chromosomes → chromatids segregate in meiosis II → four haploid nuclei.

In a sexual life cycle, haploid gametes produced by meiosis fuse at fertilization, restoring the diploid chromosome number instead of doubling it in every generation.

Chromosome number is reduced in meiosis I, even though each chromosome still has two chromatids until meiosis II.

Nondisjunction is failure of homologous chromosomes or sister chromatids to separate, producing gametes with abnormal chromosome numbers.

If an extra chromosome enters a gamete, fertilization can create a zygote with three copies of one chromosome. The phenotype depends on chromosome and gene dosage.

Trace: division error; abnormal gamete; fertilization; chromosome count; developmental consequence.

A gamete containing two copies of chromosome 21 can combine with a normal gamete to produce trisomy 21.

Nondisjunction is a chromosome-segregation error, not a point mutation in one gene.

Meiosis generates genetic diversity through random orientation of bivalents and crossing over between non-sister chromatids.

At metaphase I, each bivalent can face either pole independently. The maternal and paternal homologues therefore segregate into many possible whole-chromosome combinations.

During prophase I, non-sister chromatids of homologous chromosomes exchange corresponding DNA at chiasmata, producing recombinant chromatids with new combinations of linked alleles.

One gamete can receive a maternal chromosome carrying a short paternal segment after crossing over, plus a different random mixture of the remaining maternal and paternal homologues.

Random fertilization adds further variation but is not a meiotic process. Meiosis does not direct combinations toward future advantage.

Cell proliferation is repeated passage through growth and mitosis that increases cell number for growth, routine replacement or tissue repair.

Proliferation increases cell number only when new cells are produced faster than cells are lost. Signals regulate which cells enter and continue through the cell cycle.

Cell growth increases the size or contents of one cell; cell proliferation increases the number of cells. Uncontrolled proliferation can disrupt tissue organization.

The cell cycle is the ordered sequence G1 → S → G2 → mitosis → cytokinesis that enables cell proliferation.

G1, S and G2 together form interphase; mitosis follows interphase, and cytokinesis completes production of separate daughter cells.

Interphase is not a stage of mitosis. Checkpoint control belongs to the later cyclin Objective; this card establishes the cycle's sequence.

Interphase is a metabolically active period in which a cell grows and biosynthesizes the components needed for division.

Proteins and other cell materials are synthesized, DNA is replicated in S phase, and cytoplasm increases so the future daughter cells inherit sufficient cellular contents.

Mitochondria—and chloroplasts in photosynthetic cells—increase in number by growth and division of pre-existing organelles. Their increase is therefore part of cell growth, not manufacture from nothing.

Before a plant cell divides, it can synthesize proteins, replicate nuclear DNA and increase its chloroplast and mitochondrial populations during interphase.

Interphase is not a resting period and does not include nuclear division; active biosynthesis and preparation make division possible.

Different cyclins rise and fall in concentration during the cell cycle, and a cell passes a checkpoint only when the relevant cyclin reaches its threshold level.

Cyclin synthesis raises concentration before a transition; cyclin breakdown lowers it afterwards. This repeating pattern coordinates each checkpoint with the correct stage of the cycle.

Cyclin below threshold → checkpoint is not passed. Cyclin reaches threshold → transition can be triggered. Cyclin is degraded → its signal falls before the next cycle stage.

A graph may show one cyclin accumulating before mitosis, crossing a threshold as mitosis begins, then falling rapidly after it is degraded.

The syllabus requires changing cyclin concentrations and threshold control, not names or detailed roles of specific cyclins.

Mutations can cause uncontrolled cell division by converting proto-oncogenes into oncogenes or by disabling tumour-suppressor genes.

A clone carrying one control mutation can continue dividing and acquire further mutations, progressively weakening the systems that restrain proliferation.

An activated oncogene can keep a growth pathway switched on while loss of a tumour suppressor allows damaged cells to pass a checkpoint; together they strongly favour tumour growth.

One mutation does not guarantee cancer. Several independent controls normally limit division, so tumour development often involves accumulated mutations.

Tumours differ in division and growth rate, invasion of neighbouring tissue and capacity for metastasis; these features determine whether they cause cancer.

Metastasis occurs when cells leave a primary tumour, travel through the body and establish one or more secondary tumours.

If 24 of 200 observed tumour cells are in mitosis, the mitotic index is 24 ÷ 200 = 0.12, or 12%. Under comparable sampling conditions, a higher index indicates a larger proportion actively dividing.

Tumour size alone does not establish malignancy. Invasion and metastasis distinguish malignant cancer, while mitotic index estimates division activity only for the sampled population and time.

Gene expression is the mechanism by which information in a gene affects phenotype, most commonly through production and function of a protein.

The DNA base sequence is transcribed into mRNA, the mRNA sequence is translated into a polypeptide, and the folded protein performs a function such as catalysing a reaction. That function contributes to the cell's traits.

Gene information → transcription → mRNA → translation → protein → cellular function → phenotype.

Expression of a gene for a digestive enzyme produces mRNA, then enzyme protein; the enzyme's catalytic activity contributes to the digestive phenotype of that cell.

Possessing a gene does not mean it is expressed in every cell. Regulation can change the amount of mRNA and protein without changing the DNA sequence.

Transcription factors regulate transcription by binding specific DNA sequences such as promoters and enhancers.

Different cell types contain different combinations or activities of transcription factors, so the same genome can produce different amounts of a gene's mRNA.

An activator bound to an enhancer can contact the promoter complex and increase how often RNA polymerase initiates transcription.

A transcription factor recognizes a particular DNA sequence; it does not bind every promoter or enhancer indiscriminately.

Regulating mRNA degradation controls how long an mRNA remains available for translation and therefore how much protein can be produced.

Human mRNAs can persist from minutes to days. Shortening of the protective 3′ poly-A tail helps initiate degradation, after which nucleases break down the transcript.

Longer mRNA lifetime → more opportunities for ribosomes to translate it. Faster degradation → fewer translation rounds and a shorter-lived protein-production response.

If two cells transcribe equal numbers of mRNA molecules but one transcript is degraded within minutes while the other persists for hours, the longer-lived transcript can usually support more translation.

mRNA abundance reflects both its rate of transcription and its rate of degradation; high transcription alone does not guarantee high protein output.

Epigenesis is the development of patterns of cell differentiation in a multicellular organism from an initially undifferentiated zygote.

Cells descended from the zygote usually contain the same DNA sequence, but different genes become active or repressed. Epigenetic changes such as DNA methylation or histone modification alter how genes are read without altering their base sequences.

One zygote → repeated cell division → different epigenetic states and gene-expression patterns → different proteins → specialized cell phenotypes.

A developing muscle cell activates genes for contractile proteins while maintaining repression of genes needed only in neurons, producing differentiation without changing genotype.

Epigenetic change can alter phenotype but not genotype because the nucleotide sequence remains unchanged. It is therefore not a gene mutation.

A cell's genome is its complete genetic information, its transcriptome is the set of mRNA transcripts present at a particular time, and its proteome is the set of proteins present at that time.

A liver cell and muscle cell usually share a genome but express different genes, so their transcriptomes and proteomes differ and support different functions.

No cell expresses all its genes. Detecting an mRNA also does not guarantee that its protein is abundant or functional.

Methyl groups added to promoter DNA or to histones act as epigenetic tags that alter transcription without changing DNA base sequence.

The tags change which regulatory proteins can associate with chromatin and how accessible the promoter is to transcription machinery.

Extensive methylation of promoter cytosines can make a gene less accessible, reducing its mRNA and protein production.

Do not apply one universal rule to all methylation: promoter DNA methylation is generally repressive, while histone methylation may activate or repress.

Epigenetic inheritance passes a change in gene expression to daughter cells or offspring without changing the nucleotide sequence of DNA.

If DNA methylation or histone modifications remain in place or are re-established during chromosome replication and division, the inherited chromatin state can keep a gene active or repressed.

During mitosis, persistent tags can transmit a cell-lineage expression pattern to daughter cells. If tags remain through gamete formation and meiosis, an expression state may influence offspring.

A repressive mark maintained through repeated mitoses can help daughter cells retain the same differentiated identity even though every cell division copies the same DNA sequence.

Epigenetic inheritance is inheritance of expression state, not a change in allele sequence, and most tags are not guaranteed to persist through gamete formation.

Environmental exposure can alter gene expression by changing epigenetic tags; air pollution can modify DNA and histone methylation patterns.

Pollutants reaching lung tissue can trigger cellular responses associated with altered methylation. Changed access to regulated genes can affect inflammatory pathways and may contribute to lung disease without changing the DNA sequence.

Air-pollution exposure → altered DNA or histone methylation → altered transcription pattern → changed cell response → possible contribution to inflammation or respiratory disease.

Comparing exposed and less-exposed lung-cell samples may reveal different methylation and expression patterns associated with inflammatory responses.

An association between pollution, methylation and disease does not by itself prove that one methylation change caused the disease; exposure, dose and alternative causes must be evaluated.

During human egg and sperm development, most epigenetic tags are removed, but retained imprints can make only the maternal or paternal copy of a gene active in offspring.

Resetting prevents most acquired expression states being passed between generations. For an imprinted gene, a retained parent-specific tag silences one allele, so phenotype depends on whether the active copy came from the mother or father.

The hybrid pattern supports a parent-of-origin epigenetic explanation, but the exact imprinted genes making the largest growth difference are not established in the approved local textbook.

Studies of monozygotic twins compare genetically similar individuals to estimate how environment and epigenetic differences contribute to traits.

If twins differ despite near-identical DNA, differing environments, developmental history or epigenetic states are possible explanations. Concordance and study design determine the strength of inference.

Evaluate a twin result by checking: shared genes; shared environment; age and exposure; trait concordance; alternative causes.

Twins may both inherit risk alleles but develop different symptoms after different exposures, suggesting environment modifies expression or phenotype.

Twin differences do not prove a purely environmental cause; measurement error and non-shared biology also matter.

External chemical signals can change gene expression in both eukaryotes and bacteria, but the regulatory mechanism depends on the cell type.

Both mechanisms connect an external condition to selective protein production, so the cell makes a response only when the signal or substrate is present.

The lac operon is an inducible bacterial system and is not the mechanism used by steroid hormones in eukaryotic cells. One biochemical example is sufficient for the syllabus scope.

Solvation occurs when polar water molecules surround and interact with dissolved ions or polar solute molecules.

Water's partially negative oxygen is attracted to positive ions, while its partially positive hydrogens are attracted to negative ions. Polar solutes can also form hydrogen bonds with water.

These attractions form hydration shells, separate solute particles and keep them dispersed. Water molecules engaged around solutes have less freedom of movement than in pure water.

When sodium chloride dissolves, oxygen ends of water face Na⁺ and hydrogen ends face Cl⁻, producing oriented hydration shells.

Water does not form hydrogen bonds with every solute: ion–dipole attraction hydrates ions, while hydrogen bonding requires suitable polar groups.

Across a partially permeable membrane, net water movement is from the less concentrated solution toward the more concentrated solution.

If solution A is 0.10 mol dm⁻³ sucrose and solution B is 0.40 mol dm⁻³, A is hypotonic to B and net water movement is from A to B if water can cross but sucrose cannot.

At SL, express the comparison using solute concentration—not 'high water concentration'. Tonicity is relative and depends on solutes that do not freely cross the membrane.

Osmosis is the net movement of water across a partially permeable membrane, and the cell's environment determines its direction.

In an isotonic environment, water molecules continue crossing in both directions at equal rates. This is dynamic equilibrium, not an absence of molecular movement.

A cell placed in hypertonic solution loses water and decreases in volume because more water leaves than enters.

Always state the solution relative to the cell. 'Hypotonic' or 'hypertonic' without a comparison has incomplete meaning.

Changes in plant-tissue mass or length across a series of sucrose concentrations can be used to estimate the isotonic concentration.

Prepare equal tissue pieces, record initial mass or length, incubate them for the same time in known sucrose solutions, blot consistently, record final values and calculate change.

Plot mean percentage change against sucrose concentration. The x-intercept, where change is 0%, estimates the isotonic concentration. Replicates allow standard deviation to compare spread and standard error/error bars to compare uncertainty in means.

Positive mass change at 0.20 mol dm⁻³ and negative change at 0.30 mol dm⁻³ place the isotonic estimate between those concentrations; interpolate from the fitted graph rather than choosing the nearest raw point.

Zero mean change estimates isotonic conditions; it does not mean water molecules stopped moving. Consistent blotting, initial size and incubation time are needed for a fair comparison.

Cells without a wall can burst in hypotonic solution or shrink in hypertonic solution because their plasma membrane cannot resist large volume changes.

Freshwater unicellular organisms continually gain water from their hypotonic environment, so contractile vacuoles collect and expel excess water. Multicellular animals instead maintain near-isotonic tissue fluid around cells.

A plasma membrane can deform but does not provide the rigid mechanical restraint of a cell wall; active water removal is an adaptation, not a reversal of osmosis.

A cell wall resists expansion when water enters, converting osmotic water uptake into turgor pressure.

The wall’s rigidity balances the inward tendency of water. If water leaves, pressure falls and the membrane can pull away from the wall, producing plasmolysis.

Distinguish wall restraint from membrane transport: water direction first, then pressure and shape.

A plant cell in dilute solution becomes turgid rather than bursting because the wall pushes back as the vacuole expands.

A wall prevents unlimited swelling but does not stop osmosis or guarantee that a severely dehydrated cell survives.

Medical fluids are made isotonic with body tissues to avoid harmful net water gain or loss by cells.

An isotonic saline infusion replaces extracellular fluid without causing appreciable net movement of water into or out of red blood cells.

Isotonic means matched effective osmotic concentration relative to the tissue; it does not mean the solution has the same chemical composition as cytoplasm.

Water potential (ψw) is the potential energy of water per unit volume and is usually measured in kilopascals (kPa).

Absolute potential energy cannot be measured, so water potentials are expressed relative to pure water at atmospheric pressure and 20°C, which is assigned ψw = 0 kPa.

Adding solute makes water potential negative relative to pure water. Applying positive pressure can raise water potential. The numerical value predicts which region's water can release more potential energy by moving.

A solution at −300 kPa has lower water potential than pure water at 0 kPa; water has a tendency to move from the pure water toward the solution if a pathway exists.

Water potential is not solute concentration alone and is not an absolute store of energy. State the reference conditions and units.

The direction of net water movement is set by total water potential, not by one component in isolation.

A higher Ψ means water has a greater tendency to leave. Comparing total values incorporates both solute and pressure effects and explains why a concentrated but pressurized cell may not lose water.

Write both total Ψ values, compare them numerically, then state the direction and the condition for equilibrium.

Water moves from −0.2 MPa to −0.5 MPa, but not from a region at −0.8 MPa to −0.5 MPa; the latter direction is reversed.

‘Higher’ means numerically less negative; movement stops when the relevant potentials are equal or other forces intervene.

In cells with walls, total water potential is the sum of solute potential and pressure potential.

If ψs = −600 kPa and ψp = +100 kPa, then ψw = −600 + 100 = −500 kPa. Water tends to enter from a neighbouring region at −400 kPa.

Keep signs and units through the calculation. Pressure potential is not always zero or positive: tension in functioning xylem gives a negative value.

When plant tissue is bathed in hypotonic or hypertonic solution, water movement changes both solute potential and pressure potential until equilibrium or plasmolysis is reached.

A cell initially at ψs = −600 kPa and ψp = +300 kPa has ψw = −300 kPa. In an external solution below −300 kPa, water leaves, turgor falls and the internal potentials change.

Do not describe only solute concentration: in a walled cell, increasing pressure potential can stop net entry even while the cell sap remains more concentrated than the outside solution.

Asexual reproduction uses one parent without gamete fusion and normally produces genetically identical offspring; sexual reproduction uses meiosis and fertilization to produce new allele combinations.

A strawberry runner produces a clone suited to the parent's current habitat, whereas a seed formed after fertilization carries a new allele combination.

Asexual offspring can still differ after mutation or environmental effects; 'clone' refers to their inherited genome, not guaranteed identical phenotype.

Meiosis halves chromosome number in gametes, and fusion of two gametes restores the diploid number in the zygote.

The alternation prevents chromosome number doubling every generation. Independent assortment and crossing over also create combinations before fusion adds another random combination.

Track chromosome number through meiosis; then track the fusion event and the first embryo cell.

A diploid human cell with 46 chromosomes produces gametes with 23; fusion returns the zygote to 46.

Meiosis does not simply make ‘smaller’ cells; its defining outcome is reduced chromosome number plus variation.

In anisogamous species, the male produces smaller motile gametes and the female produces larger nutrient-rich gametes.

The distinction is based on gamete type, not on every secondary trait or an individual’s identity. Different species organise reproductive roles around these gametes in different ways.

Identify the gametes first, then infer the biological sex category used in the syllabus model.

In humans, sperm are small and motile while ova are large and non-motile, so the model labels their producers male and female.

Gamete definitions do not justify assumptions about behaviour, gender, or all reproductive biology.

Human reproductive systems link gamete production, transport, fertilization, implantation and birth through specialized structures.

A labelled diagram must show position and connections as well as names. Fertilization normally occurs in an oviduct; implantation occurs later in the endometrium.

The menstrual cycle combines ovarian and uterine cycles controlled by FSH, LH, oestradiol and progesterone through negative and positive feedback.

A sharp LH peak follows the high-oestradiol positive-feedback switch and occurs just before ovulation.

Feedback direction changes with hormone concentration and cycle stage; oestradiol is not always a positive-feedback signal.

Human fertilization begins in the oviduct when sperm and egg cell membranes fuse and ends with paternal and maternal chromosomes sharing the first zygotic mitosis.

Sperm membrane fuses with egg membrane → sperm nucleus enters while its tail and mitochondria are destroyed → sperm and egg nuclear membranes dissolve → both condensed chromosome sets attach to one mitotic spindle → chromosomes segregate to form two diploid nuclei.

This sequence brings one haploid paternal and one haploid maternal chromosome set into a diploid zygote genome while preventing paternal sperm mitochondria becoming part of the embryo.

The 23 paternal and 23 maternal chromosomes participate together in the first mitosis, so each of the first two embryonic nuclei receives a diploid set.

Fertilization is not implantation: nuclear union begins in the oviduct, while attachment to the endometrium happens later.

IVF treatment temporarily takes control of normal reproductive hormone signalling so artificial hormone doses can induce superovulation.

Normal pituitary hormone secretion is first suppressed to prevent an uncontrolled ovulation. Carefully timed FSH-like stimulation matures several follicles, and an LH-like trigger completes egg maturation before collection.

Suppress normal cycle → stimulate multiple follicles → trigger maturation → collect oocytes → fertilize outside the body → culture embryo(s) → transfer selected embryo(s) to uterus.

Producing several mature oocytes in one controlled cycle gives more opportunities for fertilization and embryo selection than the usual release of one oocyte.

Superovulation increases the number of available oocytes but does not guarantee fertilization, implantation or live birth.

Flowering-plant reproduction is sexual because male and female gametes fuse, even when one hermaphroditic flower produces both pollen and ovules.

Male gametes develop inside pollen grains in anthers; female gametes develop inside ovules in the ovary. Pollination transfers pollen to a stigma, the pollen grain develops a tube, and male nuclei travel to the ovule for fertilization.

Fusion produces a diploid zygote that develops into an embryo; the ovule develops into a seed that contains the embryo.

Pollen carried by an insect reaches a compatible stigma, grows a tube down the style and delivers a male nucleus to the egg cell in an ovule.

Pollination is transfer, not fertilization. A hermaphroditic flower still reproduces sexually when gamete nuclei fuse.

An insect-pollinated flower attracts a pollinator and positions its reproductive structures so pollen is picked up and later deposited on a stigma.

As a bee reaches nectar, the flower's anthers brush pollen onto its body; a later visit places some pollen on another flower's stigma.

For a diagram, annotate each named structure with its function; colour alone does not establish insect pollination.

Plants promote cross-pollination by separating pollen and receptive female structures in time, space or among different plants, then using animals or wind as transfer vectors.

If pollen matures before the stigma of the same flower, pollen arriving later from another plant is more likely to fertilize its ovules.

Cross-pollination increases new gene combinations but is not guaranteed on every visit; self-incompatibility is a separate genetic recognition mechanism.