Topic 3 - Cell Structure, Reproduction and Development

Cell theory states that all living organisms consist of one or more cells, cells are the basic functional units of life, and new cells arise from pre-existing cells.

All cells share a cell-surface membrane, cytoplasm, DNA and ribosomes. The rest of the ultrastructure differs: eukaryotes have membrane-bound organelles, while prokaryotes lack internal membrane compartments and have smaller ribosomes.

A bacterium and a plant cell look very different, but both contain DNA, ribosomes, cytoplasm and a boundary membrane. Those shared features support the common-cell principle; they do not imply identical cell functions.

Cell theory is a unifying model, not a claim that every cell has the same organelles. Viruses are not cells and therefore are not explained as living cellular organisms by this theory.

In a multicellular organism, specialised cells of the same type work together as a tissue; tissues combine into organs, and organs cooperate in organ systems.

Specialisation makes division of labour possible: a red blood cell transports oxygen, a muscle cell contracts, and an epithelial cell absorbs. Their organisation lets a larger structure perform a function no single cell performs as effectively.

Cardiac muscle, blood-vessel and connective tissues combine to form the heart. The heart then works with blood vessels as part of the circulatory system, so the labels describe nested levels rather than interchangeable names.

A tissue is more than a pile of similar cells: the cells must cooperate in a shared function. A single specialised cell is not itself an organ or an organ system.

Eukaryotic cells contain membrane-bound organelles whose structures support distinct functions. The nucleus stores chromatin, mitochondria make ATP in aerobic respiration, ribosomes translate mRNA, and the endoplasmic reticulum, Golgi, lysosomes and centrioles have different specialised roles.

The rough ER carries ribosomes and processes proteins; smooth ER lacks ribosomes and is involved in lipid and steroid production. The Golgi modifies and packages cargo, lysosomes digest material, and centrioles organise spindle fibres in animal-cell division.

A secreted protein follows a route from nucleus (transcription) to ribosome/RER (translation and folding), then in vesicles to the Golgi for modification and onward to the cell surface. A free ribosome instead makes a protein that remains in the cytoplasm.

Ribosomes are not membrane-bound, and plant and animal cells are both eukaryotic. Do not identify an organelle from a label alone; connect its visible structure to its function.

Ribosomes on rough endoplasmic reticulum make proteins that are destined for secretion, a cell membrane or another membrane-bound compartment. The RER folds and processes the new polypeptide in its lumen.

Transport vesicles carry the protein from the RER to the Golgi apparatus. The Golgi modifies and sorts it, then sends it in vesicles to its destination: outside the cell, into a lysosome or to another membrane system.

Insulin is a useful model of the pathway: a ribosome makes the polypeptide, the RER and Golgi process and package it, and a vesicle fuses with the cell-surface membrane to release it by exocytosis.

Free cytoplasmic ribosomes generally make proteins used inside the cytoplasm. The RER is not the same as the Golgi: one is the ribosome-studded production/folding site, the other the modification and sorting centre.

Prokaryotic cells are smaller cells without a nucleus or membrane-bound organelles. Their circular bacterial DNA lies in the cytoplasm, their ribosomes are 70S, and their cell wall contains murein (peptidoglycan).

Some prokaryotes also have plasmids, a capsule, flagella or pili. Plasmids are small DNA loops; a capsule can protect the cell; flagella support movement; pili can help attachment or DNA transfer. These structures are not present in every bacterium.

A plasmid can carry a resistance gene independently of the main circular chromosome. That explains why a bacterium may acquire a useful trait without changing the definition of its chromosome.

“Prokaryote” does not mean “no DNA” or “no membrane”: it has a cell-surface membrane and genetic material, but no membrane-bound nucleus. Size alone is not a structural comparison.

Electron microscopes resolve much smaller distances than light microscopes because electrons have a shorter wavelength. A TEM sends electrons through a thin specimen to show high-resolution internal structure; an SEM scans the surface to produce a three-dimensional view.

First decide whether the image is a 2D internal section or a 3D surface. Then use distinctive features—such as a nucleus, double-membraned mitochondrion with cristae, or stacked Golgi sacs—to propose an organelle, and check that the scale and context fit.

A high-resolution 2D image showing internal cristae is consistent with a TEM mitochondrion; a raised surface image is more consistent with an SEM. The image is a dead-specimen snapshot, not a movie of a living cell.

Magnification does not create information that resolution cannot support. Do not call every dark oval a nucleus or infer function from appearance without the structural evidence.

A light microscope focuses light through a thin specimen; staining increases contrast when transparent cell structures would otherwise blend together. Electron microscopes use electrons with a shorter wavelength and therefore resolve finer detail.

Prepare a thin, supported specimen, add a suitable stain and begin with the low-power objective. Stain choice depends on the material and question: methylene blue highlights animal nuclei, iodine reveals starch, and differential staining can separate tissue types.

A clear onion or animal-cell sample may show almost no boundaries before staining, but nuclei become distinct after a suitable dye. An electron micrograph can show organelles that a light microscope cannot resolve.

Magnification makes an image larger; resolution is the ability to distinguish nearby points. More magnification cannot recover detail that the microscope never resolved, and a stain is not a natural colour of every structure.

For the animal-cell practical, make a thin, well-prepared slide, observe it systematically with a light microscope, and record only structures that are actually visible. Use a calibrated eyepiece graticule when a scale measurement is required.

Start on low power to locate the specimen safely, then increase magnification for cellular detail. Calibrate the graticule against a stage micrometer at the objective being used; the calibration changes when magnification changes. Make a clear, proportional labelled drawing without shading or invented structures.

If 40 graticule divisions equal 10 μm, one division is 0.25 μm. An object spanning five divisions is therefore 1.25 μm, provided the same objective calibration is still in use.

Image size, actual size and magnification must use compatible units. A plan drawing shows tissue arrangement at low power; a cell drawing shows detail at higher power. Neither replaces the observation itself.

A locus is the position of a gene on a chromosome. Different alleles occupy the same locus, but genes at different loci may be linked if they lie on the same chromosome and tend to travel together through meiosis.

Linked genes do not assort independently in the simple way that genes on different chromosomes do. Crossing over can separate linked alleles, so offspring may show parental combinations more often than recombinant combinations.

If two genes are close on one autosome, gametes usually retain the parental allele combinations; a crossover between their loci creates a less frequent recombinant combination.

Linkage is about physical chromosome location, not about two traits looking similar. Sex-linked genes require the X/Y context, while autosomal linkage concerns non-sex chromosomes.

Meiosis reduces a diploid nucleus to four haploid nuclei through two divisions. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II, so chromosome number halves in the first division.

Variation arises because homologous pairs align independently at metaphase I and non-sister chromatids exchange corresponding DNA at crossing-over in prophase I. Each gamete therefore receives a different combination of parental alleles.

For a species with haploid number n, independent assortment alone permits 2ⁿ chromosome combinations. In humans that is 2²³ possible combinations before crossing-over and fertilisation add further variation.

Crossing-over swaps segments between non-sister chromatids of homologous chromosomes; it is not the same as sister-chromatid separation. Meiosis produces haploid gametes, not two identical diploid cells.

Mammalian gametes are haploid cells produced by meiosis. Their structures are specialised for meeting, penetrating and supporting the next stage of reproduction.

Sperm have a flagellum for movement, many mitochondria to supply ATP and an acrosome containing enzymes that digest the zona pellucida. An egg is much larger, contains nutrient reserves, and has a zona pellucida that can become a barrier after fertilisation.

The acrosome helps one sperm reach the egg membrane; once fertilisation begins, changes to the egg’s zona pellucida help prevent additional sperm nuclei entering.

The egg’s size is not simply “for movement” and the sperm’s acrosome is not a nucleus. A gamete is haploid; the diploid chromosome number is restored only when nuclei fuse.

Fertilisation is the fusion of sperm and egg nuclei to form a diploid zygote. It begins when a sperm reaches the egg in the oviduct and its acrosome releases enzymes that digest a path through the zona pellucida.

After the sperm crosses the egg membrane, cortical granules release their contents. The zona pellucida hardens, producing the cortical reaction that blocks other sperm. The sperm and egg nuclei then fuse, restoring the full chromosome number; the zygote divides by mitosis.

The acrosome reaction permits entry, whereas the cortical reaction prevents polyspermy. They occur at different stages and solve opposite problems: access first, exclusion second.

Fertilisation is nuclear fusion, not merely sperm contact with the egg. The first divisions of the zygote are mitotic, so they increase cell number without halving chromosome number.

In flowering plants, fertilisation follows pollination: a pollen grain germinates on the stigma and grows a pollen tube down the style to an ovule. Male nuclei travel through the tube and one fuses with the egg nucleus.

The tube grows towards the ovary, enters the ovule through the micropyle and releases the male nuclei. Fusion produces a diploid zygote; the sequence matters because pollen-tube growth delivers the nucleus to the egg rather than fertilising on the stigma.

A pollen grain can germinate only when the stigma is compatible. The tube then provides a continuous route from the pollen grain to the embryo sac, where nuclear fusion occurs.

Pollination is transfer of pollen; fertilisation is fusion of nuclei. Do not treat pollen landing on a stigma as completion of fertilisation.

The cell cycle is the regulated sequence from one cell division to the next: G1 growth and normal work, S-phase DNA replication, G2 preparation and checking, M-phase nuclear division, then cytokinesis.

After S phase each chromosome has two sister chromatids. Mitosis separates those chromatids into two genetically identical nuclei, and cytokinesis partitions the cytoplasm so each daughter cell receives one nucleus.

A root-tip cell can spend much of its cycle in interphase, then pass through prophase, metaphase, anaphase and telophase before the plant cell forms a new wall between daughters.

Interphase includes G1, S and G2; it is not a single resting stage. Cytokinesis follows nuclear division and is not one of the four named stages of mitosis.

Root-tip meristems contain actively dividing cells, so a stained squash can show the stages of mitosis under a light microscope. Acid softens the tissue, stain makes chromosomes visible, and gentle squashing spreads cells into a thin layer.

Cut a root tip, treat and rinse it, stain the tissue, squash it carefully on a slide and observe with suitable magnification. Identify condensed chromosomes and classify cells by the visible stage rather than by where you expect them to be.

A metaphase cell has chromosomes aligned at the equator; an anaphase cell has sister chromatids moving apart. Counting every stage with visible chromosomes allows a mitotic index to be calculated.

The preparation is a snapshot of a fixed sample: it cannot show a living cell progressing through time. Do not count an unclear nucleus as a stage or damage the specimen by pressing too hard.

Mitotic index measures the proportion of cells in a sample that are undergoing mitosis: number of cells with visible chromosomes divided by the total number of cells counted. Multiply by 100 for a percentage.

Count cells systematically, include prophase, metaphase, anaphase and telophase in the numerator, and include every counted cell in the denominator. Keep the unit as a proportion or state clearly that it has been converted to a percentage.

If 32 of 42 cells show visible chromosomes, the mitotic index is 32/42 = 0.76, or 76% when expressed as a percentage.

A high index means more cells were observed in mitosis in that sample; it does not automatically prove faster growth. Sampling region, preparation quality and counting decisions affect the estimate.

A stem cell can divide by mitosis and produce cells that remain stem cells or differentiate. Totipotent cells can form embryonic and extra-embryonic tissues; pluripotent embryonic cells can form embryo cell types; adult stem cells are usually more limited (multipotent).

The morula is an early embryo with highly potent cells; the blastocyst contains an inner cell mass used as an embryonic stem-cell source. Adult tissue stem cells can replace particular cell types, such as blood or intestinal epithelium.

A bone-marrow transplant uses adult stem cells to restore blood-cell production. Embryonic cells could offer a wider range of targets, but their use raises consent, embryo status, regulation and immune-compatibility questions.

“More potent” does not mean automatically safer or better. Medical decisions weigh cell source, differentiation control, rejection risk, evidence and ethical viewpoints rather than potency alone.

Cells in one organism usually contain the same genes, but they become specialised because different genes are switched on or off. Active genes are transcribed into mRNA, translated into proteins, and those proteins alter cell structure or processes.

A transcription factor can increase or reduce RNA-polymerase access to a gene. Changing which proteins are produced therefore changes a cell’s properties without changing its entire DNA sequence.

A muscle precursor and a nerve precursor can share the same genome yet express different sets of proteins, giving them different structures and jobs. Environmental signals can alter transcription-factor activity and gene expression.

Differentiation is not caused by cells losing all unused genes. Epigenetic marks can change how DNA is read without changing the base sequence; that differs from a mutation.

Eukaryotic pre-mRNA contains coding exons and non-coding introns. Post-transcriptional splicing removes introns and joins exons before the mature mRNA leaves the nucleus.

If the exons are joined in different combinations, alternative splicing produces different mature mRNAs from one gene. Translation of those mRNAs can therefore produce different polypeptides.

The same antibody gene can be spliced to include or omit an exon encoding a membrane-binding region. The resulting protein is either cell-bound or secreted, even though the original gene is the same.

Alternative splicing changes the RNA message after transcription; it does not alter the DNA base sequence. Do not confuse intron removal with DNA replication or mutation.

An organism’s phenotype results from the interaction between its genotype and its environment. Epigenetic modifications can change which genes are expressed without changing the DNA sequence itself.

DNA methylation can reduce transcription, while histone acetylation loosens chromatin so transcription factors and RNA polymerase can access genes. These changes alter mRNA and protein production, and some can persist through cell division.

Genetically similar plants can differ in height when water or mineral supply differs. A lifestyle or environmental signal can also change gene expression through epigenetic tags, but that is not the same as creating a new allele.

An environmentally changed phenotype is not automatically inherited. Distinguish a reversible epigenetic state from a mutation in the DNA sequence, and state which evidence supports inheritance across cell division.

Some phenotypes depend on several genes or several alleles as well as the environment. Small additive effects from many loci produce a range of values rather than a few discrete categories.

A single-gene trait such as ABO blood group gives discontinuous categories. A polygenic trait such as height or mass is usually continuously distributed because different allele combinations add together and environmental conditions shift the result.

If two genes each contribute a small height effect, an individual carrying more contributing alleles can be taller than one carrying fewer; nutrition and water availability can still move either genotype away from its genetic potential.

Do not assume every gene contributes equally or that a continuous phenotype is purely genetic. “Polygenic” describes many genetic contributors; it does not remove environmental influence.