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.

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 protein for export is transcribed in the nucleus, translated on an rER-bound ribosome, moved in vesicles to the Golgi and packaged for its destination. A free ribosome instead makes many proteins used in the cytoplasm.

1. A transport vesicle buds from the rER and carries the protein to the cis face of the Golgi apparatus.
2. Golgi cisternae modify the protein, for example by adding carbohydrate groups, and sort it.
3. A secretory vesicle buds from the trans face, moves to the cell-surface membrane and fuses with it.
4. Exocytosis releases the enzyme outside the cell.

A pancreatic cell follows this route to produce digestive enzymes. The enzyme is packaged before release, so it does not digest cellular material while being transported.

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.

A TEM thin section shows internal ultrastructure. The same organelle can appear circular, elongated or partial depending on where and at what angle it was sectioned; use surrounding structures and the scale bar to test the identification.

A light microscope uses glass lenses and visible light, so living or stained cells can be viewed but fine ultrastructure is unresolved. Electron microscopes use electromagnets and electrons with a much shorter wavelength, giving higher resolution. TEM shows thin internal sections; SEM scans surfaces.

Many specimens have little natural contrast. Coloured stains absorb visible light in selected structures; heavy-metal stains scatter electrons in electron microscopy. Differential uptake makes boundaries or components distinguishable, but preparation can introduce artefacts.

Keep units consistent: $\text{magnification}=\dfrac{\text{image size}}{\text{actual size}}$. A scale bar remains valid if an image is resized, whereas a printed magnification label may not.

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.

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.

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.

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.

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.

1. The pollen tube grows down a chemical gradient and enters the ovule through the micropyle.
2. The tube releases the two male nuclei into the embryo sac.
3. One male nucleus fuses with the egg nucleus to form a diploid zygote.
4. The other fuses with the two polar nuclei to form the triploid primary endosperm nucleus, which develops into nutritive tissue.

The tube provides a protected route that does not require motile sperm to cross dry air. The two nuclear fusions coordinate embryo formation with a food supply.

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.

1. Cut the final few millimetres of a young root tip.
2. Treat it with warm dilute hydrochloric acid for the specified time, then rinse.
3. Place the meristem on a slide, add acetic orcein or another suitable chromosome stain and tease the tissue apart.
4. Lower a coverslip and press vertically through folded paper to spread one cell layer.
5. Start at low power, then identify stages at higher power in several fields of view.

Prophase shows condensing chromosomes; metaphase shows them at the equator; anaphase shows sister chromatids separating; telophase shows two chromosome groups forming nuclei.

Use roots of the same age/region, consistent acid and staining times, and systematic fields. Wear eye protection and handle acid, stain, glass and heating equipment safely.

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 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.

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.

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.

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.

The ABO blood-group gene has $I^A$, $I^B$ and $i$ alleles in the population. $I^A$ and $I^B$ are codominant, while each is dominant to $i$, producing four phenotype categories from several genotypes.

For a polygenic trait, many allele combinations make small additive contributions. Recombination and random fertilisation create numerous combinations, while factors such as nutrition or sunlight shift expression. The resulting values often form continuous variation, such as height or skin pigmentation.