Cell structure, reproduction and development
Meiosis and variation
Meiosis makes four genetically different haploid cells from one diploid cell, by two divisions. Meiosis I separates homologous chromosomes (pairs carrying the same genes): in prophase I homologues pair as bivalents and cross over at chiasmata, swapping sections of chromatids; in metaphase I bivalents line up randomly (independent assortment); in anaphase I homologues separate. Meiosis II separates sister chromatids, like mitosis. Variation comes from crossing over, independent assortment ( combinations) and random fertilisation. Non-disjunction (failure to separate) gives gametes with an extra or missing chromosome, e.g. Down syndrome (trisomy 21). Meiosis halves the chromosome number so fertilisation restores diploidy.
≈ 50 minEukaryotic and prokaryotic cell structure
Eukaryotic cells have a nucleus (double envelope with pores, chromatin, nucleolus that makes ribosomes) and membrane-bound organelles: rough ER (ribosomes; makes and transports proteins), smooth ER (lipids, steroids), Golgi body (modifies, packages proteins into vesicles), mitochondria (aerobic respiration, ATP; folded cristae), lysosomes (digestive enzymes), 80S ribosomes, centrioles and microtubules. Plant cells also have a cellulose cell wall, chloroplasts and a large vacuole. Prokaryotic cells are smaller, with no nucleus (circular DNA, plasmids), 70S ribosomes, a peptidoglycan (murein) wall and sometimes a capsule and flagellum. Viruses are non-living: nucleic acid in a protein capsid, sometimes an envelope.
≈ 45 minGene expression and stem cells
Every cell has the same genes, but different cells express different ones. Stem cells are undifferentiated: totipotent (any cell, including placenta; early embryo), pluripotent (any body cell; embryonic stem cells), multipotent (a range, e.g. bone marrow → blood cells), unipotent (one type). Induced pluripotent stem (iPS) cells are made by adding transcription factor genes to adult cells. Transcription factors bind promoters to start or stop transcription; oestrogen enters cells and binds a receptor, releasing a transcription factor. RNA interference (siRNA) destroys specific mRNA, blocking translation. In prokaryotes the lac operon switches on lactose-digesting genes only when lactose is present. Epigenetics: heritable changes without changing base sequence: DNA methylation (silences genes) and histone acetylation (opens chromatin, activates). Cancer: mutated oncogenes (overactive) or tumour suppressor genes (inactive), sometimes by abnormal methylation.
≈ 50 minMicroscopy and magnification
Magnification ; convert to the same unit first (1 mm = 1000 µm; 1 µm = 1000 nm). Resolution is the ability to distinguish two close points: light microscopes resolve about 200 nm; electron microscopes about 0.5 nm because electrons have a much shorter wavelength. TEM gives 2D sections with internal detail; SEM gives 3D surface images; both need a vacuum, so specimens are dead. An eyepiece graticule is calibrated against a stage micrometer (divisions of known size, often 10 µm) at each objective. Cell fractionation: homogenise in cold, isotonic, buffered solution, then ultracentrifuge: nuclei sediment first, then mitochondria, then ribosomes.
≈ 40 minMitosis and the cell cycle
The cell cycle: interphase (G1 growth and protein synthesis; S phase DNA replication; G2 growth and checks), mitosis (nuclear division) and cytokinesis. In mitosis: prophase (chromosomes condense into two sister chromatids joined at the centromere; nuclear envelope breaks down; spindle forms), metaphase (chromosomes line up at the equator, attached to spindle fibres), anaphase (centromeres divide; chromatids pulled to opposite poles), telophase (nuclear envelopes reform). Result: two genetically identical diploid cells, for growth, repair and asexual reproduction. Mitotic index = cells in mitosis ÷ total cells. Telomeres protect chromosome ends; stem cells keep dividing; cancer results from mutations in genes controlling the cycle.
≈ 45 min