MD

1042SCG Genetics and Evolutionary Biology – Module 1 Lecture 1

Fundamental Cell Biology Background

Prokaryotes vs Eukaryotes – Key Contrasts

  • DNA location:
    • Prokaryotes – region called nucleoid (no membrane)
    • Eukaryotes – membrane-bound nucleus.

  • Membranous organelles: absent in prokaryotes; present (ER, Golgi, mitochondria, vesicles, chloroplasts in plants) in eukaryotes.

  • Chromosomes:
    • Prokaryotes – usually one circular chromosome (+ plasmids); single origin of replication; divide by binary fission.
    • Eukaryotes – multiple linear chromosomes; multiple replication origins; divide by mitosis or meiosis.

  • Cell size: prokaryotes < 5\,\mu\text{m}; eukaryotes 10–100\,\mu\text{m}.

  • Additional structures present in both (to varying extents): plasma membrane, ribosomes, cell wall (prokaryotes, plants, fungi), cytoskeleton, glycocalyx, flagella (motility), cytosol, macromolecules (RNA, proteins, lipids, polysaccharides).

  • Unique extras: fimbrae, capsule in some bacteria; centriole-containing centrosomes in animal cells; potential multicellularity in many eukaryotes.

Prokaryotic Cell Diagram Take-aways

  • Fimbriae (adhesion), glycocalyx (capsule/slime layer) confer pathogenicity/immune evasion.

  • Single circular DNA anchored at plasma membrane; ribosomes free in cytosol.

Eukaryotic Cell Organelles (selected)

  • Rough ER (ribosome-studded) – site of secretory/membrane protein translation.

  • Smooth ER – lipid synthesis, detox.

  • Golgi – post-translational modification & sorting.

  • Lysosome – catabolism via hydrolytic enzymes.

  • Peroxisome – oxidative metabolism, detoxification.

  • Cytoskeleton (microtubules, intermediate filaments, actin microfilaments) – structure, transport, division.

Flow of Genetic Information (Central Dogma)

  • DNA (in nucleus) —transcription→ pre-mRNA —processing→ mature mRNA —export via nuclear pore→ cytoplasm —translation→ polypeptide on ribosomes.

  • Ribosome localisation determines destination:
    • Free ribosomes ➔ proteins remain in cytosol.
    • Rough ER-bound ribosomes ➔ secreted proteins, lysosomal enzymes, or membrane proteins.

Interactive Practice Q&A (Answer Keys)

  1. “Where is DNA transcribed to mRNA?” ➔ Nucleus.

  2. “Proteins destined for secretion are translated in …” ➔ Ribosomes on rough ER.

  3. “The nuclear membrane is impermeable: True/False?” ➔ False (selective nuclear pores allow traffic).

The Eukaryotic Cell Cycle

  • Purpose: reproduction, growth, tissue repair. All heritable information + organelles must duplicate.

  • Duration in typical human cell: \approx24\,\text{h} (Interphase \approx23\,\text{h}; Mitosis <1\,\text{h}).

Phases of Interphase

  1. G₁ (Gap 1) – Normal metabolism, protein synthesis, cell growth.

  2. S (Synthesis) – DNA replication begins at many origins; full genome copied.

  3. G₂ (Gap 2) – Further growth, duplication of centrosomes (two centrioles each), DNA repair.

Centrosome Definition & Role

  • Microtubule-organising centre; duplicates once per cycle; seeds mitotic spindle.

Mitosis – Detailed Breakdown (PPMAT)

Stage

Hallmark Events

Prophase

Chromosomes condense; nucleolus disappears; spindle begins to form from centrosomes.

Prometaphase

Nuclear envelope fragments; spindle microtubules attach to kinetochores on duplicated chromosomes.

Metaphase

Chromosomes align on metaphase plate; every chromatid attached to opposite poles via kinetochore MTs.

Anaphase

Cohesin proteins cleaved; sister chromatids pulled to opposite poles as spindle shortens.

Telophase

Chromatids now daughter chromosomes; decondense; two nuclear envelopes reform; nucleoli reappear.

Cytokinesis***

Actin microfilament ring contracts at former metaphase plate → cleavage furrow → two genetically identical daughter cells.

Mnemonic: PPMAT (Pro-, ProMeta-, Meta-, Ana-, Telo-).

Cytokinesis (Animal Cells)

  • Driven by actin + myosin sliding; membrane invagination forms cleavage furrow → physical separation.

Binary Fission (Prokaryotic Division) – Contrast Point

  1. DNA replication from single origin proceeds bi-directionally generating two circular chromosomes.

  2. Origins anchor to opposite poles; cell elongates.

  3. Septum & new cell wall form, partitioning daughters.

Chromosome Counting & Ploidy

  • Human somatic cells: 2n=46 (23 maternal + 23 paternal).

  • Homologous chromosome pair = same gene loci; may carry variant alleles.

  • Polyploidy (>2n) common in plants (e.g.
    triploid seedless watermelon) but rare in animals.

  • Example notables: social ant species have n=1 or 2; one butterfly species \sim450 chromosomes!

Visual Vocabulary

  • Centromere – constricted region joining sister chromatids.

  • Kinetochore – protein complex on centromere where spindle attaches.

  • Aster – microtubule array radial from centrosome in early mitosis.

Lecture Summary / Key Takeaways

  • Distinctions between prokaryotic & eukaryotic cellular architecture underpin genetic & evolutionary strategies.

  • Central Dogma links molecular genetics to phenotype; localisation (nucleus vs ER) matters.

  • Cell cycle orchestrates precise duplication and segregation, ensuring genomic integrity.

  • Mitosis (PPMAT) = equal nuclear division; cytokinesis finalises physical split.

  • Ploidy concept vital for understanding inheritance patterns & upcoming Meiosis lecture.

Looking Ahead

  • Next lecture: Meiosis & Mendelian Genetics – mechanisms creating genetic diversity and the quantitative patterns of inheritance.