Chromosomes and DNA (Chapter 20)

Examination Structure

  • Total course encompasses multiple chapters; detailed marks allocation provided for each topic.

  • Marks distribution for final examination (selected highlights):

    • Homeostasis: MCQs 5 (Total 2 Marks), CRQ 1 (Marks 3), ERQ choose ONE from TWO (Marks 7); Aggregate 22 marks.

    • Support & Movement: Same pattern; Aggregate 21 marks.

    • Co-ordination & Control: Same pattern; Aggregate 21 marks.

    • Variation & Genetics: MCQs 5; remaining as above; Aggregate 22 marks.

    • Reproduction, Growth & Development, Chromosomes & DNA, Cell Cycle each carry smaller CRQ-only weightings (mostly 2–3 marks each).

    • Biotechnology, Evolution, Ecosystem, Major Ecosystems, Man & His Environment collectively add 50 marks to reach grand total 85.

  • Examination year referenced: 2024.

Mid-Term Chapters (Testable)

  • 1. Chromosomes & DNA

  • 2. Cell Cycle

  • 3. Homeostasis

  • 4. Support & Movement

  • 5. Co-ordination & Control

  • 6. Reproduction

  • 7. Growth & Development

Chapter 20 – Chromosomes and DNA

Student Learning Outcomes (SLOs)

  • 20.1.1 Define chromosomes with examples of organisms possessing different chromosome numbers.

  • 20.1.2 Differentiate among:

    • Autosomes vs. Sex chromosomes

    • Homologous vs. Non-homologous chromosomes

    • Telocentric, Acrocentric, Metacentric & Sub-metacentric chromosomes.

  • 20.1.3 Describe levels of eukaryotic chromosomal organisation.

  • 20.1.4 Describe a chromosome karyotype.

  • 20.1.5 Differentiate between heterochromatin and euchromatin.

Definition, Discovery & Nomenclature

  • Chromosomes = thread-like bodies that appear during cell division and carry hereditary information in the form of genes.

  • Discovered by German embryologist Walther Flemming in 1882 while observing rapidly dividing salamander larval cells.

  • Term derived from Greek: “chromo” (colour) + “soma” (body) – they are actually colourless but stain darkly when treated with Perkin’s aniline dye (source of initial misconception).

Chromosome Numbers Across Species

  • Chromosome count is species-specific (diploid numbers shown):

    • Penicillium: 2

    • Mosquito: 6

    • Drosophila: 8

    • Garden pea: 14

    • Frog: 26

    • Honey bee: 32

    • Mouse: 40

    • Human: 46 (i.e., 23 pairs)

    • Sugar-cane: 80

    • Ferns: up to 1000 chromosomes!

Macro-Structure of a Metaphase Chromosome

  • Consists of two identical sister chromatids joined at a common point called the centromere.

  • Kinetochore – disc-shaped protein complex within the centromere; attachment site for spindle microtubules.

  • Arms: short p-arm and long q-arm (p for “petit”).

  • Terminal regions = telomeres (satellites); composed primarily of non-coding “junk” DNA protecting coding regions.

Quick-reference parts

  • Banding patterns (light/dark) useful for cytogenetic mapping.

  • Chromatids + centromere together considered a single chromosome until anaphase.

Types of Chromosomes

A. Autosomes vs Sex Chromosomes

  • Autosomes

    • Determine somatic (non-sex) traits.

    • Humans possess 22 homologous pairs (numbered 1–22).

    • Identical in both sexes.

  • Sex chromosomes

    • Determine gender & secondary sexual traits.

    • Humans: Female XX (homologous); Male XY (non-homologous).

    • Other labeling systems in animals & plants exist (e.g., ZZ/ZW in birds).

B. Homologous vs Non-homologous

  • Homologous chromosomes

    • One maternal + one paternal member of same pair.

    • Same gene sequence; alleles occupy identical loci; similar centromere position & arm length.

  • Non-homologous chromosomes

    • Belong to different pairs; gene content & structure differ.

    • Example: Human X and Y chromosomes are non-homologous except for small pseudo-autosomal regions.

C. Based on Centromere Position

  1. Telocentric

    • Centromere at terminal end ⇒ single visible arm (I-shaped).

    • Not present in human karyotype.

  2. Acrocentric

    • Centromere very near one end ⇒ one very long & one very short arm (J-shaped).

    • Human Y chromosome is acrocentric.

  3. Sub-metacentric

    • Centromere slightly off-center ⇒ unequal arms (L-shaped).

    • Human X chromosome is sub-metacentric.

  4. Metacentric

    • Centromere centrally located ⇒ arms almost equal (V-shaped).

D. Centromere Count & Other Constrictions

  • Monocentric – normal; one centromere.

  • Dicentric / Polycentric – two or more centromeres; segregation problems during mitosis/meiosis.

  • Acentric – no centromere; usually lost during division.

  • Secondary constrictions (NORs) – regions other than primary centromere where the chromosome narrows; contain nucleolar organiser genes for rRNA synthesis.

Eukaryotic Chromosomal Organisation (SLO 20.1.3)

Chemical Composition (average)

  • DNA ≈ 40\%

  • Proteins (mainly histones) ≈ 60\%

  • Small amount of RNA present.

DNA Fundamentals

  • DNA is a polymer of nucleotides; each nucleotide = nitrogenous base + deoxyribose (pentose sugar) + phosphate group.

  • Four bases: Adenine, Guanine, Cytosine, Thymine.

Hierarchical Packing (“Billion-to-Micron Compression”)

  1. Naked DNA double helix – diameter 2\,\text{nm}; length per human cell ≈ 2\,\text{m} (≃ 7 ft).

  2. Nucleosome String (“Beads-on-a-String”)

    • DNA wraps \approx1.65 turns around an octamer of histone proteins (two each of \text{H2A, H2B, H3, H4}) giving particles 11\,\text{nm} in diameter.

    • Adjacent nucleosomes joined by linker DNA; histone \text{H1} binds externally to clamp structure.

  3. 30-nm Chromatin Fibre (Solenoid/Zig-zag)

    • Nucleosome string coils into a thicker fibre, regulated by histone tail interactions.

  4. 300-nm Looped Domains (Super-coil)

    • Fibre further loops & attaches to a non-histone protein scaffold during prophase.

  5. 700-nm Chromatid

    • Tight helical coiling of looped domains forms each chromatid.

  6. 1400-nm Metaphase Chromosome

    • Two chromatids join; most condensed state, visible under light microscope.

Visual Dimension Scale

2\,\text{nm} \rightarrow 11\,\text{nm} \rightarrow 30\,\text{nm} \rightarrow 300\,\text{nm} \rightarrow 700\,\text{nm} \rightarrow 1400\,\text{nm}

Active vs Inactive Regions

  • Euchromatin – loosely packed; transcriptionally active (“naked” DNA more accessible).

  • Heterochromatin – densely packed; transcriptionally silent; highly condensed during interphase.

Karyotype & Cytogenetic Applications (SLO 20.1.4)

  • Karyotype = ordered visual profile of paired chromosomes (by size, centromere location, band pattern).

  • Used for diagnosing aneuploidies (e.g., Down’s syndrome \rightarrow 47,\,+21), sex determination, evolutionary studies.

Telomere Significance (Wrap-up)

  • Contain repetitive non-coding sequences (e.g., \text{TTAGGG}_n in humans).

  • Protect coding DNA from exonuclease degradation & replication-end problem.

  • Progressive shortening ↔ cellular ageing; enzyme telomerase extends repeats in germline & cancer cells (ethical & therapeutic research focus).

Ethical, Philosophical & Practical Implications

  • Gene-centric view of heredity stems from chromosome theory; underpins modern genetics, biotechnology & personalised medicine.

  • Chromosomal aberrations raise bioethical debates on prenatal testing, gene editing (CRISPR), and population screening.

  • Rapid progress fuels discussions about "playing God" versus alleviating human suffering through genomic interventions.

Quick-Check Revision Cards

  • Chromosome discovered: Flemming, 1882.

  • Human diploid number: 46 (23 pairs).

  • Histone types: H1, H2A, H2B, H3, H4.

  • Most condensed mitotic structure: 1400\,\text{nm} wide metaphase chromosome.

  • Only acrocentric human sex chromosome: Y.

  • Absent chromosome type in humans: Telocentric.

(End of structured study notes)