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Chapter 19 — Chapter 19 — Chromosome and DNA
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📘Easy summary
Chromosomes (Flemming 1882; named by Waldeyer-Hartz 1888, 'coloured bodies') are thread-like structures of DNA and histone proteins. The complete set is the karyotype; somatic cells have homologous pairs; gametes have half. In interphase they are uncoiled chromatin; before division DNA coils around histones and condenses into chromosomes. After duplication each chromosome has two identical sister chromatids joined at the centromere (a DNA sequence of about 220 nucleotides) bearing a disc-shaped protein kinetochore for spindle attachment. Some have a secondary constriction (nucleolar organizer region, NOR) that makes nucleoli; the knob-like end beyond it is the satellite. Terminal ends are telomeres, which stop chromosomes attaching end to end. Types by centromere: metacentric (centre), submetacentric (slightly away), acrocentric (near end), telocentric (at end).
⭐Key points
- Sister chromatids joined at centromere.
- Kinetochore: spindle attachment.
- Four types by centromere position.
🔬Diagram
🌿Real-life example
Human chromosome 21 is acrocentric — centromere near one end.
💡Exam tip
- Telomeres protect ends.
✅Quick check
1. Spindle fibres attach to the:
2. Centromere at the very end:
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