Eukaryotic Chromosome Organization
High-Yield Summary
- Human genetic info is divided among 46 chromosomes. Chromatin structure is the multi-level packaging system that fits over 2 meters of DNA into a microscopic nucleus.
- A nucleosome = DNA wrapped around a histone octamer (2 copies each of H2A, H2B, H3, H4) — "beads on a string," beads = nucleosomes, string = linker DNA. Histone H1 stabilizes this and promotes further packing into chromatin fibers.
- Heterochromatin: tightly packed, dense, transcriptionally silent, dark under microscope. Euchromatin: loosely packed, open, transcriptionally active, light under microscope.
- Telomeres cap chromosome ends, high GC content (3 H-bonds, more stable) for protection; shorten each division (DNA polymerase can't fully replicate the lagging strand's end) — countered by telomerase, active in stem/germ cells and reactivated in cancer cells.
- Centromeres hold sister chromatids together after replication and are the spindle-fiber attachment site during anaphase; also high GC content for structural strength.
Key Terms
- Nucleosome
- DNA wrapped around a histone octamer (2× H2A, H2B, H3, H4) — the basic chromatin packaging unit.
- Linker DNA
- The DNA segment connecting adjacent nucleosomes (the "string" between "beads").
- Heterochromatin
- Densely packed, transcriptionally silent chromatin; appears dark under microscope.
- Euchromatin
- Loosely packed, transcriptionally active chromatin; appears light under microscope.
- Telomerase
- Enzyme that adds repeating sequences to telomeres, countering end-replication shortening.
- Centromere
- Chromosome region holding sister chromatids together; spindle-fiber attachment site.
Heterochromatin vs. Euchromatin
| Feature | Heterochromatin vs. Euchromatin |
|---|---|
| Packing | Tightly packed, dense / Loosely packed, open |
| Transcriptional activity | Silent (genes shut down) / Active (genes being transcribed) |
| Microscope appearance | Dark / Light |
DNA Packaging: Bare DNA → Chromosome
- 1DNA winds around a histone octamer (2× H2A, H2B, H3, H4) to form a nucleosome.
- 2Nucleosomes connected by linker DNA form the "beads on a string" arrangement.
- 3Histone H1 binds and stabilizes this structure, promoting further coiling into compact chromatin fibers.
- 4Continued coiling condenses chromatin into a visible chromosome, especially prominent during cell division.
Common MCAT Trap
- Telomere shortening happens because DNA polymerase can't fully replicate the very end of the lagging strand — not a random degradation process. This ties directly back to the leading/lagging strand mechanics.
- Telomerase being active in cancer cells isn't incidental — it's exactly why cancer cells can divide indefinitely without telomere-driven senescence.
- Both telomeres and centromeres rely on high GC content for stability/strength — the same G-C triple-hydrogen-bond logic from base pairing applies to both, for different structural jobs (protection vs. chromatid cohesion).
Quick Recall
What four histone proteins (and how many copies each) make up the nucleosome core?
Which chromatin state is transcriptionally active, and how does it appear under a microscope?
Why do telomeres shorten with each cell division?
What is the functional role of the centromere during anaphase?