W - BIOL200 - 4.6 CHROMATIN

Q: What is chromatin?

A: DNA packaged with proteins to fit in the nucleus.

Q: What helps DNA fit inside the nucleus?

A: It’s packed into chromosomes.

Q: What are histones?

A: Proteins that help package DNA.

Q: What is a nucleosome?

A: A unit of 147 bp DNA wrapped around a histone core (an octamer: 2 each of H2A, H2B, H3, H4).

Q: What’s the “beads on a string” model?

A: Chromatin looks like beads (nucleosomes) on a string (DNA helix).

Q: How is chromatin organized during interphase?

A: Into chromatin loops.

Q: What holds chromatin loops together?

A: Structural maintenance of chromosome (SMC) proteins.

Q: What does the SMC complex do?

Q: What does the SMC complex do?

Q: What does Chromosome Conformation Capture (3C) show?

A: Distant DNA regions can be close together in space because proteins hold them together.

Q: What are topological domains?

A: Folded chromatin domains (200 kb to 1.5 Mb long) that may form loops.

Q: What happens to chromatin during prophase and metaphase?

A: It condenses into chromatids.

Q: What is condensin?

A: An SMC protein complex that helps chromatin condense.

Q: What are histone modifications?

A: Post-translational changes to the C- and N-terminal tails of histone proteins.

Q: What are the main histone modifications?

A:

• Methylation on lysine (K) or arginine (R)

• Acetylation on lysine (K)

• Phosphorylation on serine (S) or threonine (T)

Q: Give examples of how histone mods affect chromatin.

A:

• H3K4me3 → decondenses chromatin → euchromatin

• H3K9me2 → condenses chromatin → heterochromatin

• Acetylation → euchromatin; deacetylation → heterochromatin

Q: Are histone modifications reversible?

A: Yes.

Q: What is epigenetics?

A: The study of gene expression changes without changing the DNA code.

Q: What is a positive feedback loop in chromatin?

A: Once condensation or de-condensation starts, it spreads by recruiting proteins (e.g. HP1 for methylation or bromodomain for acetylation).

Q: Which chromatin is transcriptionally active?

A: Euchromatin — open and decondensed.

Q: Which chromatin is silent?

A: Heterochromatin — tightly packed and condensed.

Q: What experiment shows euchromatin is active?

A: DNase cuts euchromatin but not heterochromatin.

• Erythroblast cells: chicken globin gene cut → chromatin decondensed.

• MSB cells: chicken globin gene protected → chromatin condensed.

Q: True statements about chromatin structure?

A:
✅ The nucleosome has a histone octamer.
✅ SMC proteins hold chromatin loops.
✅ Nucleosomes with a histone 3 variant interact with centromeres.

Q: Which species has the most DNA?

A: Tulips have ~10x more than humans; humans > chickens > flies > yeast.

Q: What does DNA amount correlate with?

A: Selective pressures, not organism complexity.

Q: How much of human DNA is non-coding?

A: ~45% is non-coding between genes, telomeres, centromeres.

Q: What are the two types of repetitive DNA?

A:
1.

Simple sequence DNA or satellite DNA — short perfect repeats.
2.

Interspersed repeats — long repeats and transposons.

Q: Give examples of satellite DNA.

A:

• Micro-satellite: 1–13 bp, ≤150 repeats (e.g. Huntington’s)

• Mini-satellite: 14–100 bp, DNA fingerprinting

• Satellite DNA: 14–500 bp, up to 20–100 kb, near centromeres/telomeres.

Q: What is intergenic DNA?

A: ~45% of our DNA lies between genes; includes enhancers.