Updated: 6/10/2020

Chromatin Structure

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Overview 
  •  Composition 
    • DNA
      • - charge
    • histone proteins
      • + charge from Lys and Arg residues
      • H2A, H2B, H3, H4 (core proteins)
      • H1 (linking protein)
      • post-translational modification of histone tails
        • forms a code
        • interpreted by effector proteins to regulate transcription downstream
        • histone acetyltransferase (HAT)
          • acetyl group added which blocks positive charge of histone protein and loosens interaction with DNA
          • ↑ transcription
        • histone deacetylase (HDAC)
          • removes acetyl group which exposes positive charge and tightens interactions
          • ↓ transcription
    • electrostatic attraction of DNA with histone proteins
  • Organization
    • 10 nm chromatin
      • DNA wraps around dimer of H2A:H2B:H3:H4
      • called nucleosome
      • sensitive to nuclease activity
    • 30 nm chromatin
      • nucleosomes held together by H1
      • not sensitive to nuclease activity
    • 30 nm fiber loops
      • further condensation
  • Euchromatin/heterochromatin 
    • euchromatin = accessible to transcription
      • 10 nm through 30 nm fiber loops
    • heterochromatin = not accessible to transcription
      • any greater condensation than 30 nm fiber loops
      • condensed to save room 

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Questions (2)

(M1.BC.17.4708) A 62-year-old man with small cell lung cancer undergoes radiation therapy. His oncologist explains that radiation causes DNA damage and double-strand breaks, and this damage stops the cancer cells from growing because they can no longer replicate their DNA. One key mediator of this process is a cell cycle regulator called P53, which is upregulated after DNA damage and helps to trigger cell cycle arrest and apoptosis. One mechanism by which P53 activity is increased is a certain chromatin modification that loosens DNA coiling allowing for greater transcription of the proteins within that region of DNA. Which of the following enzymes most likely causes the chromatin modification described in this case?

QID: 108463
1

DNA methyltransferase

13%

(20/153)

2

Histone acetyltransferase

58%

(89/153)

3

Histone deacetylase

19%

(29/153)

4

DNA topoisomerase

8%

(12/153)

5

Xist

1%

(1/153)

M 1 C

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