1. DNA structure, packaging, histone modification and chromatin

Question 1

What are the 3 components of DNA?

Answer 1

5 carbon sugar (deoxyribose)

Phosphate group

Nitrogenous base
- adenine and guanine = purines (2 rings)
- cytosine and thymine = pyrimidines (1 ring)

Question 2

Describe the bonding in DNA

Answer 2

Sugar-phosphate backbone

3’ carbon of sugar bonded to 5’ carbon of phosphate

H bonds between bases:
- A+T = 2 bonds
- C+G = 3 bonds

Question 3

Describe the wider structure of DNA

Answer 3

DNA complexed with histones - positively charged so have affinity for negatively charged DNA

Nucleosome = 147 bp of DNA coiled around 8 histone proteins

Chromatin = tightly packed nucleosomes
- euchromatin - ‘open’ DNA under active transcription
- heterochromatin - highly condensed, not expressed

Question 4

What is acetylation?

Answer 4

Post-translational modification of histone N terminal tail

Addition of acetyl group to lysine

Weakens interaction with DNA

Associated with activation of transcription

Question 5

What is phosphorylation?

Answer 5

Post-translational modification of histone N terminal tail

Addition of phosphate group to serine, threonine, tyrosine

Adds significant negative charge

Associated with activation of transcription

Question 6

What is methylation?

Answer 6

Post-translational modification of histone N terminal tail

Addition of methyl group to lysine and arginine

By histone methyltransferases (HMTs)

Associated with supression of transcription