DNA & RNA

Question 1

What are polynucleotide nucleic acids?

Answer 1

Linear polymers of nucleotides, i.e. DNA and RNA.

Question 2

What is the difference between a nucleotide and a nucleoside?

Answer 2

• Nucleotide = sugar + base + phosphate

- Nucleoside = sugar + base

Question 3

Why are nucleic acid molecules (nucleotides) polar?

Answer 3

Negative charge on phosphate group.

Question 4

What are the 2 types of pentose sugars?

Answer 4

• 2-deoxyribose (in DNA) - H linked to 2’ C

- ribose (in RNA) - OH linked to 2’ C

Question 5

How can nucleoside analogues be used as antiviral and anticancer drugs?

Answer 5

• Phosphorylated nucleoside analogues are similar to nucleosides - can be incorporated into growing viral DNA strand - act as chain terminators and stop viral DPol.
• Less selective analogues used as chemotherapy agents.

Question 6

Why are nucleosides rather than nucleotides used as antiviral/anticancer drugs?

Answer 6

Charged nucleotides can’t easily cross membranes

Question 7

What are the 2 types of nitrogenous bases?

Answer 7

1. Purines
   
   • Adenine
   • Guanine
2. Pyrimidines
   
   • Cytosine
   • Thymine (DNA only)
   • Uracil (RNA only)

Question 8

How are nucleotides joined together to form a DNA polymer?

Answer 8

Phosphodiester bond (phosphate to sugar

Question 9

Do DNA strands have polarity?

Answer 9

Yes: 5’ to 3’, with distinct 5’P and 3’OH

Question 10

How do nucleotides on different strands form complementary base pairs?

Answer 10

• Each nucleotide has specific +/- charges.

- When one is flipped (as in the antiparallel arrangement of DNA strands), the charges attract to form hydrogen bonds.

Question 11

How many hydrogen bonds form between G-C and A-T (or A-U) base pairs?

Answer 11

• Charges in G-C base pair are at the same distance so 3 bonds can form.
• Charges in A-T base pair are at different distance so only 2 bonds can form.

Question 12

What are the characteristics of DNA double-strands?

Answer 12

• Complimentary
• Anti-parallel
• Form a right-handed double helix containing major and minor grooves.

Question 13

Are major or minor DNA grooves more difficult for DNA-binding proteins to access and why?

Answer 13

Minor, due to steric hindrance.

Question 14

How wide and long is DNA?

Answer 14

• 2nm width

- 2m total length/cell

Question 15

How does DNA fit in the nucleus?

Answer 15

1. DNA is packaged using histones, forming complexes called nucleosomes (DNA wound 1.65x around 8 histones proteins).
2. Nucleosomes fold up to form a 30nm solenoid chromatin fibre, which forms loops averaging 300nm in length.
3. Chromatin fibres are compressed and folded to produce a 250nm-wide fibre, which is tightly coiled into the chromatid of a chromosome.

Question 16

What are histones?

Answer 16

Positively-charged proteins that strongly adhere to negatively-charged DNA.

Question 17

Give an example of a disease involving deficient DNA packaging/expression.

Answer 17

• RETT’S DISEASE
• Neurological development disorder (loss of hand coordination, mobility & speech).
• Mutation in methyl-CpG-binding protein 2 (silences transcription of certain genes) - inability to bind to correct DNA regions, or inability to recruit histone binding complexes - modifying complexes can’t repress gene - abnormal gene activation.