DNA & Chromosomes

Question 1

Give key attributes genetic storage must have.

Answer 1

Stable over time for storage but easily accessible to direct cellular processes.
Faithfully replicated.
Must control expression of traits.
Change in a controlled way.

Question 2

Give 3 key structural features of DNA.

Answer 2

High molecular weight, acidic, high amounts of phosphorus.

Question 3

Who discovered the double helix structure and through what method?

Answer 3

Rosalind Franklin through x-ray crystallography.

Question 4

Describe the structure of DNA.

Answer 4

Linear polymer of repeating nucleotide units bonded by phosphodiester bonds to a sugar, base & phosphate backbone coordinated into a double helix of antiparallel strands.
The backbones of each strand are closer to each other on one side of the helix than on the other, creating a minor & major groove.

Question 5

What does a nucleoside consist of?

Answer 5

Sugar & base.

Question 6

What are the groups on the 3’ and 5’ end of a DNA strand?

Answer 6

3' = hydroxyl group
5' = phosphate group

Question 7

What is the directionality of a DNA strand?

Answer 7

5’ to 3’.

Question 8

How are the two strands bonded?

Answer 8

Hydrogen bonds between complementary base pairs.
A & T = 2 H bonds.
G & C = 3 H bonds

Question 9

Why are the major and minor grooves in DNA important?

Answer 9

It is easier for proteins to bind to the major groove. Increases access for important cell processes that require the DNA to be unwound.

Question 10

Describe the genetic code.

Answer 10

Digital, quaternary, binary so easily stored & analysed by computer.

Question 11

Nuclear genetic material is distributed among which 2 discrete units?

Answer 11

Chromosomes.

Question 12

How is DNA packaged?

Answer 12

Helix wrapped around 8 core histone proteins to form nucleosomes.
H1 packages nucleosomes into chromatin fibre (stainable).
Looped domains of chromatin are spirally attached to protein scaffold.
Acidic, -ve non-histones are also associated

Question 13

Why do histones have a net +ve charge?

Answer 13

To facilitate binding to DNA which has a -ve charge.

Question 14

If 1 nucleotide codes for an amino acid, how many possible amino acids are there?

Answer 14

4^1

= 4

Question 15

If 2 nucleotides code for an amino acid, how many possible amino acids are there?

Answer 15

4^2

= 16

Question 16

If 3 nucleotides code for an amino acid, how many possible amino acids are there?

Answer 16

4^3

= 64

Question 17

Give evidence as to why the genetic code is non-overlapping.

Answer 17

Single base mutations only ever affect 1 amino acid.

Question 18

Describe the experiment that Crick & Brenner did using bacteriophage T4 on E.coli.

Answer 18

Generated mutants in rII gene, expressed as a large plaque.
Used planar mutagen proflavin (adds or deletes a single base).
Created a series of mutated genes which they shuffled.
Double mutations of the same type still have a mutant genotype.
Double mutations of opposite types cancelled each other out and the phenotype was wild.

Question 19

Which 3 scientists discovered which amino acid the codons coded for?

Answer 19

Nirenburg, Holley & Khorana.

Question 20

Give the 3 key features of the genetic code.

Answer 20

Degenerate, universal & non-overlapping.