DNA Structure

Question 1

Describe the structure of the double helix

Answer 1

Double helix complementary antiparallel polynucleotide chains that contain minor and major grooves acting as receptors for drugs

Question 2

How many base pairs are in DNA?

Answer 2

3x10^9 Base pairs of DNA in our cells

Question 3

What is DNA in the nucleus present as?

Answer 3

Chromatin

Question 4

How can DNA be damaged?

Answer 4

Light
Chemicals
Radiation

Question 5

DNA affected by hydrolysis

Answer 5

Spontaneous- loss of bases or hydrolysis of C to U (deaminate)
Change of base pairs in DNA causes mutations

Question 6

Radiation

Answer 6

UV light produces thymine dimers
Ionising radiation (X Ray, gamma ray)

Question 7

Chemicals

Answer 7

Change base structure

Insert themselves between bases

Question 8

How do you prevent mutation?

Answer 8

DNA repair proteins
Look for changes in base pairs
Ensure genome is maintained stable and accurate

Question 9

How did Rosalind Franklin show the double helix structure?

Answer 9

Retrieving a sample of pure DNA that was dissolved in water and added increased amounts of ethanol
The ethanol precipitates the DNA allowing them the ability to pull out some fibres causing the DNA molecules to become aligned fibres
When put through a pencil thin x ray beam, the x rays should go through the fibres but as there are repeating units of these fibres, the x rays will defract causing a series of spots onto a form
Pic - strong arcs represent normal base pairs and the X shape in the middle represents the double helix structure

Question 10

B form

Answer 10

Major and minor groove
Sugar phosphate and hydrogen bonds seen
Most DNA is in B form
10 base pairs per turn
Right hand helix

Question 11

A form

Answer 11

Right hand helix
Major and minor grooves identical in width
Structure found in double stranded RNA
11 base pairs per turns

Question 12

Z form

Answer 12

Anti clockwise turn
Z DNA 
Left hand DNA
DNA in this structure have alternating purines pyrimidines
12 base pairs per turn
negative supercoiling

Question 13

Describe the structure of a nucleotide

Answer 13

Linked together by 3’5’phosphodiester bonds/link
DNA runs from 5’ to 3’ direction
The 2 strands of DNA that are bonded together by hydrogen bonds, run antiparallel to each other

Question 14

Structure of tRNA

Answer 14

Folds into A form helix
Single stranded
Complimentary to itself

Question 15

Give examples of unusual DNA structures

Answer 15

Z DNA
Four stranded junction - holiday junction (ideal for double strand break repair and has a homologous recombination)
Tetraplex DNA - involves G rich sequences

Question 16

What are the protein structures of DNA?

Answer 16

Primary - sequence of bases (DNA sequencing)
Secondary - helical structure (A,B,Z) - X Ray, chemistry
Tertiary - DNA supercoil
Quaternary structure - interlocked chromosomes

Question 17

Structure of bacterial DNA

Answer 17

E. coli - circular
3x10^6 base pairs
Supercoiled by DNA gyrase

Question 18

What is a nucleosome?

Answer 18

Basic building block of chromatin

Question 19

What is the function of histone proteins in DNA?

Answer 19

Compresses DNA by a factor of 6
Still not compact enough for all DNA to fit in cell
Inside cells, DNA is always interacting with protein

Question 20

What is the importance in DNA repair?

Answer 20

Maintain stability as there are 50-100 different enzymes/proteins

Question 21

how is energy provided for the formation of major and minor grooves?

Answer 21

hydrolysis of water

Question 22

what is the diameter of the DNA?

Answer 22

2nm

Question 23

what does DNA gyrase with ATP cause?

Answer 23

Negative supercoiling

Question 24

how were nucleosomes first seen in an electron microscope?

Answer 24

seen as bead on strings

Question 25

how many base pairs are nucleosomes made up of?

Answer 25

146

Question 26

how many histones is it wrapped around?

Answer 26

8

Question 27

what is the difference between heterochromatin and euchromatin?

Answer 27

heterochromatin is densely packed and not expressed whereas euchromatin is not densely packed and is expressed

Question 28

what does H1 protein do?

Answer 28

H1 protein keeps the DNA in place whichis wrapped around the histones

Question 29

how can DNA be repaired?

Answer 29

DNA photolyase
DNA photoreactivation - photons cleave the C-C bond of cyclobutyl ring of thymine dimers
excision repair - endonuclease enzyme cleaves the DNA backbone on 5’ side of abnormal base, RNA ligase reseals with correct base, UvrA and UvrB recognises damage, UvrC-incision, UvrD - removes bases