DNA Structure

Question 1

Describe basic DNA structure & function

Answer 1

• DNA is the store of genetic info
• made of a double helix (has a major and minor groove which act as drug receptors)
• DNA bases encode genetic info
• DNA present as chromatin in the nucleus and can be damaged by radiation and chemicals

Question 2

What bonds are present in DNA

Answer 2

• 3’ to 5’ phosphodiester bonds

- H bonds between strands

Question 3

What are the four bases and describe their composition?

Answer 3

• Adenine, Thymine, Cytosine, Guanine
• Adenine & Guanine are purines (2 benzene rings)
• Thymine & Cytosine are pyrimidines (1 benzene ring)
• A & T form 2 H bonds between them
• G & C form 3 H bonds between them

Question 4

Describe the features of the DNA helix

Answer 4

• 2 anti parallel polynucleotide chain forms right hand helix
• based inside of helix, phosphate & sugars on outside
• diameter 2nm
• one strand is completely complimentary in sequence to other strand

Question 5

How many forms of DNA are there?

Answer 5

• 3

- A, B and Z

Question 6

Describe the A form of DNA

Answer 6

• Wider than B form
• it is clockwise and has right-handed turns
• found in RNA AND tRNA
• 20-25% shorter than B DNA
• grooves identical in width
• 11 bases per turn
• more tightly coiled than B

Question 7

Describe the B form of DNA

Answer 7

• common form of DNA found in cells
• clockwise and right-handed turns
• minor groove is 120°
• major groove is 240°
• 10 bases per turn

Question 8

Describe the Z form of DNA

Answer 8

• present in small amounts in cell
• may be found in cells under certain conditions ( high [salt] )
• 12 bps per turn
• left handed helix
• alternating purine and pyrimidine bases

Question 9

Describe bacterial DNA

Answer 9

• circular DNA
• 3 million bps
• super coiled by DNA gyrase
• also have plasmids that carry certain genes

Question 10

Describe eukaryotic DNA

Answer 10

• 3 billion bps
• complexed with histones to form chromatin
• double helix wound around 8 histone subunits

Question 11

What is a nucleosome?

Answer 11

• a basic building block of chromatin

- they compact the DNA further

Question 12

What may cause damage/ mutations in DNA

Answer 12

1) SPONTANEOUS:
- loss of bases
- hydrolysis is C to U

2) CHEMICAL
- change of base structure
- insert between bases

3) RADIATION
- UV light produces thymine dimers
- ionising radiation break DNA chromosomes to cause leukaemia

Question 13

why is DNA repair important?

Answer 13

• maintains genome stability

- 50-100 enzymes/proteins check is DNA is repaired properly

Question 14

What is an example of a disease due to failed repair mechanisms?

Answer 14

• Xeroderma pigmentosum
• defect in excision repair that deals with UV damage to DNA
• prone to skin cancer

Question 15

Describe the tetraplex DNA

Answer 15

4 stranded DNA hélices formed at telomeres and involve G rich DNA sequence

Question 16

Describe the Holliday Junction and when they form

Answer 16

• arise in DNA strand exchange (genetic recombination)
• two double stranded DNA molecules become separated into four strands
• important role in DNA repair

Question 17

Describe the 4 levels of DNA sequencing

Answer 17

1° sequence of bases
2° helical structure
3° DNA supercooling
4° interlocked chromosomes