S1: DNA Structure

Question 1

What is DNA packaged as in the nucleus?

Answer 1

DNA is present as chromatin (DNA packaged with histone proteins). It is not ‘free’.

Question 2

What do the minor and major grooves do?

Answer 2

They form receptors for drugs

Question 3

What are the 3 forms of DNA?

Answer 3

A form
B form
Z form

Question 4

Describe A-form DNA

Answer 4

A form DNA is more tightly coiled

• tRNA takes this form
• Right handed helices

Question 5

Describe B-form DNA

Answer 5

• Most common form (sugar phosphate on outside, bases in middle)
• Forms most of the DNA in eukaryotic cells
• Right handed helices
• Has major group (large group of bases exposed)
• Has minor group (small number of bases exposed)

Question 6

Describe Z-form DNA

Answer 6

• Left handed helix
• Formed when there is alternating purine-pyrimidine bases
• Present in small amounts in cells

Question 7

Can DNA interconvert between forms?

Answer 7

Yes

Question 8

Describe DNA structure on a molecular level

Answer 8

• DNA structure consists of repeating sugar-phosphate units linked by 3’ -5’-phosphodiester bonds.
• DNA has a polarity running from 5’ to 3’
• DNA helix is held together by AT and CG base pairs
• Hydrogen bonds between polynucleotide chains
• One strand is complementary in sequence to the other

Question 9

How many hydrogen bonds are between cytosine and guanine base pairs?

Answer 9

3 hydrogen bonds

bond length 1.8nm

Question 10

How many hydrogen bonds are between adenine and thymine base pairs?

Answer 10

2 hydrogen bonds

bond length 1.11nm

Question 11

Which bases are purine and which are pyrimidine?

Answer 11

Purine: Adenine and Guanine

Pyrimidine: Cytosine and Thymine

Question 12

How many rings do purine and pyrimidine bases have?

Answer 12

Purine: Double rings
Pyrimidine: Single rings

Question 13

What forms a right handed helix?

Answer 13

2 antiparallel polynucleotide chains forms a RH helix

Question 14

What form does tRNA fold into?

Answer 14

• A form helix

Question 15

What is tRNA made from?

Answer 15

Ribonucleotides

Question 16

What DNA pattern forms left handed DNA?

Answer 16

Left handed or Z DNA is formed by alternating patterns
5’….GCGCGCGCGCGC
5’….GTGTGTGTGTGTG

Question 17

What is a ‘Holliday Junction’?

Answer 17

• 4 stranded junction
• Formed when you bring DNA from two different chromosomes together
• Important in repairing damage

Question 18

What is tetraplex DNA?

Answer 18

• Formed at the telomeres (caps at end of DNA which protects the chromosomes)
• Formed by the DNA folding back on itself
• Found at the end of chromosomes

Question 19

Levels of DNA structure

Answer 19

Primary- Sequence of bases
Secondary- Helical structure (e.g. ABZ)
Tertiary- DNA supercoiling
Quaternary- Interlocked chromosomes

Question 20

What is primary sequence of DNA found by?

Answer 20

DNA sequencing

Question 21

What is secondary sequence of DNA found by?

Answer 21

X-ray and chemistry

Question 22

What is tertiary sequence of DNA found by?

Answer 22

Electron microscopy

Question 23

Give an example of quaternary structure

Answer 23

When bacteria replicate and chromosomes interlock initially

Question 24

Describe bacterial DNA

Answer 24

• Circular DNA
• Supercoiled (DNA ribbon itself is twisted in space)
  It is supercoiled in bacteria to replicate, supercoil stores energy

Question 25

What enzyme causes supercoiling?

Answer 25

Enzyme DNA gyrase

which uses ATP

Question 26

What is chromatin?

Answer 26

DNA is complexed with histones and made into a fibre

Multiple nucleosomes are coiled together, which stack on top of each other.

Question 27

What is a nucleosome?

Answer 27

Basic building block of chromatin

It is the combined loop of DNA around the histones

Question 28

What do nucleosomes look like under electron microscopy?

Answer 28

‘beads on a string’

Question 29

How many histone subunits does DNA double helix wind around?

Answer 29

8 histone subunits (2 molecules each of 2A, 2B, 3 and 4)

Question 30

What does histone 1 do?

Answer 30

Binds to the outside and linker DNA

Question 31

What does chromatin condense into?

Answer 31

30nm chromatin

This is then looped and coiled more, using other proteins to form the chromosomes.

Question 32

What causes mutations in DNA?

Answer 32

DNA damage causes mutations in DNA

• Spontaneous e.g. loss of bases or hydrolysis of C to U
• Chemicals e.g. change of base structure or insert between bases
• Ionising Radiation (X-rays, gamma rays) break DNA chromosomes to cause e.g. leukemia

Question 33

Why is DNA chemically unstable?

Answer 33

It is joined by sugars whose bonds can be hydrolysed

Question 34

What is the importance of DNA repair?

Answer 34

• DNA repair maintains genome stability