DNA Structure

Question 1

What are the main points of DNA structure?

Answer 1

• DNA is the store of genetic information
• it is made of a double helix (consists of major and minor grooves which are receptors for drugs)
• DNA bases encode genetic info
• DNA is present as chromatin in the nucleus

It can be damaged by radiation and chemicals - emphasising the importance of DNA repair.

Question 2

List the four bases, and describe them.

Answer 2

The four bases are Adenine, Thymine, Guanine and Cytosine.
Adenine and Guanine are Purines (in that they have 2 benzene rings) and Thymine and Cytosine are Pyrimidine (in that they have 1 benzene rings).
Adenine and Thymine have 2 H bonds between them.
Guanine and Cytosine have 3 H bonds between them.

Question 3

List the 3 forms of DNA, and describe them.

Answer 3

There are 3 forms of DNA: A-form, B-form and Z-form.

B-FORM DNA:
B is a long, symmetrical strand of helical DNA. We have the B form of DNA. It is clockwise and has right-handed turns.

A-FORM DNA:
The A form is chubbier than B, and more tacked up. Like B, it is clockwise and has right-handed turns. A-form is found in RNA.

Z-FORM DNA:
Z DNA may be found in cells under certain conditions, and its sometimes called left DNA (because it’s anticlockwise, and turns left when twirling). It’s formed by 5’ GCGCGC or 5’ GTGTGT

Question 4

List points giving a quick summary of the features of the DNA helix.

Answer 4

• Two antiparallel polynucleotide chains form an RH helix
• The nucleotide bases are situated on the inside of the helix, and the phosphate and sugars on the outside of the helix
• It has a diameter of 2nm (~10 base pairs per turn in the B-form)
• The polynucleotide chains are held together by H bonds between pairs of bases
• One strand is complementary in sequence to the other

Question 5

Describe bacterial DNA.

Answer 5

E.Coli DNA is circular and comprises of 3x106 base pairs.

It’s super-coiled - the DNA ribbon is itself twisted in space. The supercoiling is caused by the enzyme DNA Gyrase.

Question 6

Describe eukaryotic DNA.

Answer 6

Humans have 3 billion base pairs of DNA. They are organised into 23 pairs of linear chromosomes.
The DNA is complexed with histones - this is called CHROMATIN.

Question 7

What are some reasons as to why mutations may occur in DNA?

Answer 7

• It may be SPONTANEOUS: eg. loss of bases, hydrolysis of C to U
• It may be due to CHEMICALS: (it’s how many anticancer, etc. drugs work)
• It may be due to RADIATION: (eg. UV light causes thymine dimers, which lead to skin cancer) (eg. ionising radiation, such as X-rays, gamma rays, break DNA chromosomes to cause leukaemia)

Even some of the food we eat contains substances that can damage/change our DNA, further emphasising the importance of DNA repair.

Question 8

What is the importance of DNA repair?

Answer 8

• It maintains genome stability
• 50-100 (even up to 200) enzymes are involved

Sometimes, if DNA can’t be repaired, it’s fatal.
EG. patients with Xeroderma Pigmentosum have a defect in excision repair that deals with UV damage to DNA. They’re very prone to cancer.

Question 9

Describe a Holliday Junction.

Answer 9

A Holliday Junction is a cross-shaped structure that forms during the process of genetic recombination, when two double-stranded DNA molecules become separated into four strands in order to exchange segments of genetic information.
Homologous recombination occurs during meiosis and is characterized by the exchange of genes between a maternal chromatid and a paternal chromatid of a homologous chromosome pair. The two parent DNA molecules, which have long stretches of similar base sequences, are separated into single strands, resulting in base-pairing that leads to a four-stranded DNA structure. The Holliday junction travels along the DNA duplex by “unzipping” one strand and reforming the hydrogen bonds on the second strand.