DNA Structure

Question 1

What is DNA?

Answer 1

DNA acts as a store for genetic information

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).

Question 3

How many Hbonds between AT and GC respectively?

Answer 3

• 2 b/w A and T

* 3 b/w G and C

Question 4

List the 3 forms of DNA and describe them.

Answer 4

• A - is more tightly coiled, tRNA takes this form (but tRNA is made from ribonucleotides). Right handed helices. ‘chubbier than B form.
• B –is the most common, forms most of the DNA in eukaryotic cells. Right handed helices and B is a long, symmetrical strand of helical DNA. B-form has major group (where large group of bases exposed) and minor group (small number bases exposed)
• Z – is a left handed helix, these are formed when you have alternating purine-pyrimidine bases. It is present in small amounts in the cell, under certain conditions.

It is important to note, that DNA can interconvert between forms.

Question 5

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

Answer 5

• 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
• minor and major grooves form receptors for drugs

Question 6

Describe bacterial DNA.

Answer 6

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 7

Describe eukaryotic DNA.

Answer 7

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 8

What are the levels of DNA structure?

Answer 8

• Primary – Sequence of bases (found by DNA sequencing)
• Secondary – Helical structure [e.g. A,B,Z] (X –ray and chemistry)
• Tertiary – DNA supercoiling (electron microscopy)
• Quarternary – Interlocked chromosomes [e.g. when bacteria replicate and chromosomes interlock initially]

Question 9

How may mutations occur in DNA?

Answer 9

1. It may be SPONTANEOUS: eg. loss of bases, hydrolysis of C to U
2. It may be due to CHEMICALS: (it’s how many anticancer, etc. drugs work)
3. 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)
   o All this is because DNA is chemically unstable, it is joined by sugars, whose bonds can be hydrolysed
   o Even some of the food we eat contains substances that can damage/change our DNA, further emphasising the importance of DNA repair.

Question 10

What is the importance of DNA repair?

Answer 10

o 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 11

Give examples of unusual DNA structure and briefly describe them.

Answer 11

o Left handed or Z-DNA is formed by alternating patterns, e.g. 5’ …GCGCGCGCGCGC… or 5’ … GTGTGTGTGTGTG….
o Holliday junction – a four stranded junction, which is formed when you bring DNA from two different chromosomes together. It is important in repairing damage.
o Tetraplex DNA - which is formed at the telomeres (which protect the chromosomes). Tetraplex DNA is formed by the DNA folding back in itself, it is found at the end of chromosomes.