Session 9 - DNA Structure and Replication

Question 1

What is the difference between heterochromatin and euchromatin?

Answer 1

Heterochromatin is tightly packed DNA which is associated to histones and pocked into a solenoid. It appears black on a TEM. Euchromatin however is loosely packed and allows for gene transcription it is white on a TEM

Question 2

Describe chromosome packing

Answer 2

Double helix wraps around 8 histone proteins (beads on a string) forming a nucleosome. Nucleosomes then fold up to produce a solenoid. The solenoid then forms loops, like a snake, which the compress and coil into a chromosome.

Question 3

Considering we have 2,500 genes and E.Coli has around 4,500 genes (only 5 times less) what does this suggest?

Answer 3

Considering how much more complex we are as a eukaryote. It suggests that we must regulate out genes very effectively

Question 4

What is the difference between a nucleoside and a nucleotide?

Answer 4

Nucleoside = base and sugar
Nucleotide = Base, Sugar and Phosphate

Question 5

What are the two types of nucleotide, describe their structure and name the bases that fill these descriptions

Answer 5

Purine - 2 ring structure - Guanine and Adenine

Pyrimidine - 1 ring structure - Cytosine, Thymine and Uracil

Question 6

What characterises the 5’ and 3’ ends of DNA?

Answer 6

5' = phosphate group
3' = -OH (hydroxyl) group

Question 7

How many hydrogen bonds do each base pair make and what are they?

Answer 7

GC = 3 H bonds
AT = 2 H bonds
AU = 2 H bonds

Question 8

What is the convention for DNA nomenclature?

Answer 8

Single DNA sequence = 5’ to 3’
Duplex = top - 5’ to 3’
bottom = 3’ to 5’

Question 9

During what stage of interphase does DNA replication take place?

Answer 9

During S phase (ie synthesis phase)

Question 10

What is meant by the semi conservative model of DNA synthesis?

Answer 10

• Each old DNA strand acts as a template for a new DNA strand

Question 11

Describe the process of DNA replication

Answer 11

1- DNA primase is needed to “kick start” synthesis
2- As DNA helicase continues to unwind the double helix DNA polymerase binds to 3’ end of each strand and produces the new complimentary strand in the 5’ to 3’ direction
3- On the lagging strand (strand which discontinuous as helicase runs in opposite direction to polymerase) Ozaki fragments are formed
4- The Ozaki fragments are joined by DNA ligase