Semester 2 Week 3

Question 1

What is the structure of DNA?

Answer 1

DNA has a double helix structure made out of 2 anti-parallel sugar phosphate backbones, with nitrogenous base pairs in molecule’s interior ( A,T, C, G).

Question 2

What is every DNA nucleotide made out of?

Answer 2

Every DNA nucleotide is made out of a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group.

Question 3

Why are the DNA strands said to be anti-parallel?

Answer 3

DNA strands are said to be anti-parallel because each strand runs in opposite direction - one strand runs 5’ to 3’ and the other strand runs 3’ to 5’.

Question 4

What is chromatin?

Answer 4

Chromatin is the DNA strand itself, plus the histones that act as structural support for chromosome.

Question 5

How many Histones are in a nucleosome?

Answer 5

There are 8 Histones in a nucleosome.

Question 6

What is Euchromatin?

Answer 6

Euchromatin is the loosely packed, active form of chromatin. It enables DNA replication, transcription and gene expression.

Question 7

What is heterochromatin?

Answer 7

Heterochromatin is the inactive, highly condensed form of chromatin. It inhibits DNA replication, transcription and gene expression.

Question 8

What is histone acetylation?

Answer 8

Histone acetylation converts heterochromatin into euchromatin. Loss of Histone + charge due to acetylation which weakens their interaction with DNA, converts chromatin into loose active form.

Question 9

What is histone deacetylation?

Answer 9

Histone deacetylation converts euchromatin into heterochromatin. It restores + charge of Histone, strengthening interaction with DNA.

Question 10

Why is DNA replication said to be semi-conservative?

Answer 10

DNA replication is said to be semi-conservative because in every DNA molecule, 1 strand was from the parent molecule and one strand was newly synthesised.

Question 11

Why is DNA replication said to be bidirectional?

Answer 11

DNA replication is said to be bidirectional because replication proceeds in both directions from each origin until entire molecule is copied.

Question 12

What is the replication fork?

Answer 12

The replication fork is a Y shaped region at the end of each replication bubble where new DNA stands are elongating.

Question 12

What are Helicases?

Answer 12

Helicases are enzymes that untwist the DNA double helix structure at the replication forks.

Question 13

What is the single strand binding protein?

Answer 13

The single strand binding protein binds and stabilises the single-stranded DNA until it can be used as template strand.

Question 14

What is topoisomerase?

Answer 14

Topoisomerase corrects ‘overwinding’ ahead of replication forks by breaking and rejoining DNA strands.

Question 15

What are the two limitations of DNA polymerase?

Answer 15

The two limitations of DNA polymerase are:
1) They can only add nucleotides to a pre-existing nucleotide chain
2) They can only add nucleotides in the 5’ to 3’ direction

Question 16

What is Primase?

Answer 16

Primase is an enzyme that synthesises a small RNA primer (5-10 nucleotides long) using DNA as template strand.

This indicates where DNA synthesis by DNA polymerase should start.

Question 17

Why is there a lagging strand?

Answer 17

DNA polymerase synthesises 3@ to 5@ strand with okazaki fragments which are joined by DNA Ligase.

Question 18

What are Telomeres?

Answer 18

Telomeres are the ends of eukaryotic chromosomes that have sequence TTAGGN. They protect chromosomes from erosion and degradation. They postpone shortening of DNA molecules.

Question 19

What is the end replication problem?

Answer 19

The end replication problem is there is no Okazaki fragment for the replication of the 3’ end of chromosome. In every DNA replication round a small region of telomere can’t be replicated, so some base pairs are lost.

Question 20

What is caused by the shortening of telomeres?

Answer 20

Shortening of telomeres causes aging. Telomeres are shortened to prevent cells from becoming cancerous by limiting number of cell divisions.

Question 21

What is Hayflick’s limit?

Answer 21

Hayflick’s limit is the number of times human cell can divide before it permanently stops cell division. Permanent cell cycle arrest is called senescence

Question 22

What is Telomerase?

Answer 22

Telomerase is an enzyme that prevents shortening of telomeres by synthesising the repetitive sequence TTAGGG of telomeres using RNA primer and enzyme reverse transcriptase.

Question 23

What are the three DNA repair mechanisms for single strand repairs?

Answer 23

The three DNA repair mechanisms for single strand repair are:

1)Mismatch repair (MMR)
2) Base excision repair (BER)
3) Nucleotide excision repair (NER)

Question 24

Explain Mismatch repair.

Answer 24

Mismatch repair (MMR):
1) Mismatch detected in newly synthesised DNA
2) New DNA strand cut, mispaired nucleotide and neighbours removed
3) Missing patch replaced with correct nucleotides by DNA polymerase
4) DNA Ligase seals gap in DNA backbone

Question 25

Explain Base excision repair

Answer 25

Base excision repair (BER):
1)Deamination converts cytosine into uracil
2) Uracil detected and removed, leaving baseless nucleotide
3) Baseless nucleotide removed, leaving small hole in DNA backbone.
4) Hole filled with right base by DNA polymerase and gap sealed by Ligase.