Lecture 10: the central dogma

Question 1

Why is it beneficial for DNA to be double stranded?

Answer 1

1. it ensures that essential information is not lost. If one copy is damaged then the other strand can be used as a template for repair.
2. It reduces the dangers inherent in transmitting genetic information –> if a single chain is present, genetic information might be lost

Question 2

What phase of mitosis does DNA replication occur?

Answer 2

S phase

Question 3

How does semiconservative DNA replication work?

Answer 3

2 strands in a parent DNA molecule are separated and each strand acts as a template for the synthesis of a new daughter strand.

Question 4

DNA polymerase has the general structure of a ____________________.

Answer 4

Right hand

Question 5

DNA polymerase has the general structure of a ____________________.

Answer 5

Right hand

Question 6

Which direction does DNA synthesis occur in?

Answer 6

The growing strand is synthesized in the 5’ to 3’ direction

Question 7

DNA polymerase needs ______________ to facilitate catalysis?

Answer 7

metal ions

Question 8

What are the functions of the parts of DNA polymerase?

Answer 8

1. “Fingers” : interact with incoming dNTP and template DNA
2. “Palm” : the site where bond formation is catalysed
3. “Thumb” : positions duplex DNA

Question 9

What is the bond that DNA polymerase forms?

Answer 9

It catalyses the formation of a phosphodiester bond between the 3’ -OH of the growing strand and the 5’ phosphate group of the incoming dNTP

Question 10

What is the structure of helicase?

Answer 10

It is made u of 6 proteins arranged in a ring shape

Question 11

What is the function of helicase?

Answer 11

It unwinds the DNA double helix into 2 strands

Question 12

What is the replication fork?

Answer 12

The point where the DNA is separated into single strands

Question 13

What is the function of single stranded binding proteins (SSBs)?

Answer 13

1. SSBs coat the newly exposed DNA strands during replication
2. They prevent the 2 polynucleotide strands from joining back together.

Question 14

What is the function of primase?

Answer 14

It synthesizes short RNA primers which are needed to start the replication process

Question 15

What is the function of the sliding clamp protein?

Answer 15

1. It helps to hold the DNA polymerase onto the DNA strand during replication.

Question 16

What is the function of RNAse H?

Answer 16

It removes the RNA primers that previously began the DNA strand synthesis.

Question 17

What is the function of DNA ligase?

Answer 17

It links the DNA together to form one continuous strand

Question 18

The lagging strand is synthesized in small fragments called ______________

Answer 18

Okazaki fragments

Question 19

How long are okazaki fragments?

Answer 19

1000-5000 bp

Question 20

How does RNAase H remove the primers?

Answer 20

1. It recognises RNA-DNA hybrid helices
2. It degrades the RNA by hydrolyzing the phosphodiester bonds

Question 21

What is the origin of replication?

Answer 21

A stretch of DNA sequence in the genome at which DNA replication is initiated

Question 22

How can acyclovir act as an anti cancer agent?

Answer 22

1. It has a nucleoside like structure –> with the same nucleic acid base, but without the complete sugar ring
2. It can get incorporated into the elongating DNA strand, but more nucleotides cannot be added to it –> it prevents DNA elongation

Question 23

What is the (general) function of DNA topoisomerase?

Answer 23

It acts of DNA to reduce supercoiling stress caused by unwinding during replication.

Question 24

What are the 2 ways DNA topoisomerase can reduce supersoiling stress?

Answer 24

1. Type I: single stranded cut –> unwind –> join back together
2. Type II: double stranded cut –> unwind –> join back together

Question 25

How does camptothecin act as an anticancer drug?

Answer 25

1. It forms hydrogen bonds with DNA and tipoisomerase I, forming a stable ternary complex
2. This complex prevents DNA religation
3. Interferes with DNA replication

Question 26

How does fluoroquinolones act on bacteria?

Answer 26

1. It inhibits bacterial topoisomerase II (but not human)
2. Causes accumulation of clevage complexes hat lead to bactericidal effect