DNA REPLICATION

Question 1

What does DNA gyrase do?

Answer 1

untwists the double helix a bit at a time and cause the DNA to start to unwind

Question 2

What does DNA helicase do?

Answer 2

allows the DNA molecule to start to unzip where the hydrogen bonds between the complementary nucleotide bases break
results in a structure called a replication fork where the branching contains single-stranded DNA with exposed bases

Question 3

What does DNA polymerase do?

Answer 3

catalysed the addition of the new nucleotide bases in the 5’ to 3’ direction to the single strands of DNA and catalysed the formation of ester bonds between adjacent nucleotides

Question 4

What does DNA ligase do?

Answer 4

catalyses the reactions that join the okazaki fragments of the lagging strand into one single strand

Question 5

Lagging strand

Answer 5

forms in the opposite direction of the unzipping/direction of the DNA helicase
synthesised discontinuously so repeatedly has to start and stop again , forms short sections or fragments called okazaki fragments
3’ to 5’ direction

Question 6

Leading strand

Answer 6

acts in the same direction as DNA helicase
synthesised in one continuous strand following the replication fork without interruption
5’ to 3’ direction

Question 7

How do DNA helicase and DNA polymerase work in association with each other?

Answer 7

at the same time

Question 8

‘The 2 strands of DNA are never completely split at any time’
True or False

Answer 8

true

Question 9

At what stage of the cell cycle does DNA replication occur?

Answer 9

the S phase of interphase

Question 10

What are two strands going in opposite directions called?

Answer 10

anti-parallel

Question 11

PROCESS OF DNA REPLICATION

Answer 11

(1) gyrase untwists the double helix
this allows the DNA molecule to unwind
(2) hydrogen bonds between complementary nucleotide bases break, catalysed by helicase
(3) unzipping action results in the formation of a replication fork with exposed bases
(4) free phosphorylated nucleotides move in and line up the exposed bases according to complementary base-pairing rules
(5) polymerase catalyses the addition of new nucleotide bases in 5’ to 3’ direction, to the single strands of DNA
(6) hydrolysis of activated nucleotides releases the extra phosphate groups
(7) this supplies the energy to form phosphodiester bonds between the deoxyribose sugar of one nucleotide and the phosphate group of the adjacent nucleotide
(8) as the leading strand is synthesised in the same direction as the growing replication fork, it can be replicated continuously
(9) as the direction of the lagging strand is in the opposite direction of the growing replication fork, it is replicated discontinuously by forming short segments called Okazaki fragments
(10) okazaki fragments are joined during reactions catalysed by DNA ligase
(11) termination of the process occurs when the replication fork stops
(12) the final product is 2 DNA molecules which are identical to each other as well as to the parent DNA molecule that initiated the process
(13) the process is called semi-conservative replication because each new DNA molecule consists of two strands: one old strand plus one newly built strand

Question 12

What does semi-conservative mean?

Answer 12

2 new molecules are formed, each containing one old strand and one new strand
ratio 2:2

Question 13

If DNA was conservative, what would you expect to find?

Answer 13

1 molecule of DNA containing 2 original strands and 1 molecule of DNA containing 2 new strands
ratio 1:3

Question 14

What is DNA known as being?

Answer 14

self-replicating
semi-conservative