KA : 1.2 Replication of DNA

Question 1

What is helicase involved in during DNA replication?

Answer 1

Helicase is involved in unzipping the DNA molecule.

Question 2

What does DNA polymerase control during DNA replication?

Answer 2

DNA polymerase controls the formation of the sugar-phosphate bonds when making the new strand.

Question 3

What does DNA ligase join together during DNA replication?

Answer 3

DNA ligase joins fragments of DNA together.

Question 4

What is a primer?

Answer 4

A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand allowing polymerase to add DNA nucleotides.

Question 5

What happens prior to cell division?

Answer 5

DNA is replicated by DNA polymerase.

Question 6

Stages of DNA replication

Answer 6

1. The DNA is unwound and unzipped. Special molecules break the weak hydrogen bonds between bases, which are holding the two strands together. Giving you 2 ends with the bases now exposed at a Y-shaped replication fork.
2. DNA polymerase will add the free DNA nucleotides using complementary base pairing to the 3’ end of the primer this will allow the new DNA strand to form. A primer is needed to start replication.
   1) Leading strand is synthesised continuously. DNA polymerase adds nucleotides to the deoxyribose (3’) ended strand in a 5’ to 3’ direction.
   2) Lagging strand is synthesised in fragments. Nucleotides cannot be added to the phosphate (5’) end because DNA polymerase can only add DNA nucleotides in a 5’ to 3’ direction. The fragments are then sealed together by ligase.
3. The two new strands twist to form a double helix. Each is identical to the original strand.

Question 7

What are the requirements for DNA replication?

Answer 7

The nucleus must contain:
1. DNA (to act as the template)
2. Primers (short bits of DNA that are complementary to the DNA being copied)
3. A supply of the 4 types of nucleotide
4. DNA polymerase and ligase enzymes
5. A supply of ATP (energy)

Question 8

What is PCR?

Answer 8

Polymerase chain reaction (PCR) amplifies DNA using complementary primers for specific target sequences.

Question 9

What are the requirements for PCR?

Answer 9

1. DNA – the original strand of DNA which needs amplified.
2. Complementary primers – primers are short complementary sequences of nucleotides needed to start DNA synthesis.
3. Thermal cycler – equipment that varies the temperature of the reaction.
4. Heat-tolerant polymerase – an enzyme which will add nucleotides to the growing strand and which is not denatured by the high temperatures used in the reaction.
5. Supply of nucleotides – to synthesise the new strands of DNA.

Question 10

Stages of PCR

Answer 10

1. Heat DNA between 92˚C and 98˚C. This causes DNA to denature and strands to separate.
2. Cool DNA to 50˚C to 65˚C. This makes short primers bind to the separated DNA strands.
3. DNA heated to between 70˚C and 82˚C. This allows heat tollerant DNA polymerase (taq polymerase) to replicate the DNA.
4. Repeat cycle over and over for roughly 20 to 30 cycles.

Question 11

Uses of PCR

Answer 11

1. DNA profiling - helps identify people and solve crimes.
2. Disease detection - used for diagnosis.
3. Paternity - find out parents of a child.