More biological molecules: DNA replication

Question 1

State why DNA replicates

Answer 1

To ensure each cell has a full amount of DNA

Question 2

Outline the steps of semi-conservative replication

Answer 2

• DNA helicase breaks the hydrogen bonds between the complementary base pairs on the two polynucleotide DNA strand - making the helix unwind to form two single stands (like a zip)
• Each original strand acts as a template for a new strand to form
• Free floating DNA nucleotides become attracted to their complementary exposed bases (on the template strand)
  *DNA polymerase catalyses the condensation reactions that join the nucleotides of the new strands together
• Hydrogen bonds form between bases from the template and original strand
• MEANING each new DNA strand contains one strand from the original DNA molecule and one new strand

Question 3

Describe the function of DNA helicase

Answer 3

DNA helicase catalyses the hydrolysis reactions between the hydrogen bonds that holds the the strands of DNA together - unwinding and separating the DNA

Question 4

Describe the function of DNA polymerase

Answer 4

DNA polymerase catalyses the condensation reactions that join the adjacent free nucleotides of the new strands together

Question 5

State how is DNA replicated

Answer 5

Through semi conservative replication

Question 6

State what end of a DNA strand that the active site of DNA polymerase is complementary to

Answer 6

3’ of pentose sugar

Question 7

State which two directions polynucleotide strands in a DNA can go

Answer 7

3’ to 5’

AND

5’ to 3’

Question 8

State what direction DNA polymerase builds a new strand of polynucleotides in

Answer 8

5’ to 3’

Question 9

What is the function of DNA primase

Answer 9

It catalyses the production of short RNA molecules used a primers, indicating where DNA polymerase should start synthesising a new polynucleotide chain

Question 10

What is a leading strand in DNA replication

Answer 10

A new strand of DNA that is synthesized in the same direction that DNA helicase unwinds the two polynucleotide strands of a DNA molecule

Question 11

What is a lagging strand in DNA replication

Answer 11

A new strand of DNA that is synthesized in the opposite direction that DNA helicase unwinds the two polynucleotide strands of a DNA molecule

Question 12

What is the function of DNA ligase

Answer 12

DNA ligase facilitates the joining of Okazaki fragments together by condensation reactions by catalysing the formation of phosphodiester bonds on lagging strands

Question 13

What are Okazaki fragments

Answer 13

Short sequences of DNA nucleotides synthesized on the lagging strand during DNA replication - later joined together by DNA ligase

Question 14

What evidence validates the semi-conservative replication

Answer 14

Meselson and Stahl’s experiment

Question 15

Describe the meaning of semi-conservative in terms of DNA replication

Answer 15

DNA replication that produces 2 copies, each containing one ORIGINAL and one NEW strand.

Question 16

Compare conservative and semi-conservative replication

Answer 16

• Semi-conservative: Two copies each contain one original strand and one new strand
• Conservative: Two DNA copies are produced from one original DNA, which serves as a template. Out of these two, one is entirely new DNA, and the other is made of old DNA strands

Question 17

Describe the meaning of conservative in terms of DNA replication

Answer 17

When two DNA copies are produced from one original DNA, which serves as a template. Out of these two, one is entirely new DNA, and the other is made of old DNA strands

Question 18

Describe the procedure of the Meselson and Stahl experiment

Answer 18

• E. Coli were grown in heavy nitrogen (15N) This caused all the DNA to incorporate heavy nitrogen into their bases
• Heavy nitrogen DNA was more dense, so when it was spun in the centrifuge, it all accumulated at the bottom
• Then the cells were transferred to light nitrogen (14N) and allowed the cells to divide once (creating one new and one fused)
• DNA now has heavy and light nitrogen. When centrifuged, the DNA didn’t sink as far.
• The cells divides one more time in light N which caused 2 bands (one light and one fused)

Question 19

Explain how the Meselson and Stahl experiment demonstrated semiconservative DNA replication

Answer 19

• Each DNA molecule separate and serves as a template for a synthesis of a new complementary strand
• So each daughter molecule comprises one “old” and one “new” strand.