Nucleic Acids- DNA Replication

Question 1

How is DNA replicated?

Answer 1

Semi-conservatively

Question 2

Why is it called semi-conservative replication?

Answer 2

One strand in new DNA molecule is from original DNA molecule which means there is genetic continuity between generations

Question 3

What does DNA helicase do?

Answer 3

Breaks the hydrogen bonds between bases on the two polynucleotide DNA strands which makes the helix unwind to form two single strands

Question 4

What happens after the double helix has been unwinded?

Answer 4

Each original single strand acts as a template for a new strand and complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand

Question 5

How do the nucleotides of new strands join together?

Answer 5

Condensation reactions join nucleotides together catalysed by DNA polymerase and hydrogen bonds form between the bases on the original and new strands

Question 6

What do the new DNA molecules each contain?

Answer 6

One strand from original DNA molecule and one new strand

Question 7

How does DNA polymerase move?

Answer 7

Opposite ways along antiparallel DNA strands

Question 8

What are the antiparallel strands?

Answer 8

3’ end and 5’ end run in opposite directions

Question 9

What is DNA polymerase complementary to?

Answer 9

Active site is only complementary to 3’ end of newly forming DNA strand so enzyme can only add new nucleotides to new strand at 3’ end

Question 10

What way does DNA polymerase move down the strand?

Answer 10

Down the template strand in a 3’ to 5’ direction, DNA polymerase working on one template strand moves in opposite direction to DNA polymerase working on other template strand

Question 11

How did Meselson and Stahl validate the theory of semi-conservative replication?

Answer 11

1. Grew two samples of bacteria (heavy 15N and light 14N)
2. Sample of DNA taken from each batch of bacteria and spun in a centrifuge, heavy settled lower down centrifuge tube than light
3. Bacteria grown in 15N taken out a put in broth containing only 14N
4. Bacteria left for one round of DNA replication and then another sample was taken out and spun in centrifuge
5. DNA settled in middle showing that DNA molecules contained a mixture of heavy and light nitrogen