Chapter 3.9 - DNA replication

Question 1

What is the type of replication DNA undergoes?

Answer 1

Semi-conservative replication

Question 2

What does semi-conservative replication mean?

Answer 2

DNA replication which results in an old identical parental strand and a newly synthesised strand present in each daughter DNA molecule

Question 3

What are the seven steps in semi-conservative DNA replication?

Answer 3

1. The enzyme DNA helicase breaks the hydrogen bonds between complementary bases. This unwinds the parental double helix and separates the strands
2. Single stranded binding proteins stabilise the unwound DNA
3. Each strand acts as a template as free-floating nucleotides in the cytoplasm are attracted to their complementary bases
4. The enzyme DNA polymerase joins the free nucleotides together via condensation reactions in the 5’ to 3’ direction, this forms phosphodiester bonds to create the sugar-phosphate backbone of the new DNA strand(leading and continuous)
5. The lagging strand (3’ to 5’) is synthesised discontinuously producing Okazaki fragments
6. The enzyme DNA ligase joins the Okazaki fragments to the growing strand
7. Two identical copies of DNA are made and each copy is made up of one original DNA strand and one newly synthesised DNA strand and so is semi-conservative

Question 4

What is the role of DNA helicase?

Answer 4

It is an enzyme that catalyses the unwinding and separating of strands in DNA replication

Question 5

What is the role of RNA primase?

Answer 5

It is an enzyme that catalyses the synthesises of RNA primers which initiate DNA replication

Question 6

What is the role of single-strand binding proteins?

Answer 6

They keep the two DNA strands separated by preventing secondary structure formation

Question 7

What is the role of DNA polymerase?

Answer 7

It is an enzyme that catalyses the formation of phosphodiester bonds between adjacent nucleotides in DNA replication

Question 8

What is the role of DNA ligase?

Answer 8

It joins the ends of DNA segments together, for example the Okazaki fragments

Question 9

Describe the experimental process of finding out that DNA replicates semi-conservatively

Answer 9

1. Bacteria were grown in a medium containing 15N, so all their DNA is ‘heavy’.
2. The bacteria were transferred to a medium with 14N for one round of replication, so the lighter nitrogen was incorporated into any new DNA strands they made.
3. The DNA was extracted and centrifuged.
4. Steps 2-3 were repeated for another round of replication.
5. The distribution of heavy and light DNA was analysed to track how the DNA was replicating:
   ->The heavier bands sink lower in the test tube.
   ->The intermediate bands, made of DNA with one heavy strand and one light strand, are in the middle of the test tube.
   ->The lighter bands are higher up in the test tube.

Question 10

What were the results in the first replication?

Answer 10

• The original, heavy DNA strands separate.
• Each heavy strand acts as a template for a new complementary strand.
• New, light strands form alongside the original strands.
• Each resulting DNA molecule consists of one old, heavy strand and one new, light strand.
  -> All DNA was intermediate

Question 11

What were the results in the second replication?

Answer 11

• Both original strands and the two new strands act as templates.
• New, light strands form alongside all four templates.
• Half the resulting molecules have one, original heavy strand and one new light strand.
• The other half are completely made of light strands.
  -> 1/2 was intermediate and the other 1/2 was light