DNA REPLICATION

Question 1

Why is DNA replication important?

Answer 1

So cells get the same number and type of chromosome

Question 2

What does the semi conservative replication of DNA ensure?

Answer 2

Ensures genetic continuity between generations of cells

Question 3

Step 1 of semi- conservative DNA replication:

Answer 3

• DNA helicase breaks the hydrogen bonds between complementary base pairs
• DNA needs to be partially unzipped because otherwise bases are exposed for a long period of time

Question 4

Step 2 of semi- conservative DNA replication:

Answer 4

• each exposed polynucleotide strand acts as a template where complementary free DNA nucleotides bind by the specific base paring rule- e.g. C-G

Question 5

Step 3 of semi- conservative DNA replication:

Answer 5

• nucleotides are joined to their complementary bases on the template strand by hydrogen bonds
• the formation of phosphodiester bonds between these new nucleotides in a condensation reaction catalysed by the enzyme DNA polymerase to form the new replicated DNA strand

Question 6

Step 4 of semi- conservative DNA replication:

Answer 6

• both of the DNA molecules now contain one of the original DNA trends and one new DNA strand composed of the free DNA nucleotides
• but it is identical to the previous strand thanks to the complementary base pairs

Question 7

DNA replication is….

Answer 7

• semi conservative
• each strand in the double helix acts as a template for synthesis of a new, complementary strand

Question 8

What direction does DNA polymerase work in?

Answer 8

3’ to 5’ direction on the template strand, it cannot work continuously on the lagging strand (which rubs 5’ to 3’) so it works against the direction of helicase and forms separate strands of joined DNA strands called OKASAKI FRAGMENTS
- DNA ligaments joins together these fragments to form a fully synthesised new DNA strand

Question 9

Describe how the proportion of DNA that contained 15N changed at each divisions hen bacteria were grown on the 14N region (2 marks):

Answer 9

• decreases by 50% per generation

Question 10

The change in the proportion of DNA containing 15N is due to the way in which DNA replicates, explain how (2 marks):

Answer 10

• in DNA replication strands separate
• each strand acts as a template for the formation of a new strand

Question 11

Describe how, after the parent DNA strands separated, the second strand of DNA in region Y was formed (3 marks):

Answer 11

1) DNA helicase unwinds DNA strands
2) complementary bases bind to each other
3) by hydrogen bonds
4) by a condensation reaction

Question 12

How are the two strands of the DNA molecule held together?

Answer 12

Hydrogen bonds

Question 13

One advantage of DNA molecules having two strands?

Answer 13

Protects bases

Question 14

The change in the proportion of DNA containing 15 N and 14 N is due to the way in which DNA replicates, explain how (3 marks):

Answer 14

• In DNA replication, each old strand acts as a template for a new strand
• So that one strand in each new DNA molecule is old and one is new
• Once e.coli is transferred to the medium containing 14 N then only light nitrogen can be used to form their new strands