3.4: DNA replication

Question 1

Explain DNA replication in terms of unwinding of the double helix and separation of the strands by helicase, followed by the formation of the new complementary strands by DNA polymerase:

Answer 1

Helicase:

• Unwinds the DNA and separates the two polynucleotide strands by breaking the hydrogen bonds between complementary base pairs
• The two separated polynucleotide strands act as templates for the synthesis of new polynucleotide strands

DNA Polymerase:

• Synthesises new strands from the two parental template strands
• Free deoxynucleoside triphosphates (nucleotides with three phosphate groups) are aligned opposite their complementary base partner and are covalently bonded together by DNA polymerase to form a complementary nucleotide chain
• The energy for this reaction comes from the cleavage of the two extra phosphate groups.

Question 2

Explain the significance of complementary base pairing in the conservation of the base sequence of DNA:

Answer 2

Complementary base pairing is very important in the conservation of the base sequence of DNA. This is because adenine always pairs up with thymine and guanine always pairs up with cytosine. As DNA replication is semi-conservative (one old strand an d one new strand make up the new DNA molecules), this complementary base pairing allows the two DNA molecules to be identical to each other as they have the same base sequence. The new strands formed are complementary to their template strands but also identical to the other template. Therefore, complementary base pairing has a big role in the conservation of the base sequence of DNA.

Question 3

State that DNA replication is semi-conservative:

Answer 3

DNA replication is semi-conservative.