Chapter 6: DNA: The Molecular Basis of Life

Question 1

Base Pairing in DNA and RNA

Answer 1

1. In DNA:
   
   • A (adenine) pairs with T (thymine).
   • G (guanine) pairs with C (cytosine).
2. In RNA:
   
   • A (adenine) pairs with U (uracil).
   • G (guanine) pairs with C (cytosine).

Question 1

Steps in DNA Repair

Answer 1

The steps of DNA repair generally involve:

• Identifying the damaged section of DNA using repair protiens
• Removing the damaged nucleotides. (Exonucleases remove incorrect nucleotides).
• Replacing the damaged nucleotides with the correct ones using DNA polymerase III;(checks for errors and corrects them during replication.)
• Sealing the gap with DNA ligase.

• Mismatch Repair: Mismatch repair enzymes fix errors after replication.

Question 2

DNA replication: step one

Initiation

Answer 2

• Helicase unwinds the DNA double helix, creating replication forks.
• Single-strand binding proteins (SSBs) bind to the separated DNA strands to keep them stable and prevent them from rejoining.
• Primase synthesizes a short RNA primer to provide a starting point for DNA synthesis.
• DNA polymerase III binds to the primer and begins adding nucleotides in the 5’ to 3’ direction.

Question 3

DNA relication: step two

Elongation

Answer 3

• On the leading strand, DNA polymerase III synthesizes continuously in the 5’ to 3’ direction.
• On the lagging strand, DNA polymerase III synthesizes Okazaki fragments in the 3’ to 5’ direction.
• Each Okazaki fragment begins with an RNA primer.
• DNA polymerase I removes RNA primers and replaces them with DNA nucleotides.
• DNA ligase seals the gaps between Okazaki fragments, forming a continuous strand.

Question 4

DNA replication: step three

Termination

Answer 4

• Replication ends when the replication forks meet.
• The replication machinery disassembles.
• DNA polymerase performs final proofreading to correct any errors, ensuring the accuracy of the new DNA strands