1.7 DNA Replication

Question 1

Why is semi-conservative DNA replication important?

Answer 1

• Each new DNA molecule consists of one original and one new strand
• Ensures genetic continuity between generations of cells

Question 2

How does DNA replicate semi-conservatively?

Answer 2

• DNA helicase breaks hydrogen bonds between complementary bases, unwinding double helix
• Both strands act as templates
• Free DNA nucleotides join to exposed bases by complementary base pairing
• Hydrogen bonds form between adenine-thymine and guanine-cytosine
• DNA polymerase joins adjacent nucleotides by condensation reactions, forming phosphodiester bonds

Question 3

Why does DNA polymerase move in opposite directions along DNA strands?

Answer 3

• DNA has antiparallel strands
• So shapes of nucleotides at two ends are different
• DNA polymerase is an enzyme with a specific shape active site
• So can only bind to substrate with complementary shape

Question 4

How did Meselton & Stahl validate semi-conservative DNA replication?

Answer 4

• Bacteria grown in heavy nitrogen so heavy nitrogen incorporated into DNA bases
• DNA settled near bottom of centrifuge as contained two heavy strands
• Bacteria transferred to medium containing light nitrogen to divide once
• DNA settled in middle as contained one heavy and one light strand
• Bacteria in light nitrogen divide again
• Half DNA settled in the middle as contained one heavy and one light strand and half near top as contained two light strands