1.5.2 DNA replication ( biological molecules )

Question 1

why is semi conservative replication important

Answer 1

ensures genetic continuity between generations of cells

Question 2

describe the process of semi conservative DNA replication

Answer 2

• dna helicase breaks hydrogen bonds between complementary bases , unwinding the double helix
• both strands act as templates
• free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
• hydrogen bonds form between adenine - thymine and guanine - cytosine
• DNA polymerase joined adjacent nucleotides on new strand by condensation reaction
• forming phosphodiester bonds

Question 3

use your knowledge of enzyme action to suggest why DNA polymerase moves in opposite directions along DNA strands

Answer 3

• DNA has antiparallel strands
• so shapes of nuelcotides on two ends are different
• DNA polymerase is an enzyme with a specific shaped active site
• so can only bind to substrate with complementary shape

Question 4

name the two scientists who proposed models of the chemical structure of DNA and DNA replication

Answer 4

Watson and Crick

Question 5

Describe the work of meselson and Stahl in validating the watson - crick model of semi conservative DNA replication

Answer 5

1) bacteria growth in medium containing heavy nitrogen (N15) so nitrogen is incorporated into DNA based
• DNA extracted and centrifuged => settles near bottom as all DNA molecules contain 2 “heavy strands “
2) Bacteria transferred to a medium containing light nitrogen ( 14N) and allowed to divide once
•DNA extracted and centrifuged - settles in the middle as all DNA molecules contain 1 original heavy and 1 new light strand
3) bacteria in light nitrogen (14N) allowed to divide again
• DNA extracted and centrifuged => half settled in middle as contains 1 original heavy and 1 new light strand , half settled near top as contains 2 light strands