DNA Structure and Function

Question 1

DNA strands are antiparellel

Answer 1

• If one strand reads from 3’ to 5’, then the other reads from 5’ to 3’

Question 2

DNA strands are complementary

Answer 2

• adenine pairs with thymine and cyanine pairs with guanine

Question 3

3 models of DNA replication

Answer 3

• Conservative - one of the daughter cell is the photocopy of the parental cell; all new material
• Dispersive - The parental DNA is cut into pieces and distributed among two daughter DNAs; new material fills in the gaps
• Semiconservative - the two strands of the parental DNA are split; new material makes one complementary strand per daughter DNA

Question 4

Meselson and Stahl

Answer 4

• grew bacterial cells in an N15 environment
• after 20 minutes, these pure N15 cells were transferred to a lighter N14 environment.
• DNA samples were extracted after every 20 minutes
• The band pattern revealed that the semiconservative model accurately reflected DNA replication

Question 5

Prokaryotic DNA replication

Answer 5

• have a single circular chromosome
• replication bubble is formed, replication takes place on both ends of the fork until two circular chromosomes are formed

Question 6

Eukaryotic DNA replication (enzymes)

Answer 6

• Helicase - used to unwind DNA
• Topoisomerase - prevent supercoiling
• Single-strand-binding proteins - prevent forming of H-bonds between nitrogenous bases
• DNA primase - makes primers
• DNA polymerase III - synthesizes DNA
• DNA polymerase I - removes primers and synthesizes DNA
• DNA polymerase II - involved in repair
• Ligase - joins gaps between Okazaki fragments

Question 7

Leading vs lagging strands

Answer 7

• DNA polymerases read in the 3’ to 5’ direction
  Therefore,
  
  • the 3’ to 5’ strand is the leading strands (read in one go)
  • the 5’ to 3’ strand is read in fragments called Okazaki fragments; need multiple primers

Question 8

Telomerase

Answer 8

Telomeres are a template sequence of nucleotides that attach to the 3’ end of the lagging strand
- older cell = less telomerase activity
- telomerase activity is higher in cancer

Question 9

Mutagens

Answer 9

factors that increase the chances of mutations

Question 10

2 mechanisms of mutation control (thymine dimer)

Answer 10

1. Photoreactivation repair
   DNA photolyase attaches and breaks the bond between thymine dimers using sunlight
2. Excision repair
   Endonuclease removes the region of the thymine dimers and uses DNA polymerase and ligase to synthesize the DNA gap.