1.5.2: DNA replication

Question 1

What are the 3 hypotheses for DNA replication

Answer 1

conservative, semiconservative, dispersive

Question 2

describe and explain the Meselson and Stahl experiment 1958

Answer 2

cultured 2 populations of E-coli bacteria, one on 14 N agar (light nitrogen), one on 15 N agar (heavy nitrogen). Nitrogen because N synthesises the nitrogenous bases. The bacteria grown in 15N medium was transferred to 14N medium and samples removed after 1, 2 and 3 generations. The DNA would be extracted from the bacteria, suspended in special solution, and centrifuged. DNA would then settle and result in a line in the tube. High = 14N DNA strand, Low = 15N DNA strand. After the first generation, there was a line in the middle which meant it was one heavy and one light strand. After second generation there was one mixed and one light strand. This experiment provided evidence that DNA replication is semiconservative

Question 3

What is conservative DNA replication

Answer 3

two helices where one helix contains both original strands and one helix contains both new strands

Question 4

What is semiconservative DNA replication

Answer 4

two helices that both have one original strand and one new strand

Question 5

What is dispersive DNA replication

Answer 5

two helices that both contain the original strand which contains fragment of the new strand

Question 6

What were the controls used in the Meselson and Stahl experiment 1958

Answer 6

DNA from E-coli grown in 14N medium extracted and suspended in special solution and centrifuged to produce a line representing light DNA (14N) which was quite high
Same with the heavy DNA which produced a line representing heavy DNA (15N) which was quite low

Question 7

Describe and explain how DNA replicates semiconservatively (2 marker)

Answer 7

· Both original polynucleotide chains act as a template.
· Both new DNA double helices contain one original polynucleotide and one new polynucleotide

Question 8

Describe and explain the sequence of events for semiconservative replication in DNA

Answer 8

DNA helicase unwinds and unzips (H bonds broken) the double helix
The 2 polynucleotides have exposed bases that act as as template - this is called the replication fork
Free DNA nucleotides in the nucleoplasm align themselves against each template strand
H bonds form between complementary base pairs joining the aligned nucleotides to their exposed base pair on the template
DNA polymerase catalyses the formation of the new polynucleotide strands (condensation reactions forming phosphodiester bonds)
Enzymes rewind the DNA molecules back into a double helix
As both of the original strands acted as a template, each newly formed DNA molecule has one original and one newly formed polynucleotide strand