DNA replication

Question 1

What is semi-conservative replication? (3 pts)

Answer 1

1. The two original DNA strands separate during replication;
2. each strand then serves as a template for a new DNA strand,
3. this means that each newly synthesized double helix is a combination of one old (or original) and one new DNA strand.

(Conservative replication would mean that the strands stay together)

Question 2

Why does DNA copy itself?

Answer 2

This is part of cell division - so each new cell has a replica of the original DNA

This permits genetic continuity between the generation of cells

Question 3

What is the process of DNA replication?

Answer 3

1. DNA helicase is an enzyme that unzips the DNA helix (breaks the h-bonds)
2. The unzipped strands now act as a template for copying
3. This is done by complementary bases pairing up with the unzipped strands (A with T, C with G)
4. DNA polymerase enzyme speeds up the re-zipping of the strands (h-bonds reforming)
5. Each new DNA contains one original and one new strand (semi-conservative replication)

Question 4

Which way does polymerase move along DNA strands?

Answer 4

Opposite ways along anitparallel strands.

Each DNA strand is slightly different : one end is 3’ and the other 5’ (3 prime, 5 prime)

The strands run in opposite directions as they are ‘anti-parallel’

Cool video

Question 5

How did Meselson and Stahl provide evidence for semi-conservative replication?

Answer 5

Two samples of bacteria - one in light nitrogen (N₁₄) broth, the other in a heavy nitrogen (N₁₅) broth

*heavy nitrogen is an isotope with an extra neutron*

The reproducing bacteria picked up nitrogen for their new DNA

Sample of DNA taken from both batches and put in centrifuge; the DNA pciking up heavy nitrogen settled lower than the light nitrogen so that gives researchers a clue…

Sample of bacteria was then removed from heavy and put into the light; they were left for one replication;

Sample of that bacteria’s DNA then taken and put into the centrifuge.

HYPOTHESIS:

If the DNA did not split, the heavy DNA would settle at the bottom;

If the DNA split, one strand would be heavy, one light, so would settle half way

RESULT:

It settled in the middle,

CONCLUSION

the results indicate semi-conservative (unzipping) replication

COOL VIDEO

Question 6

What can go wrong with DNA

Answer 6

Transcription can produce mistakes in mRNA, which can result in faulty or no proteins from that dodgy mRNA strand.

Inherited ‘errors’ in DNA cause genetic conditions, which arise with the DNA is copied.

Sometimes mutations can arise in the ovary/testis that is dividing to form a an egg/sperm, hence it can be passed on

A lot of mutations have no effects as much of the DNA found in a cell does not play an active role in protein synthesis

Question 7

What is the mutation in CF?

Answer 7

A section of DNA on chromosome 7

this carries the code to make the CFTR protein

This protein has 1480 amino acids.

Hundreds of mutations lead to the pathology of CF

e.g, sometimes ATP is unable to bind and open the ion channel

othertimes, the channel opes but protein structural changes lead to reduction of Cl ions passing through

DF508 mutation is the deletion of three nucleotides causing the loss of the aa phenylalanine, the 508th aa in CTFR