UNIT 3 LESSON 3 - DNA REPLICATION

Question 1

what are the models of dna replication

Answer 1

1. conservativ : one new molecule and conserve the old.
2. semi-conservative: two hybrid molecules of one old strand and one new strand.
3. dispersive: hybrid molecules with each strand being a mixture of old and new strands.

Question 1

What is dna replication

Answer 1

dna replication is the process of copying one dna molecule into two identical molecules. happens during the s phase of interphase in the cell cycle

Question 2

who determined and how to determine the correct model

Answer 2

• matthew meselson and franklin stahl reasoned that they could test the models if they could distunguish between parent and daughter strands of DNA>
  they used 2 diff isotopes of nitrogen to label the dna (light N and heavy N).
• nitrogen is a component of dna and would be incorporated into newly synthesized daughter strands.
  dna with heavy nitrogen would be more dense than dna with light nitrogen.

Question 3

remember the meselson and stahl experiment

Answer 3

ok

Question 4

what are the important proteins and enzymes in dna replication

Answer 4

1. helicase: helps unwind the parent dna & breaks the hydrogen bonds apart
2. SSB proteins: keep the two strands apart
3. gyrase: relieves any tension from the unwidning of the double helix
4. RNA primase: synthesizes an RNA primer needed to begin the new strand (provides a 3’ end)
5. DNA polymerase III: Adds nucleotides to the 3’ end of a growing chain
6. DNA polymerase I: removes the RNA primer & replaces it with DNA and can also proofread newly synthesized dna.
7. DNA ligase: joins the okazaki fragments in the lagging strand
8. DNA polymerase II: proofreads the newly syntehsized DNA

Question 5

what is the process of replication

Answer 5

semi conservative replication has 3 phases:
1. initiation phase: a portion of the dna double helix is unwound to expose the bases for new base pairing
2. elongation phase: two new strands are assembled using the parent DNA as a template. the new DNA molecule reform into double helices.
3. termination phase: the process is completed and the two new dna molecules separate from one another. the replication machine is dismantled.

Question 6

what goes on in initiation

Answer 6

• replication starts at a specific nucleotide sequince called the origin of replication.
• enzymes/proteins involved:
  – helicase unwinds dna and breaks h bonds between the strands.
  – ssb proteins keep the two strands apart
  –gyrase helps to relieve any tension
• initiation creates a replication bubble with two y-shaped regions at each end of the unwound area.
• each y-shaped region is called a replication fork. !! know that!!

Question 7

what goes on in elongation

Answer 7

• dna polymerase III adds new dna nucleotides one ata time to create a dna stradn that is complemntary to the parental srand.
• but,,,, it cannot attach a nucletide to thin air, it only attaches new nucleotides to the free 3’ hydroxyl end of a pre-existing hcain of nucletodies.
• polymerase iii can only work in the 5’ to 3’ direciton

Question 8

what happens with the leading strand

Answer 8

rna primase assembles about 10 rna nucleotides to provide a place where dna polymerase iii can begin.
- dna polymerase iii will keep adding new dna nucletodies to the 3’ end as it moves along in the same direction as the replication fork.
dna polyermase i will remove the rna priemr and replace it with dna.

Question 9

what happens in the lagging strand

Answer 9

the lagging strand must also be built in the 5’ to 3’ direction.
- dna polyermase iii must move in the opposite direction to the replication fork.
– this results in the synthesis of the lagging strand to occur in short segments and in a discontinous matter.
–these short segments of dna are called okazaki fragments.

Question 10

the lagging stand

Answer 10

1. primase adds several small rna primers.
2. the primers are elongated by dna polymerase iii to create the okazaki fragments
3. dn polyermase i removes he primer and replaces them with dna nculeotides.
4. dna ligase joins the okazaki fragments by creating phosphodiester bonds.

Question 11

what is termination

Answer 11

termination cocurs when the synthesis of the new dna strands is complete.
the two dna moelcules seperate from each other and the replication machine is dismantled.

Question 12

correcting errors during replication.

Answer 12

• dna polymerase i and ii have proofreading abilities. these enzymes recongnize and correct errors in the newly synthesized strands of dna. this method repairs about 99 percent of mismatch errors that occur during replication.
  another mechanism for correcting dna replication errors is called mismatch repair.
  – this is when a group of proteins recongnize a mispaired nucleotide on the newly synthesized strand and replace it with a correctly apired nucleotide.
• errors that remain after dna polymerase proofreading or mismatch repiar are considered mutations once cell division occurs.