DNA Replication - WordPress.com · 2012-12-17 · DNA Replication.notebook SemiConservative...
Transcript of DNA Replication - WordPress.com · 2012-12-17 · DNA Replication.notebook SemiConservative...
DNA Replication.notebook
DNA ReplicationRecall:The Organization of DNA
• Each double helix of DNA is really long and unruly. They need to be organized in a compact manner within the nucleus.
• Eukaryotic cells have much more DNA than bacteria and viruses, and so the organization is more sophisticated.
double helix
wrapped around histone proteins
coiled around itself (chromatin)
folded back upon itself (chromosomes)
Chromosomal form appears only during mitosis, and is used in karyotypes.
DNA Replication.notebook
SemiConservative Replication
Recall:During interphase of the cell cycle, cells make an extra copy of their DNA to prepare for mitosis.
Each strand of the original double helix (parent strand)serves as a template to match the sequence of base pairs in a complimentary strand.
SemiConservative Replication
Original DNA
1st Replication
2nd Replication
DNA Replication.notebook
Phases of ReplicationThere are three phases of replication which happen simultaneously
on the DNA molecule.
Phase 1: Initiation
Phase 2: Elongation
Phase 3: Termination
DNA Replication.notebook
Replication fork
As the replication bubble expands, the junction where the two strands are still attached is called the replication fork.
Phase 1: InitiationReplication Origins: These sites are characterized by a specific sequence of base pairs. A group of enzymes recognize the sequence and bind to the site, unraveling the strands and separating them to form an opening called a replication bubble.
DNA Replication.notebook
Start Point of Bubble
RNA Primer
Helicase
DNA Polymerase
Start Point of Bubble
An enzyme called DNA helicase unwinds the double helix just ahead of the replication fork.
Within the replication bubble, DNA polymerase then binds to the parent strands within the bubble, using them as a template to add complementary nucleotides one at a time.
Once the strands have separated, a small piece of RNA called a primer attaches to the separated parent strands to begin replication.
InitiationDirection of Replication
DNA Replication.notebook
Phase 2: Elongation
DNA polymerase lengthens the new strands by adding nucleotides to the free 3’ hydroxyl end of a preexisting chain.
Therefore replication can only take place in the 5’ to 3’ direction of the new strand.
Special Circumstance:
DNApolymerase
DNA Replication.notebook
But the two strands run antiparallel....what about the strand running in the other direction?
The leading strand is able to replicate continuously. It adds nucleotides to the new strand in the same direction as the fork opens.Only one primer is required.
The lagging strand must be built in small sections. As the fork continues to open, a new fragment is begun with a primer and elongates to meet the fragment behind it. DNA Ligase links the Okazagi fragments together to form the daughter strand.
DNA Replication.notebook
Remember, in replication there are two forks, and hence two leading and lagging strands.
Original point of origin
LEADING
LEADING
LAGG
ING
LAGG
ING
DNA Replication.notebook
Phase 3: TerminationOnce the RNA primers are removed at the end, or telomere of the strand,there is a small gap that cannot be filled in.
As a result, each daughter strand is slightly shorter than the parent strand!
Fortunately, telomeres are composed of highly repetitive noncoding nucleotide sequences that act as a buffer.
Lagging Strand Issue:
DNA Replication.notebook
MistakesDespite the specific Hbond arrangement between complementary base pairs, occasionally an incorrect nucleotide is inserted into the replicated strand.
DNA polymerase can recognize when Hbonding does not occur, and rejects the nucleotide that was inserted.
If the nucleotide error is missed, a single point mutation occurs.
DNA Replication.notebook
http://www.youtube.com/watch?v=4jtmOZaIvS0
http://www.youtube.com/watch?v=AGUuX4PGlCc&feature=related
http://www.youtube.com/watch?v=teV62zrm2P0
http://www.youtube.com/watch?v=zdDkiRw1PdU&feature=related
DNA Replication clip 1
DNA Replication clip 2
DNA Replication clip 3
DNA Replication clip 4 - Real time animation