UNIT STUDY: DNA Replication Flashcards
Why is DNA Replication Important?
When a cell divides into two daughter cells, it must contain the same genetic information, or DNA, as the parent cell
When does DNA replication take plase
During the S phase (interphase) of a cells life cycle
Proposed models for DNA Replication
3 possible models proposed by Watson and Crick
1. Semiconservative (what DNA replication is): one old strand and one new strand in each daughter molecule
2. Conservative: One molecule has only old strands and one has only replicated strands
3. Dispersive: Elements of old and new strands are in both molecules
How was DNA proven to be semiconservative
By an e. coli experiment between Meselson and Stahl
Molecular Events in DNA Replication
- Initiation: A portion of the DNA doubnle helis is unwound to expose the bases for new base pairing
- Elongation phase: two new strands of DNA are essembled using the parent DNA as the template
- Termination phase: the replication process is completed and two DNA molecules are seperated from each other
What enzymes are used in DNA Replication
Gyrase (topisiomeraase) - takes tension out of double helix making it a flat shape
Helicase - brakes up H bonds between nitrogonous bases
Primase - adds primer to 5’ end so DNA polymerase III knows where to start replication
Ligase - bonds okasiki fragments together making phosphidester bonds
Single Stranded Binding Proteins - bond to outside of DNA and prevents bases from rebonding
DNA polymerase III - Attatches corresponding nucliec bases to parent strand
DNA polymerase I - removes RNA primer added by primase and proof reads DNA
DNA polymerase II - Proof reads DNA
What happens in the initiation phase
- DNA is unwound by gyrase enzymes
- Helicase breaks H-bonds holding comlimetry strands together
- SSBP stabalize the newly unwound DNA
- Replication bubbles are formed
How is DNA read vs synthesized
Read - 3’ to 5’
Synthesized - 5’ to 3’
What happens in the elongation phase
- Primase instals RNA primers
- DNA polymerase III bonds complementary base pairs together, building a complementary strand
- DNA polymerase I removes RNA primers and proof reads complimentry strand
- DNA polymerase II proof reads complimentry strands
- Ligase joins DNA fragments together creating phosphidester bonds between nucleotides
Explain the leading vs lagging strand
- Leading strand built in direction of helicase and replication fork
- In the leading strand, DNA polymerase III reconizes the RNA primer and attatches nucleotide bases until it reaches the replication fork and DNA is terminated
- Lagging stand is built in the oppisate direction of the helicase and replication form
- In the lagging strand, DNA Polymerase III builds 5’ to 3’, but then a new one must attatch to a new RNA primer and work in the oppisate direction of the helicase
- The lagging strand is slower and creates okazaki fragments
What happens in the termination phase
- When newly formed strands are complete they rewind back into a double helix structure
- Protien-DNA complex at each replication form is dismantled
- Strands fall apart, enzymes float away
- Two new DNA molecules seperate from one another
How does quality control of DNA happen
DNA polymerase I and II proofread as an exonuclease
- Backtracks over newly synthesized strand to cut out any incoreect nucleotides and replace them with matching base pair
What are telomeres
- The aglets of DNA
- Non-coding region at the end of each chromosome
- 5’-TTAGGG-3” (x3000)
- Telomeres act as buffer to protect gene coding region of DNA
- As cells reproduce, telomeres shorten (11BP lost per replication)
- Once telomeres have been degraded, genetic info is destroyed and cells undergo apoptosis
- Too much cell death causes aging
