Lecture 18: The Genetic Code Flashcards
How to synthesize new DNA?
Add the following in a test tube?
- Triphosphate nucleotides(bases with phosphate)
- DNA polymerase III(enzyme that catalyses replication)
-Template DNA strand
-Ragged Ends
What is a ragged end?
-A stretch of DNA that is partly double stranded and partly single stranded
When does DNA replication occur?
During interphase
How does DNA polymerase III work?
-Find the end of the area of double strandedness and binds to the 3’ hydroxyl group
-It then sees what nucleotide base is on the opposite strand and adds its complementary base
Importance of ragged ends?
-Without ragged ends you would only have one single strand of DNA
-A double strand is needed so that the DNA polymerase III has a 3’ hydroxyl to bind to
Role of helicase?
Enzyme that unwinds the DNA
Role of primase?
-Enzyme that adds a short sequence of complementary RNA(primer), this acts as the ragged end for DNA polymerase III
What are origins of replication? How many do eukaryotes/bacteria have?
Specific spots where replication starts
- Eukaryotes: have multiple origins of replication along a DNA strand
-Bacteria: Have one origin of replication
What is a replication bubble?
Helicases unwind DNA in different directions from the origins of replication
What is replication fork?
-Half of a replication bubble
How does primase work?
1.Pry the double strand of DNA apart(creating a replication bubble)
2.Primase as a little piece of RNA (10-20 base pairs long)( primer)
3.The primer provides the 3 prime hydroxyl that the DNA polymerase III can now use
4.Synthesis then occurs(addition of nucleotide bases)
Does primase require a 3’ hydroxyl to add RNA primer?
NO
Leading Strand vs Lagging Strand
Leading Strand: Run 3’ to 5’ towards the fork and is replicated continuously(Template strand runs 5’ to 3’)
Lagging Strand: Runs 5’ to 3’ towards the fork and is replicated discontinuously(template strand runs 3’ to 5’)
How is the lagging strand transcribe?
- Primase adds and RNA primer
- DNA polymerase III then add DNA nucleotides (Okazaki fragments)
How much primer is on the leading and lagging strand after replication?
Leading strand: Only contains one primer
Lagging strands: Contains multiple primers mixed within DNA
What does DNA polymerase I do ?
1.Rips out the RNA primers between okazaki fragments
2. Then uses the 3’ hydroxyl from the previous okazaki fragment to fill in the gap with DNA nucleotide bases
What still must be done after RNA primers are ripped out and replaced with DNA?
- The Okazaki fragments are missing phosphate bonds between them
What does DNA ligase do?
Catalyzes the formation of the phosphodiester bonds between the 3’ end of one fragment and the 5’ end of another okazaki fragment
What happens if there is a mistake?
-The DNA polymerase all have the ability to proofread the strands (they immediately remove bases that are not complimentary)
What does mismatch repair do and when does it work ?
- Repairs during recombination, when strand are being annealed back together
-Correct newly synthesized DNA and make sure bases are properly matched