DNA replication Flashcards
Where does the DNA replication occurs ?
It occurs in nucleus
When does it happen ?
During the Inter phase of cell cycle.
What the function of helicase
1) Unzips the 2 strands of DNA
What the function of DNA polymerase
1) Adds DNA bases
What the function of Primase
1) initializer
2) Makes Primer
3) Primer is a small piece of RNA
What the function of Ligase
1) Helps attaching the Okazaki fragments
3 Main DNA replication Steps
1) Initiation
2) Elongation
3) Termination
What happens during Initiation step ?
1) DNA replication begins at the point of Origin
2) At the origin, helicase unwinds the Double stranded DNA by disrupting the hydrogen bonding between the base pairs to separate the strands
3) SSB (single stranded binding proteins) bind to these DNA strands to keep them separated.
4) Primase comes in an makes RNA primer on both strands.
5) The primer always acts as the starting point of replication
What happens during Elongation step ?
1) DNA polymerases are responsible creating the new strand by a process called elongation.
2) DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand.
3) Polymerase can only add nucleotide in 5’ to 3’ direction.Therefore, 5’ to 3’ DNA strand is called Leading strand and it goes through continuous replication.
4) Whereas the 3’ to 5’ end is called lagging strand.The lagging strand begins replication by binding with multiple primers.DNA polymerase then adds pieces of DNA.These fragments are disjointed.
What happens during Termination step ?
1) Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands. These primers are then replaced with appropriate bases.
2) DNA ligase joins the Okazaki fragments
What are Okazaki fragments ?
Disjointed fragments of synthesized DNA on lagging strand.
Whats the function of Polymerase I ?
1) DNA polymerase I participates in the DNA replication of prokaryotes.
2) polymerase 1 removes the RNA primer and fills in the nucleotides which are necessary for the formation of the DNA in the direction- 5’ to 3’. It also helps in proof reading to see if there is any mistake done while replication
Whats the function of DNA Gyrase ?
Stabilizes the DNA double helix when helicase unzips the molecule at multiple sites
How the replication of DNA is semi-conservative ?
1) DNA replication is a semi-conservative process, because when a new double-stranded DNA molecule is formed:
One strand will be from the original template molecule
One strand will be newly synthesized.
2) Each new strand formed will be identical to the original strand separated from the template
The two semi-conservative molecules formed will have an identical base sequence to the original molecule
Mathew Meleson and Franklin stahl Semi-conservative model
1) Bacterial cells (E.coli) were grown in medium containing N15. N15 is a heavy isotope of Nitrogen.Since, Nitrogen is an essential part of DNA therefore, DNA had Heavy Nitrogen.
2) The cells grown in N15 media were harvested and introduced to N14 media
3) As the cells divided, they replicated their DNA
4) DNA was isolated and centrifuged to determine the composition of DNA.
5) The results after two divisions supported the semi-conservative model of DNA replication
After one division, DNA molecules were found to contain a mix of 15N and 14N, disproving the conservative model
After two divisions, some molecules of DNA were found to consist solely of 14N, disproving the dispersive model
