Ch. 11 DNA Replication (part 1) Flashcards
How is DNA replicated? (2 descriptors)
semiconservatively and semidiscontinuously
Where does DNA replication begin? How does this differ between prokaryotes and eukaryotes?
Begins at an origin of replication (ori).
Prokaryotic circular chromosomes have one ori and two replication forks.
Eukaryotic linear chromosomes have multiple ori and multiple replication forks.
What does it mean that DNA replication is semidiscontinuously?
Leading strand is continuous
Lagging strand is discontinuous
What is DNA synthesis done by?
polymerases
How do DNA pol’s extend DNA?
ONLY extend DNA 5’ → 3’ (add to the 3’ end)
What do DNA pol’s require to synthesize DNA? (2)
They require a template strand to guide new strand synthesis through base pairing.
They require a primer containing a free 3’-OH group.
What is the primer for DNA pol in cells?
RNA (placed by RNA pol)
How does proper base pairing occur with DNA pol?
Only proper base pairs can fit into the active site of DNA pol. Incorrect bases are not in line.
(get an error every 10⁴ - 10⁵ bases)
How does a nucleotide get added to a growing DNA strand by DNA pol? (3)
- the 3’-OH of the primer attacks the ɑ phosphate of the dNTP
- pyrophosphate (PPᵢ) is released
- DNA pol slides one base forward and repeats
What does the action of DNA pol require as far as enzyme structure? (2)
Two active sites…
1. insertion site (new base)
2. postinsertion site (previous position)
What feature/activity do many DNA pol’s have?
3’→5’ exonuclease activity
What is a nuclease?
enzymes that can break phosphodiester bonds
What is an exonuclease?
a nuclease that shortens DNA from the ENDS
What is an endonuclease?
a nuclease that shortens/cuts DNA INTERNALLY
What does 3’→5’ exonuclease activity do?
It removes erroneous base pairs and increases accuracy by 10² - 10³. (DNA PROOFREADING)
Where does 3’→5’ exonuclease activity occur in the DNA pol?
At a different active site than synthesis.
What feature/activity do a few DNA pol’s have?
5’→3’ exonuclease activity
What does 5’→3’ exonuclease activity do? (3)
It removes RNA primers during replication and is involved in DNA recombination and repair.
How many DNA pol’s does E. coli have?
five
What does DNA pol I of E. coli do?
It removes RNA primers and is used during DNA repair mechanisms.
What does DNA pol II, IV, and V of E. coli do?
They are expressed following extensive DNA damage and are not very accurate. (NO 3’→5’ exonuclease activity)
What does DNA pol III of E. coli do?
It is the main DNA pol for chromosome replication.
What does a DNA pol structure resemble?
A right hand
What does the “palm” of DNA pol contain?
It is where DNA sits. The pol active site.