#2: DNA Replication (Details) Flashcards
Two main steps in DNA replication
- Separating the two strands of nucleotides
2. Building the new strands of DNA
DNA helicase
hydrogen bonds between the two strands of nucleotides are broken by DNA helicase
(unzips parent chain)
Single stranded binding proteins (SSBs)
are small pr- molecules that bind to the two strands of nucleotides to prevent them from reforming their hydrogen bonds
Topoisomerases and tension
as DNA helicase unwinds the double stranded molecule of DNA, tension will develop in the molecule of DNA
Topoisomerases relieves this tension in DNA strands by cutting one or two of the strands near the replication fork, allowing them to untangle, and then rejoining the cut strand(s)
Origin of replication
site at which DNA helicase unwinds the molecule of DNA
prokaryotic cells only have one origin of replication
eukaryotic cells have multiple origins of replication
Replication Fork
Replication bubble
Replication fork: the edges of the replication bubble where the two strands of nucleotides are still joined together
the replication bubbles will continue to get larger and larger until the entire molecule of DNA has been reproduced
Problem
DNA can only be replicated in a 5’ to 3’ direction
as a result, the new strand of DNA must start at the 3’ end of the original strand
a problem arises from the fact that the two strands of DNA run antiparallel
one of the new strands of DNA will be formed as a continuous molecule (leading strand)
the other strand of DNA will be formed as a discontinuous molecule (lagging strand)
Primase
responsible for laying down an RNA primer that will be used by DNA polymerase III to build the new strand of DNA
DNA Polymerase III
responsible for adding the correct nucleotide to the developing strand of DNA
DNA Polymerase I
responsible for removing the original RNA primer and replacing it with the correct sequence of DNA nucleotides
DNA Ligase
responsible for joining together all of the different fragments in the growing strand of DNA
Continuous Strand (leading strand)
as a molecule of DNA is unwound, primase attaches an RNA primer to the 3’ end of the original DNA molecule
after the RNA primer has been attached, DNA polymerase III will start to build the new strand of DNA in a 5’ to 3’ direction
Discontinuous Strand (lagging strand)
as a molecule of DNA is unwound, primase attaches an RNA primer to an exposed 3’ end of the original 5’ to 3’ strand
after the RNA primer has been attached, DNA polymerase III will start to build the new strand of DNA in a 5’ to 3’ direction
the fragmented strands of DNA in the discontinuous strand are called Okazaki fragments
after the strands have been formed, DNA polymerase I removes the RNA primer and replaces it with the proper DNA nucleotides
once the RNA primers have been removed and replaced, DNA ligase attaches all of the fragmented pieces together into a single strand of DNA
Dealing w Errors
DNA polymerase enzymes proofread and correct errors.
DNA polymerase III usually backs up and replaces any incorrect bases with correct copy.
DNA polymerase I and II, move slowly along strand looking for distortions.
A repair complex determines the incorrect base, replaces it
DNA ligase fills in the gaps by repairing the phosphodiester bonds
