Lecture 15- DNA Replication Flashcards
How does DNA replicate? Who found out?
It replicates in a semi-conserved manner, meaning that the two strands unwind, and are both completed making 2 DNA molecules that each contain one new and one old strand.
Meselson and Stahl found that out.
What do polymerases do? How do they do it?
Polymerases catalyze the formation of a phosphodiester bond between the 5’ end of a free-floating nucleotide and the 3’ end of the DNA (OH group).
What does polymerase need to function?
Polymerase can only attach a nucleotide to an existing 3’ OH group, it therefore needs a primer to start working. The primer is attached by RNA primase.
What is the difference between endo and exonuclease activity? What do these represent?
Endo and exonuclease are methods that polymerases use to cut DNA internally. Endo is like scissors, exo is like degrading DNA from a loose end.
How does DNA get unwound for DNA polymerase III to do its job?
It is unwound by the protein helicase, which melts the hydrogen bonds between the base-pair nucleotides. Each strand is then stabilized by single strand binding proteins.
What other protein helps the unwinding of DNA?
Topoisomerase, which relieves the increased tension.
What are the two strands for DNA replication and what is the direction of replication?
There are the lagging and leading strands. They both work 5’ to 3’.
How does the lagging strand work?
Primers are constantly added by primase to the lagging strand next to helicase so that DNA polymerase can work backwards creating Okazaki fragments, which end at the previous primer. Then, DNA polymerase I uses its exonuclease activity to cut and replace the RNA primer by the appropriate DNA. Finally, DNA ligase links the gaps.
What is the replisome?
All the steps of DNA replication put together.
What is the main difference between prokaryotic and eukaryotic DNA replication?
Eukaryotic DNA replication has many origins of replication across several chromosomes.
What are telomeres?
Bits of repetitive DNA at the edges of chromosomes that protect the DNA from shrinking during lagging strand synthesis.
Why are telomeres needed?
Because of the nature of lagging strand synthesis, there is no OH present at the very end of a strand, so their primers cannot be replaced by DNA. Telomeres do not code for anything, which makes the shortening of the DNA harmless.
What if telomeres are shortened to the point that there is no more?
This doesn’t happen because of the enzyme telomerase, which extends the telomeres.
What are the two methods in which DNA can be repaired?
- Photorepair, where thymine dimers are fixed by photolyase, which absorbs light energy and disrupts the dimer.
- Excision repair, where damaged or mismatched base pairs are removed by DNA excision repair enzymes and replaced by DNA polymerase I and DNA ligase.
