Flashcards
DNA Replication
Cards in this deck (28)
- 1What did the Meselson-Stahl experiment look at?They grew bacteria in a medium containing N15 meaning the DNA of the bacteria would pick up some N15. These bacteria were then put in a medium containing N14. N15 was still present after two generations. Shows semi-conservative replication.
- 2What is the replisome?A large multiprotein complex required for DNA replication
- 3What is DNA polymerase?It is part of the replisome and is responsible for synthesising the DNA. To act, DNA polymerase requires a single stranded region of DNA as a template, deoxynucleoside triphosphate precursors and the 3'-OH group onto which the nucleotide is added
- 4What happens when an incoming deoxynucleoside triphosphate p[airs with the base on the template strand?A phosphodiester bond is formed
- 5Which direction does DNA polymerase work in?The 5' to 3' direction. It can only add nucleotides to the 3' end of a chain.
- 6What is primase?An enzyme that synthesises RNA primer which initiates DNA synthesis. The RNA primer is a sequence of 8-12 bases.
- 7What are Okazaki fragments?As the lagging strand has to be synthesises discontinuously being as DNA polymerase can only work in the 5' to 3' direction. It works in small fragments known as Okazaki fragments which are synthesises 5' to 3' and then DNA polymerase moves onto the next fragment
- 8How long are Okazaki fragments in bacteria?1000-2000 bases in length
- 9How is the RNA primer removed in the lagging strand?By the 5'-3' exonuclease activity of the DNA polymerase, then the DNA polymerase will replace the 'missing' bases
- 10What does DNA ligase do?it joins up the nicks between neighbouring Okazaki fragments using ATP
- 11What does the 'initiator protein' do?It binds to the Ori (origin of replication) and unwinds the DNA there
- 12What does Helicase do?Unwinds the DNA at the replication fork
- 13What do topoisomerases do?They relax the DNA and stop overwinding of the supercoiled DNA ahead of the replication fork. An example is DNA gyrase.
- 14What are the differences between DNA polymerase I and DNA polymerase III?DNA polymerase I removes and replaces the RNA primer.
DNA polymerase III is responsible for synthesis of the leading and lagging strands - 15What happens if a base is added incorrectly?DNA polymerase will use its exonuclease activity to remove the mismatched nucleotide
- 16What is the error rate in DNA replication?1 mistake per 10^7 bases copied
- 17Once post-replication repair is involved, what is the error rate per bases copied then?1 mistake every 10^9/10 bases copied
- 18What exactly do exonucleases do?Cleave nucleotides one at a time from the end of a polynucleotide chain. Restriction exonucleases recognise specific sequences and cleave those.
- 19Summary of bacterial DNA replication?They have one origin of replication (as their DNA is circular) and two bidirectional replication forks are formed. DNA polymerase synthesises DNA from 5' to 3'. Leading strand copied continuously but the lagging strand synthesised in Okazaki fragments.
- 20What is the rate of DNA polymerases in Eukaryotes?100-200bp per second. They are slower than their bacterial counterparts.
- 21How long are eukaryote Okazaki fragments?100-200 bases in length.
- 22Do eukaryotes also only have one Ori?No, eukaryotes have multiple origins of replication per chromosome, each generating bidirectional replication forks.
- 23What extra issue is there in eukaryotic DNA replication compared to bacterial DNA replication?The chromosomes are linear so there is a problem completing the lagging strand on the end of each chromosome
- 24What would the issue in eukaryotic DNA replication be if it wasn't for telomerase/telomeres?The DNA fragment would shorten one of the 5' ends of the daughter DNA because when the primer on the last Okazaki fragment is removed, DNA polymerase cannot fill in the gap.
- 25What are telomeres?The terminal ends of chromosomes that contain a highly repeated sequence
- 26What is telomerase and what does it do?It is a RNA containing enzyme and it can make additional copies of the repeat sequence on the 3' end, replacing those lost during DNA replication. It is a reverse transcriptase and the RNA acts as a template for DNA synthesis.
- 27Which cells have telomerase activity?Germ cells and a few other actively dividing cells. Somatic cells do not normally have telomerase activity. Somatic cells have enough teloere repeats to allow DNA replication during the organism's lifetime.
- 28What happens if telomeres become too short?It can trigger apoptosis (programmed cell death). Telomerase is also present in cancer cells as it allows the cancer cells to divide indefinitely.
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- Genetics I Flashcards on DNA Replication , created by Emma Lloyd on 26/04/2018.
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- Emma Lloyd
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