DNA replication Flashcards
what were the 3 main theories on DNA replication?
- conservative: complete transmission of DNA to daughter cells (which are perfectly identical to mother cells)
- semi-conservative:1 DNA -> generate 2 molecules of DNA (1 parental strand and a newly formed)
- dispersive: patched replication of DNA
describe meselson’s and stahl’s experiment and what it proved
proof of semiconservative replication (only replication)
- N15 (heavy isotope)
- N14 (light isotope)
- both added to bacteria through centrifugal force
- DNA will position at the density similar to its own (heavy-low, light-top)
- intermediary DNA forms after a single replication
- 2nd replication: chains form half intermediate density and half light density
- DNA is replicated in semiconservative way and the chromosomes derive from parent and new strand
what is DNA polymerase
main replication enzyme (works with other enzymes to unwind, replicate and re-close strand)
describe the actions of DNA polymerase in replication
- 5’ to 3’ nucleotide chains added
- contain multiple origins of replication
- complete replication faster for cell division
- fork of replication is asymmetrical: the speed of replication on the strands is different
- DNA polymerase always needs a free prime OH to add nucleotide chains
- RNA polymerase synthesises directly along DNA -> DNA polymerase recognises 3’ OH and starts polymerising DNA- created many times in lagging strand to initiate fragment
describe the theta models
- 1 strand has 3’-OH allows for straight replication (leading strand)
- polymerase is linear and progresses without interruption (towards the helicase splitting fork)
- 1 strand has opposite configuration so polymerase moves in the same direction in segmented strand (lagging strand, Okazaki fragments formed)
replication components: topoisomerase
prevent DNA tangling and supercoiling during replication (problem of topology) - DNA rotates on itself before the bond is recreated
replication components: helicase
moves around DNA (1000bp/s, very high speed) using hydrolysis of ATP to unzip the DNA strands in front of replication fork
replication component: single strand DNA binding protein
bind DNA where its open (single strand) to protect it - polymerase removes it when it starts the replication
replication components: DNA primase
synthesis of primer (RNA)
replication components: DNA polymerase
recognises the primase and starts the polymerisation of DNA
replication components: ligase
consuming ATP reforms the phosphodiester bond between ribose and phosphate between okazaki fragments (lagging strand)
proofreading mechanism of replication
avoid change in coding sequences in DNA
- polymerase incorporates a nucleotide and check the perfect match -> if its incorrect stops and starts degrading the DNA
- removes small piece of DNA and starts synthesising again
cell cycle
- G1 phase: where the cell lies
- S: DNA duplication (6-8 hours)
- G2: preparation for cell division
- M: mitotic division
what is the lac operon?
repressor/promoter mechanism for RNA synthesis -> create balance between proteins - happens with lactose
describe the RNA synthesis from DNA
RNA has the same sequence as the coding strand of DNA
- RNA polymerase acts from 5’ to 3’ strand
- opens DNA, reads sequence and synthesises the new RNA transcript in a single strand
what is RNA polymerase?
different polymerases act to transcribe different genes
describe the components involved in RNA synthesis
transcription unit: gene
TATA box: site of attachment for polymerase (TA bond is easier to break) (30 base pair further from TSS)
- causes DNA to bend (>90 degrees) to open the binding site for polymerase
gene regulatory proteins: regulate activity of polymerase to control levels of transcription
activator proteins -> activated by enhancer (sequence of DNA which acts as binding site for protein)
- mediator: allows interaction between simpler transcription proteins and enhancer (activates transcription)
TSS: Transcription start site
- TFIID -> necessary for transcription start
- tail in RNA polymerase: phosphorylation of ATP for energy production
what are general transcription factors?
each element has a specific sequence which triggers a general transcription factor
what is superhelical tension?
caused by unbinding in transcription
- DNA with free end: doesn’t cause alteration in structure
- DNA with fixed ends: causes supercoiling -> topoisomerase II stops superhelical structure
