Cell Biology- Hejmadi3- DNA replication

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What are the 4 requirements for DNA to be the genetic material?
1. Must carry information -genetic code, which must be stable 2. Must replicate -DNA replication 3. Must allow for information to change -mutation 4. Must govern the expression of the phenotype -gene function
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Define DNA replication
The process of duplication of the entire genome prior to cell division
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What is the significance of DNA replication?
It - preserves integrity of the genome in successive generations - replication rates vary (slower in eukaryotes due to DNA stored in very condensed form)
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Describe the basic 3 step process of DNA replication
1.Initiation -Proteins bind to DNA and open up double helix 2.Elongation -Proteins connect the correct sequences of nucleotides into a continuous new strand of DNA 3.Termination Proteins release the replication complex
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What are the 6 basic features of DNA replication?
Semi-conservative, starts at the origin, can be uni or bi-directional, semi-discontinuous, synthesis in 5'-->3', RNA primers required
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What is meant by DNA replication being semi-conservative?
the two daughter molecules will have one parental strand and one newly synthesized strand of DNA.
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What were the two other proposed models of DNA replication?
1)conservative model- 2 parental strands are kept together and so are 2 newly synthesized strands. 2)dispersive model, the strands of the daughter molecules contain pieces of the parental and pieces of newly synthesized DNA
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Who proved the correct model was semi-conservative and how?
Matt Meselson and Frank Stahl- by growing bacteria in different isotopes of nitrogen (ensure you can describe this experiment in detail - cell bio lecture 3 p2-3)
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Initiator proteins identify specific base sequences on DNA called ...?
sites of origin. (where replication starts)
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How many sites of origin do a)prokaryotes b)eukaryotes have?
a) single b)multiple
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What does DNA replication being uni or bi-directional refer to?
that replication forks can move in the same or opposite directions (usually opposite and DNA helices elongate from each origin until they eventually join echother- circular DNA)
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What does DNA replication being 'semi-discontinuous' refer to?
Leading strand synthesises continuously in direction of replication fork movement, whilst lagging strand synthesises fragments(discontinuous) in the opposite direction
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What are these fragments known as? by what and when are they joined together?
Okazaki fragments- sometime after their synthesis in a reaction catalysed by DNA ligase
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How do we know that DNA synthesis always occurs in the 5'-->3' direciton?
Due to pol1's proofreading capabilities (it can edit a DNA chain as it is synthesized so as to correct its mistakes. )
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What 3 important factors does DNA polymerase require to work?
dNTPs, a template strand, a primer
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Why is a template strand necessary?
to line up a new nucleotide in the right order so that the dNTPs can pair by complementary base pairing with the pairs on existing strand.
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What is a primer?
a place where the DNA molecule is already double stranded- has a 3’ end available on it- DNA polymerase acts by adding dNTPs onto that 3’ end.
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Describe the synthesis of the new strand briefly
The phosphorus in the 5’ phosphate group of dNTP will form a covalent bond with the oxygen in the 3’ hydroxyl group of the primer. In this process 2 phosphates and one oxygen are lost (hydrolysis), which releases energy to drive the reaction
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What bond is formed and what does it do?
A phosphodiester bond, from 5' to 3', hlds the new nucleotide onto the end of the strand
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Describe what happens if DNA polymerase makes a mistake.
the polymerase activity is inhibited and the 3’→5’ exonuclease removes the incorrect nucleotide. The polymerase activity then resumes DNA replication.
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What does DNA polymerase require in order to proceed adding new bases along the template?
a 3'OH group
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So What does RNA polymerase do?
creates a short temporary primer on the DNA template. (it requires a template like DNA polymerase does not need the -OH group of previous nucleotide)
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After a short RNA primer sequence has been created on the DNA template strand...
the RNA polymerase falls off and DNA polymerase can add onto the hydroxyl group (-OH) of the RNA primer and start synthesizing from there. The RNA is removed shortly after the DNA synthesis starts.
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Describe the function these core proteins at the replicaiton fork a)Topoisomerases b)Helicases c)Primase d)Single strand binding proteins
a) Prevents torsion by DNA breaks b) separates 2 strands c) RNA primer synthesis d) prevent reannealing of single strands
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e)DNA polymerase f)Tethering protein g)DNA ligase
e )synthesis of new strand f) stabilises polymerase g) seals nick via phosphodiester linkage
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What is the difference between DNA polymerase I and III?
a)III- after RNA polymerase has layed down an RNA primer, DNA polymerase III lays down new DNA. b)I-replaces RNA primers layed down by RNA polymerase with DNA
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Card 2


Define DNA replication


The process of duplication of the entire genome prior to cell division

Card 3


What is the significance of DNA replication?


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Card 4


Describe the basic 3 step process of DNA replication


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Card 5


What are the 6 basic features of DNA replication?


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