11778574
Description
Flashcards by J yadonknow, updated more than 1 year ago
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Created by J yadonknow
over 6 years ago
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| Question | Answer |
| Where does DNA replication begin in bacterial DNA? (3) | DNA replication beings at a particular site known as the origin of replication. This area is rich in A-T base pairings as these regions have fewer hydrogen bonds needed to break in order to unwind DNA. |
| Draw a diagram of Bacterial DNA replication. | semi-conservative replication bidirectional replication |
| What class of enzymes are responsible for DNA synthesis? | DNA polymerase |
| How many DNA Pol' does E. Coli have? Which are of most importance in replication? (3) | 5. DNA Pol' I + DNA Pol' III are of most importance |
| What does a DNA Pol' need in order to synthesise DNA? | The four dNTP's A template DNA strand A Primer |
| What is a primer? | A short piece of nucleic acid, base-paired to the template DNA strand |
| What is the function of a primer? | Acts as a starting point from which DNA Pol' may synthesise new strands of DNA Must have an accessible 3'OH group. |
| Draw a diagram of the addition of a dNTP to the template strand of DNA | S |
| Draw a diagram of the DNA Polymerase reaction | Phosphodiester bond labelled Primer labelled Template strand labelled 5' -> 3' Nucleophilic addition of O: shown |
| What is the function of DNA Pol' i? | Can degrade DNA via exonuclease activity, cleaving DNA from its exposed ends in a 5'-3' orientation. |
| Draw a diagram showing exonuclease degradation of DNA | S |
| What conditions are necessary for exonuclease degradation by DNA Pol' i? (2) | No dNTPs or Primer DNA present |
| What is "nick translation?" | When the 5'-3' exonuclease activity and DNA Polymerase activity take place simultaeneously on the same strand of DNA |
| Draw a diagram showing nick translation (4) | Phosphodioester bond missing Base being added to OH group Degradation from 5'-3' Extension of nick, nick still exists |
| What is the function of 3'-5' exonuclease activity? | Allows DNA Pol' i/iii to remove incorrect nucleotides from the newly formed DNA (proof reading) Opposes DNA synthesis |
| Draw a diagram showing 3'-5' exonuclease activity | Incorporation of incorrect nucleotide DNA synthesis stopping Removal of mis-matched nucleotide DNA synthesis resuming |
| Compare DNA Pol' i/iii (8) | Pol' i Pol' iii 5'-3' e. act. ✓ X 3'-5' proof reading ✓ X Polymerisation rate 16-200 250-1000 (nucleotides per s) Processivity* 3-200 750,000 *(no. nucl. incorporated b4 enzyme dissociates from template) |
| Why does DNA Pol' iii have a much higher processivity? | Because DNA Pol' iii will only dissociate when a whole sequence of DNA has been processed |
| What allows DNA Pol' iii to be so processive? | A hexameric B-subunit which forms a sliding clamp surrounding the template strand of DNA |
| Draw a diagram showing the appearance of a Replication fork | 5'-3' Leading strand 3'-5' Lagging strand |
| What direction does DNA synthesis occur in? | 5'-3' direction |
| Describe Stage 1 of DNA replication (5) | 1.Partial unwinding by DNA Helicase 2.Due to partial unwinding + supercoiling occurs in the template strand 3.These supercoils are corrected by DNA Gyrase (Topoisomerase) 4.single stranded DNA-binding proteins prevent strands from rebinding 5.(Diagram) |
| Describe Stage 2 of DNA replication (5) | 1.Synthesis of primers by primase occurs 2.In vivo RNA Primase is necessary, a specialised RNA polymerase 3.Unlike DNA polymerase, RNA polymerase can start from scratch and doesn't requires a short strand of DNA to operate. 4.Primers arrange themselves antiparallel to the template strand, with exposed OH groups for dNTP addition. 5.(Diagram) |
| Describe Stage 3 of DNA replication (8) | 1.DNA Pol' iii polymerises the nucleophilic addition of the exposed OH to dNTPs. 2.New DNA is synthesised 5'-3' 3.DNA Helicase causes unwinding of the DNA molecule and DNA Gyrase corrects supercoils. 5.After a gap of ~1000 nucleotides a second RNA Primer arranges itself antiparallel to the newly exposed bases. 6. New DNA is synthesised catalysed by DNA Pol' iii, with a phosphodiester gap existing between the 2 newly synthesised strands of DNA. 7. These fragments of DNA are known as Okazaki fragments. (8) Diagrams |
| Describe Stage 4 of DNA replication (3) | 1.RNA primers are removed via nick translation, which in turn is catalysed by DNA Pol' i and RNase H. 2.The primer is degraded by 5'-3' exonuclease activity and simultaneously replaced with DNA using upstream Okazaki fragments as primers 3.(Diagrams) |
| Describe Stage 5 of DNA replication (3) | 1.The gaps between Okazaki fragments are sealed using DNA ligases. 2. DNA ligases achieve this via formation of phosphodioester bonds between the DNA replacing the RNA primer as a result of DNA Pol' i polymerisation action. |
| What is AZT? | Azido-dideoxythymidine An inhibitor DNA polymerase, used to treat retroviruses such as H.I.V. |
| How do DNA polymerase inhibitors work?(5) | 1.The nucleoside of this class of inhibitors has its usual 3'-OH group replaced by N3 or H. 2.(Diagram) 3.Once incorporated into DNA, they prevent further DNA synthesis as there is no OH group available for phosphodiester bonds to form with a successive nucleotide. 4.Our bodies recognise AZT as abnormal and don't try to incorporate it during DNA synthesis. 5.(Diagrams) |
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