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DNA & RNA
Uni of Notts, Genes, Molecules and Cells, first year
| Question | Answer |
|---|---|
| Bacterial transformation | Taking in free DNA from the environment (often from dead bacteria) to gain new traits without needing to divide or mutate. This can happen naturally in "competent" bacteria or can be induced with CaCl2 or electroporation |
| Bacterial recovery | The bacteria repairs from the membrane stress, if the DNA is compatible & functional it expresses the genes & the new trait becomes stably inherited. It's either integrated into the chromosome or maintained as a plasmid |
| Griffith (1928) study: pneumonia in mice | Non-pathogenic pneumonia bacteria can become pathogenic in mice to cause lethal pneumonia in the presence of heat-treated pathogenic bacteria |
| Serotypes | Distinct variations within the same species of virus or bacteria depending on which surface antigens are expressed |
| How Avery & MacLeod (1944) proved DNA was the transforming factor of bacteria | They ran the Griffith's (1928) experiment in vitro & systematically removed every macromolecular group to see if the bacteria still transformed. They only stopped transforming when the DNA was hydrolysed |
| Life cycle of bacteriophages (5) | 1. Infection - phage injects bacterium with DNA leaving an empty shell 2. Early G phase - DNA replicated 3. Late G phase 1 - head & tail synthesised for progeny 4. Late G phase 2 - components join together 5. lysis - bacteria releases progeny by lysis |
| Hershey-Chase bacteriophage experiment | Found the life cycle of phages by marking empty "phage ghosts" with 35S radioisotope & DNA with isotope 32P to follow the shell of the phage & viral DNA in the bacteria using radiomarkers |
| Evidence for our current understanding of the DNA structure (3) | X-ray diffraction - repeating helical units every 10 bases (3.4nm) Chargraff's law - number of purines = number of pyrimidines Nucleotide structure Nucleotide structure - Antiparallel phosphodiester backbones & base pairing explain observed properties |
| How nitrogenous bases bind to pentose sugars (purines & pyrimidines) | N-H on a C-N ring (N1 for purines, N9 for pyrimidines) uses its lone pair to attack the C1 hydroxyl group on pentose & form a β-N-glycosidic bond in a condensation reaction |
| Why the type of pentose (ribose or deoxyribose) matters in DNA & RNA | Ribose has a hydroxyl on C2 making it more reactive (nucleophilic) & able to degrade faster compared to DNA for which needs a stable structure to store genetic information |
| Nucleoside naming convention | Nucleosides are nitrogenous bases bound to pentose sugars. Purines are given the suffix -osine (adenosine, guanosine) whereas pyrimidines have -idine (cytidine, thymidine, uridine) |
| Grooves in DNA structure (major & minor) | Uneven spaces formed by the helical twist, based on purine:pyrimidine ratio. The major groove is wider & better for binding drugs or transcription factors or reading DNA without unwinding it to allow regulation |
| How DNA extension occurs | DNA polymerase only catalyses the nucleophilic attack of exposed OH on pentose at the 3' end to the alpha phosphate of dNTPs. Pyrophosphate is cleaved, releasing energy to drive the reaction |
| Secondary & tertiary structures of RNA (rRNA & tRNA) | the 2nd OH on ribose allows rRNA to coil in on itself to form 2* & 3* similar to peptides to catalyse condensation of amino acids. tRNA coils to form a clover 2* structure with self-hydrogen bonding & the OH binds amino acids for transport |
| Semi-discontinuous replication | During synthesis 1 strand (leading) is continuously synthesised using its exposed 3' end while the other (lagging) is made by annealing 1-2kBP "Okazaki" fragments |
| END replication problem | An RNA primer is needed at the end of chromosomes to add an Okazaki fragment & when it's removed there are no 3'OHs to replace the base meaning the telomeres get 1 base smaller each cycle (except in sex, cancer, or stem cells with telomerase) |
| Replication fork | Point of double stranded DNA where double stranded DNA is unwound by helicase. Originates from the origin with 1 fork (unidirectional) or multiple (bidirectional) depending on frequency of weaker A-T bonds |
| How replication forks in bacteria are demonstrated | Bacteria grown in low levels of radiolabelled thymidine to show the presence of these at replication forks then observed when grown in higher concentrations of the label to show bidirectional replication |
| How bacteria proofread DNA during replication | DNA pol III adds dNTPs to 5'-3' strand, 3'-5' exonuclease removes incorrect nucleotides from the strand, DNA pol I removes RNA primers & fills gaps, DNA ligase fixes nicks & joins Okazaki fragments |
| Subunits & their functions in DNA polymerases | α - catalytic core ε (epilson) - proofreading in exonuclease θ (theta) - stabilises epilson β - sliding clamp for DNA binding γ (gamma) complex - beta clamp loading τ (tau) - coordinates subunits & dimerises adjacent bases |
| DNA supercoiling | Over or underwinding of DNA beyond the natural double helix twisting to make them more compact in cells & regulate transcription & replication |
| 2 types of DNA supercoiling | Negative (underwinding): Most common in most cells, keeps DNA functional while minimising space Positive (overwinding): Less common, can silence genes by stalling replication machinery ahead of a fork |
| How DNA supercoiling regulation enzymes | Topoisomerase I: Nicks 1 DNA strand to increase positive coiling Topoisomerase II (gyrase): Nicks both strands to introduce negative supercoils or relax positive ones |
| Bidirectional replication in circular chromosomes | The origin of transcription (OriT) has a lot of A-T bonds which melt at lower temperatures & produce 2 replication forks travelling in opposite directions using the same enzymic machinery |
Created by:
Beech47
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