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Gene Expression - Coggle Diagram
Gene Expression
the genetic code
codons are 3 nucleotides (4 different bases, 20 amino acids, must be 4x4x4 = 64 as 4x4 = 16 (Crick)
degenerate (some amino acids can be coded for by more than one codon), universal (same in all living organisms) and non-overlapping (each codon is read seperately)
Har Gobind Khorana - 1968 Nobel Prize -> deduced the genetic code, that UCUCUCUCUCUCU resulted in a repeating sequence of 2 different amino acids, first person to synthesise nucleic acids
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only 1 start codon (methionine) and 3 stop codons, third position often does not matter and the first position changes results in similar chemistry - means it could have been a 2 letter code with 16 possibilities
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RNA polymerase
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RNA polymerase (sigma subunit) recognises promotor region (-35 and -10) and DNA unwinds from the -10 position (AT rich and only 2 H+ bonds) and RNA synthesis starts at +1, synthesis begins and sigma protein is released
the bottom strand forms the template strand and is copied, the top strand has the same sequence as the new RNA so it is the coding/sense strand
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DNA accessibility
DNA methylation 5-methyl C in 5' -CG- 3', marker of inactive DNA, tightly wound so can't be transcribed
methylation promotes tighter packaging, acetylation promotes opening - gene expression
chromatin remodelling: ATP dependent, nucleosome sliding and displacement
eukaryotic transcription requires stimulation by transcription factors and activators, more regulated than bacteria
5' cap (modified gene wrong way round to STOP DEGREDATION) and a poly-A tail (lots of A bases at the end)
both modifications protect the ends of RNA from digestion by nuclease enzymes so stabilise RNA and play a role in translation of the message into proteins
genes
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Cricks 'central dogma'
describe the principle that genetic information flows only in 1 direction, from DNA to RNA to protein, held to be universally true -requires some qualification now as we can go from RNA to DNA
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bacterial DNA
circular DNA, large and so packaged by supercoiling and wrapping round some proteins, fairly open structure, default position is that genes are on - transcribed
as soon as you form RNA, you can immediately translate it into proteins at the ribosome, attached to mRNA
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tRNA adapter roles: recognises the codon, binds codon to complementary anticodon, supplies correct amino acid and must be highly accurate
bring relevant aa together, allow formation of peptide bonds