Ch. 7 RNA & Genetic Code

the central dogma states that

DNA is transcribed to RNA which is translated to protein

a degenerate code allows multiple codons to

encode for the same amino acid

start codon

AUG

stop codons

• UAA (U Are Annoying)

• UGA (U Go Away)

• UAG (U Are Gone)

redundancy and _____ allows mutations to occur without effects in the protein

wobble (thid base in the codon)

what is wobble

the fact that the 3rd base codon usually doesn’t effect which amino acid is produced therefore mutations here don’t effect the amino acid

point mutations can cause

• silent mutations - no effect on protein

• nonsense (truncation) mutations- premature stop codon

• misses mutations - produce a codon that codes for a different amino acid

• frameshift mutations- nucleotide + or - , changes reading frame of subsequent codons

RNA is structurally similar to DNA except

• Substitution of a ribose sugar for deoxyribose

• Substitution of uracil for thymine

• single-stranded (RNA) vs double-stranded (DNA)

3 types of RNA with separate jobs in transcription

• Messenger RNA (mRNA)

• Transfer RNA (tRNA)

• Ribosomal RNA (rRNA)

what does mRNA do

carries the message from DNA in the nucleus via transcription of the gene; travels into the cytoplasm to be translated

what does tRNA do

brings in amino acids and recognizes the codon on the mRNA using its anticodons

what does rRNA do

makes up the ribosomes and is enzymatically active

helicase

unwinds the DNA double helix

RNA polymerase 2 bind to the ____ box within the promotor region of the gene

TATA

_____(precursor to mRNA) is synthesized from the DNA template (antisense) strand

hnRNA

posttranscriptional modifications

• 7-methylguananylate triphosphate cap added to 5’

• polyadenosyl (poly-A) tail added to 3’

• splicing by snRNA and snRNPs in the spliceosome (introns removed in lariat structure, exons are ligated)

• gene variability (next Q)

how do prokaryotes and eukaryotes increase variability of gene products vis posttranscriptional modification

• PROK: one transcript through polycistronic genes (starting transcription in different sites → different products)

• EUK: alternative splicing → combining different exons in a modular fashion to acquire different gene products

tRNA translates the codon into the

correct amino acid

ribosomes are the factories where

translation (protein synthesis) occurs

three stages of translation

1. initiation

2. elongation

3. termination

initiation in prokaryotes occurs when

the 30S ribosome attaches to the Shine-Dalgarno sequence and scans for a start codon; it lays down N-formylmethionine in the P site of the ribosome

initiation in eukaryotes occurs when

40S ribosome attaches to the 5’ cap and scans for a start codon; lays down methionine in the P site of the ribosome

elongation involves the

addition of a new aminoacyl-tRNA into the A site of the ribosome and transfer of the growing polypeptide chain from the tRNA in the P site to the tRNA in the A site. the now uncharged tRNA pauses in the E site before exiting the ribosome

termination occurs when

the codon in the A site is a stop codon; a release factor places a water molecule on the polypeptide chain and thus releases the protein

posttranslational modifications

• folding by chaperones

• formation of quaternary structure

• cleavage of proteins or signal sequences

• covalent addition of other biomolecules (phosphorylation, carbopxylation, glycosylation, prenylation)

the Jacob-Monod model of repressors and activators explains how

operons work

operons

inducible or repressible clusters of genes transcribed as a single mRNA

inducible systems (lac operon) are bonded to a _____ under normal conditions; they can be turned on by an ____ pulling the repressor from the operator site

repressor, inducer

repressible systems (trp operon) are transcribed under normal conditions; they can be turned off by a ____ coupling with the repressor and the binding of this complex to the operator site

corepressor

transcription factors search for

promoter and enhancer regions in the DNA

_____ are within 25 base pairs of the transcription start site

promoters

_____ are more than 25 base pairs away from the transcription start site

enhancers

modification of chromatin structure affects the ability of transcriptional enzymes to access the DNA through ____ ____ (increases accessibility) or ___ ______ (decreases accessibility)

histone acetylation, DNA methylation