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24 Cards in this Set
- Front
- Back
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processes pre-mRNA needs to go through to become mRNA |
1. 5' cap 2. 3' poly(A)-tail 3. exon splicing (removal of introns) 4. RNA editing (in some cases): further editing |
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prokaryotic vs eukaryotic |
-prokaryotic: transcription and translation can occur almost simultaneously; no mRNA processing -eukaryotic: transcription in nucleus; mRNA processing in nucleus; translation in cytoplasm (ribosomes) |
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mRNA processing |
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5' cap |
-ONLY on mRNA made by RNA poly II -guanine is added at 5' to protect mRNA -added right after start of transcription -facilitates binding to ribosome in translation + prevents degradation by nucleases |
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polyadenylation |
-3' end of mRNA is modified with the addition of a poly(A) tail -cleavage + polyadenylation -AAUAAA (consensus region) represents recognition site in pre-mRNA -3' end is cleaved 11-30 nucleotides from AAUAAA -multiple adenine residues (50-250) are then added by poly-adenylate polymerase (PAP) -stability: slows down degradation |
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exon splicing |
-introns = common in eukaryotic genomes: nuclear, mitochondrial, and chloroplast -nuclear-derived pre-mRNA: need 3 consensus sequences for splicing 1. 5' splice site: GU (5' end of intron) 2. 3' splice site: AG (3' end of intron) 3. branch point: adenine 18-40 bases upstream of the 3' splice site (internal within intron) |
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spliceosome************************ |
-large structure in which splicing takes place -composed of different snRNAs and proteins: snRNPs: U1, U2, U4, U5, U6 snRNP 1. U1: binds to 5' end of intron; other snRNPs bind; 5' splicing 2. U2: binds to branch point; 5' end of intron links to adenine in branch point; forms a lariat 3. 3' end is spliced: excised intron is in the loop 4. exons are ligated together 5. introns are degraded/snRNPs are released |
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alternative processing pathways |
1. alternative splicing 2. multiple 3' cleavage sites -produce a different mRNA from a single pre-mRNA -order of introns is maintained |
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alternative splicing |
-same pre-mRNA processed different ways |
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multiple 3' cleavage sites************************ |
-3' cleavage site before polyadenylation (because transcription goes past end point) -2+ 3' splice sites exist in last exon -creates pre-mRNA of different lengths |
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prokaryotic vs eukaryotic translation |
-prokaryotes: transcription AND translation occur simultaneously in cytoplasm -eukaryotes: mRNAs must be transferred into the cytoplasm for translation to take place |
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translation components |
1. mRNA 2. ribosomes: small and large subunits wrap around mRNA 3. tRNA: anticodon binds to triplet codon of mRNA; bring in amino acid as directed by mRNA; smallest RNA molecules 4. aminoacyl-tRNA synthetase: catalyzes bonding of tRNA to an amino acid; ONLY MOLECULES WHICH CAN READ LANGUAGES OF NUCLEIC ACIDS AND PROTEINS |
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4 steps of translation |
1. binding of amino acids to tRNAs 2. initiation 3. elongation 4. termination |
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binding of amino acids to tRNAs |
-step 1 -30-50 different tRNAs, each specific to one of 20 amino acids -20 different aminoacyl-tRNA synthetases, one for each amino acid |
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aminoacyl tRNA synthetase |
-amino acid is linked to 3' end of tRNA -specific sequences within tRNA molecule dictate which amino acid is added -"charged" tRNA = ready to go to ribosome 1. amino acid interacts with ATP --> CMP gets attached 2. tRNA replaces CMP on amino acid |
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translation initiation requires: |
-mRNA -small and large ribosome subunits -three initiation factors (IFs) -initiator tRNA with N-formylmethionine (f-met) attached -guanosine triphosphate (GTP = energy) |
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translation initiation overview |
-mRNA molecule binds to small ribosome subunit -initiator tRNA binds to mRNA -large ribosome joins |
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consensus sequences in translation initiation |
-where translation begins -prokaryotes = Shine-Delgarno sequence (upstream of AUG start codon) -eukaryotes = Kozak sequence (AUG start codon included in sequence) -sequences are recognized by small ribosome subunit -place ribosome in correct position |
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ribosome subunits |
-prokaryotic: 50S + 30S = 70S -eukaryotic: 60S + 40S = 80S |
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translation initiation |
-IF3 binds to small subunit, which prevents it from binding to large subunit -this allows the small subunit to bind to the mRNA -then a charged tRNA with methionine binds with IF2 and GTP -UAC of tRNA and AUG of mRNA base-pair while IF1 binds to small subunit -ALL IFs leave, GTP is hydrolyzed, and large subunit joins to make 70S ribosome |
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translation elongation |
-increase of the polypeptide chain by one amino acid -sequence of the next codon in the mRNA dictates the type of tRNA molecule that will bind next -two binding sites in large subunit that tRNA molecules can bring in the amino acid: 1. P (peptidyl) binding site: first site, is bound to methionine 2. A (aminoacyl) binding site: second site, receives second amino acid -after both P and A sites have a bound tRNA, amino acids are linked by a peptide bond (peptidyl transferase) -P site releases its tRNA molecule -ribosome moves down the mRNA molecule -tRNA that was bound to A site is now moved to P site -A site is now open for next tRNA |
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translation termination |
-elongation of the polypeptide chain continues until the A binding site reaches a stop codon -stop codons do not specify an amino acid --> tRNA doesn't bind at a stop codon -causes the finished polypeptide chain to be released from the P site -once this occurs the ribosome releases the mRNA |
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control of translation |
-single mRNA can be translated by multiple ribosomes at once -BUT the mRNA is degraded at variable rates in the cell (few minutes to hours) -if mRNAs remained stable, then the cell wouldn't be able to turn of expression of a gene quickly |
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tRNA structure |
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