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Translation
In order to understand the message we must first know what the message in mRNA means.
- Bases that code for amino acid are called a codon (triplets eg. AUG)
- there are only 20 amino acids and about 64 possible codons
- first two nucleotides are consistent for a particular amino acid, but the third nucleotide can vary
and still code for the same amino acid
- this third amino acid is called the wobble position
- this allows for more than one codon to code for an amino acid in the case of error
mRNA
- Once the mRNA is modified, it leaves the nucleus through a nuclear pore and enters the
cytoplasm
- in the cytoplasm, it moves to the rough endoplasmic reticulum (which is covered by ribosomes)
or to free ribosomes to make protein
- once the mRNA is in the cytoplasm, the ribosome works with the tRNA to make protein
Transfer RNA (tRNA)
- capable of attaching to an amino acid at top near the acceptor stem; at bottom has anticodon
loop that is the compliment of the codon found on mRNA
- like mRNA transcribed but does not stay linear
- folds into three lobed shape
- at end of one lobe is anticodon; complimentary to codon of mRNA molecule
- at 3’ end is attachment site for amino acid; particular aa specified by mRNA codon
- tRNA molecule bound to aa = amino-acyl tRNA (aa-tRNA)
Activating enzyme
- responsible to attach correct aa to tRNA molecule
- 2 binding sites: 1 recognizes aa, other recognizes anticodon on tRNA molecule
- enzyme catalyzes reaction that attaches aa to 3’ hydroxyl group
- aa-tRNAbinds to specific codon on mRNA that matches anticodon
Ribosomes
- brings mRNA, aa-tRNA, and enzymes that build polypeptides together
- complex of proteins with rRNA (ribosomal RNA)
- rRNA form 2 subunits; large and small sub-units
- rRNA is linear strand of RNA that is always bound to proteins in ribosome assembly
- rRNA sandwiched between units
- rRNA strands in ribosome provide binding site for mRNA and 3 binding sites for tRNA
- P-site : holds one aa-tRNA with growing aa chain
- A site : holds tRNA bringing next aa to add to chain
- E site : exit site
Initiation
- sequence of nucleotides at 5’ end of mRNA (leader sequence) binds to rRNA in small
ribosomal sub-unit
- initiator tRNA binds to ribosomal-mRNA complex; has anticodon UAC
- this is complement to start codon AUG which carries methionine (aa) and starts translation
- first aa-tRNA carrying methionine begins the process by sitting on the P Site
Elongation
- mRNA codon exposed in A binding site forms base pair with incoming aa-tRNA
- enzymes in large sub-unit catalyze peptide bond between last aa in growing chain and new aa
- polypeptide chain transferred from tRNA in P site to tRNA in a site
- ribosome moves 3 nucleotides (codon) along mRNA; translocation
- this brings pp chain into P site and exposes new A site
- tRNA that was in P site now in E site and detaches from ribosome
Termination
- ribosome will eventually reach the stop codon in A binding site which has no corresponding
amino acid
- tRNA carrying pp chain stays on P site until protein called a release factor binds to A site
recognize that the ribosome has stopped and release the polypeptide chain
- the ribosome will break down into subunits and translation stops
- the protein folds into its 3-D structure and may be modified with carbohydrates or phosphates
and cleaved
- it may move to part of the cell where it is needed or packaged to leave the cell through
endocytosis
--- overall: as soon as first ribosome moved off initiation sequence of mRNA new ribosome
assembly can bind to same molecule
- this way many ribosomes can attach to 1 mRNA molecule = polyribosome
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