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The reading of the mRNA transcript and the creation of the polypeptide chain.
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Occurs on the Ribosomes:
 Composed of a small subunit (40S) and a large subunit (60S)
 Made up of ribosomal RNA and protein
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The mRNA transcript is read by the ribosome in sets of 3 nucleotides to determine the amino acid sequence.
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tRNA deliver the appropriate amino acid to the ribosome where the polypeptide chain is built.

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There are 20 amino acids found in proteins, but only 4
bases in the code (U C A G of mRNA)
Codons: sequences of three nucleotide bases used to code for an amino acid. Ex. UAC codes for Tyrosine
4 3 =64 possible codons
 therefore some amino acids can have more than one codon
 Ex. UUU UUC, UCU, UCC all code for phenylalanine
 This redundancy helps to reduce errors

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AUG: start
codon used 99% of the time (codes for Methionine)
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UAA, UAG,
UGA: stop codons

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 tRNA delivers amino acids to the ribosome
Looks like a cloverleaf
Has an anticodon – sequence of three bases that recognize (are complementary to) the mRNA codon
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 ex. The GCC codon specifies the a.a. alanine
The anticodon on the tRNA carrying alanine would be CGG
Opposite arm carries the amino acid binding site
Amino acids are added by Aminoacyl
t-RNA synthases to make aminoacyltRNA

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The small subunit of the ribosome recognizes the 5' cap on the mRNA transcript and binds to the RNA.
The ribosome will position itself at
AUG (the first codon read for every protein)
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Ensures the correct reading frame
(phase in which the mRNA is read) is used by the ribosome
The large subunit binds and the subunits act as a clamp (80S) around the mRNA.

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The large subunit of the ribosome has two sites that the mRNA moves through:
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A (acceptor) site: where new amino acids enter the ribosome
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P (peptide) site: where the polypeptide chain grows as the amino acids are combined.
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At a third site, the catalytic site, the peptide bonds are made between the amino acid in the A site and the P site.

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The AUG codon will be in the P site, tRNA carrying the corresponding amino acid
(methionine) enters this site.
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The next tRNA carrying the required a.a. enters the
A site.
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At the catalytic site (b/w
A and P) a peptide bond forms between the two amino acids.

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The ribosome moves over one codon. The second tRNA moves to the P site, a new tRNA brings the next a.a. to the A site and the a.a. is added to the chain.
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The first tRNA is released to be recycled for use with another amino acid.
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The process repeats with the polypeptide chain trailing in the cytoplasm.

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Ribosome will reach a stop codon (UGA, UAG,
UAA)
No tRNA or a.a. exists for a stop codon
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A release-factor protein helps the two ribosome subunits to fall off the mRNA and the polypeptide chain is released.

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Polypeptide chains travel through the RER and are modified to become functional proteins.
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Some amino acids may be glycosylated (sugar added) or phosphorylated (phosphate added) or altered in another way.
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Enzymes may cleave (cut) the chain at specific places.

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In codons the third base may differ between 2 codons that code for the same amino acid (UAU and UAC both code for tyrosine).
If the tRNA's anticodon is AUA it can still bind to UAC.
This flexibility allows for the correct amino acid to be added to the polypeptide chain despite errors in the gene sequence.
The proposal that tRNA can recognize more than one codon by unusual base pairing is known as the “wobble hypothesis”.