Translation

What is the genetic code?

The genetic code is mRNA code, not the DNA

Specificity

The genetic code is specific. A particular codon always codes for the same amino acid

Universality

The genetic code is virtually universal

Nonoverlapping and commaless

The code is read from a fixed starting point as a continuous sequence of bases. Three at a time.
Each codon is 3 nucleotide long
ABCDEFGHI -- ABC/DEF/GHI

Duplicative

The genetic code contains duplicate.
An amino acid may have over one triplet (codon) coding for it. Ex. Arginine is specified by 6 different codons

How many codons are there?

There are 64 codons. 61 code for amino acids and 3 codes for stop codons: UAA, UAG, UGA.
Start codon: Methionine, Met, AUG.

Informational flow

The end product is a correctly translated protein, with the correct amino acid sequence.

DNA coding strand...

... has the same sequence as mRNA except U replaces T in mRNA. Translated as 5' to 3'

Components of translation

The ribosome carries out polypeptide synthesis
tRNA: molecules that align amino acids in the correct order along the mRNA template

Amioacyl-tRNA synthetases

They link amino acids to the correct tRNAs

Components of translation

mRNA-molecules that encode the amino acid sequence information for the polypeptide being synthesized
Protein factors- They facilitate several steps in the translation process

Large subunit

catalyzes the formation of a peptide bond

Small subunit

binds mRNA and is responsible for the accuracy of translation.

Ribosomes during translation

Ribosomes are assembled in the cytoplasm. Contact with mRNA is necessary.

Ribosomes during translation

There are no ribosomes sitting around. They are all working. No ribosome in the ER are waiting to be used.

tRNA

There are 40 different tRNA molecules that bring different acids to ribosomes

mRNA

Codons, 5' to 3' orientation

tRNA

Anti-codon, 3' to 5' orientation

Wobble Hypothesis

There is flexibility in base pairing between the third base of a codon and its corresponding base in its anticodon.

Wobble Hypothesis

The tRNA can recognize more than one codon
If the codon does not match the anticodon, it doesn't matter if the amino acid is different. The anticodon match really matters

Initiator tRNA

Start codon

Initiator signals

In both prokaryotic and eukaryotic cells, translation always initiated with methionine, AUG.

Elongation

The ribosome has 3 binding sites:
A-aminoacyl P-peptidyl E-Exit sites
The initiator methyl tRNA is bound at the P site, the next aminoacyl tRNA binds to the by pairing with the second codon of the mRNA.

Stop codons

UAG. UAA. UGA.
Eukaryotes have a single release factor eRF.

Polyribosomes

More than one ribosome at a time can be involved in translating the mRNA into protein

mRNA Stability

Controls translation by regulating mRNA half-life
The length of poly A tail correlates with half-life. Longer tail= longer half-life.

Protein finishing

Protein must be finished just like the mRNA has to be finished. Common mechanisms include: Cleavage, Folding, Methylation

Chaperones

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