Translation:
The Rules for the Translation of RNA into Amino Acids Are Contained in the Genetic
↳ Genetic Code → Which amino acid will be used to build a protein
↳ Codon:A sequence of three nucleotide bases that specifies one amino acid
General Map of Translation:
1. 2. The mRNA transcript is organized into codons…
Which is recognized by tRNA
↳ Has amino acid specificity,
includes an Anticodon (denoted by
the three yellow bases in the image to
the right) which is complementary to
the mRNA codon, and noncovalently
interacts with ribosomes.
↳ Each tRNA carrying an
amino acid called “charging” which
requires ATP
(denoted by pink section at the top of
tRNA)
Practice Question: The anticodon of a particular tRNA molecule is:
a. b. Complementary to the corresponding mRNA codon
The same sequence as the mRNA codon → This is incorrect because it’s not the same,
the anticodon sequence is complementary
c. The part of tRNA that binds to the specific amino acid
3. This is all occurring through interactions with
ribosomes
↳ Ribosome holds mRNA and tRNA in correct
position to assemble a polypeptide
↳ The Translation sequence occurs at 3 Sites:
E, P, and A
→ The Steps of Translation:
1. Initiation: A complex forms around the mRNA
a. The small ribosomal subunit
b. Charged (carrying an amino acid) tRNA
-
The first amino acid in a polypeptide
is always methionine (sometimes
taken out later)
c. The large ribosomal subunit
2. Elongation:During elongation, amino acids are sequentially added to the growing polypeptide chain as the ribosome moves along the mRNA strand.
a. b. c. After the initial tRNA enters the P site in
Initiation, the next codon is “open” for another
charged tRNA in the A site
The large subunit catalyzes a two-step reaction:
i. The bond between the tRNA and its amino
acid in the P site is broken
ii. A peptide bond forms between that amino
acid and the amino acid attached to the
tRNA in the A site
Therefore, the growing peptide is now in the A
Site
d. Our ribosome complex will then shift:
To recap: The next open codon is in the A Site,
The growing peptide is in the P site, the original
tRNA (without its amino acid now) is now in
the E site
e. The next tRNA will then enter our previously
open A Site
-
The growing peptide chain is
transferred to the next amino acid
-
The tRNA in the E Site (which is
uncharged) dissoassociates to become
recharged
-
The ribosome shifts again, leaving the
A Site open again
3. Termination: Occurs when the Stop Codon is in the
A Site
-
-
The Stop Codon binds a release factor which
releases the tRNA from its polypeptide chain in
the P Site
The polypeptide separates and folds into its 3D
Shape
→ What happens to proteins after the translation process?
-
They have specified functions so they have to go to the right place
↳ A Signal Sequence on the protein indicates where the protein’s “job” is
-
Depending on this sequence, translation could be moved to:
-
the RER because the proteins ultimate job is in the endomembrane system
or exported from the cell
-
OR translation could occur in the cytoplasm to allow entrance to the
nucleus
-
Post Translational Modification
↳ Phosphorylation (addition of phosphate groups), Glycosylation (addition of sugars),
Proteolysis (polypeptide cut by proteases to remove a signal sequence)