Protein Synthesis Translation. Protein Synthesis.
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Transcript of Protein Synthesis Translation. Protein Synthesis.
Protein Synthesis
Translation
Protein Synthesis
Translation
Initiation
Elongation
Termination
Translation
From DNA blueprint Protein
Once mRNA enters the cytoplasm, it is ready to be translated
Translation is made up three stages
Translation- Initiation
Initiation occurs when a ribosome recognizes a specific sequence, the 5' cap, on the mRNA and binds to that site
This process is the initiation of translation of nucleic acid into protein
Ribosomes
In eukaryotes, the ribosome consists of two subunits
A large subunit of 60SA small subunit of 40SThe S refers to the
rates at which various components sediment when centrifuged
The rates of a measure of size
Intact ribosome (80S)
Structure of RNA
E site (Exit site)
P site (Peptidyl tRNA binding site)
A site (Aminoacyl tRNA binding site)
mRNA binding site Small
subunit
Large subunit
Ribosomal subunits are a combination of ribosomal RNA (rRNA) and protein
Translation-Initiation
The ribosomes move along the mRNA in the 5'-3' direction, adding a new amino acid to the growing polypeptide chain
The two ribosomal subunits bind to the mRNA, clamping the mRNA between them
Ribosomes
Ribosomes synthesize different proteins by associating with different mRNAs and reading their coding sequence
Reading frame: a contiguous and non-overlapping set of three-nucleotide codons
There are 3 possible reading frames in an mRNA strand
Reading Frame
In theory any mRNA can be translated in any of three different "reading frames" depending on the nucleotide sequence at which translation starts.
Thus 3 different amino
acid sequences can be obtained from a single mRNA
The Role of Transfer RNA (tRNA)
The ribosome alone cannot synthesize the polypeptide chain
The correct amino acids must be delivered to the ribosome binding site
The molecule that delivers the amino acids is tRNA
tRNA
A small single-strandednucleic acid whose structure resembles acloverleaf
At one arm of tRNA isthe anticodon
Anticodon is a sequenceof three bases that recognizes the codon of the mRNA
The opposite arm carries the corresponding amino acid
mRNA
5' 3'
tRNA
tRNA and mRNA recognize each other by complementary base pairing
tRNA
Every tRNA carries only a specific amino acid, which means that at least 20 different tRNAs are required
Recall: there is 64 possible codons
In reality, anywhere between 20-64 types of tRNA molecules are available depending on the organism
Wobble Hypothesis
Proposed by Francis CrickThe theory proposes that the first two bases in
the codon and anticodon will form complementary pairs in the normal antiparallel fashion.
However, a degree of ‘wobble’ is allowed in the base-pairing at the third position.
This hypothesis accounts for the number of tRNAs that are observed
It also accounts for the degeneracy that is observed in the Genetic Code. The degenerate base is that in the wobble position.
Wobble Theory
Crick recognized that the following base-pair schemes were possible:
5' anticodon base 3' codon base
A U
C G
G C or U
U A or G
I A or C or U
Aminoacyl-tRNA
tRNA molecule with its corresponding amino acid attached to its acceptor site at the 3' end
Acceptor site of amino acid
Translation-Initiation
The mRNA joins to the small ribosomal unit at the 5' untranslated region. This binds to a special binding site on the small ribosomal subunit.
Translation-Initiation
The large ribosomal subunit attaches to the small subunit such that the first codon is aligned at the P binding site.
Translation-Initiation
A tRNA carrying the amino acid methionine attaches to the start codon (AUG) on the messenger RNA. This initiates elongation.
The AUG codon codes for methionine, i.e, every protein initially starts with this amino acid
The tRNA that carries methionine enters the P site
Translation-Initiation
Translation-Elongation
The next tRNA carrying the required amino acid enters the A site
The tRNA and its amino acid attaches to the A binding site.
Amino end of polypeptide
mRNA
Translation-Elongation
The methionine amino acid at the P site is bonded to the amino acid at the A site
There is a Peptide bond formation between the two amino acids Peptide
bond formation
Translation- Elongation
The tRNA at the P site is now free of amino acids
Translation-Elongation
Ribosome shifts over one codon (three bases)
Ribosome moves in the 3' direction down the messenger RNA
The A binding site is now open and a vacant tRNA is in the E binding site.Ribosome ready for next aminoacyl tRNA
Translation- Elongation
A new tRNA attached to an amino acid enters the vacant A site on the ribosome and the process repeats again
Amino end of polypeptide
mRNA
Translation-Termination
The ribosome will eventually reach a stop codon
The three stop codons are:
UGA UAG UAA
Since these codons do not code for an amino acid, there are no corresponding
tRNAs
Translation- Termination
Since, the stop codons do not code for an amino acid, there are no corresponding tRNAs
Translation- Termination
A protein known as a release factor recognizes that the ribosome has stopped translating
The release factor helps in the release of the polypeptide chain from the ribosome
The two subunits fall off the mRNA and translation officially stops