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[Components of Translation]
- Translation is the process by which the nucleotide sequence of mRNA
is converveted to the amino acid sequence of a polypeptide.
- In bacteria, this process takes place in the cytoplasm.
- In the first step of the process, all the components needed for
translation come together.
- These components include mRNA, tRNA and ribosomal units.
- mRNA is the product of transcription.
- It is a single-strand of ribonucleotides that is complementary to its gene template.
- The purpose of mRNA is to carry the genetic code from DNA to the
ribosome for translation.
- mRNA is read in a series of triplets called codons.
- For example, the mRNA sequence AUGAAGCACUAC has four codons.
- Each codon corresponds to one amino acid.
- In the above code: AUG codes for the amino acid MET, AAG
codes for Lys, CAC codes for His and UAC codes for Tyr.
- The dictionary of the genetic code tells us which of the 20 amino
acids that a codon designates.
- This code is redundant or degenerate because most amino acids are signalled by several different codons.
- For example, the amino acid leu is coded for by six different codons.
- The genetic code contains 3 codons signal STOP
- AUG signals codes for Met but also signals START.
- Transfer RNA (tRNA) is a single strand of 80 ribonucleotides.
- It assumes a cloverleaf configuration because of interactions between
the nitrogenous bases.
- It fuctions as an interpreter between nucleic acid and pepetide sequences by picking up amino acids and matching them to the proper codons in mRNA.
- There are two important locations on a tRNA molecule that help it do this:
- At the bottom of the loop are three ribonucleotides grouped together in an anticodon.
- An anticodon is complementary to an mRNA codon.
- An anticodon can recognize and bind to its complementary mRNA codon.
- Some tRNAs can recognize more than one codon because there is a relaxation of the complementation rule of base pairing between the anticodon and codon in the third position.
- This relaxation is called the Wobble Hypothesis.
- At the 3’ end of the tRNA strand is where the amino acid attaches
to the tRNA molecule.
- Each tRNA carries one amino acid that corresponds to an mRNA codon.
- The proper amino acid is joined to the tRNA by the enzyme aminoacyl-tRNA synthetase.
- There is one type of this enzyme for each amino acid and the active site of each fits only the specific combination of the proper amino acid and tRNA.
- A ribosome is made of rRNA and proteins.
- A ribosome is composed of two subunits, a large subunit and a small subunit.
- These subunits join to form a functional ribosome when they attach to mRNA.
- Each ribosome has two binding sites for mRNA:
- The p-site (peptidyl site)
- The a-site (aminoacyl site)
- Now that you know the parts and functions of the components needed
for translation, we will discuss the process step by step:
- The small ribosomal subunit binds to the mRNA molecule.
- At the same time, the initiator tRNA with the anticodon UAC and
the amino acid Met binds to the start codon AUG (Remember AUG
is the start codon).
- The large subunit binds to this complex.
- The initiator tRNA is in the p-site and there is no tRNA in the a-site.
- A tRNA carrying a second amino acid approaches and enters the
empty a site.
- The anticodon of the tRNA binds to the second codon of mRNA in the a-site.
- The first amino acid is joined to the second amino acid by a peptide
- The tRNA that was in the p-site is released and the ribosome moves
along the mRNA until the second tRNA is in the p-site.
- This means the ribosome advances only one codon.
- The a-site is empty again.
- A third tRNA approaches and enters the empty a-site.
- Its anticodon binds to the mRNA codon in the a-site.
- A peptide bond is formed between the new amino acid in the a-site and the growing peptide chain.
- The tRNA is released from the p-site and the ribosome advances one
- This process continues until the ribosome reaches a STOP codon.
- The STOP codons are UAA, UAG and UGA.
- When a ribosome reaches a STOP codon, the a-site accepts a protein called a release factor instead of a tRNA.
- The release factor breaks the bond between the tRNA and the
- The polypeptide and tRNA are released.
- The released polypeptide forms a protein.