Translation

What is DNA translation?

Translation is the process of translating the sequence of a messenger RNA (mRNA) molecule to build a polypeptide or a sequence of amino acids during protein synthesis. For most purposes, a polypeptide is basically just a protein (with the technical difference being that some large proteins are made up of several polypeptide chains). Translation occurs at ribosomes, which are either free in the cytoplasm or attached to the rough ER.

The Genetic Code

In an mRNA, the instructions for building a polypeptide come in groups of three nucleotides called codons. Here are some key features of codons to keep in mind:

1. There are 61 different codons for amino acids

2. Three “stop” codons mark the polypeptide as finished

3. One codon, AUG, is a “start” signal to kick off translation (it also specifies the amino acid methionine)

These relationships between mRNA codons and amino acids are known as the genetic code.

• Main features of the genetic code

• Triplet: a sequence of three bases (a codon) is needed to specify one amino acid

• Non overlapping code: no bases are shared between consecutive codons

• Continuous code

• Degenerate: more than one codon can code for the same amino acid

• Universal: generally the same in all organisms

• The ribosome moves along the mRNA three bases at a time

Requirement of translation

1. mRNA codon

2. tRNA anticodon

3. rRNA (ribosome)

4. Various protein factors

Stages in translation

There are 4 stages in translation. These are namely; amino acid activation, initiation, elongation, and termination.

Step 1: Amino acid activation

1. Amino acid must be activated by tRNA and aminoacyl-tRNA synthetases to form aminoacyl-tRNA before being incorporated into growing protein chain.

Step 2.Initiation

Initiation requires:

1.An "initiator" tRNA carrying the first amino acid in the protein, which is usually methionine (Met)

2.initiation codon (AUG) of mRNA

3.30S ribosomal subunit

4.50S ribosomal subunit

5.initiation factors (IFs)

6.GTP, Mg2+ (side note: Guanosine-5'-triphosphate (GTP) is a purine nucleoside triphosphate. It can act as a substrate for both the synthesis of RNA during the transcription process and of DNA during DNA replication

During initiation, these pieces must come together in just the right way. Together, they form the initiation complex, the molecular setup needed to start making a new protein.

Steps in initiation process:

1. -An "initiator" tRNA carrying the first amino acid in the protein, which is almost always methionine (Met)

2. -When it encounters an mRNA, it attaches and starts to scan for a start signal.

3. -When it finds the start sequence AUG, the codon (triplet) for the amino acid methionine, the large subunit joins the small one to form a complete ribosome and the protein synthesis is initiated.

Step 3: Elongation

-Uses 3 binding sites for tRNA present on the 50S subunit of the 70S ribosome: P (peptidyl) site, A (aminoacyl) site,

E (exit) site.

Steps in elongation:

1.A new tRNA-amino acid complex enters the codon next to the AUG codon.

2.If the anticodon of the new tRNA matches the mRNA codon, base pairing occurs and the two amino acids are linked by the ribosome through a peptide bond

3.On the other hand, if the anticodon does not match the codon, base pairing cannot happen and the tRNA is rejected. Then, the ribosome moves one codon forward making space for a new tRNA-amino acid complex to enter.

4.This process is repeated several times until the entire polypeptide has been translated.

Step 4: Chain Termination

Translation ends in a process called termination.

Termination happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site. Stop codons are recognized by proteins called release factors, which fit neatly into the P site (though they aren't tRNAs). Release factors mess with the enzyme that normally forms peptide bonds: they make it add a water molecule to the last amino acid of the chain. This reaction separates the chain from the tRNA, and the newly made protein is released.

For more understanding for this subtopic,take a look on this video