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Lesson Objectives:

• Where does RNA translation take place?
• Understand how mRNA along with (tRNA) and ribosomes work together to produce proteins in translation.
• Learn about the stages of translation in protein synthesis.

Translation

Translation is the process in which the genetic code in mRNA is read to make a protein. In this process:

• Ribosomes floating in the cytoplasm or ER synthesize proteins after the process of transcription of DNA to RNA.
• Along with the mRNA template, several other molecules such as ribosomes, tRNAs, and various enzymatic factors contribute to the process of translation.

Components of Translation

1. Messenger RNA (mRNA)

• This is a single-stranded molecule that is always complementary to the specific DNA template.
• It is a product of the process of transcription and is composed of codons.
• Each codon is a chain of three nucleotide bases specific to a particular amino acid to be added to the polypeptide sequence.
• mRNA interacts with the ribosomal RNA (rRNA), recognizing the start and stop codons present within it.

2. Transfer RNA (tRNA)

• A single strand of tRNA is made up of approximately 80 ribonucleotides.
• Short sequences of three ribonucleotides are called anticodons since they are complementary to the codons on the mRNA.
• When the complementary codons bond, the tRNA carries the amino acid and adds it to the polypeptide chain.

3. Aminoacyl tRNA synthetases

• They are the enzymes that aid in the attachment of the amino acid to the corresponding tRNA.
• There is a unique synthetase for every amino acid, with a unique active site only fit for a certain amino acid-tRNA combination.

4. The Ribosomes

• The mRNA moves to a ribosome after it leaves the nucleus.
• Ribosomes exist as separate subunits and are composed of protein and rRNA.
• The ribosome will read the nucleotide sequence from the 5′ to 3′ direction on binding to an mRNA and synthesises the corresponding protein from amino acids in an N-terminal (amino-terminal) to C-terminal (carboxyl-terminal) direction.

Ribosomal Sites for Protein Translation.

The larger of the two ribosomal subunits have three active sites: E, P, and A.

These sites are important in the catalytic activity of ribosomes and are formed by the rRNA molecules present in the ribosome.

• A site (the aminoacyl-tRNA binding site): This is where the incoming aminoacyl-tRNA binds during elongation.
• P site (the peptidyl-tRNA binding site): This is where the tRNA linked to the growing polypeptide chain is bound.
• E site (the exit site): This is the binding site for tRNA prior to its release from the ribosome and after its subsequent role in translation.

Activation of Amino Acids:

• The amino acid and the A residue of the CCA sequence at the 3′ end are covalently bound during synthesis of the aminoacyl-tRNA.
• Each tRNA molecule is known to carry a single amino acid.
• An enzyme named aminoacyl-tRNA synthetase catalyzes the attachment of an amino acid to a tRNA.
• A different aminoacyl-tRNA synthetase exists for every amino acid, making 20 synthetases in total.

Initiation

• In all mRNAs, the first codon translated is the start codon or initiation codon, AUG that codes for methionine.
• The first amino acid of a new protein is N-formylmethionine (fMet) in prokaryotes
• The small subunit binds to the 5′ side at the start of the mRNA.
• It then proceeds to read the mRNA in the 5′–>3′ direction until the encounter of the START codon (AUG) while the large subunit attaches to the initiator tRNA, that carries methionine (Met), binds to the ribosome’s P site.
• Proteins called initiation factors (IFs) are required to initiate protein synthesis, and three initiation factors, namely IF-1, IF-2, and IF-3, are essential in prokaryotes.

Elongation

The ribosome catalyzes every process that occurs in the three sites by shifting one codon at a time.

• A charged tRNA enters the complex with every step, the polypeptide becomes one amino acid longer, and an uncharged tRNA departs.

Binding of AA-tRNA at A-site

• The second tRNA carrying the following amino acid comes into the A-site and recognizes the codon on mRNA. This binding is facilitated by EF-TU and utilizes GTP.
• Post binding, GTP gets hydrolyzed and EF-TU-GDP is released
• EF=TU-GDP then enters into the EF-TS cycle.

Peptide bond formation

• The amino acid present in t-RNA of P-site, i.e., Fmet, is transferred to t-RNA of A-site forming peptide bonds. This reaction is catalyzed by peptidyltransferase.
• Now, the t-RNA at P-site becomes uncharged.

Ribosome translocation

• After peptide bond formation, the ribosome moves one codon ahead along the 5′-3′ direction on mRNA so that dipeptide-tRNA appears on the P-site and the next codon appears on the A-site.
• The uncharged tRNA exits from the ribosome and enters the cytosol.
• EF-G-GTP (translocase enzyme) is required in ribosome translocation. This changes the 3D structure of the ribosome and catalyzes the 5’-3′ movement.
• The codon on A-site now becomes acknowledged by other aminoacyl-tRNA.
• The dipeptide on the P-site gets transferred to the A-site to form tripeptide.
• This process continues and results in a long polypeptide chain of amino acids.

Termination

• Eventually, one of three Stop codons (UAG, UAA, and UGA) becomes positioned in the A site.
• The stop codons are recognized by the next protein called release factor (RF-1, RF-2, and RF-3), which hydrolyses and causes the release of all components, i.e., the 30S, 50S, mRNA, and polypeptide separates.
• RF-1 recognizes UAA and UAG while RF-2 recognizes UAA and UGA while RF-3 dissociates the 30S and 50S subunits.
• After several ribosomes complete translation, the mRNA is degraded to reuse the nucleotides in another transcription reaction.

FAQs:

1. What is translation in cell and molecular biology?

Translation is the process in which the information present in the genetic material called messenger RNA (mRNA) is decoded to synthesize proteins.

2. What are the four steps of translation?

The four steps of translation are activation, initiation, elongation, and termination.

3. What is meant by translation in biology?

Translation is the process in which the information present in the genetic material called messenger RNA (mRNA) is decoded to synthesize proteins. Amino acids join to form proteins. Translation takes place within the ribosomes of a cell.

4. What is an example of translation in biology?

An example of how the codons are read during translation is methionine is synthesized when the codon AUG is read.

5. Why is DNA translation important?

Proteins are important building blocks of the body. DNA translation allows the information present in the codons to be translated into amino acids, which make up proteins.

6. How does DNA translation work?

It occurs in three basic steps, namely initiation, elongation, and termination.

7. What occurs during translation?

During translation, amino acids are synthesized while decoding the codons, which are nucleotide bases such as adenine, thymine, guanine, and cytosine.

8. What are the steps in the process of translation?

The basic steps in the process of translation are initiation, elongation, and termination.

9. What is translation in DNA replication?

DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules.The process by which DNA is copied to RNA (specifically mRNA) is called transcription, and that by which RNA is used to make proteins is called translation.

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