tRNA – the Adapter Molecule

tRNA is also known as transfer RNA is a subtype of RNA, tRNA help in the protein synthesis process. tRNA carries the amino acid to the ribosome, which is the molecular machine that assembles the protein, and ensures that the amino acid is incorporated into the growing protein chain in the correct order. Different types of tRNA present, each with a different sequence and structure allow it to recognize a specific amino acid. Specific amino acids are incorporated in the protein sequence via specific tRNA.

What is RNA?

RNA

Ribonucleic acid (RNA) is a single-stranded, non-hereditary (except-retroviruses) molecule that helps in the synthesis of proteins. The nitrogenous bases present in RNA are A (Adenine), U (Uracil), G (Guanine), and C (Cytosine), in which A pairs with U and G with C. The function of ribonucleic acid is to convert genetic information from genes into amino acid sequences of protein. It is classified into three types- messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) on the basis of the function and molecular size of the nucleic acids.

What is t-RNA?

t-RNA stands for transfer RNA. As its name suggests, it transfers the specific amino acid from the amino acid pool to the mRNA to form a polypeptide to make proteins. It is also known as an adapter molecule as it connects the messenger RNA (mRNA) molecule and polypeptide chain. You can relate it to your mobile adapter which connects your charger cable to the switchboard. So, here your charger cable has an analogy with tRNA, cable with mRNA, and switchboard with polypeptide chain. Each tRNA carries a particular amino acid.

tRNA Structure

tRNA Structure

tRNA molecule has two ends 5’ and 3’ ends. The 5’ end consists of a phosphate group which is attached to the 5th carbon atom of ribose sugar while the other 3’ end has a free OH group on the 3-carbon atom. So, there are two types of structure of tRNA- 2D clover leaf model and 3D, L shape model.

Cloverleaf Model

tRNA molecule consists of 70-80 nucleotides that fold like a clover leaf. A 2D t-RNA molecule appears like a cloverleaf. As clove has the following ends, the t-RNA molecule also has four ends:

1. Acceptor end: It is called the acceptor end as it accepts the specific amino acid. It is made up of 7-9 nucleotides. It has two ends- 3′ and 5′. It is the 3’ end that consists of a base triplet CCA with OH group whereas the 5′ end consists of a phosphate group. At this OH at the 3′ end, the COOH group of amino acids joins.
2. Anti-codon end: It is made up of 5 base pairs and consists of codons that are complementary to the codon present on mRNA, therefore known as an anticodon. Anticodon is the triplet base sequence in tRNA which binds with the codon at the time of translation. There is specific tRNA for a particular amino acid. Base pairing between the codon and anticodon helps in the synthesis of proteins. There are no tRNAs for stop codons but have specific tRNAs for initiate codons. 
3. Enzyme site/DHU end: It is present on the lateral side of the molecule that recognizes aminoacyl-tRNA synthetase that activates the amino acids and catalyzes the binding of a specific amino acid to a tRNA molecule. It consists of 3- 4 base pairs and is called a D loop because it consists of a modified nucleotide called dihydrouridine.
4. Ribosome recognition end: It is on the other lateral side of the molecule. It is meant for attachment to a ribosome.
5. TΨC Loop end: It is known as T arm.  It consists of 4-5 base pairs and a loop consisting of modified uridine called pseudouridine.
6. Variable Loop: This loop is of variable size and ranges from 3-21 base pairs. It is present between the anticodon and TΨC loop. It recognizes tRNA molecules.

L-shaped model

3D tRNA looks like an L-shaped molecule that has two functional ends: 

• The acceptor stem: Site of attachment of specific amino acid (3’ end of the molecule)
• Anticodon loop: Site where codons are read on mRNA (5’ end of the molecule)

t-RNA Functions

1. It helps in the synthesis of proteins.
2. It helps in aminoacylation which is the first step of protein synthesis. 
3. It transfers the specific amino acid from the amino acid pool to the mRNA to form a polypeptide to make proteins.
4. It helps amino acids link with mRNA to form proteins. 
5. It helps in the recognition of specific amino acid and carry them to the ribosomal unit for translation. 
6. The ribosome site, where the polypeptide chain grows has three binding sites of tRNA-aminoacyl (A site), peptidyl (P site), and exit (E site).
7. It consists of anticodon which decodes the amino acid code for a specific amino acid present in mRNA. 

How does tRNA work?

1. The anticodon site of tRNA has complementary nucleotides that code for specific amino acids. For example-AUG is coded for methionine amino acid and the tRNA anticodon site has a complementary code for it which is UAC.
2. Once, the amino acid is coded, now it has been sent to the DHU site that recognizes the methionine amino acid from the amino acid pool. 
3. This is done with the help of the enzyme aminoacyl-tRNA synthetase.
4. After recognition, methionine will be accepted at the carrier end of tRNA and finally sent to the ribosome recognition site where it will get attached to the ribosome.

FAQs on tRNA

Q1: Where is tRNA located?

Answer:

Transfer RNA is present in the cytoplasm of the cell.

Q2: What does it mean when tRNA is charged?

Answer:

tRNA carries amino acid on its arm and transfers it to the A site. tRNA carrying amino acids is known as changed tRNA.

Q3: What is the function of tRNA?

Answer:

tRNA has dual functions, first, it reads the codons and second, it binds with specific amino acids.