2.27.3 Transfer RNA (tRNA)

Definition: tRNA is the smallest of the RNA molecules and it consists of 75 to 90 nucleotides.

Key Points:

• Smallest of the RNA molecules.
• Consists of 75 to 90 nucleotides.
• A tRNA is a single-stranded molecule.
• It shows a duplex appearance at some regions where complementary bases are bonded to one another.
• It shows a flat cloverleaf shape in two-dimensional views.
• Its 5' end always terminates in Guanine base.
• Its 3' end is always terminated with base sequence of CCA.
• Amino acid is attached to tRNA at the 3' end.
• The nucleotide sequence of the rest of the molecule is variable.

• D loop – Recognizes the activation enzyme.
• Anticodon loop – Middle loop; contains the anticodon.
• T loop (θ loop) – Recognizes the specific place on the ribosome for binding during protein synthesis.

• The middle loop in all the tRNA is composed of 7 bases.
• The middle three of these form the anticodon.
• The anticodon is complementary to a specific codon of mRNA.

• There is at least one tRNA molecule for each of the 20 amino acids found in proteins.
• Sixty tRNA have been identified.
• However, human cells contain about 45 different kinds of tRNA molecules.
• Each tRNA transports a specific amino acid from cytoplasm to the surface of ribosome for protein synthesis.

Structure:

Comparison of mRNA, rRNA, and tRNA

Summary Table of RNA Types

Intermediate Level

mRNA in Detail

• Every three nucleotides = one codon.
• One codon = one amino acid.
• 64 codons in total.
• 61 codons code for amino acids.
• 3 stop codons: UAA, UAG, UGA.
• Start codon: AUG (Methionine).

rRNA in Detail

• Prokaryotic: 70S (50S + 30S)
• Eukaryotic: 80S (60S + 40S)

tRNA in Detail

• D loop: Recognizes the activation enzyme.
• Anticodon loop: Contains the anticodon (7 bases; middle 3 form anticodon).
• T loop (θ loop): Recognizes the specific place on the ribosome.

Comparison of mRNA, rRNA, and tRNA

Important Points

• 1. mRNA consists of a single strand of variable length.
• 2. Its length depends upon the size of the gene, as well as the protein for which it is taking message.
• 3. For example, for a protein molecule consisting of 100 amino acids, the mRNA will have the length of 300 nucleotides.
• 4. Every three nucleotides in mRNA encode a specific amino acid.
• 5. Such triplets of nucleotides along the length of mRNA are called codons of genetic codes.
• 6. mRNA is about 3 to 4% of the total RNA in the cell.
• 7. mRNA takes the genetic message from the nucleus to the ribosome in the cytoplasm to form particular protein.
• 8. This process is known as translation.
• 9. rRNA is transcribed by the genes present on the DNA of the several chromosomes.
• 10. It is called rRNA because it eventually becomes part of ribosome.
• 11. The rRNA is packaged with a variety of proteins into ribosomal subunits.
• 12. The base sequence of rRNA is similar from bacteria to higher plants and animals.
• 13. rRNA has the largest size among the RNA.
• 14. Approximately 80% of total RNA contents of a cell are rRNA.
• 15. It is a part of ribosome where protein synthesis takes place.
• 16. In other words, rRNA provides a platform for protein synthesis.
• 17. tRNA is the smallest of the RNA molecules and it consists of 75 to 90 nucleotides.
• 18. A tRNA is a single-stranded molecule but it shows a duplex appearance at some regions where complementary bases are bonded to one another.
• 19. It shows a flat cloverleaf shape in two-dimensional views.
• 20. Its 5' end always terminates in Guanine base.
• 21. Its 3' end is always terminated with base sequence of CCA.
• 22. Amino acid is attached to tRNA at this end.
• 23. The nucleotide sequence of the rest of the molecule is variable.
• 24. tRNA has three loops:
• - D loop – Recognizes the activation enzyme.
• - Anticodon loop – Middle loop; contains the anticodon.
• - T loop (θ loop) – Recognizes the specific place on the ribosome.
• 25. The middle loop in all the tRNA is composed of 7 bases, the middle three of which form the anticodon.
• 26. The anticodon is complementary to a specific codon of mRNA.
• 27. There is at least one tRNA molecule for each of the 20 amino acids found in proteins.
• 28. Sixty tRNA have been identified.
• 29. However, human cells contain about 45 different kinds of tRNA molecules.
• 30. Each tRNA transports a specific amino acid from cytoplasm to the surface of ribosome for protein synthesis.

Remember

• - mRNA = messenger; carries message; codons; 3–4% of total RNA
• - rRNA = ribosomal; forms ribosome; platform for protein synthesis; ~80% of total RNA
• - tRNA = transfer; brings amino acids; anticodon; cloverleaf; 75–90 nucleotides
• - mRNA = single strand; variable length; 300 nucleotides for 100 amino acids
• - rRNA = largest RNA; packaged with proteins into ribosomal subunits
• - tRNA = smallest RNA; 5' guanine; 3' CCA; D loop, anticodon loop, T loop
• - Anticodon = complementary to mRNA codon
• - Three types of RNA = mRNA, rRNA, tRNA

Common Mistakes

• 1. Confusing mRNA, rRNA, and tRNA – mRNA carries message; rRNA forms ribosome; tRNA brings amino acids.
• 2. Thinking mRNA is the most abundant RNA – rRNA is the most abundant (~80%); mRNA is the least abundant (3–4%).
• 3. Forgetting that mRNA has codons – mRNA has codons (triplets of nucleotides).
• 4. Confusing codons and anticodons – Codons are on mRNA; anticodons are on tRNA.
• 5. Thinking rRNA is a single molecule – rRNA is packaged with proteins into ribosomal subunits.
• 6. Forgetting the 5' and 3' ends of tRNA – 5' end is guanine; 3' end is CCA.
• 7. Thinking tRNA has only one loop – tRNA has three loops: D loop, anticodon loop, T loop.
• 8. Forgetting the number of nucleotides in tRNA – 75–90 nucleotides.
• 9. Thinking all tRNAs are the same – There are at least 20 different tRNAs, one for each amino acid.
• 10. Confusing the size of RNA molecules – rRNA is the largest; tRNA is the smallest; mRNA is variable.
• 11. Forgetting the function of rRNA – rRNA provides the platform for protein synthesis.
• 12. Thinking mRNA is stable – mRNA is less stable than rRNA and tRNA.
• 13. Forgetting that tRNA has an anticodon – The anticodon is complementary to the mRNA codon.
• 14. Confusing the percentage of RNA types – mRNA = 3–4%; rRNA = ~80%; tRNA = ~15%.

Quick Revision

• mRNA consists of a single strand of variable length.
• Its length depends upon the size of the gene, as well as the protein for which it is taking message.
• For example, for a protein molecule consisting of 100 amino acids, the mRNA will have the length of 300 nucleotides.
• Actually every three nucleotides in mRNA encode a specific amino acid, such triplets of nucleotides along the length of mRNA are called codons of genetic codes.
• mRNA is about 3 to 4% of the total RNA in the cell.
• mRNA takes the genetic message from the nucleus to the ribosome in the cytoplasm to form particular protein.
• This process is known as translation.
• rRNA is transcribed by the genes present on the DNA of the several chromosomes.
• It is called rRNA because it eventually becomes part of ribosome.
• The rRNA is packaged with a variety of proteins into ribosomal subunits.
• The base sequence of rRNA is similar from bacteria to higher plants and animals.
• rRNA have largest size among the RNA.
• Approximately, 80% of total RNA contents of a cell are rRNA.
• It is a part of ribosome where protein synthesis takes place.
• In other words rRNA provides a platform for protein synthesis.
• tRNA is the smallest of the RNA molecules and it consists of 75 to 90 nucleotides.
• A tRNA is a single stranded molecule but it shows a duplex appearance at its some regions where complement entry bases are bonded to one another.
• It shows a flat cloverleaf shape in two dimensional views.
• Its 5' end always terminates in Guanine base while the 3' end is always terminated with base sequence of CCA.
• Amino acid is attached to tRNA at this end.
• The nucleotide sequence of the rest of the molecule is variable.
• tRNA has three loops.
• The middle loop in all the tRNA is composed of 7 bases, the middle three of which form the anticodon; it is complementary to specific codon of mRNA.
• The D loop recognizes the activation enzyme.
• Theta (θ) loop recognizes the specific place on the ribosome for binding during protein synthesis.
• There is at least one tRNA molecule for each of the 20 amino acids found in proteins.
• Sixty tRNA have been identified.
• However, human cells contain about 45 different kinds of tRNA molecules, each transports a specific amino acid from cytoplasm to the surface of ribosome for protein synthesis.

60-Second Revision

• mRNA = messenger; carries message; codons; 3–4% of total RNA
• rRNA = ribosomal; forms ribosome; platform for protein synthesis; ~80% of total RNA
• tRNA = transfer; brings amino acids; anticodon; cloverleaf; 75–90 nucleotides
• mRNA = single strand; variable length; 300 nucleotides for 100 amino acids
• rRNA = largest RNA; packaged with proteins into ribosomal subunits
• tRNA = smallest RNA; 5' guanine; 3' CCA; D loop, anticodon loop, T loop
• Anticodon = complementary to mRNA codon
• Three types of RNA = mRNA, rRNA, tRNA
• Warning: mRNA has codons; tRNA has anticodons. rRNA is the most abundant; mRNA is the least abundant. tRNA has three loops. tRNA ends are 5' guanine and 3' CCA. Do not confuse the functions of the three types of RNA.