Question

What three forms of RNA are found in both prokaryotes and eukaryotes? Why are these three common to all organisms (describe their cellular functions)?

Answer 1

RNA:

Ribonucleic acid, they are the messengers of gene to carry out protein synthesis. RNA has 4 nucleotides Adenine, guanine, cytosine and uracil.

In all prokaryotes and Eukaryotes there are 3 forms of RNA present, they are

1. Messenger RNA - mRNA
2. Ribosomal RNA – rRNA
3. Transfer RNA – tRNA

mRNA:

• mRNA is the messenger RNA, it is formed form the DNA through the process called transcription. The segment which codes for protein is alone transcribed as mRNA.
• The sequence of mRNA is read as codon. Codon is the triplet code which codes for amino acids.
• At 5’ and 3’ end there are non coding regions present.
• AUG is mostly the start codon and UAG, UGA and UAA are the stop codon.

rRNA:

• Ribosomal rRNA, there are 2 subunits present which is differentiated by the sedimentation co- efficient.
• In prokaryotes it is 70S – subunits are 30S and 50S.
• In eukaryotes it is 80S – subunits are 60S and 40S.
• rRNA binds to mRNA to participate in protein synthesis – translation.
• It has A site where the tRNA brings the amino acid at this site.
• It has P site, once the amino acid attaches at the A-site and rRNA moves one step forward along mRNA as the amino acid reaches the P site, when the A site is empty tRNA brings the next amino acid at A-site and a peptide bond is formed between the 2 amino acids, and the protein synthesis continues until it reaches the stop codon.
• In prokaryotes both transcription and translation takes place together.
• In Eukaryotes the transcription takes place in nucleus and translation takes place in cytoplasm.

tRNA:

Transfer RNA, it carries amino acids based on the codon present on the mRNA to the rRNA A site for the protein synthesis.

There is different tRNA each to code for 20 different amino acids.

It has clover leaf structure.

It has 5 arms, they are

1. Acceptor arm
2. T?C arm
3. Anti codon arm
4. D arm
5. Extra arm

Acceptor Arm:

It always ends with CCA at the 3’ end.

tRNA with amino acid is called Amino acyl tRNA.

T?C arm:

It is named T?C arm because it has 3 nucleotides T and C and a modified psi base-Psuedouridine .

This arm is involved in binding to the rRNA.

Anticodon arm:

This is present opposite to the acceptor arm.

It has a triplet codon which is complementary to the mRNA triplet codon, because of this it can bind to the mRNA.

D arm:

It is named based on the dihydrouridine modified base present in the loop.

This arm helps in recognizing the site of enzyme amino acyl tRNA synthetase to add amino acid to the acceptor arm.

Extra arm:

Short extra arm is present between the T?C arm and the anticodon arm. The function of this arm is not known.