Types of RNA MCQ - Practice Questions with Answers

1. Ribosomal RNA (rRNA): 

• It is the most abundant RNA (70-80%) of the total.
• It is a constituent of ribosomes. 
• Depending upon their sedimentation coefficient, rRNAs of eukaryotes are of four types:
  • 28S
  • 18S
  • 5.8S
  • 5S
• Prokaryotes have three types of rRNAs:
  • 23S
  • 16S
  • 5S
• 50S subunit of the prokaryotic ribosomes contains 23S rRNA, 5S rRNA and 32 proteins.
• 30S subunit of the prokaryotic ribosomes contains 16S rRNA and 21 proteins.
• 60S subunit of the eukaryotic ribosomes contains 28S rRNA, 5S rRNA, 5.8S rRNA and 50 proteins.
• 40S subunit of the eukaryotic ribosomes contains 18S rRNA and 33 proteins.
• rRNAs combine with the protein molecules and give rise to the ribosomes.
• 3’ end of the 18S rRNA (16S in prokaryotes) is complementary to the 5’ cap region of the mRNA.
• 5S rRNA provides a binding site for tRNA.

2. Transfer RNA (tRNA):

• tRNAs constitute about 15% of the total RNA in the cell.
• tRNAs have a clover leaf structure which is stabilized by strong hydrogen bonds between the nucleotides. 
• They normally contain some unusual bases in addition to the usual 4, which are formed by methylation of the usual bases. 
• The secondary structure of most tRNA is composed of four helical stems:
  • Acceptor stem: It has an amino acid binding site that lies at the 3’ end and has a CCA-OH group.
  • TΨC-stem: This site attaches the tRNA to the ribosome.
  • DHU stem: It is the binding site for the aminoacyl tRNA synthetase enzyme.
  • Anticodon stem: It has 7 bases out of which 3 bases form the anticodon for recognising and attaching to the codon on the mRNA.
• tRNA is an adaptor molecule which is meant for transferring amino acids to ribosomes for synthesis of polypeptides.

3. Messenger RNA (mRNA):

• The mRNA carries the message from the DNA, which controls all of the cellular activities in a cell.
• It constitutes 2-5% of the RNA content of the cell. 
• It carries information of the sequence of amino acids in a particular polypeptide chain in the form of genetic code.
• Three adjacent bases (codon) specify a particular amino acid.
• There is a methylated region at the 5’ end of the mRNA that functions as a cap for the attachment with the ribosome. 
• The cap is followed by an initiation codon.
• There is a small region at 3’end made up of polyadenylated sequence, forming the tail.
• Eukaryotic mRNA carries the information for one gene at a time. They are called monocistronic. 
• Prokaryotic mRNA may carry the information from multiple genes at a time. They are called polycistronic.
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