VPB 321: Animal Biotechnology (2+1)

RNA STRUCTURE

RNA structure

• Primary structure is the same as that of DNA, a polynucleotide chain with 5’------- 3’ sugar phosphate linked.
• But generally it exists as a single polynucleotide chain rather than a double helix of antiparellel strands. However base pairing can take place within and between RNA molecule.
• RNA occurs as a single stranded molecule and in RNA, thymine is replaced by uracil. There are three types of RNA,
  • mRNA,
  • tRNA and
  • rRNA
  • Details of RNAs

mRNA (messenger RNA)

• mRNA has the same function in all the cells but there are difference in the synthesis and structure of prokaryotic and Eukaryotic mRNA
• In prokaryotes, mRNA is translated and transcribed in the single cellular compartment and the two processes occur simultaneously. Bacterial mRNA is unstable and therefore translated into proteins.
• In a eukaryotic cell, synthesis and maturation of mRNA occur in the nucleus. Only after maturation, mRNA is exported to the cytoplasm, where it is translated by ribosomes. Eukaryotic mRNA is more stable and continues to be translated for several hours.
• Monocistronic: Some mRNAs code for a single gene.
• Polycistronic: Some mRNAs which carry sequences coding for several proteins tRNA structure and function.

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tRNA (transfer RNA)

• tRNAs are the adaptor molecules that deliver amino acids to the ribosome and decode the information in mRNA. The clover leaf structure is a common secondary structure of tRNA molecule. tRNA is a small molecule whose polynucleotide chain is only 75- 85 bases long.
  • Acceptor arm.
  • T Y C arm is named for the presence of a base Pseudouridine (Y stands for pseudouridine – modified base).
  • Anticodon arm always contains the anticodon triplet in the center of the loop.
• D – arm is named for its content of the base dihydrouridine.
• When tRNA is charged with the amino acid corresponding to its anticodon, it is called aminoacyl tRNA. The process of charging a tRNA is catalyzed by a specific enzyme, aminoacyl tRNA synthetase.

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rRNA (ribosomal RNA)

• Ribosomal RNA (rRNA) is a component of the ribosomes, the protein synthetic factories in the cell. Eukaryotic ribosomes contain four different rRNA molecules: 18 s, 5.8 s, 28 s, and 5 s rRNA.
• Three of the rRNA molecules are synthesized in the nucleolus. rRNA molecules are extremely abundant. They make up at least 80% of the RNA molecules found in a typical eukaryotic cell.
• Synthesis of the three nucleolar rRNA molecules is unusual because they are made on one primary transcript that is chopped up into three mature rRNA molecules. These rRNA molecules and the 5 s rRNA combine with the ribosomal proteins in the nucleolus to form pre 40 s and pre 60 s ribosomal subunits. These pre-subunits are exported to the nucleus where they mature and assume their role in protein synthesis.
• The rRNA molecules have several roles in protein synthesis. First, the 28 s rRNA has a catalytic role, it forms part of the peptidyl transferrase activity of the 60 s subunit. Second, 18s rRNA has a recognition role, involved in correct positioning of the mRNA and the peptidyl tRNA. Finally, the rRNA molecules have a structural role. They fold into three-dimensional shapes that form the scaffold on which the ribosomal proteins assemble.

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