BOTANY Higher Secondary Second Year - Textbooks Online

specific for a particular amino acid. In bacterial cell, there are more than
70 tRNAs and in eukaryotic cells the number is even greater. There are
four or five tRNAs specific for a particular amino acid and these are
called isoacceptor tRNAs.
Structure
The tRNA has a cloverleaf like structure. It is synthesized in the
nucleus on a small part of DNA. In 1965, R.W. Holley suggested the
cloverleaf model of tRNA. Though tRNA molecule consists of a single
strand, it assumes clover leaf like structure through folding. There are
three folds in the clover leaf tRNA. It has four arms namely anticodon
arm, D arm, T C arm and aminoacid acceptor arm. The tRNA molecules
are made up of 73 to 93 ribonucleotides. The acceptor arm carries an
aminoacid. The anticodon arm has three anticodon nucleotides, which
will join with the complementary codon in mRNA during protein synthesis
i.e. three nucleotides in the tRNA pairs with three nucleotides of mRNA.
In certain tRNAs in addition to these four arms an extra arm called variable
arm occurs as shown in the figure. The aminoacid acceptor and the
anticodon arms are oriented in opposite directions.
Ribosomal RNA
This is found in the ribosomes. The rRNA represents about 40 to 60
per cent of the total weight of the ribosomes. Relatively it constitutes
about 80 per cent of the total RNA of the cells. They are produced in the
nucleus. They are the most stable forms of RNA. They consist of single
strand of nucleotides. At some regions, the strand is folded.
Comparison between DNA and RNA
DNA RNA
1. It contains a 5C deoxyribose. 1. It contains five carbon sugar ribose
2. It contains adenine, guanine, 2. It contains adenine, guanine,
cytosine and thymine cytosine and uracil.
3. It mostly occurs as a double- 3. It often occurs as a single stranded.
stranded helix.
4. It is often much longer. 4. It is shorter.
5. It is more stable. 5. It is less stable.
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