Q.Describe the structure of R.N.A. and give the importance of R.N.A.
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Start your 14-day free trial to unlock the full solution →RNA is a single-stranded nucleic acid central to gene expression, and the genetic material of some viruses. OR: the genetic code is a degenerate triplet code; Darwin's theory explains evolution through natural selection.
Structure of RNA: RNA (ribonucleic acid) is generally a single-stranded polynucleotide, though it can fold back on itself to form secondary structures (as seen in tRNA). Each nucleotide of RNA consists of a ribose sugar, a phosphate group, and a nitrogenous base, which may be a purine (Adenine or Guanine) or a pyrimidine (Cytosine or Uracil — Uracil replaces the Thymine found in DNA). Successive nucleotides are linked by 3′–5′ phosphodiester bonds, forming the sugar-phosphate backbone. There are three principal types of cellular RNA: messenger RNA (mRNA), which carries the genetic code from DNA to the ribosome as a template for protein synthesis; ribosomal RNA (rRNA), a structural and catalytic component of ribosomes; and transfer RNA (tRNA), an adapter molecule with a characteristic clover-leaf structure that carries specific amino acids to the ribosome during translation.
Importance of RNA: RNA serves as the genetic material in several viruses (e.g., TMV, QB bacteriophage, and retroviruses such as HIV). It plays a central role in gene expression, acting as the intermediary (mRNA) that carries the transcribed genetic code from DNA to the ribosome for translation into protein; tRNA and rRNA are themselves indispensable for carrying out translation. RNA (in the form of ribozymes) can also act as a biological catalyst. Because RNA is more chemically reactive and versatile than DNA, it is believed to have been the first genetic material to have evolved (the RNA world hypothesis), with the more chemically stable DNA evolving later for long-term storage of genetic information.
OR:
(a) Salient features of the genetic code: (i) It is a triplet code — a sequence of three nucleotides (a codon) specifies one amino acid. (ii) Of the 64 possible codons, 61 code for amino acids, while 3 codons (UAA, UAG, UGA) are stop/non-sense codons that do not code for any amino acid and signal the termination of translation. (iii) A codon codes for only one amino acid, so the code is unambiguous and specific. (iv) The code is degenerate — since 61 sense codons specify only 20 amino acids, some amino acids are coded by more than one codon. (v) The codon is read in the mRNA in a contiguous, non-overlapping fashion. (vi) The genetic code is nearly universal — for example, UUU codes for phenylalanine in organisms ranging from bacteria to humans (with a few exceptions, such as in mitochondria). (vii) AUG has a dual function: it codes for methionine and also acts as the start/initiator codon for translation.
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