Explanations on Deoxyribose Nucleic Acid and Ribose Nucleic Acid.
Deoxyribose Nucleic Acid (DNA):
Watson and Crick (1953) have proposed a model for the structure of DNA molecule which is now usually accepted by all. According to this model called as Watson Crick Model, the DNA molecule is a double helix structure consisting of two long polynucleotide chains coiled round each other around an imaginary axis and running opposite to each other. (Fig. 9.14).
Each polynucleotide chain consists of thousands of nucleotide units.
The back-bone of the two helices of polynucleotide chain consists of deoxyribose phosphates while the bases are present on the inner sides.
The bases of the one polynucleotide chain are complementary to the bases of the other polynucleotide chain and are joined together by hydrogen bonds (Fig. 9.16).
The base pairing is very specific (Fig 9.15), The complementary bases are:-
Adenine and Thymine
Guanine and Cytosine
The ratio of purine and pyrimidine bases is 1: 1.
The distance between two subsequent base pairs in the polynucleotide chain is 3.4 Å.
Each turn of the two polynucleotide chains is completed after 10 base pairs i.e., a distance of 34 Å.
The distance between the axis and the sugar phosphate region is about 10 Å. The helical coiling is right handed.
(1. D.N.A. molecules are gigantic. Their molecular weights are in the region of several millions. 2. In some bacteriophages i.e., the viruses that attack bacteria, the DNA is single stranded.)
There are different forms of DNA in living organisms viz., A, B, C, and rarely D and E. In all these forms, the helical coiling of DNA molecule is right handed. These forms differ in number of base pairs (bp) per turn, diameter of the helix and similar other minor details. Most common and usual form of DNA found in living organisms is B form which is called as B-DNA. The preceding account of the structure of DNA in-fact pertains to the B-DNA.
Recently, another form of DNA has artificially been synthesized in which helical coiling of DNA is left handed and the phosphate backbone of the polynucleotides follows a zig-zag course. This form of DNA has therefore, been designated as Z-DNA. Some of the other important features of Z-DNA one: 1. Each turn of the two polynucleotide chains is completed after 12 base pairs i.e., a distance of about 45 Å. 2. Distance between two subsequent base pairs is 3.7 Å. 3.
The distance between axis and sugar-phosphate is 9 Å. 4. Alternate deoxy-ribose sugar units in the polynucleotide chain have inverse orientation (one such unit having ethereal oxygen facing upward and the other subsequent unit facing downward).
Ribose Nucleic Acid (RNA):
RNA is a single stranded structure consisting of only one polynucleotide chain. If sometime the complementary bases come very close to each other, hydrogen bonds are established between them to give polynucleotide chain a helical appearance like DNA.
RNA consists of the following bases:
Adenine and Uracil
Guanine and Cytosine.
The ratio of purine and pyrimidine bases is not 1:1.
The pentose sugar is β-D-Ribose.
The size of RNA molecule is very small in comparison to the DNA molecule. Molecular wt. of RNA may range from several thousands to some lakhs.
There are 3 different forms of RNAs in plant cells:
(i) Messenger RNA (m-RNA):
Molecular wt. of m-RNA is higher among different types of RNAs usually varying from 5- 10 lakhs. These constitute 5-10% of the total RNA is the cell.
m-RNA is synthesized in nucleolus and after taking genetic information from DNA goes into the cytoplasm and helps in the formation, of specific protein. m-RNA are short lived.
Sequence of 3 bases of nucleotides in m-RNA molecule constitutes a codon. Actually the genetic information obtained from the DNA is encoded in codons which are specific (for details see genetic code).
(ii) Ribosomal RNA (r-RNA):
r-RNA is found in ribosome which act as template for the synthesis of proteins. It is of most stable kind and constitutes about 80% of the total RNA in the cell.
(iii) Transfer or Soluble RNA (t-RNA or s-RNA):
The structures of many t-RNA molecules are known in quite detail. These are comparatively very small with a molecular weight of about 25000. Basic structure of all t-RNA molecules is on the clover leaf pattern. Clover leaf model of t-RNA is given in Fig. 9.17.
t-RNAs are found in cytoplasm and consist of only about 80 bases. These constitute 10- 15% of total RNA in the cell.
t-RNAs contain many unusual bases and nucleotides. These are e.g., pseudouridine (Ψ), dihydrouridine (DHU), inosine (I) etc. Methylation of bases is also common.
All t-RNA molecules contain Guanine (G) at 5′ end. The 3′ end always ends in the base sequence Cytosine-Cytosine-Adenine (CCA). During protein synthesis this end in fact picks up the amino acid and transfers it to the growing polypeptide chain and hence these RNAs are called as t-RNAs or transfer RNAs.
These t-RNAs are also called as s-RNA or soluble RNAs because they are soluble in IM NaCl.
t-RNA molecules are folded in a clover leaf pattern with three or more double helical regions (like DNA) terminating in loops.
Three important loops of t-RNA are:
(i) Anticodon loop
(ii) Amino acyl synthetase binding loop
(iii) Ribosomal binding loop.
Anticodon loop consists of 7 bases. At the free end, 3 unpaired bases constitute the anticodon which is complementary to codon in m-RNA. Aminoacyl synthetase binding loop consists 8-12 bases. Because of the presence of dihydrouridines in this loop, it is also known as DHU loop.
The ribosomal binding loop consists of 7 bases. In contains sequence of GTΨC and hence is also called as GTΨC loop.
There are different t-RNA molecules with specific anticodons to pick up specific amino acids. However, many t-RNAs may be specific to a particular amino acid or a single t-RNA species may recognise several amino acids.
The t-RNA molecule whose structure was first given by Holley in detail is Yeast alanyl- t-RNA (Fig. 9.18).