Swiss biochemist, Friedrich Miescher (1869) discovered and isolated nucleic acids from the pus cells. By 1938, it became evident that nucleic acids are of two types:- Deoxyribose nucleic acid (DNA) and Ribose nucleic acid (RNA).
•DNA is found in chloroplasts and mitochondria. It is the hereditary material in most of the organisms. The nucleic acids are among the largest of all molecules found in living beings.
They contain three types of molecules
a) 5 carbon sugar,
b) Phosphoric acid and
c) Nitrogen containing bases.
Three join together to form a nucleotide of nucleic acid.
Fuelgen (1924) showed that chromosomes contain DNA. He established that nucleic acids contain two pyrimidine bases-cytosine and thymine, and two purine bases-adenine and guanine
Wilkins and co-workers showed that the purine and pyrimidine bases are placed regularly along the DNA molecules at a distance of 3.4 Ao.
DNA is composed of : Sugar molecule (It is a pentose sugar of deoxyribose type), Phosphoric acid (also called phosphates when in chemical combination), Nitrogen containing bases (these are nitrogen containing organic ring compounds).
Principally bases are of two types: (a) pyrimidine bases and (b) purine bases.
Pyrimidine bases are single ring (monocyclic) nitrogenous bases. Cytosine, Thymine and Uracil are pyrimidines.
Purine are double ring (dicyclic) nitrogenous bases Adenine and guanine are purines.
Erwin Chargaff (1950) estimated the relative amounts of the four nitrogenous bases in DNA. They observed that the pyrimidine and purine always occur in equal amount in DNA. They also found that the base ratio i.e. A+T / G+ C. It may vary in the DNA of different groups of animals and plants but A+T/ G+C ratio remains constant for a particular species.
1. Structure of DNA :
DNA is a very long chain made up of alternate sugar and phosphate groups. The sugar is always deoxyribose and It always joined to the phosphate in the same way, so that the long chain is perfectly regular, repeating the same phosphate-sugar sequence over and over again. Each sugar of the sugar-phosphate chain has a ‘base’ attached to it and the base is not always the same. This unit which consists of a sugar, phosphate and a base is called nucleotide.
N.B. + Sugar + Phosphate = Nucleotide
The nitrogenous base and a sugar of a nucleotide form- a molecule, nucleoside.
N.B. + Sugar = Nucleoside
Thus, nucleoside does not contain phosphate group.
Four types of nucleosides are found in DNA molecule. In a nucleoside, nitrogenous base is attached to the first carbon atom (C-1) of the sugar. When a phosphate group gets attached with that of the carbon (C-5) atom of the sugar molecule a nucleotide molecule is formed.•A single strand of DNA consists of several thousands of nucleotides one above the other.
The phosphate group of the lower nucleotide attached with the 5th carbon atom of the deoxyribose sugar forms phospho-di-ester bond with that of the, 3rd carbon atom of the deoxyribose sugar of the nucleotide placed just above it.
Single long chain of polynucleotides of DNA consists of one end with sugar molecules not connected with another nucleotide having C-3 carbon not connected with phosphate group, similarly the other end having C-5 of the sugar is not connected with any more phosphate group. These two ends of the polynucleotide chain are called as 3′ and 5′ ends respectively. The single polynucleotide strand of DNA is not straight but helical in shape.
The DNA molecule consists of such two helical polynucleotide chains which are complementary to each other. The two complementary polynucleotide chains of DNA are held together by the weak hydrogen bonds. Adenine always pairs with thymine, and guanine with cytosine ( a pyrimidine with a purine). Adenine-thymine pair consists of two hydrogen bonds and guanine-cytosine pair consists of three hydrogen bonds Thus, if the sequence of bases of a polynucleotide chain is known, that of the other can be determined.
Watson and Crick did not conduct any experiment on DNA. Crick was expert in physics, X-ray crystallography and Watson in viral and bacterial genetics. They only analyzed and comprehended the results of experiments performed by scientists like R. Franklin, M. Wilkins, etc.
DNA Model: According to Watson and Crick, DNA molecule consists of two strands twisted around each other in the form of a double helix. The two strands i.e. polynucleotide chains are supposed to be in opposite direction so end of one chain having 3’ lies beside the 5′ end of the other.
One turn of the double helix of the DNA measures about 34Ao. It consists paired nucleotides and the distance between two neighbouring pair nucleotides is 3.4Ao. The diameter of the DNA molecule has been found be 20Ao.
There are certain organisms like Bacteriophage ϕ x 174 and several bacterial viruses which possess single stranded DNA. The phi X 174 bacteriophage is a single-stranded DNA virus that infects Escherichia coli, and the first DNA-based genome to be sequenced. This work was completed by Fred Sanger and his team in 1977.
Ribonucleic Acid (RNA): RNA is a single stranded polynucleotide. Found in the living organisms is Ribose nucleic acid. In most of the organisms it is not found to be hereditary material. But in certain organisms like tobacco mosaic virus, it is the hereditary material.
In some cases, RNA is double stranded e.g. Reovirus and wound tumour virus. The nucleotides of RNA have ribose sugar (C5H10O5)instead of the deoxyribose sugar (C5H10O4)as in the case of DNA.
In case of RNA, Uracil substitutes thymine of DNA. Purine, pyrimidine equality is not found in RNA molecule. RNA strand is usually found folded upon itself in certain regions or entirely. These foldings helps in stability of the RNA molecule.
Most of the RNA polynucleotide chains start either with adenine or guanine. Three types of cellular RNAs have been distinguished:
(a) messenger RNA (mRNA) or template RNA,
(b) ribosomal RNA (rRNA),
(c) transfer RNA (tRNA) or soluble RNA.
mRNA:
It carries genetic information for arranging amino acids in definite sequence. It is a linear polynucleotide. It accounts 3% of cellular RNA. Its molecular weight is several million. mRNA molecule carrying information to form a complete polypeptide chain is called cistron. Size of mRNA is related to the size of message it contains. Synthesis of mRNA begins at 5’ end of DNA strand and terminates at 3’ end.
rRNA:
It form 50-60% part of ribosomes. It accounts 80-90% of the cellular RNA. It is synthesized in nucleus. Kurland (1960) discovered it. It gets coiled here and there due to intrachain complementary base pairing.
tRNA:
tRNA molecules are much smaller consisting of 70-80 nucleotides. It is also single stranded but to number of complementary base sequences after pairing, it is shaped like clover-leaf (Holley,1965). Each tRNA can pick up particular amino acid.
Following four parts can be recognized on tRNA
1) DHU arm ( Dihydroxyuracil loop / amino acid recognition site
2) Amino acid binding site
3) Anticodon loop / codon recognition site
4) Ribosome recognition site.
In the anticodon loop of tRNA, three unpaired nucleotides are present called as anticodon which pair with codon present on mRNA. The specific amino acids is attached at the 3′ end in acceptor stem of clover leaf of tRNA.
