6. Biomolecules - part 04 - Nucleic Acids

 

6. Biomolecules - part 04 - Nucleic Acids


D. Nucleic Acids :
  • Fuelgen (1924) showed that chromosomes contain DNA. 
  • He established that nucleic acids contain two pyrimidine (cytosine and thymine) and two purine (adenine and guanine) bases. 
  • 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 : 
  1. Sugar molecule (It is a pentose sugar of deoxyribose type) 
  2. Phosphoric acid (also called phosphates when in chemical combination) 
  3. Nitrogen containing bases (these are nitrogen containing organic ring compounds). 
  • Principally bases are of two types:
  1. pyrimidine bases
  2. purine bases
  • Pyrimidine bases :  are single ring (monocyclic) nitrogenous bases. Cytosine, Thymine and Uracil are pyrimidines. 
  • Purine bases : are double ring (dicyclic) nitrogenous bases Adenine and guanine are purines.
  • Erwin Chargaff (1950) estimated the relative amounts of the four nitrogenous bases viz. adenine, thymine, cytosine and guanine 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 may vary in the DNA of different groups of animals and plants but A+T/ G+C ratio remains constant for a particular species.

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. 
  • The nitrogenous base and a sugar of a nucleotide form- a molecule, 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 and 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.
  • 3' end : 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.This end of the polynucleotide chain is called as 3' end
  • 5' end :  the other end having C-5 of the sugar is not connected with any more phosphate group. this end of the polynucleotide chain is called as 5' end
  • 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.

Do you know ?
  • 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.

Ribonucleic Acid (RNA) :


  • Another nucleic acid 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.
  • Like DNA, ribose nucleic acid also consists of polynucleotide chain with the difference that it consists of single strand. 
  • In some cases e.g. Reovirus and wound tumour virus, RNA is double stranded. 
  • The nucleotides of RNA have ribose sugar instead of the deoxyribose sugar as in the case of DNA.
  • In case of RNA, Uracil substitutes thymine of DNA. 
  • Purine, pyrimidine equality is not found in RNA molecule because of its single stranded structure. 
  • 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: 
  1. messenger RNA (mRNA) or template RNA
  2. ribosomal RNA (rRNA)
  3. transfer RNA (tRNA) or soluble RNA.

messenger RNA (mRNA) or template RNA :

  • mRNA 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.

ribosomal RNA (rRNA)

  • rRNA 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.

transfer RNA (tRNA) or soluble RNA :

  • 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.

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