NUCLEIC ACIDS (DNA and RNA)

NUCLEIC ACIDS (DNA and RNA)

Introductio

Nucleic acids are important component of cells, which control all the activities of cell.

Discovery

Nucleic acids were first isolated in 1869 by F. Miescher from the nuclei of pus cells. Due to their isolation from nuclei and their acidic nature, they were named as nucleic acids.

Types of nucleic acids

There are two main types of nucleic acids i.e,

1. Ribonucleic acid (RNA)
2. Deoxyribonucleic acid (DNA)

Their chemical composition is described as follows;

Nucleotide

Nucleotides are units of nucleic acids, which are linked to each other by ester linkage to form polynucleotide chains

• DNA is made of deoxyribonucleotides.
• RNA is made of ribonucleotides.

Chemical composition of nucleotide

Typically a nucleotide is composed of three components i.e.

• Pentose sugar
• Nitrogenous base
• Phosphoric acid

(i) Pentose Sugar

• Pentose sugar in RNA is ribose(C5H10O5)
• Pentose sugar in DNA is deoxyribose (C5H10O4)
• (ii) Nitrogenous Bases
  Nitrogenous bases are of two types:
  1. Single-ringed pyrimidines, which include cytosine (C), thymine (T) and uracil (U).
  2. Double-ringed purines, which include adenine (A) and guanine (G).
  (iii) Phosphoric Acid
  Phosphoric acid (H3PO4) has the ability to develop ester linkage with OH group of pentose sugar.
  In a typical nucleotide, the nitrogenous base is attached to position 1 of pentose sugar while phosphoric acid is attached to position 3 of pentose sugar in front and position 5 of pentose sugar behind it. Since phosphate forms a double ester linkage with pentose sugar, the linkage is known as phosphodiester linkage.

  Formation of a nucleotide

  A compound, nucleoside is formed by combination of a base and a pentose sugar. A nucleoside and a phosphoric acid combine to form a nucleotide. As we know pentose sugar in RNA is ribose while in DNA is deoxyribose. Deoxyribose is formed from ribose by removing one oxygen from OH group at carbon number 2. ATP is also an important nucleotide used as an energy currency by the cell.

  
  

  Deoxyribonucleic acid (DNA)

  DNA is heredity material. It controls the properties and potential activities of a cell.

  Location in cell

  DNA occurs in chromosomes, in the nuclei of the cells and in much lesser amounts in mitochondria and chloroplast.

  Chemical composition of DNA

  • DNA is made of four kinds of nucleotides;
  • d-adenosine monophosphate (d-AMP)
  • d-guanosine monophosphate (d-GMP)
  • d-cytidine monophosphate (d-CMP)
  • d-thymidine monophosphate (d-TMP)
  • These nucleotides are united with one another through phosphodiester linkages in a specific sequence to form long chains.
  • Two nucleotides join to form dinucleotide e.g nicotinamide adenine dinucleotide (NAD)which is important coenzyme in several oxidation-reduction reactions in the cell.
  • Three nucleotides join to form trinucleotide.

  

  Ratio of Bases in DNA

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  In 1951, Erwin Chargaff provided data about the ratios of different bases present in DNA. This data suggested that adenine and thymine are equal in ratio and so guanine and cytosine.

  Source of DNA	Adenine	Guanine	Thymine	Cytosine
  Man	30.9	19.9	29.4	19.8
  Sheep	29.3	21.4	28.3	21.0
  Wheat	27.3	22.7	27.1	22.8
  Yeast	31.3	18.7	32.9	17.1

  Physical structure of DNA

  • Maurice Wilkins and Rosalind Franklin used the technique of X-ray diffraction to determine the structure of DNA.

  • At the same time James D. Watson and Francis Crick built the scale to determine the structure of DNA.
  • All the data thus obtained strongly suggested that DNA is made of two polynucleotide chains or strands. The two strands are coiled round each other in the form of double helix.
  • Coiling of two strands is opposite i.e. they are coiled antiparallel to each other.
  • The two chains are held together by weak hydrogen bonds.
  • Adenine is opposite to thymine and there are two hydrogen bonds between them.
  • Guanine is opposite to cytosine and there are three hydrogen bonds between them.
  • The two strands are wound around each other so that there are 10 basei of chromosomes. The amount of DNA in germ line cells (sperms and ova) is almost half to that of somatic cells.

  Type of cells	Amount of DNA/nucleus in picogram in Chicken	Amount of DNA/nucleus in picogram in Carp
  Red Blood Cells	2.3	3.3
  Liver Cells	2.4	3.3
  Kidney Cells	2.4	3.3
  Sperm Cells	1.3	1.6

  Ribonucleic acid (RNA)

  RNA is also a polymer of ribonucleotides.

  Location in cell

  RNA is present in the nucleolus, in the ribosomes, in the cytosol and in smaller amounts in other parts of the cell.
• Writer:shoaib akhtar 
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