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
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 Rosalind Franklin 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 base pairs in each turn of about 34 Angstromunits (one Angstrom = 10-10 = 1/1,000,000,000 of a centimeter).
Amount of DNA
Amount of DNA is fixed for a particular species as it depends upon number 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 |
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