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STRUCTURE OF DNA
PROPOSED BY JAMES WATSON AND FRANCISCRICK
DNA stands for deoxyribose nucleic acid. This chemical substance is present in the nucleus of all living cells . DNA controls all the chemical changes which take place in cells. The famous double helical structure was proposed by James Watson and Francis Crick in the year 1953. Nine years later both received Nobel Prize for this discovery.
STRUCTURE OF DNA
DNA is a very large molecule made up of a long chain of sub-units. The sub-units are called nucleotides. Each nucleotide is made up of a sugar called deoxyribose a phosphate group -PO4 and an organic base
Ribose is a sugar, like glucose, but with only five carbon atoms in its molecule.
Deoxyribose is almost the same but lacks one oxygen atom.
The most common organic bases are ADENINE, GUANINE ,THYMINE, AND CYTOSINE ← They are divided into two groups ← Pyrimidines and purines ← Pyrimidines (made of one 6 member ring) ← Thymine ← Cytosine ← Purines (made of a 6 member ring, fused to a 5 member ring) ← Adenine ← Guanine.
← In fact, the DNA usually consists of a double strand of nucleotides . The ‘backbones’ of DNA molecules are made of alternating sugar and phosphates ← The ‘rungs on the ladder’ are made of bases that are hydrogen bonded to each other
The bases always pair up in the same way
Adenine forms a bond with Thymine
Adenine forms a bond with Thymine
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and Cytosine bonds with Guanine
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The paired strands are coiled into a spiral called A DOUBLE HELIX
The strands run opposite of each other. The 5’ end always has the phosphate attached
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The sequence of bases in DNA forms the Genetic Code. A group of three bases (a triplet) controls the production of a particular amino acid in the cytoplasm of the cell. The different amino acids and the order in which they are joined up determines the sort of protein being produced.
For example
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CYTOSINE, GUANINE AND ADENINE FOR ALANINE
This is known as the triplet code
Each triplet codes for a specific amino acid
CGA - CAA - CCA - CCA - GCT - GGG - GAG – CCA
ALA VAL GLY GLY ARG PRO LEO GLY
A sequence of triplets in the DNA molecule may code for a complete protein.Such a sequence forms a gene.There may be a thousand or more bases in one gene
Conclusion
The Structure of DNA helped us to know how DNA is responsible for coding the characters of an Individual and there by responsible for Heridity and Variation.
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