Indira Gandhi My Favourite Leader

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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.

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

and Cytosine bonds with Guanine

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


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...
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