Almost all the DNA of eukaryotic cells is found in the linear chromosomes of the nucleus. Small amounts of circular DNA are found in cytoplasmic organelles such as mitochondria & chloroplast.
The bases in DNA have carbon-nitrogen ring structures (nitrogenous bases)
Adenine & Guanine (double ring nitrogenous bases) are purines.
Thymine and Cytosine (single ring nitrogenous bases) are pyramidines
- Adenine (A)-6-amino purine,
- Cytosine (C)-2 -oxy-4-aminopyrimidine,
- Guanine (G)-2-amino-6-oxopurine,
- Thymine (T)-2, 4-dioxy-5-methylpyrimidine
- Sugar + base=Nucleoside (The combination of a base with the sugar constitutes a nucleoside)
- Sugar + base (Nucleoside) + phosphate=Nucleotide
- In DNA, the sugar is a form of ribose called deoxyribose (it is missing the oxygen in the 2’ carbon atom)
He found that in DNA the concentration of adenine always equalled the concentration of thymine and the concentration of guanine always equalled the concentration of cytosine ie. the amount of purine=the amount of pyramidine in a given DNA molecule.
A/T or G/C=1, A=T, G=C
A+T=C+G (No. of AT pairs need not equal to the number of GC pairs)
Bonds in DNA : Hydrogen Bonds and Phosphodiester Bonds
- Adenine is paired to Thymine by 2 hydrogen bonds
- Cytosine is paired to Guanine by 3 hydrogen bonds
DNA contains many mono deoxyribo nucleotides covalently linked by 3’5’-phosphodiester bonds. The resulting long, unbranched chain has polarity, with both a 5’end and a 3’end that are not attached to other nucleotides.
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