Human Genetic lec.1

LEC.1
Dr. Liqaa M. Al-Sharifi
Human Genetics
Structure of nucleic acids: DNA & RNA
Nucleic acid is composed of a long polymer of individual molecules called
nucleotides. Each nucleotide is composed of a nitrogenous base, a sugar molecule and
a phosphate molecule.
The nitrogenous bases fall into two types, purines and pyrimidin, the purines include
adenine and guanine; the pyrimidine include cytosine, thymine and uracil.
There are two different types of nucleic acid, ribonucleic acid (RNA), which contains
the five carbon sugar ribose, and deoxyribonucleic acid (DNA), in which the hydroxyl
group at the 2 position of the ribose sugar is replaced by a hydrogen (i.e. anoxygen
molecule is lost, hence deoxy ).
DNA and RNA both contain the purine bases adenine and guanine and the
pyrimidine cytosine, but thymine occurs only in DNA and uracil is found only in
RNA.
RNA is present in the cytoplasm and in particularly high concentrations in the
nucleolus of the nucleus. DNA is found mainly in the chromosomes.
The DNA moleculeis composed of two chains of nucleotides arranged in a double
helix. The backbone of each chain is formed by phosphodiester bonds between the 3
and 5 carbons of adjacent sugars, the two chains being held together by hydrogen
bonds between the nitrogenous bases , which point in towards the center of the helix.
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