DNA

What is DNA?

DNA, DeoxyriboNucleic Acid is a long polymer that carries the heriditary informantion of most of organisms, DNA is found in the nuclei of eukaryote cells and in the nucleiod of bacteria, we found also the DNA in some organells ( mitochondria and cell plastids (chloroplasts)) and in viruses.

Structure of DNA

The main structure of DNA is double helix which is composed of two antiparallel strands, these strands are made up of the sugar 2-deoxyribose, phosphate, and purine and pyrimidine bases.

 Each phosphate is linked to the 3′ position of the preceding deoxyribose and to the 5′ position of the following deoxyribose.

There are four bases found in the DNA:

Adenine (A)

Tthymine (T)

Guanine (G)

Cytosine (C)

Adenine, A, is always paired with thymine, T.

Cytosine, C, is always paired with guanine, G.

Each 2-deoxyribose is linked to one of four bases in the 1′ position via a covalent glycosidic bond.

we call a nucleoside a base linked to a sugar

we call a nucleotide a base linked to a sugar and a phosphate

Function

DNA play a great role in:

  • Cell living and growing
  • Maintain and propagation of genetic information from generation to generation
  • Synthesis the components of the cells

Replication of DNA

Each strand of the DNA contains all the information needed to form a new double stranded molecules.

after separtion of the double helix each strand is used as a template to form the new strand of DNA, this is carried out by enzymes called DNA polymerases, All DNA polymerases synthesize new DNA in a 5′ to 3′ direction.

Transcription

Transcription is the processus of recopyin genetic information in DNA to form RNA (RiboNucleic Acid) by using RNA polymerases, the result RNA is used to make proteins by the cellular protein-synthesizing (translation) machinery.

After the synthesis of RNA, the DNA reverts to its double helix form.

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