A phosphate group
- Each nucleotide consists of a five-carbon sugar to which is attached a
phosphate group and a nitrogenous base.
- This nucleic acid consists of four kind of nucleotides.
- The four differ only in their component base, which is adenine, guanine, thymine or
cytosine. Base nucleotide
Adenine (A) deoxyadenosine 5/ monophosphate dAMP
Guanine (G) deoxyguanosine 5/ monophosphate dGMP
Thymine (T) deoxythymidine 5/ monophosphate dTMP
Cytosine (C) deoxycytidine 5/ monophosphate dCMP
- A polymer of nucleotides is called a
polynucleotide.
- The linkage between the nucleotides in a
polynucleotide is called a phosphodiester bond.
- An important feature of the polynucleotide is
that the two ends of the molecule are not the
same.
- One end of the chain ends with a phosphate
linked to the 5' (five-prime) carbon of the sugar
and is called the 5' end. The other end of the
chain ends with an hydroxyl group linked to the
3' (three-prime) carbon of the sugar and is
called the 3' end. 
- The DNA molecules of cell actually consist of two polynucleotides that spiral around an imaginary axis
to form a double helix.
- The two polynucleotides, or strand are held together by hydrogen bonds between the paired bases.
- Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This is
called complementary base pairing.
- A = T , G C