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The Structure of DNA
Deoxyribose nucleic acid (DNA) is a polymer with repeating subunits called
nucleotides. Each of these nucleotides has three subunits: deoxyribose (sugar
molecule), a phosphate, and a nitrogenous base. The nitrogenous base is the only
thing that differs between different nucleotides. There are four possible bases:
Adenine, Cytosine, Guanine and Thymine.
The sugar and phosphate groups are joined together by condensation reactions
forming covalent bonds. If two nucleotides join together a disaccharide is formed.
Therefore, DNA is a polynucleotide.
Since the sugar phosphate groups are identical all the way along the chain the
"sugar phosphate backbone" is invariant. Variation is produced by the bases. It is in
this base sequence that the genetic code lies.
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The DNA molecule has two polynucleotide strands held together by hydrogen
bonding between the pairs of nitrogenous bases. These always pair in the same way:
Adenine with Thymine and Cytosine with Guanine.
The two polynucleotide
strands run in opposite directions. The DNA molecule has been referred to as a
"double helix" or as a "twisted ladder." In 1953 the structure of DNA was suggested
by Watson and Crick, this is still the accepted idea today.…read more