Revision:DNA Structure

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Outline DNA Nucleotide structure

Outline DNA Nucleotide structure in terms of sugar (deoxyribose), base and phosphate.

DNA consists of units called nucleotides. There are four different nucleotides. Each one consists of a deoxyribose 5-carbon sugar, a phosphate group, and a nitrogenous base- adenine, guanine, cytosine, or thymine. The DNA molecule consists of 2 strands twisted together into a double helix, much like a twisted ladder. The phosphate and the sugar molecules make up the sides of the ladder. The phosphate group is joined to the deoxyribose sugar in the nucleotide on the 5' end carbon and to the 3' end carbon of the deoxyribose of the other sugar. The nitrogenous bases make up the steps of the ladder.

State the names of the four bases in DNA

1. Adenine
2. Guanine
3. Cytosine
4. Thymine

Outline how DNA nucleotides are linked together by covalent bonds into a single strand

The bases of one chain are attracted to the bases of the other chain by means of hydrogen bonds. Adenine and guanine are purines, which are double-ringed, and cytosine and thymine are pyrimidines, which are single-ringed. Adenine can only pair with thymine and cytosine only with guanine. This is known as the base-pair rule. A and T form a soluble hydrogen bond, C and G a triple one. A and T are complementary; G and C also. This is known as the base-pair rule.

Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds

• A=T: adenine forms two hydrogen bonds with its complementary base, thymine • G=C: guanine forms three hydrogen bonds with its complementary base, cytosine • Polymers coiled around, forming double helix • Hydrogen bonds between A=T and G=C hold the two strands of DNA together

Draw a simple diagram of the molecular structure of DNA

P-S-P-S-P

    |     |

    B    B

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