3.3: DNA structure

Question 1

Outline DNA nucleotide structure:

Answer 1

A nucleotide is made of the sugar deoxyribose, a base (which can be either adenine, guanine, cytosine or thymine) and a phosphate group.

Question 2

State the names of the 4 bases in DNA:

Answer 2

Adenine, Thymine, Guanine and Cytosine.

Question 3

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

Answer 3

• Nucleotides a linked into a single strand via a condensation reaction
• The phosphate group (attached to the 5’-C of the sugar) joins with the hydroxyl (OH) group attached to the 3’-C of the sugar
• This results in a phosphodiester bond between the two nucleotides and the formation of a water molecule
• Successive condensation reactions between nucleotides results in the formation of a long single strand.

Question 4

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

Answer 4

DNA is made up of two nucleotide strands. The nucleotides are connected together by covalent bonds within each strand. The sugar of one nucleotide forms a covalent bond with the phosphate group of another. The two strands themselves are connected by hydrogen bonds. The hydrogen bonds are found between the bases of the two strands of nucleotides. Adenine forms hydrogen bonds with thymine whereas guanine forms hydrogen bonds with cytosine. This is called complementary base pairing.