3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate(2)

Outline means to give a brief account or summary. .


3.3.2 State the names of the four bases in DNA (1).

State means to give a specific name, value or other brief answer without explanation or calculation.

NB: As of July 2011 there are now be more than four bases. The new addtions to the list tend to be modifications of the bases above (e.g. adding methyl groups to cytosine) and are invloved in processes that modify transription/ translation or silnce genes. The details are not examined on this course.


3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand (2).

Outline means to give a brief summary.








The image is of one polynucleotide chain.












3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds (3).


complementary means matching, is different from complimentary, which means being nice. You may recall that in 1950 Edwin Chargaff working in Columbia University USA had determined that the mass of the bases(in a DNA speciment) formed ratios of A:T and G:C. This held true when taking samples from individuals within a population or when comparing species across large classification divides. In the cell of any organism the mass of Adenine seems to be about the same as the mass of Thymine. The mass of Cytosine seems to be about the same as the mass of Guanine.

Three years later the significance of Chargaff's Rule was realize by Watson and Crick at the Cavendish Laboratory in Cambridge, England. Watson an American geneticist and Crick an English physicist began model building DNA based on a collection of results from other researchers, including Chargaff. The model building technique uses the principles of chemistry such as molecular structure and bond angles as then developed by Linus Pauling. Together with the data from X-ray crystallography studies (the combined work of Wilkins and Franklin ) they began to build DNA and part of that process involved the pairing of the bases in the centre of the helix.

Explain means to give a detailed account of causes, reasons or mechanisms.

The molecular distance from Adenine combined with Thymine is the same as the molecular distance between Guanine combined with cytosine. This gave a uniform distance that could fill the centre of the helix.

The complementary bases are formed (A-T, G-C) when hydrogen bonded occur between the two bases in a pair.

Refer to the diagram and notes in the next section..


3.3.5 Draw and label a simple diagram of the molecular structure of DNA(1) .

Draw means to be able to represent by means of pencil lines.

This image of DNA shows the arrangement of the two polynucleotide chains but not the helical shape which can be seen in the space filled model below.

This is image shows:

a)The number followed by the prime (') determined the carbon in deoxyribose free from bonding to another nucleotide.

b) Note that the two chains are in opposite directions 3' to 5' is parallel to 5' to 3' chain.

a) Adenine hydrogen binds to Thymine

b) Cytosine hydrogen bonds to Guanine





The three-dimensional structure of DNA was discovered in 1953 by Watson and Crick in Cambridge, using the experimental data of Wilkins and Franklin in London, for which work they won a Nobel prize. Ms Franklin however died before the award and the Nobel Prize is never awarded posthumously.

The main features of the structure are:


Click4Biology: 3.3 DNA Structure

DNA Structure

DNA is a right handed double stranded helix of uniform diameter. The two strands of polynucleotide are anti-parallel.


3.3.1 Outline of nucleotide structure

3.3.2 Names of the bases

3.3.3 Polynucleotides

3.3.4 Base pairing

3.3.5 DNA molecular structure







Recommended reading: Watson, J. D. (1999) Double Helix. London: Penguin Books. Has edition has an introduction by Steve Jones.