Nucleotides and Nucleic Acids

Nucleotides and Nucleic Acids

(a) state that deoxyribonucleic acid (DNA) is a polynucleotide, usually double stranded made up of nucleotides containing the bases adenine (A), thymine (T), cytosine (C) and guanine (G)

Nucleic acids come in two forms: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).


(b) state that ribonucleic acid (RNA) is a polynucleotide, usually single stranded, made up of nucleotides containing the bases adenine (A), uracil (U), cytosine (C) and guanine (G)

(c) describe, with the aid of digrams, how hydrogen bonding between complementary base pairs (A to T, G to C) on two antiparallel DNA polynucleotides leads to the formation of a DNA molecule, and how the twisting of DNA produces its ‘double-helix’ shape

The five organic nitrogenous bases are grouped into purines and pyramidines. Pyramidines are smaller than purines.

  • Purines: Adenine (A) and Guanine (G) – Ag (Silver) is PURE metal = PURINES
  • Pyramidines: Thymine (T), Uracil (U) and Cytosine (C)

DNA is found in the nucleus. The molecule is twisted into a double helix in which each of the strands are antiparallel to each other, meaning the strands always run in opposite directions to each other. It has two sugar phosphate backbones attached to one another by complementary bases. These bases pair in the centre of the molecule by means of hydrogen bonds. The chains are always the same distance apart because the bases pair up in a specific way. Where a pyramidine appears on one side, a purine appears on the other.


(d) outline, with the aid of diagrams, how DNA replicates semi-conservatively, with reference to the role of DNA polymerase

Replicating DNA:

  1. The double helix unwinds.
  2. Hydrogen bonds between the bases are broken apart to ‘unzip’ the DNA to form two single strands, exposing the bases.
  3. Hydrogen bonds form between free DNA nucleotides and exposed bases through complementary base pairing.
  4. Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone using the enzyme DNA polymerase.
  5. Each new DNA molecule consists of one conserved strand plus one newly built strand. This process of DNA replication is described as semi-conservative replication.

(e) state that a gene is a sequence of DNA nucleotides that codes for a polypeptide

A gene is a sequence of DNA nucleotides that codes for a polypeptide (protein)


Proteins are made from amino acids. Different proteins have a different number and order of amino acids. Each amino acid is coded for by a sequence of 3 bases in a gene.  It’s the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein. Different sequences of bases code for different amino acids.


(f) outline the roles of DNA and RNA in living organisms (the concept of protein synthesis must be considered in outline only)

There are 3 forms of RNA: