Genes and the Genetic Code

This lesson covers:

  1. How DNA is organised into chromosomes
  2. How DNA is stored in eukaryotes and prokaryotes
  3. The difference between introns and exons 
  4. The features of the genetic code 

DNA is condensed into chromosomes

DNA molecules are incredibly long and must be tightly packed up to fit within the nucleus of a eukaryotic cell. 

Diagram showing DNA molecule winding around histones to form a DNA-histone complex, which further coils to form chromatin and condenses into a chromosome.

DNA molecules are wound around proteins known as histones to form a DNA-histone complex.


These complexes coil further to pack the DNA into chromosomes whereby each chromosome contains just a single molecule of DNA.

DNA in eukaryotes and prokaryotes

Eukaryotic and prokaryotic cells store DNA in slightly different ways.

Prokaryotic cells: 

  • Store DNA in the cytoplasm.
  • Contain shorter, circular DNA molecules.
  • Do not associate the DNA with histones.
  • DNA does not contain introns.

Eukaryotic cells: 

  • Store DNA in the nucleus.
  • Contain long, linear DNA molecules.
  • Associate the DNA with histones.
  • DNA contains introns.

Eukaryotic cells also contain DNA in their mitochondria and chloroplasts. This DNA, like in prokaryotes, is short, circular, and not associated with histones.

DNA contains genes

A gene is a short section of DNA that codes for a polypeptide (a protein).

Illustration showing a gene with exons and introns in a DNA sequence.

In eukaryotes, genes consist of regions known as exons and introns:

  • Exons - Sections of DNA that do code for amino acids.
  • Introns - Sections of DNA that do not code for amino acids. Remember it as if introns are interfering. 

Each gene is located at a specific position along a chromosome known as a locus. 

The complete set of genes within a cell is known as the genome. 


The full range of proteins that a cell is capable of producing is known as the proteome.

The genetic code

The genetic code refers to the sequence of bases that code for amino acids.

Diagram showing the genetic code with DNA triplets coding for amino acids glycine, serine, and valine.


Scientists found that each amino acid is coded for by a sequence of three DNA bases, known as a triplet.

The genetic code has the following features: 

  1. Universal - Each DNA triplet codes for the same amino acid in all organisms (with a few minor exceptions). 
  2. Non-overlapping - Each base in the DNA sequence is only read once (e.g. CGTATC is read as CGT and ATC).
  3. Degenerate - Most amino acids are coded for by more than one triplet (e.g. ACA and ACG both code for cysteine).

How to interpret tables that show codons for amino acids

The genetic code can be summarised into a table as shown below. The code is often shown as mRNA bases rather than DNA, so the base uracil (U) is shown instead of thymine (T).

Table showing codons for amino acids with mRNA bases including uracil instead of thymine.

Each amino acid is coded for by a sequence of three mRNA bases known as a codon (similar to a DNA triplet).


We can see from the table above, that many amino acids are coded for by multiple codons. For example, the amino acid phe (phenylalanine) is coded for by codons UUU and UUC.