Genome

The genome is the totality of genetic information present in every cell within an organism

• This includes all genes as well as non-coding DNA sequences (introns, promoters, short tandem repeats, etc.)
  

• It also includes all non-nuclear sequences – such as plasmid DNA (in prokaryotes) and organelle DNA (in eukaryotes)
  

Genomes vary in both size (total amount of DNA) and organisation (total number of chromosomes) 

• Genomes also vary in base sequence – with variation between species being much larger than variation within a species.
  

Organisms in the same species share most of their genome (same genes), but differences will exist within the base sequence (different alleles) 

• For example, all humans share approximately 99.9% of their genetic makeup, with the differences creating the variation seen in each individual
  

Genome Size

Genome size can vary greatly between organisms

• Viruses and bacteria tend to have very small genomes, while most eukaryotic organisms have larger genomes
  

• Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid
  

Due to this significant diversity, genome size is not a valid indicator of genetic complexity

• Small animals (guinea pigs) and large animals (elephants) can have similar genome sizes 
  

• Some animals can have a small genome size but large numbers of chromosomes (and vice versa)
  

• One of the largest animal genomes belongs to the Australian lungfish (43 Gb), while the Japanese canopy plant has a genome size of 149 Gb
  

Genome Comparisons