Genomes (HL IB Biology)

Genomes

• The term genome can be defined as follows:

All of the genetic information in an organism

• This refers to the DNA present within every cell of an organism
• This includes genes that code for proteins as well as non-coding DNA sequences
• Mitochondrial DNA and chloroplast DNA are included in the genome of eukaryotic cells
• In a prokaryote cell, plasmid DNA is included in the genome

Unity and diversity of genomes

• Advances in technology have allowed scientists to determine the entire base sequence of the genes within an organism's genome
  • Determining the base sequence of DNA is referred to as DNA sequencing
• Genome-wide comparisons can now be made between individuals and between species
• Comparisons show that there is a high level of genome similarity within, and even between, species
  • Humans share around 99.9 % of their DNA with other humans
  • Humans share around 99 % of their DNA with chimpanzees
• Humans in fact share all of their coding genes with other humans; the differences between individual humans are due to possessing different alleles of genes
  • Different alleles arise due to mutations
• Differences between DNA sequences that involve a single base change are known as single nucleotide polymorphisms (SNPs)
  • Because SNPs are areas of difference between individuals who share 99.9 % of their DNA, scientists will often use SNPs to determine ancestry or disease risk

Single nucleotide polymorphism diagram

A difference in a single nucleotide is known as a single nucleotide polymorphism, or SNP. Most SNPs have no effect on the protein produced, but a few may lead to production of an altered protein.

Comparing eukaryotic genomes

• Eukaryotic genomes can vary in size; this is determined by the mass of DNA present in a nucleus
  • Size variation means that some organisms will have genes that others lack
  • E.g. plants need genes that code for enzymes involved in photosynthesis, while humans do not need these genes
• Eukaryotic genomes can also vary in base sequence
  • DNA sequence data can be compared using information from online databases
  • Scientists all around the world enter information into such databases, allowing anyone to retrieve the information for analysis
  • Examples of such databases include GenBank and NCBI
  • Databases allow you to:
    • Select a specific gene to compare
    • Select species to compare
    • Choose whether to compare amino acid sequences or DNA base sequences
• Comparison of eukaryotic species sequence data shows that closely related individuals have more similar genomes than distantly related individuals
  • E.g. individuals of the same species have more similar genomes than individuals of different species

Databases contain information on DNA, RNA and protein sequences, as well as protein structure. This information can be retrieved for analysis of variation between genomes