Higher Biology Unit 1: 1.7 Evolution

Learning Intentions To be able to describe the process of gene transfer in different organisms

Success Criteria I can explain how genes are transferred in eukaryotic organisms I can explain how genes are transferred in prokaryotic organisms I can explain how genes are transferred in viruses

Evolution Evolution is the process of gradual change in the characteristics of a population of organisms that occurs over successive generations Evolution occurs as a result of variations in the population’s genome These variations take the form of changes in the frequency of certain genetic sequences (how often alleles appear)

Inheritance Genetic sequences of DNA are passed on from parent to child This is known as vertical transfer This can occur by either asexual or sexual reproduction

Sexual reproduction During sexual reproduction two organisms with different genomes produce offspring with a different genome to either parent This increases variety within a population Vertical inheritance in this manner can be shown using a family tree in humans

A family tree allows us to see inherited characteristics passing down from one generation to the next (vertically).

Asexual Reproduction During asexual reproduction a parent produces a genetically identical copy of itself (a clone) No variation will result in successive generations unless mutations occur

Vertical gene transfer Transfer of genes from parent to offspring is relatively safe The genes passed from parent to child have allowed the parents to survive and reproduce so must offer some benefits

Horizontal Transfer In prokaryotes genetic material can transfer horizontally from one cell to another This means that genetic information can be exchanged within the population and not just passed from parent to offspring

Rapid evolutionary change is believed to have occured in prokaryotes due to horizontal gene transfer (HGT) occurring Prokaryotes were able to acquire beneficial genes from other cells rather than waiting for the gene to evolve over time With HGT there is no guarantee that the gene received will be beneficial

The need for HGT decreased as multicellular organisms reached higher levels of complexity Vertical inheritance allowed for the formation of distinct lineages with their own sets of genes

MRSA HGT still occurs in prokaryotes Certain bacteria exchange plasmids that carry genes for resistance to antibiotics form one bacteria species to another This has resulted in strains of bacteria resistant to several antibiotics One of these is methicillin-resistant Staphylococcus Aureus (MRSA)

HGT between species Some prokaryotes are able to transfer genes into eukaryotes The soil bacterium Agrobacterium tumefaciens infects wounded plant cells with a plasmid which integrates a sequence of its DNA into the genome of the plant A harmless version of this plasmid can be used in genetic engineering to improve crop plants

HGT in viruses Some viruses transfer their DNA horizontally into the genome of a host organism This allows viruses to reproduce within host cells but also lie dormant Examples include herpes, and HIV

Success Criteria I can explain how genes are transferred in eukaryotic organisms I can explain how genes are transferred in prokaryotic organisms I can explain how genes are transferred in viruses