[9.1] mutations
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how might gene mutations arise?
• during DNA replication when errors are made with the inclusion of new bases • gene mutations occur spontaneously, but mutation rate is increased by mutagenic agents • chemical mutagenic agents are called carcinogens
what is the effect of a gene mutation?
• mutations can result in a different AA sequence in the encoded polypeptide • they can be advantageous (variation), cancerous, or harmless
examples of (chemical) mutagenic agents
• radiation: UV radiation, x-rays • chemicals: cigarette smoke, barbecuing, benzoyl peroxide • infectious agents: HPV, heliobacter pylori
what is a substitution mutation?
when one base is swapped for another (ATGCCT -> ATTCCT)
what is a deletion mutation?
when one or more bases are removed (ATGCCT -> ATCCT)
what is an addition mutation?
when one or more bases are added (GGCTA -> GGGCTA)
what is an inversion mutation?
when a sequence of bases is reversed (AATTGCG -> TTAAGCG)
what is a translocation mutation?
when a sequence of bases is moved from one location within the genome to another
what is a duplication mutation?
when one or more bases are repeated (GTCAAG -> GTCAAAAG)
what impact does a mutation have on the protein made?
• mutation has (eg.) removed a base and caused a frame shift • from the point on the mutation, mRNA will be transcribed differently • all amino acids after this point will now be in a different shape • now the hydrogen/ionic/sulfur bonds within the protein are in different positions • the protein then has a different shape and is non-functional (could be cancerous)
are mutations still problems for introns?
• yes for addition or deletion (frame shift) • no for substitution (possibility to not create a different protein due to degenerate code)
what is an example of a genetic disease caused by a gene mutation?
• sickle cell anaemia • caused by a substitution of CTT to CAT (glutamine to valine) • recessive • if you have this allele you will be resistant to malaria so is often found in places where there is malaria
what is an oncogene?
a gene that when mutated or expressed at high levels, helps turn a normal cell into a tumour cell
how does the imbalance between two genes cause cancer?
• proto oncogenes stimulate cell division • tumour suppressor genes inhibit cell division • when the interaction between these 2 genes becomes unbalanced, cancer can result
what are chromosome mutations?
changes in the structure or number of whole chromosomes
what happens when there are changes in whole sets of chromosomes?
• occur when organisms have three or more sets of chromosomes rather than the usual two • this condition is called polyploidy • mostly occurs in plants
what happens when there are changes in the individual number of chromosomes?
• sometimes individual homologous pairs of chromosomes fails to separate during meiosis • this is known as non-disjunction and usually results in a gamete having either one more or one fewer chromosome • example: down’s syndrome, where individuals have an additional chromosome 21