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Genetics. Mutation – change in the nucleotide base sequence of a genome; rare Not all mutations change the phenotype Two classes of mutations 1. Base substitution eg point mutation G TTCAAG - wild type (normal) A TTCAAG - mutant(abnormal) Silent mutation - PowerPoint PPT Presentation
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Mutations of GenesMutations of Genes
Mutation – change in the nucleotide base sequence of a genome; rare
Not all mutations change the phenotypeTwo classes of mutations
◦1. Base substitution◦eg point mutation
GTTCAAG - wild type (normal) ATTCAAG - mutant (abnormal)
◦Silent mutation No change in amino acid sequence
Mutations of GenesMutations of Genes
Missense mutationNew amino acid
ALA-PHE-LEU-TRY-STOP PHE-PHE-LEU-TRY-STOP
Non-sense mutation – a stop codon is inserted into protein sequence Truncated protein Worse than missense
MutationsMutations
2. Frameshift mutation◦Insertion or deletion of one or more bases◦ATT GCC AGG TAA (convert to mRNA)◦ATG CCA GGT AA_◦Worst type of all types of mutations◦If it happens at the end of a gene it may not be
as bad
mutationsmutationsMissense mutation eg. sickle cell
◦results in a codon that codes for a different amino acid. The resulting protein may be nonfunctional
Nonsense mutation eg Cystic fibrosis ◦Stop codon,truncated protein
Frameshift insertion eg. Tay-Sachs diseaseFrame shift deletion CCR5
◦Both cause a shift in the reading frame
Causes of mutationsCauses of mutations
Spontaneous◦Happens during replicationMore often in prokaryotes than eukaryoteseukaryotes have better repair mechanisms
MutagensMutagens
Radiation Ionizing radiation (x-rays) – induces breaks in
chromosomesNonionizing radiation (UV light) – induces
thymine dimers Chemical Mutagens
Nucleotide analogs – disrupt DNA and RNA replication and cause point mutations Eg. 5-bromouracil pairs with guanine Caffeine not a strong mutagen – effect fetal
development Alkylating agents- used for cancer treatment
Identifying Mutants, Mutagens, Identifying Mutants, Mutagens, and Carcinogensand Carcinogens
Mutants – descendents of cell that does not successfully repair a mutation
Wild types – cells normally found in nature Methods to recognize mutants
Positive selection Survival of the fittest
Negative (indirect) selection selective removal of rare alleles that are
deleterious. Ames test
assess the mutagenic potential of chemical compounds
Genetic Recombination and TransferGenetic Recombination and Transfer
Exchange of DNA segments composed of homologous sequences
Recombinants – cells with DNA molecules that contain new nucleotide sequences
Vertical gene transfer – organisms replicate their genomes and provide copies to descendants
Horizontal gene transfer – donor contributes part of genome to recipient; three types◦Transformation◦Transduction◦Bacterial Conjugation
Genetic Recombination and TransferGenetic Recombination and Transfer
Exchange of DNA segments composed of homologous sequences
Recombinants – cells with DNA molecules that contain new nucleotide sequences
Vertical gene transfer – organisms replicate their genomes and provide copies to descendants
Horizontal gene transfer – donor contributes part of genome to recipient; three types◦Transformation◦Transduction◦Bacterial Conjugation
TransformationTransformationTransforming agent was DNA; one of conclusive
pieces of proof that DNA is genetic materialCells that take up DNA are competent; results
from alterations in cell wall and cytoplasmic membrane that allow DNA to enter cell
Transposons and TranspositionTransposons and Transposition
Segments of DNA that move from one location to another in the same or different molecule
Result is a kind of frameshift insertionTransposons all contain palindromic
sequences at each endSimplest transposons are insertion sequences
which have no more than two inverted repeats and gene for transposase
Complex transposons contain one or more genes not connected with transposition (e.g. antibiotic resistance)
TransformationTransformationTransforming agent was DNA; one of conclusive
pieces of proof that DNA is genetic materialCells that take up DNA are competent; results
from alterations in cell wall and cytoplasmic membrane that allow DNA to enter cell
Gene FunctionGene Function
Genotype – set of genes in the genomePhenotype – physical features and functional
traits of organism
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