GeneticsGenetics

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

Figure 7.20

Effects of MutationEffects of Mutation

Figure 7.9

Genetic CodeGenetic Code

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

Figure 7.24

DNA RepairDNA Repair

Figure 7.24

DNA RepairDNA Repair

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

Figure 7.29

Griffith’s ExperimentsGriffith’s Experiments

Figure 7.30

Generalized TransductionGeneralized Transduction

Specialized transductionSpecialized transduction

Figure 7.31

Bacterial ConjugationBacterial Conjugation

Figure 7.31

Bacterial ConjugationBacterial Conjugation

Figure 7.32

Bacterial ConjugationBacterial Conjugation

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

Transfer of Genetic InformationTransfer of Genetic Information

Transcription – information in DNA is copied as RNA nucleotide sequences

Translation – polypeptides synthesized from RNA nucleotide sequences

Central dogma of genetics◦DNA transcribed to RNA◦RNA translated to form polypeptides