Mutations + Recombination = Genetic Variation!! Chapter 16: Mutations Mutations generate genetic variants These variants are then subject to recombination Mutations + Recombination = Genetic Variation!!

Consequences of Mutations

What are the effects of point mutations on gene products? What would you see in a Northern? In a Western?

Are mutations random or induced? Luria and Delbruck used the “fluctuation test” They were interested in determining which hypothesis was correct Conclusion: Mutation can occur in any cell at any time; occurrence is random. (Nobel Prize, 1969)

Replica Plating Repeated identical patterns suggest that the mutation occurred before exposure to phage, and not as a consequence of exposure!

What can cause spontaneous mutations? Errors in DNA replication Transitions: base pairs mismatch (tautomeric shifts results in mispairing) Transversions: base pair mismatch with the normal keto base forms

What can cause spontaneous mutations? Errors in DNA replication: Frameshift mutations (Also known as indel mutations)

What can cause spontaneous mutations? Spontaneous lesions from DNA damage! (depurination, deamination, and oxidative damage)

Trinucleotide repeats may lead to disease…why?

How can we induce mutations? Use base analogs Cause incorrect base pairing

How can we induce mutations? Use intercalating agents which mimic base pairs and slip in between the base pairs…promote indel mutations

How can we induce mutations? Promote base damage UV light Ionizing radiation Promotes large strand breaks Aflatoxin B1 Binds to guanine and generates an apurinic site

Ames Test to Determine Mutagenicity

Ames Test identifies type of mutations Three strains treated with aflatoxin Which one generates revertents?

What type of mutations does aflatoxin cause in bacteria? TA100: base-pair reversion TA1538 and TA1535: frameshift mutations

Biological Repair Mechanisms Direct reversal of damaged DNA Base-excision repair Nucleotide-excision repair Mismatch repair

Direct reversal of damaged DNA

Base-Excision Repair Carried out by DNA glycolylases, generate apurininc or apyrimidinic (AP)sites AP endonuclease nicks strand Deoxyribophosphodi-esterase removes more DNA DNA polymerase fills in the gap with new DNA

Nucleotide-Excision Repair Used to repair base damage or transcription blocks Autosomal recessive diseases, Xeroderma pigmentosum and Cockayne Syndrome result from defects in this system

Mismatch Repair Recognize mismatch base pairs Determine which base is the incorrect one Excise the incorrect base and repair DNA

Error Prone Repair Known as translesion synthesis, requires a bypass polymerase

Repair of Double-Strand Breaks What conditions can cause double strand breaks? There are two possible situations: Nonhomologous end joining Homologous recombination

Non-homologous End Joining

Homologous Recombination Damage corrected by synthesis-dependent strand annealing (SDSA) Uses sister chromatids as templates

DSB and Meiotic recombination Similar process to synthesis dependent strand annealing (SDSA) Requires Spo II, Rad51, Dmc1

Why can mutations lead to cancer? Two types of mutations associated with cancer Oncogenes are activated Tumor suppressor genes are inactivated Genes that control the cell cycle or inhibit apoptosis are considered proto-oncogenes Tumor Suppressor genes often participate in regulation of cell cycle, activate cell apoptosis, or repair of damaged DNA

Chapter 17: Changes in chromosomes

Chromosome Numbers Changes in chromosome sets are known as aberrant euploidy Changes in parts of chromosome sets is known as aneuploidy

Polyploids More common in plants Correlation between the number of chromosome sets and size of organism Autopolyploids: multiple chromosomes from one species Allopolyploids: sets of chromosomes from two or more different species

Agricultural Applications Monoploid plants provide a way to select for desired traits

Agricultural Applications Bananas are sterile triploids Autotetroploid grapes are larger than diploid grapes

Wheat Genome is HUGE!

Aneuploidy Mostly due to nondisjunction during meiosis or mitosis

Abnormal numbers of sex chromosome Turner Syndrome Result of only one sex chromosome (XO)

Abnormal numbers of sex chromosome Klinefleter Syndrome The result of an extra X chromosome (XXY)

Abnormal Number of Autosomes Down syndrome Results from an extra copy of chromosome 21

Can Down syndrome be inherited?!?

Cancer From Translocation