Genetics 275 Notes Week 7

Transcription

1 Cytoplasmic Inheritance Genetics 275 Notes Week 7 Criteriafor recognition of cytoplasmic inheritance: 1. Reciprocal crosses give different results -mainly due to the fact that the female parent contributes the cytoplasm (and therefore the organelles) to the zygote 2. Non-mappability cannot determine what chromosome a gene is on because it isn t on a nuclear chromosome 3. Non-mendelian segregation 4. Indifference to nuclear substitution backcrossing to a wildtype parent doesn t rescue mutant phenotype 5. Infectious-like transmission 6. Ultimately, want molecular/physical proof that a gene resides on organellar DNA Nature of organellar genomes: 1. circular 2. DNA isn t complexed with traditional set of histones 3. Variable in size due to different amounts of DNA without known function 4. Can usually be detected physically when total DNA is centrifuged due to difference in AT:GC ratio 5. Can carry out protein synthesis as they encode some proteins, rrnas and trnas -most encoded proteins are subunits of enzymes involved in electron transport 6. Organellar protein synthesis is relatively sensitive to antibiotics that affect bacterial processes -hints at prokaryotic origin 7. Complete DNA sequence is known for organelles of many species Example of Indeterminant Number: -number of chloroplast DNAs per beet cell 2. ~ 40 chloroplasts/cell 3. DNA is organized into nucleoids ~ 4-18 nucleoids/chloroplast 4. ~ 4-8 chloroplast DNAs/nucleoid therefore a single cell may contain 40 x 18 x 8 = 5760 chloroplast DNAs Surprises in Organellar Genomes: b. Interrupted genes may occur (may have introns) -surprise because prokaryotes don t have introns c. Heterogeneity in size d. Genetic code is slightly different even though prokaryotes, eukaryotes and viruses all use the same code [T79] cartoon of a mitochondria

2 [T80] yeast and human mtdna [T81] chloroplast genome Inheritance Pattern: b. yeast haploid or diploid, each parent contributes cytoplasm n x n grande petite small colonies due to mito mutations ade- ade+ α mating type a mating type a/α ade+/ade- Grande cytoplasm is dominant Meiosis α ade+ α ade- a ade+ a ade- 4 grande: 0 petite c. a higher plant where the female contributes all the cytoplasm -score for green vs. white (where mutation in chloroplast gene results in inability to make chlorophyll) -phenotypes: all green all white (lethal) variegated (mix of green and white) -flowers were crosses in all combinations -reciprocal crosses differed -phenotype of female parent determines phenotype of offspring (maternal inheritance) [T83] cross results [T84] cytoplasmic inheritance importance in industry Maternal Effect Inheritance -only affect the next generation

3 -involves gene products being inherited, not genes -def n when the female packages gene products (ie. RNA, protein, pigments) into the egg and these products may be utilized by the embryo even when the substance cannot be made by the embryo because the embryo is genetically mutant -example: ma-l (X-linked, maroon-like) eyes in Drosophila female ma-l/ma-l (brown) x ma-l+/y male (red) F1 (expected) ma-l+/ma-l females (red) ma-l/y males (brown) F2 ma-l/ma-l ma-l+/ma-l ma-l/y ma-l+/y (all F2 s have red eyes because red eyed mothers can make and package wildtype pigment therefore pass it on to offspring) Testcross brown red brown red and brown F2 s to ma-l flies [T85] Maternally-influenced inheritance progeny shows phenotype of mother s genotype Polygenic Inheritance -trait influenced by many genes -example: fruit weight in tomatos -cross truebreeding small and large tomatos (with some variation) -F1 is intermediate -F2 has lots of variation (covers entire weight spectrum) [T86] number of genotypic classes expected with different numbers of genes (with no environmental component) [T87] with environmental component -example: truebreeding tomatos + allele gives 4 grams additional weight, - allele has no effect, 0 or 1 + allele gives no fruit, first 2+ alleles gives 10 grams large a/a; b+/b+; c+/c+; d+/d+; e+/e+; f/f (34 grams) x small a+/a+; b/b; c/c; d/d; e/e; f+/f+ (18 grams) F1 heterozygous at each gene (26 grams) F alleles [T89] F2 results

4 Eukaryotic Transposable Elements (TE) -TE s are pieces of DNA that move around -can be considered biological mutagens because they can insert into genes and knockout function Characteristics: 1. exist in all eukaryotic cells that have been well studied 2. may be used as tools for cloning a. tagging [T90] b. vectors in making transgenic organisms 3. some similarities with prokaryotic TE s eg. Transposition to a new site generates a short repeated sequence at the target site 4. also differrences with prokaryotic TE s eg. So far no prokaryotic TE s has been found to have an RNA intermediate (which is common in eukaryotic TE s) -not known whether TE s normally play a functional role in the eukaryotic genome or if they are merely pieces of selfish DNA that exists only to propagate itself -Drosophila a several families of TE s -as much as 10% of Drosophila genome may be composed as TE s [T91] Examples of 3 of the categories of TE's in Drosophila: 1. copia-like elements - lots of transcripts 2. fold-back elements 3. P elements - up to 50 copies/genome -most copies have internal deletions -4 ORF's - encode transposase enzyme and a repressor to inhibit moving (equilibrium is reached between transposase and repressor to stop movement) Examples of Human TE's: 1. LINES - long interspersed nuclear elements > 1000 nts, > 10 5 copies/genome 2. SINES - short interspersed nuclear elements <500 nts, <10 5 copies/genomes

5 -both are retroposons and they transpose/move via an RNA intermediate -best know LINE is L1H - likely junk DNA (no known use) -6 kb and contains a polya signal followed by polya (indicates it arose from RNA) -contains 2 genes: 1. pseudogene 2. a reverse transcriptase (RNA DNA) -a known SINE in humans is the Alu family (contains an Alu restriction enzyme site) -300 nts, 5x10 5 copies/genome -contains 2 complete copies of 7S snrna and a pseudogene [T92] RNA intermediate in yeast TE P elements -cause hybrid dysgenesis when you cross a lab female with a wild male -progeny exhibit sterility, high mutation rate, male recombination, chromosome aberrations, non disjunction (all in germline only) -when you cross a lab male and a wild female, progeny is okay -problems due to lab female cytoplasm not having repressor (because lab strains do not have P elements) therefore wild male's P elements can spread and cause defects Biotechnology 1. tagging 2. transgenics (see T93) Nature of Crossing Over and Recombination -historical - discovery of linkage -discovered by Bateson and Punnet using Sweet peas purple, long x red, round PPLL ppll F1 PpLl F2 They saw PL 220 (> expected) Pl 22 (< expected) pl 21 (<expected) pl 55 (> expected) -didn t see 9:3:3:1 ratio -saw increase in parental phenotypes, and decrease in recombinant phenotypes -testcross resulted in increase in parental phenotypes, and decrease in recombinant phenotypes (same as F2) [T94] relationship between crossing over frequency and physical distance Note: a cross made in coupling has one parent with both dominant characteristics (ie. PPLL) and the other with both recessive characterisitics (ie. ppll)

6 -a cross made in repulsion has both parents with one mutant and one dominant characteristic (ie. PPll and ppll)