lecture 19 Flashcards
where are balanced translocation and meiosis happening
person x person - translocation arose during meiosis and inherited to offspring
person somatically normal
gonad = meiosis, happens here
50% of gametes have non viable genomes
what is the dynamic genome
Discovery of transposable elements through study of “unstable” alleles
Impact of transposable elements on the genome
Genomic surveillance mechanisms that limit transposon mobility
how were transposable elements discovered
through study of unstable alleles - careful genetic study
what is maize used as
system for studying inheritance of traits - pigment genes
Describe biology of maize
each pollen grain contains 2 identical haploid sperm cells - looked at traits inside cells
people figured out that physical substance for inheritance that determines traits
sperm cell n + central cell 2n
endosperm = 3n, supporting structure for growing plant, traits seen here
embryo 2n
sperm cell + egg cell lead to 2n embryo
focusing on traits we can already see in endosperm
describe inheritance of traits of maize
coloured alureone C = important for production of purple pigment in maize
c is recessive allele that is unable to make pigment
C1 is dom neg inhibitor allele that represses pigment production
C1>C>c
normal 3:1 phenotypic ratio
C/C homo
C/c
hetero
certain traits mapped to chromosomes - 9, coloured, shrunken, bronze, waxy b
what led to discovery of transposable elements - experiment
Careful observation of “unstable” pigment gene alleles
Experiment: use pollen from a C1 (dominant negative allele) strain to fertilize C/C ovules
predicted result = all colourless kernels
no pigment produce but lead to spotted kernels, some have blue sectors
spot = sector of cells that has regained ability to make pigement
what led to discovery of transposable elements - experiment, hypothesis 1 and interpretation
Experiment: look for breakage in chromosomes of the pollen parent
Result: high rate of breakage in this strain
hypothesis 1: C1 allele is lost in some cells of developing endosperm
interpretation: loss of c1 allele is caused by chromosome breakage
what is observed genetic instability due to
chromosome breaks at specific locations
Describe a - ds stable without ac
ds = chromosome breaking locus
describe b - with ac, ds breaks the chrom causing blue sectors uncovering c
onlu mutant at c
C1 allele lost = no longer blocks c function
describe c - with ac, ds breaks the chrom causing bronze, waxy and shrunken sectors (waxy and shrunken not visible)
recessive alleles = traits visible in aleurone kernal
result = all had spots but also all always has all 3 Pheno
break always occurs at same place - cell breakpoint dx
how can be describe what is happening in these sectors
mosaicism since not all the cells are affected - some cells break did not occur and give rise to yellow colour and some cells = break occurs
cells lose dom C1 allees so can make pigment agains
Failure to complement = if break uncover recessive muatnt pehno
what happens in related strain and what does this prove
breaks occur in dif locations = evidence for movement of these genetic elements
with ac = ds excise and jumps
= in some strains, ds has moved to new location, new location can be inferred from phenotype of sectors
with ac ds breaks chrom causing blue sectors with normal starch, but complemented
= in these sectors pigment is restored but other traits remain unchanged
describe Evidence for movement of these genetic elements
multiple jumps and unable to map ac element
50% of kernels, without ac, ds stable and the c allele gives blue kernels
50% of kernels = with ac, ds breaks chrom causing off white sectors by uncovering c
rare = ds jumped into creating new c-m1 allele- ds excision gives blue sectors, spot regained ability to make blue pigment, cannot map where ac element is ~1/4000 = rare, yellow = c/c ds and blue = C/C ds w/ ds jumped out
what can transposable elements be
non autonomous = ds
= requires ac to jump out transpose or excise
autonomous = ac
= jumps out on own, is a TE that encodes its own transposition machinery
what does spot size depend on
when during endosperm development c gene became active
jump out early to restore c function = large spot
jumps out late = small spot
TE can mostly be anywhere but also some popular spots
describe 2 classes of transposable elements
class 1 = retrotransposons (eukaryotes)
class 2 = dna transposons (prokaryotes and eukaryotes)
are transposable elements found in all organisms
yesssss
studies in maize found TEs genetically but later studied in yeast and drosophila that identified actual pieces of dna that are mobile and discovered how they move
transposons or TEs are ancient = found in all organisms = bacteria, plants, yeast, multicellular animals
describe class 2 transposon - ds and ac elements in maize
multiple transposon fams in maize, ac and ds in same fam each has transposase version that is specific for that fam
ac element = gene that encodes an enzyme that catalyses a reaction between the 2 repeats of dna, ds lacks transposase gene if has ac = transpsoase can help ds element, brings together and catalyzes cleavage event
releases intervening sequence, stays associated with transposase enzyme whioch can now cataluyze same nbreaking and rejoining = integrates to new location
describe class 2 transposon - p element in drosophila
encodes a single protein - the transposase
inverted repeats = 31 bp flanks transpose gene
Resembles transposons found in bacteria
describe class 1 retrotransposon
similar to retroviruses
first identified in years as a repeated sequence in genome
found in many organisms
line (long interspersed element) is a retrotransposon found in human genome
ltr, gag protein (antigen present in viral coat), pol = polymerase gene to replicate genome, env = viral encelope important to make cells produce more virsu
how do retrotransposons move
through rna intermediate
describe movement of retrotransposon - generally
move via rna intermediate
transposition mediated by rt encoded by retrotransposon pol gene (not transposase)
Replicative copy paste mechanism
describe movement of retrotransposon - specific pathway
same flanking sequences - help with insertion
transcription by cellular rna pol
proteins and pol transcribes transposon genome and explorted and translation
pol gene encodes rt and makes double stranded transposon genome which jumps to another chrom
whats missing = env gene
