Chapter 10 Flashcards
Chromosomes are species-specific
genome content, chromosome number in a nucleus, and relative size and shape of each chromosome
are species-specific
closely related species tend to have similar numbers
Chromosome territories
- specific regions where chromosomes are partitioned during interphase
- don’t occupy the same territory in each nucleus
- once situated, don’t stray till mitosis
Karyotypes
- organized display of chromosomes
- arranges in descending order of size
- can identify abnormalities in number or structure
Chromosome structure
centromere at the center of each chromatid
short p arm
long q arm
Four chromosome shapes
- metacentric
- p and q same length
- centromeres in the middle - submetacentric
- short arm and long arm
- seen in humans - acrocentric
- centromere near end of chromosome
- also seen in humans (ex. y) - telocentric
- no p arm
- centromere at the end
FISH - fluorescent in situ hybridization
- uses fluorescent molecular probes to detect a target sequence
- can use diff wavelength/colour probes
- can be used to identify each chromosome in a cell
Chromosome banding technique
- older than FISH
- identifies chromos based on shape, size, and banding patterns
- stop cell during metaphase
- dye
Giemsa banding used for humans
Human chromo banding patterns
letters/numbers used to identify major and minor band regions
begins at centromere then goes outwards
Euchromatin
light regions
chromatin is less compact
higher gene expression
Heterochromatin
dark regions
chromatin is more compact
lower gene expression
Dipteran flies chromosome banding
discovered by Balbiani
polytene chromosomes are produced in salivary glands
stay together while replicating
produce distinct bands when stained
used to help map genes, identify mutations, etc
Nondisjunction
the failure of chromosomes and sister chromatids to properly separate during cell division
can lead to abnormalities in chromosome number
can occur in meiosis 1 or 2
Aneuploid
abnormal chromosome counts
most don’t survive gestation
ex.
- autosomal trisomies (13/18/21 = patau/edwards/down)
- sex monosomy or trisomy
Trisomy vs monosomy
tri
2n+1
mono
2n-1
after fusion with a normal gamete
Meiosis 1 nondisjunction
- homologs fail to separate into two cells
one daughter has an extra homolog (X), one lacks a homolog
results in 4 abnormal gametes
Meiosis 2 nondisjunction
homologs separate
sister chromatids fail to separate
one daughter cell lacks a homolog
results in 2 abnormal gametes
Trisomy 21
down syndrome
most well known aneuploidy in humans
link between maternal age and aneuploidy
Robertsonian translocation
two non-homologous chromosomes fuse to form a large chromosome
Robertson translocation and down syndrome
way for down syndrome to occur not as a result of a random non-disjunction
fusion between chromos 14 and 21
= only one chromo at 21
gamete with robertson (asymptomatic) + normal gamete
= down syndrome
Polyploidy
the presence of three or more sets of chromosomes
ex. triploids (3n), tetraploids (4n)
common in plants
Two types of polyploidy
Autopolyploidy
- occurs from the duplication of chromos within a species
Allopolyploidy
- combining chromo sets from different species
Causes of polyploidy (3)
- meiotic nondisjunction
- leads to diploid instead of haploid - mitotic nondisjunction
- doubles chromo number so all cells have doubled count - combination of the two
Autopolyploidy example
bigger strawberries have more chromosomes
Allopolyploidy example
species with different chromo number reproduce together
60 and 62
gamete fusion = 2n = 61
mitotic nondis = 2n = 122
is fertile
Chromosome breakage
causes mutation by loss, gain, and rearrangement of chromosomes
can have an effect or no effect
Chromosomal deletions
break occurs at chromo break point
= both DNA strands severed
Terminal deletion
chromo tip missing
telomere lost
entire arm or part of it breaks off
ex causes cri du chat
Acentric
lacking a centromere
usually lost during cell division
Interstitial deletions
internal chromosomal deletions
Issues when chromo reattaches after breaking
can cause inversions and translocations
if no critical/regulatory regions mutated, maybe no pheno consequences
Chromosome inversion + 2 types
reattachment of the wrong end
- paracentric
- centromere outside of inversion - pericentric
- centromere within inversion
Chromosome translocation
reattachment to a non-homologous chromosome
Recombination between inverted regions
- inversion usually suppresses recombination bc need homology to be exchanged
BUT
- can still occur using a physical inversion loop
- can lead to chromo breakage and large deletions
Unequal crossover
takes place between two homologs
results in a partial duplication of one homolog and a partial deletion on the other
rare
Williams-Beuren syndrome
result of partial duplication of PMS gene due to unequal crossovers on chromo 7
naive, outgoing, intellectual disabilities, heart issues
Deletion mapping
compare unknown phenotype to known mapped mutations
recessive mutations
relies on pseudodominance aka hemizygosity
Pseudodominance
if an organism has a recessive allele on one chromosome, and a deletion on the other, the recessive allele is expressed
Notch gene deletion mapping
- drosophila notch = developmental gene
- affects wing phenos
deletion mapping showed it’s in a region on the X chromo
Period gene deletion mapping
- drosophila notch = circadian rhythm clock
gene within loci on X chromo
pair with genetic markers for yellow and white genes
