Post on 01-Jan-2016
transcript
Changes within chromosomes
Can occur within a chromosome or between non-homologous chromosomes
Types of changes
- deletions
- duplications
- inversions
- translocations
Chromosome deletions
- Loss of a piece of an arm or the lose of an arm from a chromosome
Types:- terminal, from the end of a chromosome- intercalary or intersitial, from within the chromosome
Chromosome deletions
Origin of deletions:- movement of transposible elements- radiation- genetically induced- product of recombination events with heterozygous inversions- product of meiosis with heterozygous translocations
Movement of transposible elements
- It is speculated that when a conservative transposible element moves a section of the chromosome that the element is leaving can be lost.
- This loss can either result in a terminal deletion or a intercalary deletion
transposible element
reconnection does not occur
terminal piece lost
Radiation:
High energy radiation can cause a physical break in the chromosome resulting in a deletion
Deletions caused by radiation can be either terminal or intercalary
Terminal deletion by radiation
Intercalary deletion by radiation
Protocol for producing deletions by radiation
- radiate pollen of dominant genotype
- pollinate recessive genotype
- screen progeny for recessive phenotype
- plants showing recessive phenotype could be the result of a deletion
Genetically inducing deletions
In certain interspecific crosses it was found that the presence or absence of a chromosome from one of the species could induce deletions
The genes that induce the deletions are called gametocidal genes because the deletions may lead to non-functional gametes
Behavior of deletions in meiosis
- reduced chromosome pairing
- as the deletion size increases pairing frequency decreases
- result is a decrease in the amount of recombination
- terminal deletions breed true
indicating restoration of telomeres
Determining the location of chromosomes using deletions
- molecular
- Southern blot
- PCR
Set of deletion lines
A A A a a a gene
expression
Set of deletion lines
location of gene
A A A a a a gene
expression
Southern blot analysis
- presence or absence of a marker would determine the chromosome and chromosome location of the molecular marker
- if the marker was linked to a gene then the gene location would now be known
Dis
om
ic
Nu
lli
1A
Nu
lli
1B
Nu
lli
1D
Tel
o 1
DS
Tel
o 1
DL
Del
1D
S-1
Del
1D
S-2
Del
1D
S-3
Southern blot results with marker used as a probe – what is the chromosome location of the probe?
Dis
om
ic
Nu
lli
1A
Nu
lli
1B
Nu
lli
1D
Tel
o 1
DS
Tel
o 1
DL
Del
1D
S-1
Del
1D
S-2
Del
1D
S-3
Gene is on the short arm of chromosome1D in the region covered by deletion 1DS-3
PCR approach
Have primers for a marker, EST or gene of interest
Identify chromosome, chromosome arm and deletions by the absence of product
Dis
om
ic
Nu
lli
4A
Nu
lli
4B
Nu
lli
4D
Tel
o 4
BS
Tel
o 4
BL
Del
4B
L-1
Del
4B
L-2
Del
4B
L-3
PCR results with primers - what is the chromosome location of the probe?
Dis
om
ic
Nu
lli
4A
Nu
lli
4B
Nu
lli
4D
Tel
o 4
BS
Tel
o 4
BL
Del
4B
L-1
Del
4B
L-2
Del
4B
L-3
PCR results with primers indicate the product is located on chromosome 4BL in the region of deletion 2.
Chromosome duplications
Regions of a chromosome that are repeated resulting in an increased dosage (number) of some genes
example: isochromosome, one arm duplicated and one arm is missing
A B AA
Normal chromosome Isochromosome
Types if duplications
- intra-arm where the duplication is in the same arm
- inter-arm where the duplication is in the other arm of the
chromosome
- non-homologous duplication where the duplication is on a different chromosome
Origin of duplications
natural
- oblique synapsis (unequal crossing-over)
- inversions
- translocations
induced
- radiation
- chemicals
Inversions
A chromosome inversion is a change of the linear sequence of genes on a chromosome
Two types of inversions can occur:
- paracentric where the inversion does not involve the centromere- pericentric where the inversion does involve the centromere
Formation of a paracentric inversion
B
D
C
C
A
D
B
A
Formation of a pericentric inversion
An inversion in theory may not be detrimental since no genes are gained or lost due to the inversion
Gene function could be affected if the break occurs in the middle of a gene or in its promoter region
Gene function could be affected by moving into or away from a region of heterochromatin
inverted
normal
In individuals heterozygous for an inversion recombination is reduced especially for the genes close to the breakpoints
This can play a role in speciation by allowing genes within the inversion to be inherited as a block of genes
The reason for the low level of recombination of genes within the inversion for individuals heterozygous for the inversion are the problems caused when crossing-over occurs
Crossing-over within the inversion can lead to duplication and deletions
In the case of paracentric inversions such a cross-over can result in dicentric and acentric chromosomes in meiosis
X
Prophase I – paracentric inversion
*showing only the chromatids involved in the cross-over
dicentricbridge
acentricfragment
Early anaphase I
dicentricbridge breaks
acentricfragmentlost
Late anaphase I
dicentricbridge breaks
acentricfragmentlost
Late anaphase I
The presence of chromosomes with duplications and deletions leads to non-viable gametes
If two of the four chromatids are involved in the cross-over then 50% of the gametes will be non-viable
The presence of partial sterility is one method for identifying individuals that are heterozygous for an inversion
To get proper pairing of the complimentary regions a loop structure has to form
If a cross-over occurs within the looped area (region with the inversion) then duplications and deletions can be produced
• Would we get any acentric fragments in pericentric (where centromere is involved in the cross over) inversion ?
X
Prophase I – pericentric inversion
*showing only the chromatids involved in the cross-over
A B C D E
A B C D E
F G H I J
F G H I J
A B C D E E D C B A
F G H I JJ I H G F
Late anaphase I
While there are no dicentric bridges or acentric fragments two of the chromosomes will have duplications and deletions
An interesting affect of the inversion is that genes within the inversion would be inherited as a block of genes because only chromatids not involved in the cross-over would result in a viable gamete
The inheritance of blocks of genes and the partial sterility found in individuals heterozygous for the inversion can lead to speciation
For speciation to occur it would require the presence of several inversions
With each additional inversion the probability of viable gametes in a hybrid between a normal by an inversion genotype decreases while fertility of individuals having two normal or two inversion parents stays at 100%
Combine the increased sterility with the inheritance of blocks of genes and differences between the two genotypes becomes great enough that they can not successfully mate and produce fertile hybrids (i.e. become two species)
An example of this is tomato and diploid potato
Based on molecular marker studies it was shown that the genes for the two species were generally on the same chromosomes but the gene order was inverted on several of the chromosomes when the two species were compared