Chromosome Structure Changes Flashcards
What are the four basic types of chromosome rearrangements?
Deletion = loss of chromosome segment
Duplication = doubling of chromosome segment
Inversion = orientation within chr reversed
Translocation = segment moved to different site
What parts of the chromosome are required for survival of cell after meiosis?
Centromere and 2 telomeres
Why is a lack of centromere problematic for meiosis?
Chromosome is acentric so there’s nowhere for the spindle to attach causing DNA to be lost
What is 2 centromeres problematic?
Chromosome is dicentric so it isn’t incorporated into progeny cell
What happens if telomere is lost?
It impacts DNA stability and replication.
What is the “.” in the chromosome abbreviation AB.CDEFG?
The centromere
What is deletion?
Loss of a chromosome segment. AB.CD__G
Is deletion viable?
In many cases it isn’t viable especially when multiple genes are lost. (gene dosage effect)
What are the effects of duplications?
Can be advantageous.
What does it mean to be homozygous for duplication?
Individuals that are homozygous for duplication carries duplication on both homologous chromosomes.
How are gene duplications detected?
There is a characteristic loop structure
How do gene duplications take place?
Chromosomes do not align properly resulting in unequal crossing over. This results in a gene moving from one chromosome to the other.
Unequal crossing over between chromosomes containing 2 copies of the gene takes place resulting in another gene being added to the chromosomes.
How do homologous chromosomes meet when one has a duplication?
The duplicated gene loops out of the chromosome
How do inversions take place?
For inversion chromosome must break in 2 places.
What is the effect of gene inversions?
May break gene in 2 parts.
If between genes it can cause inverted gene order and mis-expression of gene.
What are the types of inversions?
Paracentric (does not include centromere)
Pericentric (Includes centromere)
How is the result different for inversions if the individual is homozygous for a particular inversion compared to being heterozygous?
If homozygous the homologous chromosomes can pair and separate normally.
If heterozygous an inversion loop forms resulting in a dicentric chromosome and an acentric chromosome which are not viable.
How does a heterozygous paracentric inversion result in viable and non viable gametes?
A dicentric chromatid is produced due to formation of bridge between chromatids.
What is the overall effect of inversions on fertility?
Individuals with inversions are semi-sterile due to 50% of their sperm/eggs being non-viable.
What is a translocation?
Movement of genetic material between non-homologous chromosomes. or within the same chromosome
What is a non-reciprocal translocation?
Material moves from one chromosome to another without recpiprocal exchange
What is a reciprocal translocation?
2 way exchange
What are the effects of translocations?
Can physically link genes that were previously on different chromosomes and this changes gene expression as genes may be under control of different regulatory sequences. (eg philadelphia chromosome)
Chromosomal breaks may take place within a gene,
Deletions frequently accompany translocations eg: Robertsonian translocation
What is a robertsonian translocation?
Translocation that is identical to down’s syndrome caused by reciprocal translocation between chromsome 21 and 14.
Why does the Robertsonian translocation cause symptoms of Down’s syndrome?
The translocation adds expression of chromosome 21 for a third time due to its attachment to chromsome 14. This overexpression increases chromosome 21’s gene dosage resulting in symptoms of Down’s syndrome.
What are the possible phenotypes resulting from the Robertsonian translocation?
25% Down’s syndrome
25% translocation carriers
25% normal
25% lethal (non-viable)