Chromosome rearrangements Flashcards
(24 cards)
What is a polytene chromosome and where are they found?
Giant chromosomes that form when multiple rounds of replication produce many sister chromatids which stay fused together without cell division (endomitosis). Commonly found insect salivary glands e.g. drosophila
What is the functional role of polytene chromosomes?
Increase nucleus volume and cause cell expansion, and also permit high levels of gene expression e.g. chromosomes of the drosophila larval salivary glands undergo many rounds of endomitosis to produce large quantities of adhesive mucoprotein (“glue”) before pupation.
Describe Fluorescent in Situ Hybridisation
Structure of a protein is used to work out the base sequence that coded for it. Fluorescent DNA probes complement the target sequence. DNA in the sample is denatured to make it single-stranded, & the probe hybridises its matching sequence. Unbound probes are washed away, and the sample is examined under a fluorescence microscope. The fluorescent signal indicates the presence and location of the target sequence.
What are the uses of FISH
used to study chromosomal abnormalities, gene mapping, and gene expression in both research and clinical diagnostics.
What are HeLa cells?
Henrietta lacks - unusually aggressive cervical cancer - tissue samples unconsensually taken and shared as first successfully cultured human cell line - termed ‘immortal’ as cells can divide indefinitely. Used in first ever human-animal cell hybrid experiment.
Describe Somatic Cell Hybridisation
Mouse and HeLa cells are cultured with Sendai virus, which binds both with surface fusion proteins and causes them to fuse into a heterokaryote w/ two nuclei. These cells spit out human chromosomes and this was how the chromosome coding the MHC complex was found. (no. 6)
How are Hybrid cells sorted from the culture following SCH?
Poison is applied to the culture. Resistance is conferred by two enzymes jointly - A+B.
Mouse line is A⁻ B⁺ [dies]
Human line is A⁺ B⁻ [dies]
Half the hybrid cells are A⁺ B⁺ [survive]
Describe Radiation Hybrid Mapping
High X-ray doses are used to fragment desired (Human probs) DNA, which is introduced into rodent cells capable of incorporating, maintaining and replicating these fragments for division (not recombination). Closer loci more likely to be on the same fragment, so likelihood of being found in the same hybrid cell lines indicates loci proximity.
What cells are often used for Radiation Hybrid Mapping and why?
Chinese Hamster Ovary (CHO) cells are already engineered or selected for robustness and adaptability to carry foreign DNA. They survive and replicate as the fragments incorporated are small and non-functional in their new environment.
Name the different chromosome rearrangements
Deletion, Duplication, Inversion, Translocation.
Describe Deletion (Chromosome rearrangment) and give examples
A section of the chromosome is removed.
Examples include Drosophila notch wing and Cri-du-Chat.
Describe the importance of Notch Wing
Results from a deletion in the Notch locus, involved in cell signalling and thus development. Notch deletion results in many cancers - highly expressed in Leukaemia patients.
Describe Cri-du-Chat syndrome
Deletion at the end of chromosome 5 results in small head, mental problems, wide eyes, heart, hearing and sight defects, difficulty speaking and a distinctive cat-like cry.
How does deletion mapping work?
If a deletion in a heterozygote for an allele causes the expression of the recessive, then we know the locus is within the region of deletion.
Describe Duplication (Chromosome rearrangment) and give examples
A section of chromosome is doubled up. e.g. led to Globin and heavy chain and odorant-receptor gene families; Huntington’s disease in humans
What is a gene family?
A cluster of genes that are functionally similar, usually the result of duplication.
What effects can duplication have on recombination?
Can cause mispairing of homologous chromosomes during meiosis, where homologous sequences are shifted out of sync, and crossing over can cause further duplication in some gametes in a vicious cycle and a deletion in others.
What is the relevance of duplication in Huntington’s?
Dominant neurodegeneration shown at differing ages. Due to duplication of CAG repeats in westerners. Normal range 10-26, Premutation in danger - 27-41 (Most fine, but tends to go up in the next gen). Affected 36-121. Similar in Spinocerebellar ataxia
What are the symptoms of spinocerebellar ataxia?
Unsteady walk, poor coordination, Speech abnormality, food regurgitation in the mouth.
Describe Paracentric Inversion (Chromosome rearrangment) and give examples
A section of chromosome has been rotated and replaced in position. During crossing over an inversion loop in one chromosome allows it to pair correctly with its homologue. This leads to a dicentric and acentric fragment. The former has two centromeres and is torn apart, the latter with none is left behind; meiosis prevented. As such inversions are crossover suppressors.
How does Pericentric inversion differ to paracentric and what effects does this have?
Crossing over creates chromatids with duplications and deletions of genetic material (no dicentric/acentric fragments because each fragment still has one centromere). These recombinants are typically unbalanced and non-viable, but
structurally different from paracentric cases.
Describe Translocation (Chromosome rearrangment) and give examples
Two non-homologous chromosomes exchange parts. E.g. used in attempts to control tsetse and mosquito fly populations in Africa by producing sterile males. Often associated with cancers e.g. chronic myeloid leukaemia (9-22 translocation) and Burkitt’s lymphoma (8 cell cycle gene -14 antibody gene translocation).
How does position effect variegation work?
chromosomal rearrangements move a gene from euchromatin near hetero chromatin (typically found near centromeres or telomeres). Hetero chromatin can spread into neighboring regions, partially or completely silencing the gene. This doesn’t always happen uniformly in every cell - causing variegation.
Give an example of positive effect variegation
In Drosophila (fruit flies), a gene for red eye color placed near heterochromatin may be active in some cells and silenced in others, leading to a mottled, patchy eye color—some red, some white. That patchiness is the “variegation.”
