Chromosome Rearranegements Flashcards
Polytene chromosomes
Methods of mapping chromosomes
Oversized chromosomes which have developed from normal chromosomes and are found in Drosophilia salivary glands
Fluorescent in situ hybridisation (FISH)
Methods of mapping chromosomes
Using structure of a protein you can workout its DNA code
Complementary fluorescent DNA probe will bind to these regions only and therefore can be used to indetify the location of the gene
Somatic cell hybridisation
Methods of mapping chromosomes
HeLa cells (derived form cervical cancer cells in Henrietta Lacks) fuse with other human and mouse cells using a Sendai virus
The virus facilitates the fusing of the cells.
Successful hybrid cells containing a genome of both cell lines can be selected using a poison which needs to enzymes to break down; one enzyme is found in the human genome, and one is found in the mouse genome. After fusion, human chromosomes get spat out.
Radiation hybrid mapping
Methods of mapping chromosomes
Desired chromosomes are broken into segments with X-rays, after which they are implanted in rodent cells, which clone the chromosomes. Loci which are far apart are more likely to be in different segments than loci that are close together.
Deletion
A section of chromosome is removed (*=centromere)
ABCDEF Goes to ACDEF
E.g. notch wing in Drosophilia, Cri-du-chat in humans (tip of chromosome 5 lost)
Duplication
A section of chromosome is doubled up
A B C * D E F goes to A B B C * D E F
E.g. Globin and odorant-receptor gene families; Huntington’s disease in humans
When chromosomes bearing tandem duplication (perhaps repeated many times) pair up during
meiosis, there is very precise pairing of particular DNA sequences; but duplications can cause
mispairing – like a zipper; teeth get into the wrong holes.
A B C B C D Homozygote mispairs A B C B C D
A B C B C D A B C B C D
Also look at when chiasma forms in the booklet
And image of Drosophilia bars
Bar EYE in Drosophila: cross the bar-eye flies together. Sometimes get a mix of normal round-eyed flies and “double-bar” (eye reduced to a slit). The visible bands in the polytene chromosomes show
that bar has a duplicated segment; double bar has gained an extra copy of a duplication, while the normal eyed flies have lost one to return to wild type phenotype.
This “slippage” means that there is an intrinsic tendency for the size of a tandem duplication to
increase or decrease Helps explain why there are many repeats of BCBCBCBC in DNA.
Inversion
A section of chromosome has been rotated and replaced in position.
*A B C D E
*A D C B E Must be pairing A with A; only way is to make a loop
Look at diagram in the booklet
Gives a dicentric and an acentric fragment – first breaks, second lost at cell division, i.e., inversions
are CROSSOVER SUPPRESSORS; prevent crossover between inverted and normal segments of chromosome.
Now, consider heterozygote for an inversion containing the centromere: A B * C D → A C * B D
In a heterozygote, if there is a crossover between * and C, abnormal gametes will be formed: A B * C D (normal), A C * B D (inverted), A B * C A and D B * C D. The last two have an excess of one gene (A or D) and a deficiency of the other (D or A). Sometimes, if small, these inversions produce viable gametes and abnormal children. If the duplicated or deficient sections are large, the gametes are lethal and there is no medical problem
Translocation
Two o non-homologous chromosomes exchange parts.
A B C * D E F and p q r s t goes to A B C * D s t and p q r E F
e.g. used in attempts to control tsetse and mosquito fly populations in Africa. Often associated with cancers e.g. chronic myeloid leukaemia (9-22 translocation) and Burkitt’s lymphoma (8-14 translocation).
