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Lecture 9

Lecture 9 - Changes in chromosome structure

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Human Biology
Michelle French

Tuesday, February 3, 2009 -Todays lecture carries on talking about chromosomal abnormalities but specifically, changes in the structure of chromosomes as opposed to last lecture which was on changes in chromosome number. -Here we see a representation of the many different types of changes in chromosome structure that you can have. We will go through these in the lecture. -Here you have the wild-type sequence which is the sequence youd normally find in a population. You can have different types of changes. For example, one section can be deleted or alternatively, one section can be duplicated. And presumably, both of those will have some kind of phenotypic changes although not always. -As well, theres translocation. Notice now that theres an extra bit from another chromosome. So presumably, the blue part would be exchanged out and a new pink part is added in. Translocation is relocating genetic material or moving pieces from one part to another. -Theres also inversion as well which is the change in the order of genetic material. So, in the example, the brown & the green pieces are swapped. -When people are studying this kind of thing, it helps to have model organisms to look at. It helps to have chromosomes that you can readily see. -1 type of chromosome that can be easily seen are these polytene chromosomes found in the salivary glands of Drosophila. Normal size chromosomes would be something like the small bubble to the left but these polytene chromosomes are gigantic. They arise because theres duplication of the DNA but no actual division of the DNA into the 2 cells so you get these large cells with these large chromosomes that you can actually visualize under a microscope. -Another benefit of polytene chromosomes is that they have a banding pattern thats specific for each chromosome so it allows you to identify regions on the chromosome. -In deletion, a piece of the DNA is taken out. -You can imagine different ways that this can occur. For example, x-rays can cut double-stranded DNA. And you can see that this red chunk can be removed and youll end up with a deletion. Those deletions could be relatively small within a gene (intragenic) or they could be really large. Weve talked about the idea of small deletions within a gene earlier so we will focus now on these large deletions and the implication of those. -Large deletions could conceivably remove several genes or maybe a big chunk of chromosome. These large multigene deletions, if theyre in the homozygous state, in other words, if both copies of that chromosome have this deletion, then those are usually lethal because youre losing genes, youre losing the products of those particular genes that are being deleted. These things can be called Del for deletion or Df for deficiency. In other words, its either deleted or deficient. -In the case of a heterozygote, meaning that there is 1 chromosome with a big deletion and 1 normal chromosome, then it depends on the genes that are missing. In some cases, that wont be a problem bc the other copy of those genes will take over and produce enough products. And that would be an example of haplosufficiency; all of the genes that are deleted are haplosufficient in the sense that you only need 1 copy. -But, in other instances, there would be a situation of haploinsufficiency
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