Tuesday, February 3, 2009
-Today’s 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
-Here you have the wild-type sequence which is the sequence you’d
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, there’s translocation. Notice now that there’s 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.
-There’s 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
-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 there’s
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 that’s 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 you’ll end up with a deletion. Those deletions could be
relatively small within a gene (intragenic) or they could be really large.
We’ve 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 they’re in the
homozygous state, in other words, if both copies of that chromosome
have this deletion, then those are usually lethal because you’re losing
genes, you’re 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, it’s 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 won’t be a problem b/c 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