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L3 - Chromosomal
Rearrangements l - Deletions,
Duplications & Inversions
Chromosomal Rearrangements and Changes in
Chromosome Number
In this section we are going to talk about chromosomal rearrangements and
changes in chromosome numbers.
We noticed when we spoke about karyotyping how all placental mammals had
the same DNA content, yet they had different chromosome numbers.
So chromosomes have clearly been rearranged in terms of their order, with
respect to which chromatin is in the same neighbourhood with which other bits of
chromatin.
And we also had chromosomes being broken and therefore more chromosome
numbers in some species versus less chromosome numbers in other species.
These chromosomal rearrangements and changes in chromosome number have
reshaped genomes over evolutionary time.
For example, if we looked at mice and humans, we have this interesting
evolutionary paradox where mice and humans have very, very different
karyotype numbers.
They have different chromosomal patterns, and they have different numbers of
chromosomes. They’re clearly two very different species.
However, if you looked at a certain genetic block on any chromosome in human,
and you looked at the sequences neighbouring that genetic region, if one were
to find the same genetic sequence for the mouse and looked at its neighbouring
sequences, you’d find very close genetic similarity over thousands and
thousands of base pairs.
And these similar blocks are called syntenic blocks, or blocks of synteny.
L3 - Chromosomal Rearrangements l - Deletions, Duplications & Inversions 1
, So one has great synteny between mouse and human chromosomes, yet the
chromosome number and the banding pattern is different.
So, how do we explain this? In fact, people have thought about it in another way.
One could reconstruct the mouse genome by taking the human genome and
breaking it in about 350 fragments, each of about 16 megabases, and just paste
it into a different order.
So you’ve got the same genetic kind of foundation, it’s just built in a different
way.
So in a sense, you can almost think of these synteny blocks as blocks of Lego,
and you’re building a structure. So the structure one builds as a mouse I clearly
mouse-like, but the blocks are the same. So on this picture on the far right of the
slide we can see mouse chromosome one, and it has a particular sequence. And
the sequence of mouse chromosome one, you can find in giant blocks of
synteny across human chromosomes. So, for example, on the human
chromosome, for the mouse you’ve got what would be chromosome eight, a bit
of chromosome six, a bit of chromosome two, some chromosome five, a bit of
chromosome 18, more chromosome two, and then finally chromosome one is on
the mouse chromosome one. And this is indicative of the chromosomal
rearrangement that’s occurred over evolutionary time, and defines the species.
Different Types of Chromosomal Rearrangements
So we’re going to talk about a range of different chromosomal rearrangements.
Deletion: Removal of a segment of DNA
The first kind of chromosomal rearrangement we’ll talk about is where you delete
a bit of DNA material, a bit of chromatin.
So if you look at the picture, there is a chromosome that has eight chromosomal
regions.
Now, be clear that each number does not represent a gene. It’s a giant
chromosomal region. Remember, chromosomes have megabases of DNA.
So each block represents a large chromosomal region. So the picture has eight
chromosomal regions, and then note chromosomal region number four is
deleted in the part of the diagram on the righthand side.
So clearly, there’s a deletion and you’ve lost genetic material.
L3 - Chromosomal Rearrangements l - Deletions, Duplications & Inversions 2
