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Good afternoon.
My name is Scott Hawley,
from the Stowers Institute of
Medical Research in Kansas City, Missouri.
And I'm here to do a couple of things.
I have an easy job, which is to talk
to you about the genetic analysis of
meiosis in Drosophila
melanogaster females.
And I have a hard job,
which is to make you care.
And the reason it's
difficult to make you care,
is that whenever I mentioned the word
meiosis, I watch people's eyes glaze over.
They've had meiosis in Montessori school
that had five, six, seven, eight,
nine times.
And frankly, they're really tired of it.
And the reason they're tired of it
has to do with this next slide.
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If you look at this next slide,
which I took out of a textbook,
the main problem with this slide is if you
didn't understand meiosis before you saw
this slide it's kind of
hopeless thereafter.
As you see upon the top,
there's all this chromosomal fettuccini.
We even added a mushroom,
which is actually the nucleolus.
And just to make sure you could digest it
all, we even added a replication fork.
I'm not sure there's all those Greek
names leptotene, zygotene, pachytene,
diplotene, diakinesis,
they're all Greek for prophase.
And if you look down in
the middle of the bottom row,
I don't know what that
middle chromosome is doing.
But it's not metaphase II.
In other words, it's really impossible
to try and understand what's
going on in meiosis, by looking at
a complicated, overdrawn slide like this.
And that's why I wanna try and simplify
meiosis for you before we even begin.
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This slide shows the meiosis the way the
people in my laboratory think about it.
It's an idea that we've taken
from the late Barbara McClintock,
who simplified meiosis back in the 1930s,
by pointing out that if you get rid of all
that Greek, if you get rid of
all those complicated diagrams.
You can think about meiosis as a process
where three things have to happen.
Chromosomes have to pair, they have
to match up along their length, so
that the copy of chromosome one that was
obtained from the organism's father,
lines up against the copy of chromosome
one that was obtained from the organism's
mother, and so on.
So they have to pair.
They have to match by homology
along their entire length.
Once they pair, they have to
undergo exchange or crossing over.
If you went to private school or
recombination or chiasma formation,
I don't care what term you use,
as long as you realize that exchange or
chiasma formation serves the vital
function of interlocking paired homologs.
That's what meiosis does it makes
sure that what was paired remains
stuck together as these chromosomes go
throughout the ballet, which is meiosis.
And then finally,
at the first meiotic division,
chromosomes have to
segregate from each other.
They have to disjoin if you will.
So that each homologue goes to opposite
poles, and we end up with two daughter
cells, each of which contains
a haploid complement of chromosomes.
If we're talking about humans,
that means we have to go from a cell
with 46 chromosomes, to two daughter
cells each of which have 23.
If you're talking about my organism,
which is Drosophila melanogaster females,
they start with eight chromosomes at
the beginning of meiosis and the two
products of the first meiotic division,
each of which each has four chromosomes,
usually one of each homologue if
everything has gone correctly.
The second meiotic division we're not
going to worry about very much it really
is a haploid mitosis.
Unfortunately, as you're going
to see in the next slide,
this is rather a simplified
description of meiosis,
because we can't count on
exchange always occurring.
In this slide, we look at
the frequency of recombination for
