Hi, bio people! We're going to follow up on our discussion of mitosis and meiosis by
talking about chromosomal abnormalities. These are all terms that you should be
familiar with by the end of this screencast, and we're going to start off
by talking about karyotypes, and then move on to nondisjunction, Down syndrome
and something called amniocentesis. Remember that DNA is coiled up very
tightly into something called a chromosome. A segment of the DNA that
codes for a specific group of proteins is called a "gene". You have a lot of genes
that all determine different traits that you have. So, for instance, you have a
certain number of genes that code for how tall you, are what color hair you
have, or what color eyes you might have. Somatic or "body" cells are diploid.
"Diploid" means that they have 23 pairs of chromosomes. This is different from sex
cells like sperm and egg cells because sex cells are haploid. They have 23 single
chromosomes rather than having 23 pairs. This means that body cells have a total
of 46 chromosomes, whereas sex cells have a total of 23. This is what is called a
karyotype; it shows all of the different chromosomes that were inside one person's
cell, and matches them up with their pair. Because we have 23 pairs of chromosomes,
here we can determine that this actually came from a body cell rather than from a
sex cell. Each pair is made up of two sister chromatids; 23 pairs in all.
Now, notice this last pair. The 23rd pair has two that looked pretty different from
one another, which is different than any of the other preceding pairs. This is
because the last set of chromosomes, the X X chromosomes or the X and the Y
chromosomes, determine the person's gender. if the person has an X chromosome and a
Y chromosome, they are biologically male. If they have two X chromosomes, they are
biologically female. Notice how the Y chromosome is very short and stubby
compared to the X chromosome. If we look at this other person's karyotype, we can
see that their x and y chromosomes look a little bit different. Here we can see
they have two X chromosomes and no y chromosomes and
so this tells us this person is female. It's very important that all of your
body cells should have 23 pairs of chromosomes. If there's a different
number, too many or too few, then you have what's called a chromosomal abnormality.
For instance if you have an extra copy of chromosome number 21 and have 3
copies instead of 2, we call it trisomy 21. This is what
causes Down syndrome. Before giving birth parents can choose to have their baby
undergo something called an "amniocentesis". This is where they take a
small sample of the cells and fluid surrounding the baby and test it to see
if it will have chromosomal abnormalities. Things like trisomy 21
will usually show up on an amniocentesis before the baby is born. However this
also raises a number of ethical questions. Should people then be allowed
to terminate the pregnancy if they decide they are not able to care for a
Down Syndrome child or a child with a serious chromosomal abnormality? This is
an ethical question rather than a "scientific" question but it's important
to keep ethics in mind as we continue to discuss this topic. Remember that in
mitosis, a diploid parent cell splits into two identical diploid daughter
cells. In meiosis we go through two rounds of division transforming a
diploid parent cell into four different daughter cells each of which are haploid.
Each of these has genetic variation thanks to two procedures called crossing
over and independent assortment. Every single daughter cell is different from
the other daughter cells, as well as being different from the parent. Meiosis
consists of two rounds of cell division each one consisting of "PPMAT" prophase, prometaphase
metaphase, anaphase, and telophase, just like in mitosis. However since there are
two rounds of it, we go from having a parent cell to having four haploid
daughter cells so we go from having a parent cell that looks like this with
two red chromosomes and two blue chromosomes to having four daughter
cells each of which have one red and one blue chromatid. This is what happens in
normal cell division. However we're now going to take a look at what happens
when it goes abnormally. In a nutshell, abnormal chromosome division occurs when
you wind up with the wrong number of chromosomes in one of the resulting
daughter cells. Instead of having one of each color, you wind up with too few or
too many. Before we dive into the abnormal procedure please remember this
is a simplified diagram. You have 23 pairs of chromosomes but here I've only
drawn red and blue to keep things simple. Normally half of the chromosomes wind up
on one side and half of them wind up on the other side, but sometimes this
doesn't happen and we wind up with too many chromosomes on one side and too few
on the other side. So what if the cells split like this instead? This means that
after meiosis one and one round of cell division we would have one blue
chromosome on the left hand side and 3 chromosomes on the right hand side.
Usually the problems with cell division occurred during anaphase when the
chromosomes are not pulled to the correct side of the cell. If we were to
proceed through meiosis 2 even if everything went perfectly from that
moment forwards we would wind up with all four daughter cells having an
abnormal number of chromosomes instead of each one having one of each kind. You
have some that have too few and some that have too many. So, this is what
happens if the problem occurs during meiosis 1. Now let's see what would
happen if the problem occurred during meiosis 2, rather than meiosis 1. We're
going to assume at the first round of division meiosis 1 prophase metaphase
anaphase and telophase 1 went perfectly. We wound up with exactly the right
number of chromosomes in the first round of cells.
If cell division proceeds correctly from here we'll wind up with four haploid
daughter cells, each with the same number of chromosomes but if one of them
divides incorrectly, we're going to wind up with an incorrect number of
chromosomes in one of the daughter cells or possibly even two. So, let's just say
that this cell right here on the left is where the problem occurred. Usually the
problem occurs during anaphase where one chromosome failed to separate into two
sister chromatids, and so too many of them are on one side and two fewer on
the other side. This is what the daughter cells would look like after both rounds
of meiosis if there was nondisjunction. Nondisjunction simply means the sister
chromatids failed to separate correctly. Even if this cell over here on
the right divides perfectly, you still have half of your cells that are not
correctly formulated in terms of the number of chromosomes that they have.
Nondisjunction is much more common in older individuals, so the older someone
is, the more sticky their chromatids become, and the less likely they are to
separate. This is why older parents are more likely to have children with
chromosomal abnormalities. Nondisjunction can occur either in meiosis 1 meiosis 2
or in both. However we can see here that having it occur in meiosis 2 is slightly
better than having it occur in meiosis 1. When we look at meiosis 1 nondisjunction,
every single daughter cell down here has an incorrect number of chromosomes.
However if the first round goes correctly, only half of them have the
wrong number of chromosomes and so the resulting child would have at least a
50% chance of having no chromosomal problems whatsoever but a 50% chance of
having something like Down syndrome. I think that's all for now. I'll see you
all in class. Make sure you bring questions!