GENETICS
Unless otherwise noted* the artwork and photographs in this slide show are original and © by Burt Carter.
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2 – Meiosis, gametes, and sexual reproduction.
Most cells in your body contain 46 separate strands of DNA (23 matched pairs) in cells that
are not in the cell cycle (in “Gap 0”). That number is doubled in the Synthesis stage of the
cell cycle before the DNA condenses into chromosomes.
Notice that during anaphase each of the chromosomes is pulled in half by the mitotic
spindles. This is very important. Each daughter cell doesn’t get half the chromosomes, it
gets one side of each chromosome, and the chromosome pairs have lined up on the spindle
to assure that each daughter cell gets half of each or the original chromosomes – a
“chromatid”.
Thus for a short time in Synthesis, gap 2, and up to anaphase the cell has 92 strands of DNA.
There are only two other cases of a normal human cell having some number other than 46
(2x23) DNA strands.
Hewitt
Red blood cells don’t have DNA at all. They don’t have nuclei or other organelles.
They must be very small to get through the capillaries and so everything has been
scrapped that is not absolutely essential for their work.
This, of course, means that red blood cells cannot replicate themselves. They are
divided from cells in bone, specifically from the marrow which is rich in blood vessels
partly for this reason.
The other cells that don’t have 46 DNA molecules likewise cannot replicate and must
be made by divisions from other cells, specifically those in the testes and ovaries.
These are the sex cells or gametes, and they have only half the DNA of a typical cell.
Cells with all 46 full strands of DNA are called diploid because they have 23 full pairs of
chromosomes (“di” = “2”). Gametes are haploid because they have exactly half that
amount (“haploid” = “single”). They have only one of each of the pairs in a “regular”
cell.
How these cells are created is in many ways similar to mitosis but with a couple of
extra steps that are tweaked somewhat in their operation.
Cell division that creates haploid cells is called meiosis.
Hewitt
Meiosis I proceeds similarly to the way mitosis would – prophase, metaphase, anaphase,
and telophase (each labeled “I” to signify that it is a step in meiosis I), but with some
interesting twists. It begins with the normal parts of the cell cycle as in mitosis, including
DNA replication in synthesis.
One of the important differences is that during prophase the homologous pairs of
chromosomes align next to each other so that their strands are pretty closely matched.
Then they swap out portions of those strands! This is called crossing over and it
introduces some variability into the strands that wouldn’t occur in mitosis. This happens
because the codes on those strands are now not exactly as they were before. This is
called recombination. The next slide elaborates on this. Otherwise prophase is like in
mitosis – spindles are forming and the nuclear membrane is falling apart. Metaphase is
likewise similar to the way it works in mitosis.
a
b
c
d
e
As we will see soon, each chromosome of the homologous pair comes from each parent. Let’s say that the blue
ones came from the father and the red ones from the mother of the owner of these chromosomes.
During crossing over the homologous pair (a) comes into close proximity (b). Then a compound causes each to
break (c) at the homologous point – that is, in such a way that the DNA in one of the short pieces is exactly
homologous to the DNA in the other – the same genes are present in both. Then the pieces are swapped between
the two chromosomes (d), which move apart (e).
Notice that the red chromosome is now not entirely DNA from Mom – a little piece of it is Dad’s DNA. The blue
one likely is not entirely Dad’s DNA. That little piece is Mom’s contribution. Each of these DNA molecules is now
not like either of the ones at the beginning of the process – they will code for proteins in a new way.
Crossing over doesn’t always happen to every chromosome. It doesn’t
always happen at the same place, and so on. Each time it produces a new
combination of genes for the offspring that might ultimately result from
the combination of the daughter cell with another gamete.
This is one source of the huge genetic variation that occurs in most
natural populations.
Hewitt
After crossing-over in prophase the homologous chromosomes separate a little, but not to far. They remain aligned
in such a way that the spindles they attach to in prophase and metaphase attach to full chromosomes – not the
opposite halves of the same chromosome.
This means that the daughter cells of meiosis I do not have homologous pairs of chromosomes. Instead they have
only one version of each pair, for a total of 23 chromosomes.
Be sure you realize that they are 23 different chromosomes. One version of each of the homologous pairs has been
put into each daughter cell.
The daughter cells will have some of the chromosomes from Dad and some from Mom (plus the crossed-over bits).
Which are which is different every time. Because there are a lot of ways to sort 23 things 2 by 2, this introduces yet
more variability into the daughter cells. Between crossing over and this, the number of possible genomes that can
result from this is far larger than the number of people that have ever lived, or that probably ever will!
Hewitt
The final difference occurs in telophase I and cytokinesis. Instead of the spindle apparatus falling apart it reforms in
each of the daughter cells, setting them up to divide yet again.
Hewitt
Meiosis II also proceeds as mitosis would – prophase, metaphase, anaphase, and telophase, but again with a twist.
Because the DNA was not replicated, in metaphase II the chromosomes that align on the spindles are not
homologous pairs as in mitosis. Meiosis I has already separated the chromosomes so that ther aren’t homologous
pairs anymore.
In metaphase II the spindles in each of the two daughter cells attach to either side of the chromosomes, just like in
mitosis (and not like in meiosis 1).
Hewitt
In anaphase 2 the chromosomes are separated at their centromeres, just like in mitosis. Each spindle moves one
chromatid from each chromosome into opposite halves of the two daughter cells. Again, some of these will be
chromatids that came (mostly) from Mom and (mostly) Dad into separate ends of the cell.
Make sure you realize that there is a full complement of chromatids here – one from each of the homologous pairs.
Hewitt
Cytokinesis of the two daughter cells produces four cells from the original one. Each cell has one chromatid of
each of the 23 required. However, these cells cannot function and divide because they require both
chromosomes of each pair.
Some of the chromosomes in each cell are from Mom and some from Dad, but it’s time to redefine “Mom and
“Dad” now. These cells are destined to be part of the raw material for the next generation. They are now “Mom”
or “Dad” themselves!
Most of them, of course, will not show up in the next generation. They will be “wasted” in that sense, but
remember that sexual reproduction is a hit-or-miss proposition and there have to be millions and millions of
these just to make sure that one of them combines with another gamete.
If these cells are from a “Dad” then all four will survive for the short term and their cells will become sperm cells.
Some may have y-chromosomes, some may not.
If they are from a “mom” then only one survives to become an egg. There can be no y chromosomes in them
because Mom’s original cell had no y’s in them.
+1 haploid cell from Dad
1 haploid cell from Mom
Fertilization occurs when a sperm
from Dad adds its chromatids to and
egg from Mom.
The resulting cell now has 23
homologous pairs of DNA strands –
half of each pair from each parent.
Hewitt
Hewitt
The fertilized egg is a fully functional cell that can begin to divide and grow into an animal – in our example, a
human. Each daughter cell of each of the billions of divisions awaiting it will be instructed exactly what to do
when the time comes by the DNA it has gotten from its parents.
This introduces yet a third type of variability into the mix. Now DNA from two entirely separate individuals has
been mixed together. In the offspring some of those genes will be expressed more or less like they were in Dad
and others more or less as they were in Mom, but not in their entirety like either of the parents.
So this particular combination of those instructions will be absolutely unique – no other individual has ever had,
nor will ever have, the same set of instructions. (Except in identical twins, and even then the genes wind up
being expressed slightly differently).