Mitosis and meiosis detail
Mitosis and Meiosis
You
started off as a fertilized cell inside your mom, called a zygote. Now,
you’re a thriving community of hundreds of millions of cells, all
working together towards a common purpose: to keep you alive. How did so
many cells come from just one?
Generally
speaking, the answer is straightforward: many cells come from just one
by repeated cell division. Your first form as a zygote split to make two
cells. Then those cells split, making four...and so on and so forth,
until you became the living, functioning organism you are today.
There are two ways cell division can happen in humans and most other animals, called mitosis and meiosis.
When a cell divides by way of mitosis, it produces two clones of
itself, each with the same number of chromosomes. When a cell divides by
way of meiosis, it produces four cells, called gametes. Gametes are
more commonly called sperm in males and eggs in
females. Unlike in mitosis, the gametes produced by meiosis are not
clones of the original cell, because each gamete has exactly half as
many chromosomes as the original cell.
The concept of a chromosome
A chromosome is a thread-like object (scientists literally called them threads or loops when they were first discovered) made of a material called chromatin. Chromatin is made of DNA and special structural proteins called histones.
One way to think of a chromosome is as one very long strand of DNA,
with a bunch of histone proteins stuck to it like beads on a string.
Chromosomes
are stored in the nuclei of cells. If you compare the diameter of a
cell nucleus (between 2 and 10 microns) to the length of a chromosome
(between 1 and 10 centimeters, when fully extended!), you can see that a
chromosome must be scrunched up into a very small package in order to
fit inside a nucleus. Actually, the average chromosome is about a
thousand times longer than a cell nucleus is wide. The situation is a
bit like how a very long snake can coil up into a tight ball.
The
basic construction of chromosomes (made of chromatin) and structure
(long but scrunched up) is the same in all animals. The difference is
that each species has its own set number of chromosomes. For instance,
all human cells (except gametes) have 46 chromosomes. Cells of nematodes
(worms), other than gametes, have 4 chromosomes. The number of total
chromosomes in the non-gamete cells of a particular species is called
the diploid number for that species. The diploid number of humans is 46, and the diploid number of nematodes is 4.
The total number of chromosomes in the gametes of a particular species is referred to as the haploid
number of that species. This number is always half of the diploid
number. For instance, the haploid number in humans is 23, and the
haploid number in nematodes is 2.
The concept of mitosis
The
purpose of mitosis is to make more diploid cells. It works by copying
each chromosome, and then separating the copies to different sides of
the cell. That way, when the cell divides down the middle, each new cell
gets its own copy of each chromosome.
The phases of mitosis
In
the first step, called inter-phase, the DNA strand of a chromosome is
copied (the DNA strand is replicated) and this copied strand is attached
to the original strand at a spot called the centromere. This new structure is called a bivalent chromosome. A bivalent chromosome consists of two sister
chromatids (DNA strands that are replicas of each other). When a
chromosome exists as just one chromatid, just one DNA strand and its
associated proteins, it is called a monovalent chromosome. Here is a
drawing of what happens in a nematode nucleus (diploid number 4) during
interphase, with individual chromatids represented as numbers, sister
chromatids as the same number, and the centromere represented as a “-”.
The
second and third steps of mitosis organize the newly created bivalent
chromosomes so that they they can be split in an orderly fashion. A lot
of care has to be taken with this process, because unequal splitting of
chromosomes creates malfunctioning cells. Down syndrome is one disease
that results from unequal splitting of chromosomes.
In the second step, pro-phase,
the bivalent chromosomes condense into tight packages. Imagine the
difference between a slinky fully stretched out, and a slinky that has
been pressed back together. That's what happens to chromosomes during
pro-phase: they get pressed together into tight packages.
In the third step of mitosis, called meta-phase,
each chromosome lines up in a single file line at the center of the
cell. By this point in time, the membrane enclosing the nucleus has
dissolved, and mitotic spindles have attached themselves to each
chromatids in all the chromosomes. Here is a diagram of what a nematode
cell nucleus looks like after pro-phase and metaphase.
In the fourth step, ana-phase,
the mitotic spindles pry each chromatid apart from its copy, and drag
them to the opposite side of the cell. Four bivalent chromosomes become
two groups of 4 monovalent chromosomes.
Once
anaphase is over, the heavy lifting of mitosis is complete. In the
final phase, telophase, membranes form around the two new groups of
chromosomes, and the mitotic spindles that provided the power to create
these groups are disassembled. Once mitosis is complete, the cell has
two groups of 46 chromosomes, each enclosed with their own nuclear
membrane. The cell then splits in two by a process called cytokinesis,
creating two clones of the original cell, each with 46 monovalent
chromosomes.
The concept of meiosis
The
purpose of meiosis is to make haploid gametes. In order to explain the
difference between mitosis and meiosis quickly and easily, consider the
following analogy: You own a restaurant, and you keep 46 cookbooks on
hand, to store all the recipes you need to make the food you sell. If
you opened a new restaurant that you wanted to make the same food as the
one that already exists, what would you do? Copy all 46 cookbooks, and
take them to the new restaurant. That's like what happens in mitosis.
Consider that the cookbooks are chromosomes, each containing lots of
recipes that cells use to make “dishes,” called proteins. When cell
division occurs, each cell wants to ensure that each new cell can make
the same proteins as the original. So each of the chromosomes are copied
and evenly distributed to both new cells—both cells get a copy of each
“cookbook.”
Meiosis is different. Whereas as mitosis makes a new cell with the same number of chromosomes, meiosis is a reductive type of cell division: it results in cells with fewer chromosomes.
The phases of meiosis
Meiosis is split into two separate parts, called meiosis I and meiosis II.
Meiosis I
starts with the copying of chromosomes and their condensation into
compact forms (just like mitosis). The metaphase of meiosis I is
different, though: Instead of lining up in single file, the bivalent
chromosomes line up two-by-two. These groups, called homologous
chromosomes, are what separate during the anaphase of meiosis I (compare
this to the anaphase of mitosis, where chromatids separate).
If
we look at the anaphase of meiosis I in nematodes (diploid number 4),
the result is two groups of two bivalent chromosomes, rather than two
groups of four monovalent ones. This difference in chromosome number in
the post-anaphase groupings is really the only big difference between
meiosis I and mitosis. Membranes form around the two groups, during
telophase, and then the cell splits down the middle creating two
non-clones. Each clone has half the number of chromosomes as the initial
cell.
Meiosis
II applies the process of mitosis to the two cells created by meiosis
I. Since the chromosomes already exist in the bivalent form, interphase
is skipped. The result is four cells, called gametes, each with two
monovalent chromosomes.
Consider the following
What happens when cell division goes wrong?
Aneuploidy is a catchall term that refers to mistakes in the number of chromosomes in an organism (the prefix “eu-” essentially means “normal”, and adding “an-” in front --- “an-eu-” --- means “not normal”). For example, a human somatic cell with 46 chromosomes, or a nematode somatic cell with 4, or a human gamete with 23, is “eu-ploid” --- it has the right number of chromosomes. On the other hand, a human somatic cell with any other number of chromosomes --- 47, for instance --- is “an-eu-ploid” --- it has the wrong number of chromosomes.
Aneuploidy is a catchall term that refers to mistakes in the number of chromosomes in an organism (the prefix “eu-” essentially means “normal”, and adding “an-” in front --- “an-eu-” --- means “not normal”). For example, a human somatic cell with 46 chromosomes, or a nematode somatic cell with 4, or a human gamete with 23, is “eu-ploid” --- it has the right number of chromosomes. On the other hand, a human somatic cell with any other number of chromosomes --- 47, for instance --- is “an-eu-ploid” --- it has the wrong number of chromosomes.
One
well-known medical condition that involves aneuploidy is Down syndrome.
People born with Down syndrome have an extra copy of chromosome 21 ---
instead of 2 homologous chromosomes, they have three. This mistake in
chromosome number is called trisomy 21.
How
trisomy 21 happens is a lot easier to explain than how it causes Down
syndrome. During meiosis II, bivalent chromosomes are supposed to
separate. Trisomy 21 is caused when this separation doesn’t occur. In
humans, this means that one of the four gametes produced has the normal
selection of 22 monovalent chromosomes, plus the bivalent version of
chromosome 21. If this gamete is fertilized, a zygote is created with
one extra chromosome.
This
mistake has profound consequences. The physical and mental development
of people with Down syndrome is curbed, and results in a spectrum of
handicaps ranging in severity from mild to severe. But why? Clearly,
having an extra set of the genes contained in chromosome 21 does
something to development, but pinpointing the effects is very difficult.
The puzzle of trisomy 21 is all the more vexing when you compare its
effects with those of other, more benign aneuploidies. Women with
so-called “triple X syndrome,” which occurs when a female’s somatic
cells have three X chromosomes instead of two (for a total of 47
chromosomes), are, in most cases, visually and clinically
indistinguishable from women without the “syndrome.” Similarly, men with
“XYY syndrome” have an extra Y chromosome, but no distinguishable
symptoms.
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