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THERE ARE TWO TYPES OF CELL DIVISION: MITOSIS AND MEIOSIS
- One diploid "parent" cell divides to produce two genetically
identical "daughter" cells.
- This type of cell division is used for
- asexual reproduction in unicellular organisms
- Even some multicellular species reproduce via PARTHENOGENESIS ("virgin
- a CLONE is a group of genetically
identical organisms produced via asexual means. (It is not an individual organism produced via
cloning.) Examples are...(fill 'em in here!)
- In times of stress, even species that ordinarily reproduce asexually
may revert to sexual reproduction. Why might this be an advantage to the
- somatic (body) growth in multicellular organisms
- it occurs throughout the body, wherever there is growth
- One diploid "parent" cell divides to produce four genetically
unique "daughter" cells which will then be processed into gametes (the sex
cells we usually know as "ova" and "sperm")
- This type of cell division is used for
- sexual reproduction
- it requires the halving of the genetic material (DNA) in preparation
for recombination with an equal amount of DNA from the same species
- it (usually) occurs in specialized regions of the body known as
- ovaries (female) and testes (male) in animals
- archegonia (female) and antheridia (male) in plants
If you don't remember the various parts of the cell, please refer to the
notes and diagrams in Lecture Three.
Of great importance to understanding cell division is to note what happens
to the CHROMOSOMES. Recall that in eukaryotic cells, there are multiple,
linear chromosomes, each containing a single strand of double-helix DNA.
This is organized around a complex structural matrix of protein and RNA,
which we'll see later.
When you've seen diagrams of chromosomes in the past, you've probably seen
them represented as an "X". This is not how chromosomes exist in the
normal cell cycle. Ordinarily, they exist as diffuse CHROMATIN.
When the cell is preparing to divide, everything inside it doubles,
including the chromosomes. But the duplicated chromosomes do not
immediately separate from one another. They are still joined at the region
known as the CENTROMERE. The two identical copies of the formerly single
chromosome, still joined at the centromere, are known as SISTER
These, in turn, are classified by the location of the centromere:
In non-metacentric chromosomes:
Note: the centromere is the location of the kinetochore,
the physical structure to which special protein fibers called SPINDLE
FIBERS will attach during cell division, and eventually pull the duplicated
- The short arm of a (non-metacentric) chromosome is called the p arm
- The long arm of a (non-metacentric) chromosome is called the q arm.
In humans and many other species, chromosomes can be classified as either
SEX CHROMOSOMES or AUTOSOMES (the "regular" chromosomes). In mammals, the
sex chromosomes are named "X" and "Y".
Humans have a total of 46 chromosomes (2 sets of 23, one set from each
parent). Here's what they look like, specially photographed and stained.
- Females have two X chromosomes (XX sex chromosome genotype)
- Males have one X and one Y chromosome (XY sex chromosome genotype)
If you're observant, you've noticed that there are two chromosomes of each
"color" and shape, and they can be arranged to reveal the HOMOLOGOUS PAIRS. One homolog of each pair was
"donated" to this human by each of his parents.
In that photograph, we see what is meant by the term DIPLOID. There are
TWO members of every homologous pair, meaning that there are two complete
sets of chromosomes in the nucleus of every diploid cell in this diploid
If we were to photograph the chromosomes of the GAMETES of this human, the
picture might look something like THIS. The
gamete is HAPLOID, and has only ONE set of chromosomes; one member of each
MITOSIS - A closer look at asexual cell division
This process has (somewhat arbitrarily) been divided into phases that are
easily recognizable under the microscope, if the cell is caught and
stained at those particular moments:
One parent cell gives rise to two, genetically identical
mito - Greek for "thread"
(referring to the threadlike appearance of the chromosomes during division)
sis - Greek for "the
- INTERPHASE - The normal "life cycle" stage of the cell, when it is
simply doing what cells do: living, growing and metabolizing. Near the
end of interphase, when the cell reaches a critical size and gets internal
signals that it's time to get ready to divide, three important phases
- Gap 1 (G1) - increased protein synthesis
- Synthesis (S) - the cell duplicates all of its DNA exactly
- Gap 2 (g2) - return to increased protein production, with many of the
proteins involved in cell division now being manufactured
- PROPHASE -
- The chromosomes first become visible due to supercoiling. They
are already visible as "X"--two identical SISTER CHROMATIDS produced during S phase,
but still joined by the centromere.
- The nuclear envelope disintegrates, joining the nucleoplasm & cytoplasm
- The nucleoli disappear
- METAPHASE -
- The chromosomes line up along the equatorial plate of the cell
- Proteinaceous spindle fibers attache to the centromeres from either
pole of the cell
- ANAPHASE - The sister chromatids are pulled apart at the centromere
by the retraction of the spindle fibers, with one member of each sister
chromatid pair moving into a new cell.
- TELOPHASE -
- A return to interphase-like conditions
- nuclear envelope re-forms around the chromosomes
- chromosomes relax into chromatin again
- cells "pinch off" the plasma membrane to form two separate cells
- spindle fibers disappear
- what we once called "sister chromatids" are now fully-fledged
"chromosomes," which contain all the instructions for the cell's
A few important notes on the relevance of mitosis to CANCER
G1 Phase is important in cancer studies. During
late G1, cells pass through a "G1 checkpoint". The specific
"messages" the cell receives from its internal and external environment
will cause a healthy, normal cell to either enter a
state of suspended animation ("G0" Phase) or, more commonly, proceed into normal mitosis.
Cancer cells often have a defect in the GI checkpoint,
and rather than going into either G0 or mitosis, they enter a state of
wild, unequal division, leading to cancer.
Initiation of cell division is controlled by the
relative concentrations of two proteins in the cell
- cell division cycle protein #2 (cdc2)
(This one tends to have a constant concentration)
- cyclin protein
(degraded during part of interphase, and so has variable concentration).
When cyclin concentration is high,it combines with
cdc2 to form a complex with the handy name "CdkG1-cyclin complex".
A critical concentration of this complex will initiate MITOSIS.
In mammals, a gene known as p53 appears to be a "tumor suppressor
The product encoded by the p53 gene is a protein involved
in the control of transcription of other proteins--including cyclin and cdc #2!
Mutant versions of the p53 are often associated with loss of control
over mitosis. A cell with a mutant p53 often becomes immortal
and highly proliferative--which means CANCEROUS.
MEIOSIS - A closer look at sexual cell division
One parent cell (2n) divides to produce
four haploid daughter cells which are then processed into gametes.
meio - "less"
Hence, meiosis is "the act of making less"
Meiosis I - Reduction Division
In this first meiotic series, the DNA is actually halved, with only
one (duplicated) member of each homologous pair going to each new daughter
Meiosis II - Equational (mitotic) Division
In this second meiotic series, the duplicated single homologs separate, with
one sister chromatid of each homologous pair going to each new daughter
Prophase I - the physical and chemical events are very similar to
those of mitosis, with one *very* important difference: CROSSING OVER.
1. nuclear envelope and nucleoli disappear
Metaphase I - spindle fibers arrange homologs along
the metaphase plate at the cell's equator, this time with homologs lined up
2. spindle fibers begin to form
3. chromosomes begin to supercoil
4. homologs come to lie next to each other (synapsis is just starting)
5. CROSSING OVER occurs: non-identical homologs actually TRADE PIECES!!
6. by the end of Prophase One, sister chromatids are no longer identical, as they have undergone crossing over.
(Note: some animals stop here, including humans.
Meiosis does not continue until fertilization or ovulation.)
7. spindle fibers attach to kinetochores.
Anaphase I - spindle fibers separate homologs, carrying
them to opposite poles, but sister chromatids are still connected
at the centromere.
- Telophase I (if it occurs; some species skip this step) is a backwards
progression to the interphase-like conditions--just the same as in mitosis.
However, in Meiosis, this process is sometimes called "interkinesis"
(literally "between movements")
- The "equational" division is physically
the same as mitosis, though the genetics of the cells are different.
- This time, there is only one member of each homologous pair to
separate as sister chromatids into the newly formed cells.
- The result: FOUR non-identical new cells, which will become gametes.
After meiosis, gametogenesis occurs to make the new
haploid cells into gametes.
The generalized animal scenario:
1. Testes contain 2n SPERMATOGONIAL cells, which
constantly renew themselves via mitosis. At some point, some
will mature and enter into meiosis to become...
2. Primary spermatocytes [2n --> 2(n + n)]. These diploid
cells undergo meosis I to become two...
3. Secondary spermatocytes (n + n). These haplolid cells undergo meiosis
II to become four spermatids (n).
4. Further spermatogenesis results in the typical
1. Ovaries contain 2n OOGONIAL cells, which do not
renew themselves. At some point, these enter into meiosis I to
2. Primary oocytes [2n --> 2(n + n)] undergo meosis
I. One will become a polar body, the other will become a...
3. Secondary oocyte (n + n) undergo meiosis II
to become one ovum and one to three polar bodies (n).
4. Further oogenesis results in the typical cytoplasm-rich
A few notes of interest. What can happen during sexual
"POLYSPERMY" is the (abnormal) fusion of more than one sperm with a single egg.
- In animals, this is almost always lethal: the resulting zygote has
more than two sets of chromosomes (a condition known as POLYPLOIDY), and cannot properly develop and grow.
In plants, however, polyploidy can result in the formation of new species!
What is the normal number of chromosomes?
EUPLOIDY - is the term used to describe the normal number of
chromosomes expected in a given species. (from the Greek eu, meaning "true")
ANEUPLOIDY - more or fewer than the normal number of chromosomes per
homologous pair (from the Greek an - "away" )
Two of several ways these can happen:
- monosomy - one too few of a homologous pair
- trisomy - one too many of a homologous pair
- nullisomy - homologous pair is entirely missing.
Note that AUTOSOMAL ANEUPLOIDIES are far more devastating than sex c'some
abnormalities; the latter are often survivable, and some persons with sex
c'some abnormalities can lead totally normal lives and produce normal
(We'll return to this later, when we talk about mutations.)
- nondisjunction - two homologs migrate to the same new gamete, leaving the
other one "blank". Thus one new gamete has an extra chromosome, and the
other has one too few.
- lagging c'some - spindle fibers pull one member of a homologous pair
too slowly, and it is left out of the nucleus when the nuclear membrane
forms. This can happen either during meiosis I or II.