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Mitosis and Meiosis
Chapters 8 & 10
Learning Goals O Quiz #6: December 6th
O Describe what happens during interphase
O Identify steps of mitosis/meiosis by picture and
function
O Explain the diseases that occur when mitosis and
meiosis go wrong, such as cancer and nondisjunction
Cell Reproduction O The life-cycle of the cell is broken into two
sections: Interphase and Mitosis
O In interphase, the cell is living its life and
performing it’s normal functions.
O In mitosis, the cell is dividing and creating a
daughter cell.
O At the end of mitosis are two 100% identical
daughter cells
O The “parent” no longer exists
Interphase
Interphase is the majority of a cell’s life
Interphase is three separate stages
During the first stage the cell has just been born. It is
growing and developing proteins which will build the
organelles in the cell
During the second stage, the cell copies all of it’s DNA
(which it will pass to it’s daughter cell)
During the third stage, the cell performs its necessary
functions and prepares for mitosis
First Stage of Mitosis Prophase
During prophase, the nucleus and nuclear envelope seem to disappear
◦ What is the chromatin inside the nucleus?
◦ (The DNA of the cell)
The chromatin coils up into unique structures called chromosomes
Each chromosome is attached to its identical copy of DNA made during interphase, and held together by a centromere
Each organism has chromosomes of specific shapes and shades
Second Stage of Mitosis O Metaphase
O In metaphase, the centromeres of the
chromosomes line up in the center of the cell
and attach to spindle fibers
O Spindle are cytoskeleton that are shaped like
a football
O The spindle are attached to an organelle
called a centriole
Second Stage of Mitosis O The centrioles are located where the two
daughter cells will eventually form
O Each chromosome lines up in the very center
of the cell
O Each chromosome is attached to two spindle
fibers, which will pull the chromatids to each
centriole
Third Stage of Mitosis O Anaphase
O In anaphase, the centromeres split and each
identical chromatid begins to move to one of
the centrioles
O The chromatids reach each centriole by
walking along the spindle fibers
Fourth Stage of Mitosis O Telophase
O In telophase, chromatids have reached each
centriole.
O The nucleus and nuclear envelope begin to
reappear
O A new membrane begins to form and the two
daughter cells become visible
Cytokinesis O The last step, which is separate from the rest
of mitosis, is cytokinesis.
O The two daughter cells have formed, but they
are still attached to each other like conjoined
twins
O In animal cells, the two cells get pinched by
the plasma membrane until the two cells are
forced apart
O In plant cells, the cell wall and the plasma
membrane has to be built like a brick wall
Apoptosis A cell can only undergo so many divisions before it
becomes too risky to reproduce
Apoptosis is also known as programmed cell death,
because the cell will undergo reactions to cause its
own destruction
First, the DNA will fragment into pieces and all envelopes
and membranes will blister
Then, enzymes that have been dormant in the cell since
the beginning of interphase first division are activated
These enzymes destroy the organelles and membrane of
the cells and deposit the cell parts into the blood stream
for recycling
http://www.youtube.com/watch?v=JBekXg5LxSg
Cancer
O Cancer is a disorder that disrupts the process of cell
reproduction
O Carcinogenesis (development of cancer) is gradual and
can occur for years before a cell becomes cancerous
O Some of the following are characteristics that identify a
cancerous cell from non-cancerous
O 1. No differentiation.
O Cancerous cells have lost function. They are simply
excess tissue.
Cancer
O 2. No contact inhibition.
O Normal cells do not add press against neighboring cells.
Cancerous cells expand no matter how much room they
have, causing neighboring cells stress
O 3. Formation of tumors
O Cancer cells pile on top of each other, constantly
dividing without end. They never undergo apoptosis
O 4. Cancer cells invade
O Cancer can produce enzymes that seep into the blood
stream and invade other cells, causing the cancer to
spread
Cancer O Why does cancer spread?
O Sometimes cancer is “benign” which means your body
located the cancer and contained it inside a capsule so it
can’t spread from one cell to the next.
O Even this is rare. You and I get cancer all the time. Our body
fights it off and we never notice.
O If a tumor does grow, it typically damages the capillaries
between the cell and the blood vessels, so the cell can’t
receive any nutrients.
O You’d think this would kill the cancer…
O In order to survive, the cancer cell has learned to either
spread its proteins to other cells (metastasizing) or
rebuild new blood vessels into the tumor (angiogenesis).
Meiosis O Back to genetics…
O How do you end up with only one allele for each gene
from your parents?
O The sex cells of your body, called gametes, (eggs or
sperm, for us) have a process that ensures which
alleles offspring receive are completely random.
O Their division is different than every other cell in your
body
O The division of gametes in organisms is called meiosis
Homologous Chromosomes O Homologous Chromosomes are two similar
chromosomes
O They are similar because they contain the
same GENES but not necessarily the same
ALLELES (versions of a gene)
O Humans have 23 pairs of homologous
chromosomes for a total of 46 chromosomes.
O We get one chromosome of each pair from
our mom and one chromosome from our dad
Homologous
Chromosome
Pair
From Mom
From Dad
Diploid vs Haploid O Most cells in our body are diploid (2n), meaning they
have two sets of each chromosome
O A set from mom and a set from dad.
O The gametes in our body, however, are haploid (n),
meaning they have only one set of each chromosome
O **Two haploid gametes fertilize to produce one
diploid zygote**
O Meiosis is the process of creating these haploid
gametes.
Meiosis I O Meiosis is broken into two sections: meiosis I
and meiosis II
O At the start of meiosis there is one diploid (2n) cell
O By the end of meiosis there will be four haploid (n) cells
O These four haploid cells will be the gametes for the organism
Prophase I O Prophase I looks like prophase, for the most
part
O Organelles disappear, chromosomes appear
O One difference though is a process called
crossing over
O Crossing over is when homologous
chromosomes actually trade sections of
chromosomes
Prophase I O Why would cells do this?
O Increases randomness of which alleles are found
on each chromosome
O In prophase I the organelles disappear, the
chromosomes for each cell appear, and each
pair of homologous chromosomes undergoes
crossing over
Metaphase I O Just like in mitosis’ metaphase, the
chromosomes line up in the center of the cell
in metaphase I of meiosis.
O One difference: Each homologous
chromosome lines up next to each other in the
cell
Anaphase I O In anaphase I, instead of chromatids
separating as in mitosis, the homologous
chromosome pairs separate from each other
O Whole chromosomes move to each centriole
O This step ensures each gamete will hold only
one copy of each chromosome
Telophase I O Unlike in mitosis, the cell will not undergo a
full division
O Instead, a small membrane will be built to
ensure the chromosomes do not go between
each new cell
O The cell is now ready to enter Meiosis II
Meiosis II (Similar to Mitosis) O Prophase II
O Chromosomes appear and nuclear envelopes disappear again
O Metaphase II
O Chromosomes line up at the center of the cell at the metaphase plate
O Anaphase II
O Individual chromatids separate and move toward separate centrioles
O Telophase II
O Cell parts reform, and cytokinesis occurs
O Four new daughter cells have been produced
Genetic Variation O How does meiosis account for the randomness of
which gene the offspring will receive?
O In Prophase I, crossing over mixes which genes are found on which chromosome
O In Anaphase I, separating chromosomes ensures each cell will only receive 1 allele/gene
O In Meiosis II, the separation of chromosomes into chromatids ensures each cell only has ½ of the necessary DNA for an organism
O This will have to mix with the DNA of the other parent, further adding to the randomness
O Because of this, can you guess the number of possible genetic combinations you can have?
4,951,760,200,000,000,000,000,000,000
Nondisjunction O Nondisjuction is when homologous
chromosomes fail to properly separate
O Nondisjunction occurs if the homologous chromosomes or chromatids do not separate.
O The results can be gametes with an extra set of chromosomes or a missing set.
O Sometimes the gametes can still fertilize. Sometimes they cannot. If they can, however, the resulting zygote will have too many or too few chromosomes.
Nondisjunction O Trisomy 21
O An extra #21 chromosome
O Result: Down Syndrome
O XXY
O The male has an extra X chromosome
O Result: Klinefelter’s syndrome
O XO
O The female is missing an extra sex chromosome
O Result: Turner’s Syndrome
