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Biology
Mitosis and Meiosis Lecture Notes
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Learning Goals
o Quiz #6: December 6th
 Describe what happens during interphase
 Identify steps of mitosis/meiosis by picture and function
 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
 The “parent” no longer exists
Interphase
o Interphase is the majority of a cell’s life
o 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
o Prophase
o During prophase, the nucleus and nuclear envelope seem to disappear
 What is the chromatin inside the nucleus?
 (The DNA of the cell)
o The chromatin coils up into unique structures called chromosomes
o Each chromosome is attached to its identical copy of DNA made during interphase, and held
together by a centromere
o 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
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
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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 Siamese 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
o A cell can only undergo so many divisions before it becomes too risky to reproduce
o 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
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.
 Cancerous cells have lost function. They are simply excess tissue.
o 2. No contact inhibition.
 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
 Cancer cells pile on top of each other, constantly dividing without end. They never
undergo apoptosis
o 4. Cancer cells invade
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Cancer can produce enzymes that seep into the blood stream and invade other cells,
causing the cancer to spread
Why does cancer spread?
 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.
 Even this is rare. You and I get cancer all the time. Our body fights it off and we
never notice.
 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.
 You’d think this would kill the cancer…
 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 (eggs or sperm, for us) have a process that ensures which alleles
offspring receive are completely random.
 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
 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
Diploid vs Haploid
o Most cells in our body are diploid (2n), meaning they have two sets of each chromosome
 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
 Organelles disappear, chromosomes appear
o One difference though is a process called crossing over
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Crossing over is when homologous chromosomes actually trade sections of chromosomes
Why would cells do this?
 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
 Chromosomes appear and nuclear envelopes disappear again
o Metaphase II
 Chromosomes line up at the center of the cell at the metaphase plate
o Anaphase II
 Individual chromatids separate and move toward separate centrioles
o Telophase II
 Cell parts reform, and cytokinesis occurs
 Four new daughter cells have been produced
Genetic Variation
o How does meiosis account for the randomness of which gene the offspring will receive?
 In Prophase I, crossing over mixes which genes are found on which chromosome
 In Anaphase I, separating chromosomes ensures each cell will only receive 1 allele/gene
 In Meiosis II, the separation of chromosomes into chromatids ensures each cell only
has ½ of the necessary DNA for an organism
 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?
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Nondisjunction
o Nondisjuction is when homologous chromosomes fail to properly separate
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Nondisjunction occurs if the homologous chromosomes or chromatids do not separate.
The results can be gametes with an extra set of chromosomes or a missing set.
Sometimes the gametes can still fertilize. Sometimes they cannot. If they can, however, the
resulting zygote will have too many or too few chromosomes.
Trisomy 21
 An extra #21 chromosome
 Result: Down Syndrome
XXY
 The male has an extra X chromosome
 Result: Klinefelter’s syndrome
XO
 The female is missing an extra sex chromosome
 Result: Turner’s Syndrome