Mitosis and Meiosis

Chapters 8 & 10

Learning GoalsO 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 ReproductionO 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 MitosisO Metaphase

O In metaphase, each chromosome lines up in the very

center of the cell

O Each chromosome has a centromere that is attached

to two spindle fibers

O Spindle are cytoskeleton that are shaped like a

football and attached to an organelle called a centriole

O The centrioles are located where the two daughter

cells will eventually form

Third Stage of MitosisO 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 MitosisO 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

CytokinesisO 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

CancerO 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).

MeiosisO 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 ChromosomesO 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 HaploidO 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 IO 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 IO 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 IO 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 IO 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 IO 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 IO 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 VariationO 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

NondisjunctionO 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.

NondisjunctionO 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

Extra Credit Mitosis QuestionO You may check your answers with me ahead of time for a yes

or no response as many times as you like.

O At the beginning of mitosis, there is one cell. At the end of

mitosis there are two cells.

O These two cells are identical in every way to their parent

cell.

O Absolutely no construction, synthesis, diffusion or osmosis

occurs during mitosis.

O Question: Where does the cell get the materials for building

a 2nd identical cell if it can’t build or accept any materials

during mitosis?