Unit 2

Cell DivisionMitosis and Meiosis I and II

Mitosis Word Meanings• Chromosome

– Etymology: Greek : Chromo = colored soma = body• Centromere

– Centro= center mere= part• Inter

– Etymology: Latin: between• Pro-

– Etymology: Greek: before• Meta-

– Etymology: Greek: after• Ana-

– Etymology: Greek: up• Telo-

– Etymology: Greek : end• Cytokenesis

– Etymology: Greek: Cyto = cytoplasm kenesis = movement

I. Reasons why a cell needs to divide. 1. Destroyed cells need to be replaced.2. Cell size

a. Most living cells are between 2 and 200 micrometers in diameter (3 feet = 1 meter = 1 million micrometer)

b. Limitation of cell sizei. Surface area to volume ratio

1. Increase cell size will increase volume of cell2. As cells grow, the amount of surface area becomes

too small to allow materials to enter & leave the cell quickly enough by diffusion.

ii. Diffusion (see unit 1)1. fast & efficient over short distances2. If cell gets too large, would decrease diffusion.

iii. The nucleus1. DNA in the nucleus is copied to make proteins

which run the cells activities in the cytoplasm.2. Cell size is limited by the amount of proteins the

DNA is able to make to control the cell.

II. DNA Packing (occurs before cells divide)a. DNA is a very long thin molecule (can’t see it)

i. In order to fit in the nucleus it has to get really compact (= packed in closely).

ii. The DNA does this by wrapping around proteins called Histones, and folding in on itself

i. This is known as Chromatin. (We can see this)b. When Chromatin is wrapped up on itself and takes on a

distict shape (looks like an “X”) it is known as a Chromosome (We can see this too)

III. Chromosomes1. Human cells have 46 Chromosomes

a. Humans get 23 from the mother and 23 from the father.b. Before the DNA wraps up into a chromosome, the DNA gets

copied.c. Before cell divides each DNA piece winds up into a c’some

i. One side of the chromosome is the original chromosomeii. The other side of the chromosome is the copy of the original

chromosome.d. When a Chromosome is paired we call each side a

Chromatid. (You have 2 Chromatids for each Chromosome)a. Chromosomes pairs are held together by a centromere.b. When the chromatids separate into separate nuclei we call

them chromosomes again.

Chromatids

Centromere

After DNA Replication2 pairs per type of

chromosome

Before DNA Replication1 pair per type of

chromosomeHomologous PairsIndividual chromosomes

• 1 from each parent Dad Mom

IV. Cell Cyclei. Interphase – before cell division

1. G1 phasei. cell growth ii. production of new organelles

2. G0 (G zero)i. period when cell is not preparing to

divide. ii. Here cells lack growth factorsiii. Cells stay in the G0 until there is a

reason for them to divide. iv. Ex. Muscle, brain, nerve cells

3. S phase = Double the DNAi. DNA replication (duplicate)ii. Go from 46 to 92 single chromosome

molecules per cell in humans

4. G2 phasei. cell prepares to divide

ii. M phase = Mitosis – Cell Division1. Prophase

i. chromosomes shorten, thicken, and become visibleii. nucleus and other organelles break down and disappeariii. Organelles called centrioles begin to make spindle fibers

2. Metaphasei. spindle fibers become present and attach to

centromeres of chromosome pair.ii. chromosomes line at center of cell (metaphase plate

= equator)

3. Anaphase i. The Chromatids are pulled apart towards polar ends

of the cell.

4. Telophasei. New nuclei form around each set of chromosomes ii. two new cells start to take shapeiii. chromosomes begin to make proteins

5. Cytokinesis a. In animal cells

i. Animal cells undergo cytokinesis by pinching off along their equator

ii. This works because their membrane is flexible.

b. In plant cellsi. Plant cells cannot do the same as animal cells

because of their cell wall. ii. Plant cells send vesicles filled with cell wall material

to their equator where they fuse together.iii. This fusion creates cell plates that fuse with the cell

wall separating the two cells.

V. Canceri. It is the disease where the

body’s cells DO NOT STOP mitosis.

ii. Results in tumors forming throughout the body.

a. A tumor is a bunch of cells in one area of the body that keep dividing.

b. These cells can then move on to other parts of the body causing tumors to develop in other regions.

iii. Cancer kills by causing your organs to malfunction.

VI. Cell Differentiationi. Unspecialized cells develop into mature forms and

functionsa. Each cell has a full set of DNAb. BUT, each type of cell uses only specific genes on the DNA to

carry out its functions

ii. Stem Cellsa. Can develop into a variety of cell typesb. Will divide by mitosis into either two stem cells, or, a stem

cell and a specialized cell.

1.___________________________ 2._________________________

3.___________________________ 4.__________________________

Mitosis Pop Quiz

Meiosis Word Meanings• Homologous

– Etymology: Greek homologos : to agree– having the same position, value, or structure

• Tetrad– Etymology: Greek– a group or arrangement of four

• Dyad– Etymology: Late Latin dyas, from Greek, from dyo – : pair; two individuals

• -ploid– having or being a chromosome

• Di-– Etymology: Latin, from Greek; akin to Old English twi-– twice : two : double

• Ha-– Etymology: Greek :single

• Somatic cells and Gametes– Somatic cells are body cells.

• Make up most of your body cells. • These are diploid (2n)

– Have two sets of chromosomes.– 1 set from father & 1 set from mother

– Gametes = sex cells(sperm/egg cells)• These are haploid (n)

– Have only one set of chromosomes

– Types of Chromosomes• Autosomes – contain genes not

associated with sex• Sex chromosomes – directly control

sexual traits.

Meiosis – The creation of sex cells

Interphase (G1, S, G2)• All your cells start off as diploid cells

= Diploid (2n): cell with two of each kind of chromosome (In humans = 46 [23 from each parent])

• G1 – Cell grows; DNA is long and stringy and is not visible. (so the DNA can be copied)

• S – DNA gets replicated (copied).• G2 – Cell continues growing and prepares to

divide.

Meiosis IProphase I 1. The chromatin wind-up, creating chromosomes

which can now be seen.2. Centriols create Spindle Fibers 3. Nuclear envelope disappears 4. Each chromosome pair then actively seeks

out its homologous chromosome pair. – Homologous pairs are about the same size and shape– This is called a tetrad (four chromatids) – Crossing Over will now happen.

• where two non-sister chromatids exchange genetic material.

After DNA Replication2 pairs per type of

chromosome

Before DNA Replication1 pair per type of

chromosomeHomologous Pairs – each individual chromosome has been copiedIndividual chromosomes

• 1 from each parent Dad Mom

Prophase I

Metaphase I.

1. The spindle fibers attach to the centromere on each chromosome pair.

2. Homologous chromosomes are lined up on the metaphase plate (equator) by the spindle fibers.

3. 2 rows of 23 chromosome pairs = 46 pairs total

Anaphase I 1. The tetrad gets pulled apart = homologous chromosomes

separate.– CHROMATIDS DO NOT GET PULLED APART YET!

2. Each homologous chromosome moves to opposite poles.

Telophase I / Cytokenesis1. Individual nuclear envelopes begin to surround

the separate chromosome pairs2. Cytokenesis separates the one cell in two.

END OF MEIOSIS ITwo cells are created each with 23 chromosome pairs

Meiosis IIProphase II1. Each dyad (2 Chromatids = 1 Chromosome pair)

are connected by a centromere.2. Nuclear envelope disappears3. The centrioles create spindle fibers again.

Metaphase II1. The spindle fibers attach to the centromere on

each dyad (1 Chromosome pair = 2 Chromatids).2. The dyads are lined up on the metaphase plate

(equator) by the spindle fibers. 3. 1 row of 23 chromosome pairs in each cell.

Anaphase II1. The individual sister chromatids from each dyad

get pulled apart by the spindle fibers2. Each sister chromatid ends up on opposite poles

of the cell

Telophase II / Cytokenesis1. The shape of the cell changes, beginning to form

two cells.2. New nuclei form around each set of chromosomes3. The cytoplasm of both cells divides once again.4. Four gametes (= sex cells) are now created

END OF MEIOSIS IIFour unique haploid cells are created, each with a half set of chromosomes compared to the original (Original Parent cell had 46, each daughter cell has only 23 chromosomes).= Haploid (n): cell with only 1 of each kind of chromosome (in humans = 23 chromosomes)

Mitosis: Chromosome Number Flow

46 individual chromosomes

2n

46 chromosome pairs

46 individual chromosomes

2n

46 individual chromosomes

2n

Interphase Mitosis

Meiosis I & II Chromosome Number Flow

46 individual chromosomes

2n

46 chromosome pairs

23 chromosome

pairs

23 chromosome

pairs

Interphase Meiosis I

23 individual chromosomes

n

23 individual chromosomes

n

23 individual chromosomes

n

23 individual chromosomes

n

Meiosis II