The Cell Cycle, Mitosis, and MeiosisThe Cell Cycle, Mitosis, and Meiosis I. Cell Division

A. The Nature of Cell Division

1. Multicellular organisms need new cells to grow, develop and to replace

worn out and dead cells.

2. When a cell is getting ready to divide, one trigger to initiate this event is the balance between the ratio of cytoplasmic volume to plasma membrane area. 3. The cell membrane grows in two dimensions because the membrane width never changes- it is always two cells thick: A = l x h

Area (cell membrane) = length x width 4. The cytoplasm volume grows in three dimensions: V = l x h x w

Volume (cytoplasm) = length x width x height

5. So for cytoplasm V = l x h x w and for the cell membrane is A = l x h

6. Let’s look at this mathematically: 2

Cell Membrane A = 1 mm x 1 mm = 1 mm 3 Cytoplasm V = 1mm x 1 mm x 1 mm = 1 mm 1/1 = 1 : 1

ratio 2

Cell Membrane A = 2 mm x 2 mm = 4 mm 3

Cytoplasm V = 2 mm x 2 mm x 2 mm = 8 mm 2/4 = 1 :2 ratio

2

Cell Membrane A = 3 mm x 3 mm = 9 mm 3

Cytoplasm V = 3 mm x 3 mm x 3 mm = 27 mm 1/3 = 1 : 3 ratio

7. As the cell grows, the cell membrane material increases at a slower rate than the cytoplasm, triggering cell division.

8. Cells may be programmed to stop mitosis after 52 divisions.

9. When this occurs the cell is said to be senescent and has reached its Hayflick limit.

10. Cells stop dividing because the telomeres, protective bits of DNA on the end

of each chromosome required for replication, shorten with each copy.

11. As telomeres shorten, the cell dies.

II. The Cell Cycle

A. Stages of the Cell Cycle

III. A Closer Look at the Nucleus A. Chromatin 1. Inside the nucleus there are 46 pieces of chromatin. 2. Chromatin is threadlike and composed of DNA, proteins, and a trace of RNA. 3. DNA is so abundant that it must be tightly packed in the nucleus.

4. The nucleosome structure compacts DNA in the nucleus -Chromatin compacts the DNA in the nucleus by forming the nucleosome -Each nucleosome is composed of 8 positively charged histone proteins, core DNA, and one H1 histone -Linker DNA connects the many nucleosomes together like beads on a string

The H1 histones holds the nucleosome structure together

5. Further chromatin compaction occurs when nucleosomes combine to form the solenoid structure.6. The solenoids then compact into chromosomes.

B. Sister Chromatids

1. When chromatids become visible during mitosis and meiosis, they appear as connected sister chromatids. 2. Sister chromatids are identicle and connected by a centromere. 3. Each sister chromatid represents an individual chromosome. 4. The kinetochore area of the centromere connects with kinetochore fibers during mitosis. 5. The kinetochore fibers are composed of microtubules and function to move the chromosomes during mitosis.

C. Homologous Chromosomes

1. Homologous chromosomes are chromosome pairs that are similar but not identical. 2. Humans have 46 chromosomes. 3. The 46 chromosomes make up 23 pairsof homologues. 4. One homologue is inherited from dad and the other from mom during fertilization.

D. Human Karyotype

1. A karyotype represents the 46 human chromosomes lined up by homologues. 2. The sister chromatids are barely visible. 3. Karyotypes are used to study chromosomes.

IV. Mitosis

A. Mitosis Characteristics 1. After the cell leaves G2 , it enters into mitosis. 2. Mitosis is nuclear division.

3. The purpose of mitosis is to copy the cell’s DNA so that each

daughter cell, upon cell division, has an identical copy of its DNA. 4. After mitosis, the cell divides.

5. Cell division is called cytokinesis.

6. The stages of mitosis are: Interphase (not part of mitosis) Prophase Metaphase Anaphase Telophase

7. Sometimes mitotic stages are represented by IPMAT.

B. Mitosis Stages

Stages of mitosis in plant and animal cells

Interphase

Animal Plant

Prophase

Animal Plant

Metaphase

Animal Plant

Anaphase

Animal Plant

Telophase

Animal Plant

Can you identify the phases of animal cell mitosis?

Can you identify the phases of plant cell mitosis?

V. Abnormal Cell Division

A. Malignant Tumors as a Result of Mutations

1. The tumor begins to develop when a cell experiences a mutation that

makes the cell more likely to divide than it normally would.

2. The altered cell and its descendants grow and divide too often, a

condition called hyperplasia. At some point, one of these cells

experiences another mutation that further increases its tendency to divide.

3. This cell's descendants divide excessively and look abnormal, a condition

called dysplasia.

4. As time passes, one of the cells experiences yet another mutation. This cell

and its descendants are very abnormal in both growth and appearance. If

the tumor that has formed from these cells is still contained within its tissue

of origin, it is called in situ cancer.

5. If some cells experience additional mutations that allow the tumor to

invade neighboring tissues and shed cells into the blood or lymph, the

tumor is said to be malignant. The escaped cells may establish new tumors

(metastases) at other locations in the body.

B. Cancer Cells are Immortal

1. Cancer cells have unique features that make them "immortal." 2. The enzyme telomerase is used to extend the cancer cell's life span.

3. Telomeres of cells shortens after each division eventually causing the

cell to die, telomerase extends the cell's telomeres. This is a major

reason that cancer cells can accumulate over time creating tumors.

C. Cancer Cells Do Not Display Contact Inhibition

1. The cells in our bodies are governed by growth control mechanisms

cell senescence (aging).

2. Cell aging puts a limit on the number of times a cell can divide: the

more a cell has divided, the less likely it will be to divide again.

3. Contact inhibition is a growth mechanism that will cell growth

4. Cancerous cells typically lose this property and thus grow in an

uncontrolled manner even when in contact with neighboring cells.

D. Types of Cancer Cells

Breast cancer cells Lung cancer cells

Cervical cancer cells Prostate cancer cell

VI. Male and Female Gametes

A. Male Sperms

1. Haploid cell - 23 chromosomes

2. Produced in great numbers in the seminiferous tubules of the

paired testes

3. Composed of the heat, midpiece and tail

4. Short-lived and highly motile

B. Female Eggs (Ova)

1. Haploid cell - 23 chromosomes

2. Produced in paired ovaries

3. Large cells containing what is needed to produce a new individual

4. Nonmotile

VII. Meiosis

A. Meiosis I (cells are diploid) 1. Prophase I

-homologues synapse- synapsis -synaptonemal complex -chiasmata -crossing over

2. Metaphase I

-paired homologues move to the equator

3. Anaphase I

-homologues begin to separate

4. Telophase I

-homologues separate into separate cells

B. Meiosis II (cells are haploid)

1. Meiosis II

-meiosis II occurs the same manner as mitosis

2. End result of meiosis II is the generation of haploid gametes

Homologous pairs of chromosomes in prophase I

synapsis chiasmata crossing-over

C. Spermatogenesis

D. Oogenesis

E. Egg and Sperm Unite- Fertilization

F. Fertilization

G. Zygote formation leads to restoration of the diploid number which leads to mitosis