Unit #6 Cell Division:

Mitosis

Do we get larger because our cells increase in size or because we

produce more of them?

The cells of an adult are no larger than the cells of a child – there are just more of

them!

Why do cells divide?

A cell cannot grow forever. Eventually it must divide. There are three

reasons why cell growth is limited:

• The cell’s DNA would not be able to meet the needs of large cells

• Nutrients would not enter fast enough

• Wastes would not exit fast enough

Therefore, a cell divides into two identical cells to prevent it from

becoming too large.

Before cell division, a cell copies all of its

DNA so both new cells have a complete set of

genetic information.

Chromosomes

Chromosomes

In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next

is carried by chromosomes. Chromosomes- are made up of DNA and holds the

cell’s genetic information (genes).

The cells of every organism have a specific number of chromosomes.

• Fruit fly cells have 8 chromosomes

• Carrot cells have 18 chromosomes.

• Human cells have 46 chromosomes

• Cows have 60 chromosomes

Well before cell division, each chromosome is replicated

Because of this, when they become visible at the beginning of cell division, each chromosome

consists of two identical “sister” chromatids as shown below.

Checkpoint

The sister chromatids are identical because

1. one is a copy of the other.

2. they are both copies of a third chromatid that came before them.

3. they are both made of DNA and protein.

Biologists divide the events of mitosis into Four phases:

Depending on the type of cell, the four phases of mitosis may last anywhere from a

few minutes to several days.

1.Prophase

2.Metaphase

3.Anaphase

4.Telophase

Interphase-Resting

G2: cell growth

G1: cell growth and differentiation

prophase

metaphase

anaphase

telo

ph

ase

cyto

kine

sis

mitotic celldivision

S: synthesis of DNA; chromosomes are duplicated

Remember

• Before Mitosis a cell copies all of its DNA (Chromosomes ) so both new cells have a complete set of genetic information.

Prophase• chromosomes become visible (condense)• nuclear envelope disappears• centrioles move to opposite sides of the cell. Plant cells do not have centrioles • spindle begins to form and attaches to centromere

Checkpoint

The function of the spindle is to

a. break down the nuclear envelope.

b. help form a new nuclear envelope

c. help separate the chromosomes.

Metaphase• chromosomes line up at the middle of the cell

Anaphase• Sister chromatids separate at centromere into

individual chromosomes• Chromosomes move toward poles of cell • Anaphase ends when chromosomes stop moving

Check Point

• What happens during anaphase?

A. The chromosomes become visible.

B. The chromosomes line up across the center of the cell.

C. The sister chromatids separate.

Telophase• Chromosomes become less condensed• Nuclear envelope forms around each cluster of

chromosomes• Spindle disappears• Start of cytokinesis

Mitosis is complete.

However, cell division is not complete. Video

Cytokinesis

• occurs during telophase• cytoplasm divides

In most animal cells, the cell membrane is drawn inward until the cytoplasm is pinched

into two nearly equal partsVideo

In plants, a structure known as the cell plate forms

between the two cells.

Mitosis Animation

1.  Name the main events of the cell cycle.

2. Describe what happens during each of the four phases of mitosis.

3. What are chromosomes made of?

4.  How is cytokinesis in plant cells similar to cytokinesis in animal cells? How is it different?

Mitosis Review

CleavageCleavage is the name for the process of mitosis

that occurs in cells that forms a developing embryo.

Cleavage involves all the steps of mitosis, but after cell division, it quickly goes through mitosis again.

This results in a mass of many small undifferentiated cells. These cells will lead to the development of an embryo from a fertilized egg

gastrula

Once a gastrula forms, cells start to arrange themselves in different layers. The different layers of cells will differentiate into different tissues, which

will form organs.

When the different tissues form, the mass of cells is called an embryo.

Differentiation

• In most multicellular organisms, not all cells are alike. For example, cells that make up the human skin are different from cells that make up the inner organs. Yet, all of the different cell types in the human body are all derived from a single fertilized egg cell through differentiation.

Cellular differentiation is a concept from developmental biology describing the process by which cells acquire a "type".

1. The outer layer of cells is called the ectoderm. It will differentiate into nerve cells and skin cells

2. The middle layer of cells is called the mesoderm. It will differentiate into cells that form the muscular system, circulatory system, excretory system and endocrine system

3. The inner layer of cells is called the endoderm. It will differentiate into cells that form the digestive system, pancreas, liver and lungs

What is cancer?• Uncontrollable cell division resulting in a malignant and

invasive growth or tumor.• If untreated -Metastasis is the spread of cancer from its

primary site to other places in the body (e.g., brain, liver).