# Mitosis

### Introductory Concepts

##### Chromatin, Chromosomes

Chromatin is DNA and associated proteins.

When the cell is not dividing, individual pieces of chromatin called chromosomes may be condensed into 10 nm, 30 nm or 300 nm fibers. During cell division, DNA becomes further compacted and chromosomes such as those in the photograph below become visible. Click here for more information about chromosome structure.

Below: Human chromosomes (female)

##### Haploid, Diploid

Diploid cells (2N) have two complete sets of chromosomes.   The body cells of animals are diploid.

Haploid cells have one complete set of chromosomes.  In animals, gametes (sperm and eggs) are haploid.

##### Homologous Chromosomes

Homologous chromosomes are two chromosomes that are the same. This happens because diploid organisms have two of each chromosome. Each of the pairs is a homologous pair. One of the homologous chromosomes was inherited from the individual's mother and the other one was inherited from the individual's father. For example, the two chromosomes #1 are homologous. However, a chromosome #1 and a chromosome #2 are not homologous because they are different chromosomes.

Genes

A small segment of DNA that contains the information necessary to construct a protein or part of a protein (polypeptide) is called a gene.  Genes are the unit of inheritance.

### Types of Cell Division

A cell divides by pinching into two. Each of two daughter cells produced contains genetic material inherited from the original (parent) cell.

#### Why Divide?

Single-celled organisms divide to reproduce.

Cell division in multicellular organisms enables the organism to grow larger while the cells remain small.  A large surface:volume ratio is due to small cell size.

Organisms with many cells can have cells which are specialized for different functions and tasks.  For example, red blood cells are specialized for carrying oxygen but neurons (nervous tissue) are specialized for conducting signals from one cell to another.

Some cells of multicellular organisms must divide to produce sex cells (gametes).

##### Mitosis

Mitosis produces two daughter cells that are identical to the parent cell.  If the parent cell is haploid (N), then the daughter cells will be haploid.   If the parent cell is diploid, the daughter cells will also be diploid.

N $$\rightarrow$$ N

2N $$\rightarrow$$ 2N

This type of cell division allows multicellular organisms to grow and repair damaged tissue.

##### Meiosis

Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell.

2N $$\rightarrow$$ N

Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid.

Meiosis is necessary in sexually-reproducing organisms because the fusion of two gametes (fertilization) doubles the number of chromosomes.

Meiosis involves two divisions producing a total of four daughter cells.

### Chromosome Structure and Replication

A chromatid is a single DNA molecule.

Double-stranded chromosomes have two chromatids; normally, each one is identical to the other. The point where the two chromatids are attached is called the centromere.

#### Chromosome Doubling vs DNA Synthesis

Splitting chromosomes into two will double their number because each chromatid is identical. In the diagram below, two chromosomes are shown on the left. Splitting these chromosomes produces four chromosomes as shown on the right side of the arrow.

DNA replication occurs when a single-stranded chromosome produces a second chromatid. The diagram below shows four chromosomes on the left and four replicated chromosomes on the right.

### Overview of the Cell Cycle

 Interphase (G1 and G2) Chromosomes are not easily visible because they are uncoiled. Prophase The chromosomes begin to coil. The spindle apparatus begins to form as centrosomes move apart. Prometaphase The nuclear membrane disintegrates. Kinetochores form on the chromosomes. Kinetochore microtubules attach to the chromosomes. Metaphase The chromosomes become aligned on a plane. Anaphase The chromatids separate (The number of chromosomes doubles). Telophase The nuclear membrane reappears. The chromosomes uncoil. The spindle apparatus breaks down. The cell divides into two. G1 Interphase The chromosomes have one chromatid. G2 Interphase The chromosomes are replicated. Each one has two sister chromatids.

### The Cell Cycle

The cell cycle alternates between interphase and mitosis as diagrammed below.

Mitosis has these four phases: prophase, metaphase, anaphase, and telophase.

#### Prophase

During prophase, chromosomes begin condensing as DNA becomes coiled. The genes cannot function (produce mRNA and therefore protein) when the DNA is coiled. Coiling facilitates movement.

The nucleolus disappears.

A system of microtubules needed to move the chromosomes begins to form during prophase. The microtubules, also called spindle fibers, form from an area of the cell called the centrosome. During interphase, the cell has one centrosome but just before prophase, the centrosome duplicates, producing a second centrosome.  During prophase, microtubules radiate from each centrosome. Some of the microtubules extend from one centrosome toward the other.

The entire complex of centrosomes and spindle fibers is called the spindle apparatus.

Each centrosome  of an animal cell contains two centrioles. Plant cells do not have centrioles but they do form spindle fibers.

#### Prometaphase

The nuclear membrane becomes fragmented.

Protein plates called kinetochores form on the centromeres of each chromosome.

Kinetochore microtubules attach to the kinetochores.

Click on the image above to enlarge it.

#### Metaphase

During metaphase, the chromosomes move to the center of the cell (diagram below, photograph above). This line of chromosomes is referred to as the metaphase plate.

The structures in the diagram below are referred to as the spindle apparatus. Kinetochore microtubules are attached to the chromosomes. Polar microtubules are not attached to chromosomes but overlap each other. Asters are short microtubules that radiate from the centrosomes. The spindle apparatus can be seen on the drawing of a cell in metaphase below.

Metaphase ends when chromosomes split, thus doubling the number of chromosomes.

#### Anaphase

When the chromosomes split at the end of metaphase, the chromosome number is doubled. For example, the number of chromosomes and chromatids during each phase in a human cell is:

 Phase # Chromosomes # Chromatids Prophase 46 92 Metaphase 46 92 Anaphase 92 92 Telophase 92 92
##### Chromosome movement

Microtubules lengthen and shorten by the addition or removal of tubulin dimers. Click here for details in the chapter on cells.

Kinetochore microtubules shorten in the region of the kinetochore, pulling the chromosomes apart.

Polar microtubules push against each other and thus, push the two centrosomes apart. This, in turn, also pulls the chromosomes apart.

The chromosomes move toward poles of cell.

Cytokinesis (division of the cell) begins in anaphase. A cleavage furrow forms as actin filaments underneath the plasma membrane constrict in a band called the contractile ring. Two cells will be produced as this process continues.

#### Telophase

Telophase begins when chromosomes reach the poles of the daughter cells.

Many of the events in telophase are the reverse of prophase.  The chromosomes uncoil, the nuclear membranes around daughter nuclei appear, the spindle apparatus breaks down, and the nucleolus reappears.

Cytokinesis is completed as telophase ends.

#### Interphase

This is the non-dividing phase of the cell cycle.

During interphase, the nucleus is visible and the chromosomes are uncoiled and invisible.

Interphase includes G1, S and G2.

##### G1

Each chromosome has one chromatid.

The cell grows in size.

Synthesis of organelles occurs.

##### S

This is when DNA synthesis occurs.

##### G2

Each chromosome has two chromatids.

The synthesis of enzymes and other proteins in preparation for mitosis occurs during this period.

##### Cells that permanently leave the cycle

Some cells leave the cell cycle and enter a phase called G0. Examples: skeletal muscle, nerve cells

Some cells that leave the cell cycle are able to reenter the cycle when needed. Other cells permanently leave the cycle.

### Mitosis in Plants

Plants form a spindle apparatus as animals do but plants lack centrioles.

Instead of furrowing, vesicles derived from the Golgi apparatus fuse at the equator to form a cell plate. The vesicles contain materials necessary to construct a cell wall between the cells.

### Practice

How many chromosomes are there in each of the three diagrams below? How many chromatids?

If a parent cell had 6 chromosomes, how many during each phase listed below?

 Phase prophase metaphase anaphase telophase G1 interphase G2 interphase

If a cell had 4 chromosomes that were single-stranded, how many chromosomes and chromatids during each phase listed below?

 Phase prophase metaphase anaphase telophase G1 interphase G2 interphase

Draw each phase of mitosis (prophase, metaphase, anaphase, telophase) in a cell that has 2N = 4 chromosomes. Show the following in your drawings: chromosomes, kinetochore microtubules, and nuclear membrane.

### Control of the cell cycle

#### Telomeres

Mammalian cells typically divide only about 50 times.

This limit is set by the presence of repeated sequences of DNA at the tips of the chromosomes called telomeres

In young cells, the sequence TTAGGG is repeated hundreds or thousands of times but each time the cell divides, it loses 50 to 200 of these repeats. Cells that have divided many times have fewer of these repeats left.

When the telomere is reduced to a certain size, the cell will no longer divide.

Telomeres are restored to their original length by an enzyme called telomerase. This enzyme contains a single strand of RNA that is used to synthesize the telomeres.

Telomerase is usually found in cells involved in the production of gametes. It is not normally found in somatic cells.

#### Checkpoints

The cell cycle (diagram below) has three main checkpoints that function to stop the cycle. At the checkpoints, the cells receive a signal to go ahead and proceed with the next part of the cycle.

At each checkpoint, certain conditions are checked before the cycle proceeds.  At the G1/S checkpoint, the presence of necessary molecules needed to synthesize DNA is checked. If they are not present, the cell cycle stops. The presence of extracellular molecules such as growth factors is checked at this point. If growth factors are required, the cell cycle will stop unless growth factors are present.

At the G2 checkpoint, the cell checks to see if all of the DNA has been replicated and it also checks for damaged DNA. If DNA replication is not completed or if the DNA is damaged, the cycle stops.

At the metaphase-anaphase checkpoint the cell checks to see that all of the chromosomes are attached to spindle fibers.

Most cells stop dividing in G1. Cells that progress past the G1 checkpoint usually undergo mitosis.

#### Cyclins and Cyclin-dependent Kinases

While checkpoints function to stop the cell cycle when needed. The operation of the cell cycle is regulated by proteins called cyclin-dependent kinases (Cdk) and other proteins called cyclins. Cyclins are named because the level of cyclins fluctuates (cycles) as the cell cycle progresses.

Kinases are proteins that activate other proteins by transferring a phosphate group from ATP to the protein being activated. Kinases are normally inactive and must be activated before they can activate other proteins. Cyclin-dependent kinases become activated by combining with a protein called cyclins.

Four major types of cyclins are involved in regulating the cell cycle. G1-Cdk stimulates the cell to move past the G1 checkpoint and prepares the cell for  the S phase. G1/S-Cdk enables passage into the S phase S-Cdk initiates DNA replication M-Cdk initiates mitosis

As the name implies, the level of cyclin fluctuates (cycles). The activated cyclin-kinase complex activates enzymes that destroy the cyclin, causing the level of cyclin to decrease. At low cyclin levels, kinases are not activated and the cell cycle is halted. High cyclin levels are needed for the cell cycle to proceed.

### Prokaryotes

Prokaryotic cells do not undergo mitosis. When the cell divides, the circular chromosome replicates itself (DNA synthesis) and the cell pinches into two.

This process is called binary fission.