Lecture 4 mitosis

Cell cycle and MitosisCell cycle In order for a living organism to

grow their individual cells must:1. Increase in size2. Make exact replicas of all their

genetic material, and3. Go through a process of division

resulting in two daughter cells each with one complete copy of the entire genome

Mitosis The eukaryotic cell of higher

organisms, ranging from yeast to humans, has its genetic material (DNA) packaged into a membrane bound nucleus

In eukaryotic cells, the process of cell proliferation, the cell cycle, is characterized by four distinct phases as illustrated in the following diagrams:

Cell cycle

Cell cycle cont…

1. Gap 1 (G1) Phase - Cell are active and growing.

Cells are receptive to signals to begin DNA synthesis

2. Synthetic (S) Phase - Cells are actively replicating DNA

3. Gap 2 (G2) Phase - Cells are actively preparing

for mitosis.

Cell cycle cont… Cells contain twice the normal

amount of DNA4. G0 - Cells in G0 have exited

mitosis and are quiescent (inactive).

These resting cells may be reactivated and enter the G1 phase upon receipt of the appropriate stimuli.

Mitosis Mitosis a nuclear division produces two

identical daughter cells by replicating and dividing the original chromosomes during prophase, prometaphase, metaphase, anaphase, and telophase

This is followed by the division of the cell referred to as cytokinesis

Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle

Mitosis

The parent cell duplicates its chromosomes, providing both daughter cells with a complete copy of genetic information

When an eukaryotic cell divides into two, each daughter or progeny cell must receive:

Mitosis cont..,

1. A complete set of genes (for diploid cells, this means 2 complete genome, 2n).

2. A pair of centrioles (in animal cells).

3. Some mitochondria and, in plant cells, chloroplasts as well.

Mitosis cont…4. Some ribosomes, a portion of the

endoplasmic reticulum, and perhaps other organelles.

There are so many mitochondria and ribosomes in the cell that each daughter cell is usually assured of getting some.

But ensuring that each daughter cell gets two (if diploid) of every gene in the cell requires the greatest precision.

Mitosis cont…The answer:

1. Duplicate each chromosome during the S phase of the cell cycle.

2. This produces dyads, each made up of 2 identical sister chromatids.

These are held together by a ring of proteins called cohesins.

Human chromosomes

Mitosis cont…3. Condense the chromosomes into a

compact form. This requires ATP and a protein

complex called condensin.4. Distribute these equally between

the two daughter cells.5. Separate the sister chromatids. Steps 3-5 are accomplished by

mitosis.

Mitosis cont… It distributes one of each duplicated

chromosome (as well as one centriole) to each daughter cell.

It is convenient to consider mitosis in 5 phases: Prophase, Metaphase, Anaphase and Telophase.

When a cell is not engaged in mitosis it is said to be in interphase.

Interphase

Interphase

1. Prophase Chromatin in the nucleus begins to

condense and becomes visible in the light microscope as chromosomes.

The two centrosomes of the cell, each with its pair of centrioles, move to opposite "poles" of the cell.

Some fibers cross the cell to form the mitotic spindle.

The chromosomes become shorter and more compact.

Prophase

2. Prometaphase (Late prophase)

The nuclear envelope disintegrates because of the dissolution of the laminins that stabilize its inner membrane.

A protein structure, the kinetochore, appears at the centromere of each chromatid.

With the breakdown of the nuclear envelope, spindle fibers attach to the kinetochores as well as to the arms of the chromosomes.

Prophase/prometaphase

2. Prometaphase cont…

For each dyad, one of the kinetochores is attached to one pole, the second (or sister) chromatid to the opposite pole.

Failure of a kinetochore to become attached to a spindle fiber interrupts the process.

Metaphase

At metaphase all the dyads have reached an equilibrium position midway between the poles called the metaphase plate.

The chromosomes are at their most compact at this time.

Metaphase

4. Anaphase

The sister kinetochores suddenly separate and each moves to its respective pole dragging its attached chromatid (chromosome) behind it.

Separation of the sister chromatids depends on the breakdown of the cohesins that have been holding them together.

Anaphase

4. Anaphase cont…

It works like this:1. Cohesin breakdown is caused by a

protease called separase (also known as separin).

2. Separase is kept inactive until late metaphase by an inhibitory chaperone called securin.

4. Anaphase cont…

3. Anaphase begins when the anaphase promoting complex (APC) destroys securin (by tagging it for deposit in a proteosome) thus ending its inhibition of separase and allowing Separase to break down the cohesins.

Metaphase/anaphase

5 Telophase

A nuclear envelope reforms around each cluster of chromosomes and these return to their more extended form.

Telophase

6. Cytokinesis Mitosis is the process of separating

the duplicates of each of the cell's chromosomes.

It is usually followed by division of the cell.

Thus a special term, cytokinesis, for the separation of a cell into two.

Mitosis animation