The Cell Cycle and Mitosis Why do Cells

The Cell Cycle and Mitosis

Why do Cells Divide • Cells divide from one cell into two daughter cells through a process called cell division or mitosis. • Cells divide for 3 principal reasons: 1. Reproduction 2. Growth 3. Repair/Regeneration

1. Reproduction • Cells divide to make new daughter cells. They start from one, and at the end of mitosis there are two. • This increases the overall number of cells!

2. Growth • As organisms grow they have to produce many, many more cells. • Human beings start from a single cell (just ONE!) but then end up with 50 to 75 trillion cells. • So we have to make a few more cells along the way – we do this via cell division.

3. Repair • After we stop growing we continue to use cell division frequently. • This is because many of our cells die or are damaged and need to be replaced. • Every second we shed millions of cells in our intestine and from the surface of our skin. • It is said that on average we replace every cell in our body every 7 years (excluding nerve and cardiac cells)

The Cell Cycle

The Cell Cycle • Cells can be in either one of two states: 1. They are quiescent. They are “quiet. ” These cells are just going about their day to day functions. 2. Cells that need to divide, ie go through mitosis, go through a cyclical process, called the cell cycle. - Note that cells that are quiescent are not in the cell cycle.

Parts of the Cell Cycle • There are two major phases to the cell cycle: interphase and mitosis. • In interphase the cells go through a series of phases which allows the cell to grow, duplicate its DNA and prepare for mitosis. • In mitosis the cell divides and one cell becomes two cells.

Interphase • There are 3 parts of interphase: G 1, S and G 2. • In G 1 the cells grow. This is because they need to get big enough to go through mitosis. If they weren’t big enough the daughter cells would die.

Interphase • In S the cells replicate all of their DNA so that they have 2 copies. This is so that at the end of mitosis, when you have two daughter cells, each has a copy of all the genes. • In G 2 the cells get ready to go through mitosis and divide.

Interphase • During interphase the cell carries out normal life functions. – It grows, makes proteins, does cellular respiration, moves, and does any specialized functions. • Throughout interphase the DNA is in the long, spaghetti like strands, called chromatin.

Mitosis • At the end of Interphase the cells enter Mitosis. • The purpose of mitosis is to take one parent cell and produce 2 genetically identical daughter cells. • Note: mitosis is considered the division of the genome/nucleus. The physical division of the cell into two daughter cells is called cytokinesis and is not part of mitosis. – However it follows mitosis. • After mitosis and cytokinesis is done the cells can become quiescent or enter interphase again.

MITOSIS

Mitosis and Cytokenesis Interphase ↓ Prophase ↓ The phases of Metaphase ↓ Mitosis Anaphase PMAT ↓ Telophase ↓ Cytokinesis ↓ Interphase (repeats again)

Prophase • Prophase: – Chromatin in the nucleus condenses into thick coiled structures called chromosomes. – At this point there are two copies of each chromosome (remember that we went through S phases where all the genes were duplicated). – Each chromosome contains two copies, called sister chromatids. – The chromatids are held together by a centromere.

Prophase • Also happening in prophase: – The nuclear membrane begins to break down. – Spindle fibres (which begin in late interphase) attach to the chromosomes at the centromeres.

Metaphase • The chromosomes line up on the metaphase plate in the middle of the cell. They are moved there by the spindle fibres.

Anaphase • The centromere in the middle of the chromosome breaks. • Each chromatid is pulled to opposite sides/poles of the cell by the spindle fibres.

Telophase • In telophase the chromosomes/chromatids begin to unwind back to chromatin. • The nuclear membrane begins to reform to make two new nuclei in the same parent cell. • Remember! Mitosis is the division of the nucleus/genome. The cell has physically not divided yet. That’s to come.

Telophase

Cytokinesis • Cytokinesis is not part of mitosis. – It refers to the division of the cytoplasm to create two new daughter cells. • In animal cells cytokinesis happens when the plasma membrane pinches until two cells are formed.

Homework • Your task will be to observe mitosis in a prepared slide of an onion root tip and find the structures that will allow you to identify each phase.