MITOSIS and the CELL CYCLE

Cell Cycle vs. Mitosis

• Cell cycle= life cycle of a cell; growth, division, etc.

• Mitosis= one step of the cell cycle; division.

Why do our Cells Need to Divide?

• 1. cell survival- remember surface area to volume ratio determines how quickly nutrients are able to get through the cell.

• 2. growth- for an organism to grow we need new cells!

• 3. repair- dead or damaged cells need to be replaced

• 4. replenishment- some cells get used up when doing their job, such as red blood cells, skin cells, intestinal lining cells, etc.

How do our Cells Know when to Divide?

• Proteins!– Specifically hormones which send messages to

receptor proteins on our cells which tell them that it is time to begin to divide.

The Cell Cycle

• The cell cycle is the fancy name for a cell’s life cycle.

• It includes four (4) phases, one (1) of which is mitosis.

• Cells will undergo this same cycle over and over again.

Key terms• Chromosome- structure made of

highly coiled or condensed DNA.• Chromatin – the uncoiled form of

DNA.• Sister chromatids – two identical

sections (copies) of DNA joined together at the centromere.

• Pictures on next slide

Often confused terms• 1. Single strand of

chromatin (DNA’s normal structure in the nucleus)

• 2. Two sister chromatids joined at centromere (After replication)

• 3. Replicated or double stranded chromosome after condensation. Each half is still considered a sister chromatid (During the cell cycle) Gap1 Synthesis Prophase

1 2 3

What is the cell cycle?• All cells pass through a series of

events called the cell cycle.• Some cells cycle continually, other

cell types only a limited number of times before they die.

Cell CycleOnly one hour out of 22 is spent in active division (Mitosis)Depending upon the type of cell undergoing the cycle.

Relationship of Stages• Interphase

consists of:• Gap 1 (G1)• Synthesis (S)• Gap 2 (G2)

• Mitosis consists of:

• Prophase• Metaphase• Anaphase• Telophase

The Four Stages of Cell Cycle

• 1. G1 phase – this phase has a main focus of cell growth. – Near the end of G1 the cell is checked to see that it is

large enough to survive its environment. • 2. S phase – this phase has a main focus of DNA

replication (synthesis).• 3. G2 phase – this phase prepares the cell for

mitosis. – At the end of G2

the cell is checked to ensure that the DNA has been copied properly before proceeding w/ mitosis.

Interphase

• 1. Interphase is made up of G1, S, and G2

– To the naked eye it appears nothing is happening, but that isn’t true!

• 2. DNA is in chromatin form (46 pieces)• 3. Nucleolus is visible

– The nucleolus is part of the nucleus that is involved in making ribosomes.

• 4. The cell is performing normal cellular functions.

Interphase

• 5. DNA replicates later in this phase (during the S portion)

• 6. After replication, DNA remains in chromatin form, but made up of two chromatids (still 46 total pieces of chromatin, but double the amount of actual DNA).

Mitosis• Mitosis is the 4th and final stage of the cell

cycle.

• It, itself is divided into 5 steps (PMAT):– Prophase– Metaphase– Anaphase– Telophase– Cytokinesis

Prophase• Prophase is the first step of mitosis (cell division)…• Early prophase is the first part of mitosis:

– 1. DNA starts to condense from chromatin to chromosomes (so it can move!)

– 2. Nuclear membrane begins to disappear– DNA may still appear to be in nuclear shape, though.

– 3. Nucleolus may or may not be there, but the cell is slowly breaking it down.

– 4. centrioles begin to move to the opposite ends of the cell.• Centrioles are organelles that are found in animal cells that help

to move the “spindle-fibers” and control the movement of chromosomes and nuclear material.

Prophase

• By the time Prophase is nearing the middle to end of its time…– 1. DNA is totally in chromosome form.– 2. The nuclear membrane is complete gone –

chromosomes are no longer contained and look more scattered.

– 3. Nucleoli are totally gone (no need for ribosomes)– 4. Centrioles have moved to the opposite ends of the cell

and begin producing spindle fibers.– 5. DNA is in chromosome form, made of 2 chromatids

(46 chromosomes, 92 chromatids)

Metaphase• Metaphase is the second stage of mitosis• During early metaphase…

– 1. spindle fibers reach out from the centrioles and connect to the centromeres of the chromosomes to begin moving them toward the center of the cell.

• Near the end of metaphase…– 1. Chromosomes are lined up in the middle of the

cell by the spindle fibers.– 2. At the end of the phase, DNA is still in

chromosome form and made of 2 chromatids (46 chromosomes, 92 chromatids).

Anaphase• Anaphase is the 3rd stage of mitosis• During early anaphase…

– 1. Chromosomes are pulled apart at the centromeres causing the chromatids to be separated.

– 2. At the end of this phase, DNA is still in chromosome form, but it is no longer made of 2 chromatids, instead each chromatid is now an individual chromosome (92 chromosomes, 0 chromatids)

Late Anaphase/Early Telophase

• Before telophase (the 4th stage of mitosis) really gets going…– 1. Chromosomes are clearly pulled apart from

each other, heading towards opposite ends of the cell.

– 2. At this point in time we have 92 chromosomes, 46 going in one direction, 46 in the other.

Telophase• The 4th stage of mitosis• During telophase…

– 1. Chromosomes have reached the opposite ends of the cell

– 2. A nuclear membrane has started to form around the groups of DNA (creating a new nucleus for each new cell)

– 3. The cell begins to pinch (animal cell) or a cell plate begins to form.

• In animals this is called a cleavage furrow

– 4. DNA is starting to uncoil back into chromatin for use by the new cells.

– 5. Nucleoli are being formed

Cytokinesis

• Occurs at the same time as Telophase– 1. Cell completely pinches (animal cell) and

becomes 2 new cells. OR Cell plate completely divides the cell in half (plant cell) to form a new cell wall, thus dividing the cell into 2 new cells.

DNA has uncoiled back into 46 pieces of chromatin. There would be 46 pieces in each new cell (just like we started with!)

Finished Product

• The entire process is one cell dividing into 2 cells.

• The two new cells are genetically identical to each other and the original cell (which is now gone).