CELL DIVISION mitosis

•How many cells are you composed of?

•When an organism grows bigger do you get more cells or just bigger cells or both?

•When do your cells divide the fastest? Slowest?

•Do cells ever stop dividing?

•Are all cells capable of division and replacement?

Why does a cell divide?

-As a cell absorbs nutrients and gets larger, the volume of the cell increases faster than the surface area.

-Therefore, the demands of the cell (the volume) exceed the ability of the cell to bring in nutrients and export wastes. Solution? Divide into two smaller cells

When is cell division occurring?

GROWTH -increase number of cellsREPAIR -replace lost cells due to injury, diseaseCANCER – Abnormally high rates of cell division due to mutation

Different kinds of cells divide at different rates:Yeast cell – 2 hoursAmoeba – a few daysHuman embryo cell – 15-20 minutesHuman adult cell – 8 hours to 100 days

.

AgingAll cells die after a certain number of divisions (programmed cell death). At any given time some cells are dividing and some cells are dying.

Childhood Cell division > cell death

Adulthood Cell division = cell death

Aging Cell division < cell death

Cell Cycle = 90% Interphase and 10% Mitotic Phase

THE CELL CYCLE: 3 phases: Interphase-Mitosis-Cytokinesis

Interphase- 90% of the time.

   G1: Little new cell absorbs nutrients and grows larger. Does protein synthesis, its job.

   S phase: Synthesis of new DNA (DNA replication) for daughter cells in preparation for mitosis.

   G2: Cell continues to grow, does protein synthesis, its job. Gets too large, needs to divide.

How long is one cell cycle? Depends. Eg. Skin cells every 24 hours. Some bacteria every 2 hours. Some cells every 3 months. Nerve cells, never. Cancer cells very short.

Programmed cell death: Each cell type will only do so many cell cycles then die. (Apoptosis)

MITOSIS

Equal distribution of the 2 sets of DNA amongst the 2 daughter cells.

4 Stages:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

As cell enters mitosis from interphase it has 2 complete sets of chromosomes because of replication in the S phase. Each set must be re-arranged and distributed into the 2 new daughter nuclei. This is mitosis.

 

-Chromatin condenses (coils) into chromosomes. Sister chromatids joined by centromere.

-Nuclear membrane dissolves.

-Centrioles divide and move to opposite poles forming spindle between them.

Prophase

Metaphase

-Sister chromatids line up on metaphase plate.

-Centromeres lock on to spindle fibre

Anaphase

-Centromeres divide.

-Spindle fibres contract pulling sister chromatids apart to poles.

-New nuclear membranes form around new nuclei

Telophase:

CYTOKINESISCytoplasm splits into 2 cells.

-Animal cells: Cleavage furrow forms from outside in.

-Plant cells: Division plate forms from inside out.

 

Cell now returns to interphase . The chromosomes uncoil back into chromatin. The whole cell cycle starts over again…..

At any point in time the cells in a tissue will be at different stages in the cell cycle.

The Guarantee of Mitosis…

Why is this so important?

The 2 daughter cells formed are identical to each other and identical to the mother cell.

In Mitosis, each daughter cell is exactly the same as the original mother cell.

2n = 46

4n = 92

2n = 46

Bacteria Reproduce via Binary Fission

Mitosis is also an ASEXUAL form of reproduction. These are other examples of the uses of mitosis to create new organisms asexually:

Budding of Yeast

Propogation of plants by cuttings

Runners from plants like strawberries

Homologous pairs of chromosomes:

Each chromosome has a certain gene sequence on it. Eg. Chromosome #! Has insulin, foot size, and lactase on it. You have a chromosome one from your mom and one from your dad. So you have 2 genes for each trait. One from your mom – one from your dad. A homologous pair is a pair with the same gene sequence – one from mom, one from dad.