Chapter 10 : Mitosis & Cancer

Mitosis

Cell division is needed for growth, reproduction and to replace dead cells. Mitosis : a type of cell division that leaves two identical copies of a cell. The chromosome number is unchanged.

Most cells divide by mitosis eg skin, blood, kidney. Nerve and brain cells rarely divide after childhood, so nerve and brain damage is usually permanent. Research

Cell cycle Fig 10-5.
Interphase ( where the cell grows and DNA replicates ) is followed by Mitosis ( where the cell divides into two ).

Stages of Mitosis Fig 10-6.
Prophase : the spindle ( made of microtubules ) forms. The chromosomes are visible scattered at random.

Metaphase : chromosomes line up in the center of the cell.

Anaphase : centromeres divide.The chromosomes move to opposite ends of the cell ( V shape ).

Telophase : two nuclei form. The cytoplasm divides ( cytokinesis ). In plants, the cytoplasm separates from the center of the cell towards to outside. In animals, the cell membrane pushes inwards from the outer edge, so the center is the last part that divides.

Control of cell division.
Normal cells only divide if : a) the correct growth factor is present.
b) each cell is not completely surrounded by other cells.

Proteins inside the cell control the cell cycle. Cyclins in the nucleus control the replication of DNA. The 2001 Nobel Prize was awarded to the people who discovered cyclins.
Protein kinases switch on proto-onco genes that start mitosis. A mutation in a proto-onco gene can turn it into an oncogene that causes cancer ( uncontrolled cell division).

Tumor-suppressor genes prevent mitosis, and even make abnormal cells kill themselves. Most cancers have mutations in these tumor-suppressor genes (eg gene p53 ). Protective genes

Cancer

Cancer cells keep on dividing, even if surrounded by other cells. A benign tumor is limited to one site and cannot spread. In a malignant tumor cancer cells have spread around the body in the blood. Metastasis means that the cancer has spread.

Causes of Cancer

1) Environment :

Carcinogens ( chemicals which cause cancer ). Diet
Radiation.
Ultra-violet ( UV ) light causes skin cancer.
Virus (the virus HPV causes cancer of the cervix)

2) Genetics : some cancers have a high genetic risk.
High risk : Prostate, Colon, Breast, Skin, Ovary.
Low risk : Lung, Pancreas, Testicles, Uterus.

Cancer treatment

1) Surgery : removes cancer cells. This is best for benign tumors.

2) Chemotherapy : chemicals that kill dividing cells. Standard for malignant tumors. Problem: current chemotherapy causes many side effects, because the chemicals kill all cells that happen to be dividing: cancer cells and regular cells like skin, blood and hair cells. So your hair falls out, you feel nausea etc. In 2001 a new type of chemotherapy was approved, that targets only cancer cells: Cancer pill

3) Radiation : kills all the cells in one spot. Standard for malignant tumors.
Problem: the radiation kills all cells in the area, whether they are cancerous or not, so causes side effects. Also radiation can itself cause new cancer to start.

Cancer in the US.

Type of Cancer	Annual deaths	5 year survival	Early detection
Prostate	39,000	93 %	Exam: age 40+ Blood: age 50+
Breast	44,000	85%	Self exam : 20+ Mammogram : 40+
Colon	57,000	62%	Exam : 40+ Blood : 50+
Lung	160,000	14%	No test yet

Cancer research

Most research is done using human cells grown in the lab called HeLa cells. These cells came from Henrietta Lacks. Cancer cells have telomerase : an enzyme that repairs the telomeres on the ends of the chromosomes.

Cancer researchers are developing:
a) inhibitors against telomerase.
b) inhibitors that stop cancer cells from producing new blood vessels.
c) medications that boost the immune system. Cancer drug

Reproduction and Meiosis

Asexual reproduction - offspring are identical ( clones )
- only 1 parent ; cells divide by mitosis. Fig 9-9.
- examples : bacteria, banana.

Sexual reproduction - offspring vary.
- 2 parents ; sex cells have one set of chromosomes, compared to two sets in a normal cell.
- gametes ( sex cells ) are produced by meiosis.

Diploid cells - have two sets of chromosomes ( 2n ).
- examples skin, stomach, liver. These regular body cells are also called somatic cells.
- in humans, diploid cells have 46 chromosomes.

Haploid cells - have one set of chromosomes ( n ).
- examples sperm and egg (called the gametes or sex cells).
- in humans, haploid cells have 23 chromosomes.

Comparison of Mitosis and Meiosis Fig 10-16.

Animation (Click on Mitosis vs Meiosis).

Mitosis                        Meiosis

One cell division            Two cell divisions

One cell produces 2 cells    One cell produces 4 cells	

No pairing                   Chromosomes pair up in Prophase I		

No crossing over             Crossing over occurs

Chromosome number unchanged  Chromsome number halved in Anaphase I		

Produces diploid cells       Produces haploid cells
eg skin, blood, heart etc    eg sperm or egg

Sources of Genetic Variation

A) Independent assortment
Picking 23 chromosomes from 46 in humans.
In a species with 3 chromosomes in gametes it gives 8 genetic combinations.
This gives roughly 8 million combinations in humans.

B) Crossing over
Changes the combinations of genes that are inherited. May give thousands of different combinations (exact number is unknown).

C) Random fertilization
The sperm that fertilizes the egg is chosen at random.
In a species with 3 genetically different gametes it gives 9 combinations.
In humans (over 8 million different gametes) it gives at least 70 trillion (70,000,000,000,000) combinations. World population

Last edited November 2009, by David Byres, dbyres@fscj.edu