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
Interphase (where the cell grows and DNA replicates) is followed by Mitosis (where the cell divides into two).
Stages of Mitosis
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 the outside by forming a cell plate. In animals, the cell membrane pushes inwards from the outer edge making a cleavage furrow, 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. Everyone has some of these tumor-suppressor genes (for example gene p53 ). Most cancers have mutations in these tumor-suppressor genes. As you get older the chance of mutations increases, so cancer is much more common in older people. Protective genes
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.
1) Environment :
2) Genetics : some cancers have a high genetic risk.
High risk : Prostate, Colon, Breast, Skin, Ovary.
Low risk : Lung, Pancreas, Testicles, Uterus.
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.
|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|
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
Asexual reproduction - offspring are identical (clones)
- only 1 parent ; cells divide by mitosis.
- 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.
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
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 August 2014, by David Byres, David.Byres@fscj.edu