5.1 What is cancer?
Cancer cells keep on dividing, even if surrounded by other cells. Cell division means that one cell divides into two daughter cells.
In cancer, cells divide out of control.
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. Fig 5.1 So cancer cells are different from regular cells because they: - Divide too rapidly.
- Invade surrounding tissues.
- Move to other parts of the body.
5.2 Cell division
Cell division is needed for growth, asexual reproduction (where one parent produces identical clones) and to replace dead cells. Before cells divide they must copy their original DNA. The DNA is organized into chromosomes Fig 5.5. Most of the DNA is in a long chromatid, and the chromosome is held together by a centromere.
During DNA replication the two strands of DNA separate, and an enzyme called DNA polymerase adds new nucleotides to each existing strand, using the rule that Adenine is always opposite Thymine and Guanine is opposite Cytosine. Fig 5.7. Replicated chromosomes have two chromatids. Fig 5.8.
5.3 Cell cycle
Interphase ( where the cell grows and DNA replicates Fig 5.9. ) is followed by Mitosis ( where the cell divides into two ).
Stages of Mitosis Fig 5.10.
Prophase : the spindle ( made of microtubules ) forms.
The chromosomes are visible scattered at random.
Metaphase : chromosomes line up in the center of the cell.
Mitosis stages
Anaphase : centromeres divide.
Chromosomes move to opposite ends of the cell ( V shape ).
Telophase : two nuclei form.
Cytoplasm divides ( cytokinesis ). Fig 5.11.
Animal cells divide from the outside towards the center, plant cells divide from the center outwards.
5.4 Cancer
Normal cells will only divide if the correct growth factor is present in the blood. The cell cycle is controlled by genes called proto-oncogenes (literally "before cancer" genes). These are normal genes that allow a cell to divide. A mutation, or change in the gene, can convert it to an oncogene which causes cancer. You have some protection against cancer. 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 cells need more oxygen and nutrients than regular cells, because they are growing so fast. So they release chemicals that cause angiogenesis: new blood vessels grow towards the cancer. Cancer cells often produce an enzyme telomerase which makes the cells immortal.
5.5 Cancer treatment
Cancer is often detected using biopsy, where are small sample of cells are removed to check for cancer. Mammograms can detect breast cancer by highlighting any dense areas (tumors) in the breast.
Cancer treatment
1) Surgery : removes cancer cells. This works best for benign tumors.
2) Chemotherapy : chemicals that kill dividing cells.This is 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. This is 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.
5.6 Reproduction and Meiosis
Diploid cells - have two sets of chromosomes ( 2n ). Fig 5.23.
- examples skin, stomach, liver.
- in humans, diploid cells have 46 chromosomes.
Haploid cells - have one set of chromosomes ( n ).
- examples sperm and egg.
- in humans, haploid cells have 23 chromosomes.
Comparison of Mitosis and Meiosis Fig 5.28.
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 Chromosome number halved in Anaphase I
Produces diploid cells Produces haploid cells
eg skin, blood, heart etc eg sperm or egg