Cell Division (Cancer (Cancer occurs when cells lose the ability
Cancer occurs when cells lose the ability
to control the rate and the number of times
mitosis takes place.
A tumour results when one or more cells lose aibility
to control the rate of mitosis.
Benign tumors are not life-threatening.
Mailignant tumours are called cancers.
Cancers are dangerous because :
-Cells never stop dividing.
-Invade other tissues.
-May spread from one body part
Caused by carcinogens eg cigarette smoke.
Treatment = Surgery, radiation,
Mitosis is a form of nuclear division in which one nucleus divides to form two, each containing the same number of chromosomes with identical genes.
Stages of Mitosis
-Period between cell divisions when cell is carrying out its function.
-Nuclear membrane starts to break down.
-Centrioles migrate north & south, send out spindle fibres.
-Cell organelles duplicate.
-Nuclear membrane broken down.
-Chromsomes thicken even more.
-Fibres attach to chromosomes.
-Each chromosome has 2 fibers attached, 1from each side of the cell.
-Fibres contract, Chromsomes pulled apart.
-Each strand is pulled to opposite end of cell.
-Hence, identical set of genes pulled to each end of cell.
-Nuclear membrane forms around each of the 2 sets of chromosomes.
-Chromosomes elongate within each nucleus.
-Cytoplasm splits (cytokinesis).
Functions of Mitosis
1)Reproduction in unicellular (single cell) organisms.
2)Growth & repair in multicellular organisms.
Haploid & Diploid cells
A haploid cell has one set of chromsomes.
Symbolised by letter n.
Only haploid cell in humans = sex cell/gamete.
Meiosis is a form of nuclear division in which the four daughter nuclei contain half the chromosome number of the parent nucleus.
Functions of Meoisis
-Meoisis halfs the chromosome number.
-Two haploid (n) cells/gametes join to form a
diploid (2n) zygote.
-Fertilisation restores chromosome number to 46.
2)New combinations of genes formed.
-Cells resulting from meoisis are not identical.
Chromatin = structure of DNA when not dividing.
Chromosome = single strand of DNA & is dividing.
Homologous chromosomes = chromosome pairs that carry genes to control the same characteristics.
A gene = section of DNA that contains the instructions for the formation of a protein.
Crossing Over = the passing of genetic
material during Meoisis that leads to
continous variation in gametes.