CELL DIVISION

CHAPTER 5

Significance of mitosis

Growth- allows a zygote to produce more cell in order to grow

Repair and replacement- allow the multicellular organism maintain its tissues, example skin cells and blood

Asexual reproduction- clone

Somatic cells have 2 sets of chromosomes: contains a diploid number of chromosomes (2n). 1 set inherited from

each parent

Human: have 23 pairs of chromosome. 2n = 46

Homologous chromosomes

Cell Cycle

Cell cycle - The period that extends from the time a new cell is produced until the time the cell completes a cell division.

Mitosis: nuclear division Cytokinesis: cytoplasmic

division

The cell cycle is divided into two major phases:

1. Interphase2. Mitosis

Cell grows

Cell prepares to divide

Interphase

The period between division, divided into 3 sub phases (G1, S and G2):

i.G1- cells grow rapidly and new organelle are synthesisii.S - synthesis of DNA and chromosomes are replicatediii.G2 - cells prepare for mitosis, synthesis protein and mitotic spindle begin to form

Mitosis

Mitosis is a division of the nucleus to produce two new daughter cells containing chromosomes identical to the parent cell.

Mitosis is a continuous process and divided into 4 main phases based on the appearance and behavior of the chromosomes.

1. Prophase, P2. Metaphase, M3. Anaphase, A4. Telophase, T

ProphaseEarly prophase Chromosome condense and appear

shorter and thicker and become visible in a light microscope

Each chromosome now consists of a pair of sister chromatids joined together at centromere.

Nucleolus disappears & nuclear membrane disintegrates.

Paired centrioles move to opposite ends of the cell

Late prophase Nuclear membrane disappears Spindle form

Centrioles

Centromere

Metaphase

Spindle fibres are fully form

Sister cromatids line up at the spindle equator/ metaphase plate.

Two sister chromatids are still attached to one another at the centromere.

At the end of metaphase, the centromers divide.

Anaphase

Anaphase begin with the separation of the centromers

The sister cromatids are drawn to opposite poles of the cell by shortening of spindle fibres.

Once separated, chromatids are referred to as daughter chromosome.

End anaphase: the poles of the cell have complete and equivalent sets of chromosome.

Telophase

Telophase begin when the two sets of daughter chromosomes have reached the two poles of the cell.

The spindle fibres disintegrate, the nuclear membrane forms around each set of daughter chromosomes and the nucleoli reappear.

The nucleolus also re-forms in each nucleus.

The chromosomes uncoil and become less visible under the light microscope.

Mitosis process is now completed.

Cytokinesis process of cytoplasmic division

to form two daughter cells.

Different in animals and plants. Animal cells, the cytoplasm

contracts to pull the plasma membrane inwards, forming groove called a cleavage furrow

In plant cells, cytokinesis in cells is markedly different.

A cleavage furrow does not form. Instead, membrane-enclosed vesicle gather at a plant cell’s equator between the two nuclei.

Vesicle fuse to form a cell plate

The importance of controlled mitosis

Controlled mitosis: the ability of the cell to divide at its own rate and time.

Crucial to normal growth, development and maintenance for the perpetuity of living things.

Eg: in plants controlled mitosis occurs at shoots results in growth of plants.

in human Malphigian layer of skin divides actively to replace dead and damaged cells.

The effects of uncontrolled mitosis in living things

Mutation is the change in the DNA structure of the cell.

This change in the DNA corrupts the coded genetic instructions for mitosis control.

This leads to uncontrolled mitosis, which is the non-stop division of cells, producing a mass of new daughter cells, called tumour.

Uncontrolled mitosis

Causes of Cancer

Genetic- some forms of cancer like prostate, colon, breast, skin, ovary are suspected to be inherited from the parents

Carcinogens- these are chemicals which affect genetic activity and cause cancer, e.g. of carcinogen a diesel exhaust, cigarette smoke, hair dyes, soot, arsenic, benzene and formaldehyde.

Radiation- excess exposure to x-ray, gamma-rays and ultra violet rays lead to increase cancer risk.

Viruses- some viruses (such as the HPV and HIV-1) cause cancer.

Application of Mitosis in Cloning

Mitosis is used to improve the quality, to produce new species and to ensure uniformity in the traits.

Cloning: process of artificially creating a new individual that is genetically identical to an existing individual.

Technique in application of mitosis in cloning tissue culture.

Application on mitosis in cloningPlant tissue

Skin tissue