Cell Continuity.

Cell cycle

• The changes that occur over the life of a cell.

• Involves growth, DNA synthesis and division.

• How long a cell lives depends on its structure and function.

• All cells go through the same.

Stages of Cell Cycle

1. Interphase.

2. Mitosis.

3. Cytokinesis.

Interphase

• A period of growth and preparation for the next division. DNA is copied during this interphase.

• Longest phase in cell cycle = 90% of a cells life is in interphase.

• Cells are very active in Interphase, produces new mitochondria, chloroplasts etc and chemicals needed for growth.

• The chromosomes duplicates itself.

Mitosis

A form of nuclear division in which one nucleus divides to form two nuclei, each containing identical sets of chromosomes, called daughter cells.

Cytokinesis

The division of the cell into two cells.

Cell Continuity

All cells develop from pre-existing cells.

3 Steps to form a new Cell

• Produce materials it will need.

• Grows larger.

• Reproduces to form a new cell.

Chromosomes

• Structures in nucleus, made of DNA & protein.

• Not dividing = chromatin (long thin threads).

• When dividing = chromatin forms a number of clearly distinguishable chromosomes → i.e chromatin to chromosomes.

• Each species has a definite no. of chromosomes, humans = 46 chromosomes → in somatic cells.

Haploid

• A haploid cell has one set of chromosomes, i.e has only one set of each type of chromosomes in the nucleus.

• Symbolized by letter ‘n’ and the number of chromosomes in the cell is given as n=23.

• In humans, eggs and sperm are haploid n=23.

Diploid

• Has 2 sets of chromosomes, i.e it has 2 of each type of chromosome in the nucleus.

• Chromosomes are in pairs in diploid cell, called homologous pairs.

• Symbolized as ‘2n’ and total no. of chromosomes in cell is given as 2n=46.

In diploid cells …

1 chromosome from the homologous pair comes from the mother and the other comes from the father.

Human karyotype

The number and visual appearance of the chromosomes in the cell nuclei of an organism or species.

2 Forms of Nuclear Division

1. Mitosis - 1→2.

2. Meiosis.

Key Point on Mitosis

• Mitosis occurs in all eukaryotic cells (i.e cells with membrane-bound nuclei).

• Mitosis is technically the division of a cell’s nucleus.

• Mitosis occurs in distinct stages (prophase, metaphase, anaphase and telophase) and results in the production of two daughter cells.

• These daughter cells are genetically identical - same chromosomes, DNA and genes.

• If a cell was originally diploid, then the resulting daughter cells will also be diploid.

• If the original cell was haploid, then the resulting daughter cells would be haploid.

STAGES OF MITOSIS

1. Interphase

2. Prophase

3. Metaphase

4. Anaphase

5. Telophase

Prophase

• At the end of the Interphase, chromosomes contract and become visible.

• Each chromosome appears as a duplicated strand.

• Spindle fibres appear in cytoplasm.

• Nuclear membrane and nucleolus starts to break down.

Metaphase

• Nuclear membrane broken down.

• Chromosomes thicken even more.

• Chromosomes line up across the centre of cell.

• Spindle fibres attach to chromosomes.

• Each chromosome has 2 fibres attached, 1 from each side of the cell.

Anaphase

• Fibres contract, chromosomes are pulled apart.

• Each strand is pulled to opposite ends of the cell.

• Hence, identical sets of genes are pulled to each end of the cell.

Telophase

• Nuclear membrane forms around each of the 2 sets of chromosomes.

• Chromosomes elongate within each nucleus.

CYTOKINESIS

• Animal

Cytokinesis occurs whereby the two cells split apart. Cytoplasm splits in two. In an animal cell the cell membranes constricts (a cleavage furrow is formed) to form 2 cells.

CYTOKINESIS

• Plant

In a plant cell vesicles migrate to the equator. These vesicles contain all the cellulose and other chemicals required to make the cell wall and cell membrane. The vesicles fuse to form a cell plate. The cell plate becomes the middle lamella and the cell walls and membrane are laid down on each side of it.

Functions of Mitosis

In Unicellular Organisms

Method of reproduction for Amoeba reproduction that does not involve the joining of 2 cells is called asexual reproduction.

Functions of Mitosis

In Multicellular Organisms

• Produces new cells, not new individuals.

• Responsible for growth & renewal and repair of cells.

Binary Fission

• The division of bacterial cells and is a form of asexual reproduction.

• Bacteria can complete binary fission in less that 20 minutes.

Cancer

The uncontrolled mitotic division of normal cells.

Benign tumours

Benign means kind, they are not life threatening and do not invade other tissues e.g warts, skin tags.

Maligant tumours

• Uncontrolled multiplication of abnormal cells.

• Invade other cells and move around the body.

• Movement of these cells is called Metastasis.

Causes of Cancer

• When normal genes are altered to form cancer causing genes called oncogenes.

• Brought about by cancer causing agents called carcinogens e.g cigarette smoke, asbestos fibres, ultraviolet radiation and some viruses.

• Most cancers can be cured by radiation, chemotherapy and surgery.

Other Causes of Cancer

Smoking cigarettes.

Being overweight or obese.

Exposure to harmful UV radiation from the sun or sunbeds.

Some viruses, including the human papiloma virus (HPV).

Treatment of Cancer

• Surgery: If the tumour is benign and confined to a specific tissue, then surgery may be an option.

• Chemotherapy: A cocktail of chemicals is administered to the patients, usually over a period of weeks, killing actively dividing cells.

• Radiotherapy: This involves exposing the tumor to high energy radiation.

• Bone marrow transplant: This transplant is a possible treatment for some blood cancers, including leukaemia.

Meiosis (only occurs in diploid cells)

A form of nuclear division in which the daughter nuclei contain half the chromosome number of the parent nucleus.

Role of Meiosis in Living Things

• Production of male and female gametes in animals (sperm and eggs).

• Production of the female gamete (ovum) in plants and of the pollen grain.

• Creating variation in the offspring after sexual reproduction (driving force behind evolution).

• Ensuring the diploid number is maintained after sexual reproduction.

Functions of Meiosis

2 functions in Multicellular Organisms

• Allow sexual reproduction without increasing the number of chromosomes in the offspring.

• Allows new combinations of genes.

What parts of the human would you expect meiosis to occur in?

Meiosis occurs in the ovaries and testes to produce gametes called eggs and sperm so there are 23 chromosomes in each egg and sperm.

Comparison: Mitosis

1. Chromosome number stays the same.

2. Occurs in somatic cells.

3. Two daughter cells are made.

4. Diploid cells produced, if mitosis occurs in a haploid cell, haploid cells are produced.

Comparison: Meiosis

1. Chromosome number halved

2. Reproductive cells.

3. 4 daughter cells.

4. Haploid cells produced.