CELL DIVISIONCHAPTER 5

5.1Understanding the Mitosis5.2Understanding the Meiosis5.3The movement of chromosomes during mitosis and meiosis

5.1MITOSISCell division helps the body to produce new cells in order to replace cells that are worn out and damaged.The continuous production of new cell for the growth through a process called mitosis.

Mitosis involves nuclear division to produce two daughter cell, each containing the same number and genetically identical chromosomes as the parent cell.Mitosis occurs in all somatic cells ( all body cell except the reproductive cell).

Significant of MitosisThe number of chromosomes present in each cell is constant for the species. Example , a human cell has 46 chromosomes.

Somatic cell contain two set of chromosomes, one set inherited from each parent. These cells contain a diploid number of chromosomes (2n). For human (2n)=46.

Chromosomes exist in pairs. The chromosomes in each pair have the same structural features and are referred to as the homologous chromosomes. Each member of the pair is called a homologue.Gametes contain only one set of chromosomes or one of each pair of chromosomes and are said to contain haploid number of chromosomes.Mitosis ensure that the new cell are genetically identical to the chromosomal number of the species.

The cell cycle

The cell cycleIt is the period that each cell undergoes from the time the cell is produced until the cell completes cell division.The cell cycle can be divide into the following phase:G1 phaseS phaseG2 phaseM phase (Mitosis cell division)

Interphase

InterphaseInterphase is the stage for cells to grow larger and prepare for cell division.Interphase is divided into three stages:G1 phase (gap or growth phase 1)The cell growth by producing proteins and new cytoplasmic organelles.The chromosomes appear as thread-like structures called chromatin.S phase (DNA synthesis)DNA is synthesised at this stage and undergoes the replication process.

Each duplicate DNA contains two identical sister chromatids joined together by centromeres.

G2 phase (gap or growth phase 2)The cell continues to grow.The cell prepares for cell division.

The M phaseCell division takes place in the M phase.Consists of two parts:MitosisCytokinesisThe nucleus divides during mitosis followed by division of cytoplasm called cytoinesis.Mitosis is divided into:ProphaseMetaphaseAnaphasetelophase

Prophase The chromosomes condense, that is the shorten and thicken and finally becomes visible. Each chromosomes consist of sister chromatids attached at a point called the centromere. The spindle fibres begin to form and extend between the centrioles.The nucleolus disappear, the nuclear membrane breaks down/disintegrates.centrioles migrate to opposite poles of the cell.Centrioles are absent in plant cells.

MetaphaseThe spindle fibres are fully formed.All chromosomes are arranged with their centromers along the equator of the spindle.Two sister chromatids are still attached at the centromers.

Anaphase Anaphase begins with the separation of the centromers.The sister chromatids are drawn to opposite poles of the cell by contraction of the spindle fibres.Once the sister chromatids are separated they are referred to as daughter chromosomes.Anaphase ends when the chromosomes reach the two opposite poles of the cell.

Telophase Telophase begins when the two sets of daughter chromosomes have reached the two poles of the cell.The chromosomes appear as thread-like structure (chromatin) and are not visible.Spindle fibres disappear A new nuclear membrane formsThe nucleolus reforms

CytokinesisCytokinesis is the process of cytoplasmic division to form two daughter cells.Cytokinesis usually begins before nuclear division is completed.In animal cell, cytokinesis formA groove called a cleavage furrow.

Cleavage furrow

In plant cell cells, cytokinesis start with the formation of cell plate at the equator of the cell.The vesicles fuse, producing the cell wall.The cell plate extends outwards to the existing cell wall, and separate the two daughter cells.

The Importance of mitosisTo replace dead cell.To repair or regenerate damaged cells.To increase the number of cells thus allowing growth and development in living organisms.The basic of asexual reproduction in unicellular organisms.

Uncontrolled mitosis in living thingsWhat happen if the cells fail to divide or begin to divide at the wrong time or even divide uncontrollably?Cancer is a genetic disease caused by uncontrollably mitosis due to severe disruption to mechanism that control the cell cycle.A tumor is a lump or growth of tissue made up from abnormal cells.

Two type of tumorBenign tumorMalignant tumor

SPM 2005, PAPER 2 QUESTION 2

5.2Meiosis

5.2MeiosisProcess of nuclear division to produce four daughter cells, each containing half the number of chromosomes of the parent cell.The daughter cell recieves one set chromosomes from each pair of homologous chromosomes and is called a haploid cell.Gamates that are produces form meiosis are haploid (n).Parent cell that carries out meiosis is diploid (2n).

The genetic materials in the gamates differs from the parent cell and from each other.In animal and human meiosis occurs in Testes in males to produce spermsOvaries in females to produce ovumIn plant meiosis occurs inAnthers of flowers to produce male gamates in pollenOvaries of flowers - to produce egg cells in ovules

The significant of meiosisTo allow traits inheritances in offspring.To maintain diploid number in each generation.To ensure the production of haploid gamates in sexual reproduction .To produce genetic variation among offspring.

The process of MeiosisMeiosis involves two nuclei division:Meiosis I (the first meiotic division)Meiosis II (the second meiotic division)

Meiosis IMeiosis IIProphase IProphase IIMetaphase IMetaphase IIAnaphase IAnaphase IITelophase ITelophase II

The Stages of Meiosis IInterphase (same like Mitosis)

Prophase IChromosomes condense become shorter, thicker and visible.Each chromosomes consist two sister chromatids.Homologous chromosomes pair up through a process called sinapsis.Each pair of homologous chromosomes consisting of four chromatids is called a bivalent or a tetrad.The non-sister chromatidsof homologous chromosomes exchanges segments of DNAin a process called crossing over.

The points where the chromatids cross over are called Chiasmata. Crossing over results in new combination of genes and is an important sources of variation.Nucleolus and nuclear membrane dissapper.The two pairs of centrioles which spindle fibre radiate move to opposite poles.

Metaphase Ithe chromosomes are line up side by side as tetrads on the metaphase plate/ homologous chromosomes line up at the equator of the cell.Chromosomes of each pair is attached to the spindle fibre.Centromers does notdivide

Anaphase IThe chromosomes of each homologous pair move to opposite poles of the cell.Each chromosomes still consist of two sister chromatids which move as a single unit.The spindle fibre pull the homologous chromosomes apart.

Telophase IThe chromosomes fibres disappear arrive at opposite poles of the cell.Spindle fibre disappear Nuclear membrane reformsNucleolus reappear Cytokinesis occurs to produce two haploid daughter cell because the nucleus contain only one set of chromosomes.

Meiosis II follows immediately after cytokinesisNo Interphase (DNA replication does not occur again and the chromosomes remain in a condense state)

Meiosis II results in the separation of sister chromatids

Prophase IIThe nuclear membrane of the daughter cells disintegrate again.The spindle fibre reform in each daughter cell.

Metaphase IIThe chromosomes, each made up of sister chromatids, line up at the equator.The sister chromatids of each chromosomes face opposite poles.Each sister chromatid is attached to the spindle fibre at the centromers.

Anaphase IIThe centromers separate.The chromatids separate and move face opposite of the cells.

Telophase IINucleoli and Nuclear membrane reform.Spindle fibre break down.Cytokinesis occur and produce four daughter cell.Each with half number of chromosomes as the parent cell.

Similarities and Differences between Mitosis and Meiosis I and Meiosis II

AspectMitosisMeiosis IMeiosis IIPlace it occurSomatic cellReproductive organsSynapsis of homologous chromosomesDoes not occurOccurs during prophase INo synapsisCrossing over between non-sister chromatidsDoes not occurOccur during Prophase INo crossing overNumber of divisionOneOne & TwoNumber of daughter cells224Number of chromosomes Diploid (2n)Haploid (n)Haploid (n)

AspectMitosisMeiosis IMeiosis IICytokinesisOccur onceTwice Occur onceOccur onceGenetic content of daughter cell All daughter cell are genetically same as parent cellAll daughter cell are genetically different as parent cell

AnaphaseSister chromatids split and move towards opposite poles I- Homologous chromosomes separate and move towards opposite polesII- sister chromatids split and move towards opposite polesMetaphase Sister chromatidsLine up at the equatorial plateI- Homologous chromosomes Line up at the equatorial plateII- sister chromatids Line up at the equatorial plate

5.3Appreciating the movement of Chromosomes during Mitosis and Meiosis

Mutation A sudden changes in structure, arrangement or number of DNA in the chromosome.

Downs syndromeHas one extra chromosomes at chromosomes 21.Pairs of chromosomes do not separate properly during meiosis which produce an ovum containing 24 chromosomes.Weak body resistancesMental retardedTypical facialSlanting eyesShorter stature Prone to diseaseShorter life.

Turners syndromeGenetic disorder of women caused by the absences of the second sex chromosomes.22+ XOLack of ovaries and menstrual cycleWomen sterile Lack secondary sexualcharacteristicShort

Klinefelters syndromeGenetic disorder affecting men Extra X chromosomes so their sex chromosomes are XXYUnderdeveloped testesInfertilityBreast enlargement