Task (1): Each one of you, please think of FIVE (5) questions you would like to know about cell division. You’re free to be WILD in your question
Task (2): Each group please compose all question and make it into a group list
Task (3) Presentation
Task (4) Which question that you think is very creative?
Cells in living things do not last forever, for they… Wear out after some time Get damaged (through cuts, by ultraviolet radiation or by hazardous environmental pollutants) Grow old naturally and die
Importance of new cells produced are genetically identical to their parent cells: Continue with the specific cell functions of their parent cells within a particular tissue Avoid disrupting the stable internal environment of life or its processes Produce offspring that have the complete functions of an adult organism (in asexual reproduction) to ensure the survival of that species
Significance of mitosis Nucleus contains chromosomes. Each chromosome consists of a long DNA molecule which carries genes in a linear sequence Gene determines the individual characteristics of an organism
Significance of mitosis The characteristic number of chromosomes is referred to as the chromosomal number of the species Exp: Onion cell – 16 chromosomes Exp: Fruit fly - 8 chromosomes
Human genetics = 46 chromosome (2n)23 pairs of chromosome
Significance of mitosis Somatic cells have two sets of chromosomes, one set inherited from each parent. Each cell contains a diploid number of chromosomes (2n) In humans, each set consist of 23 chromosomes. Typical human somatic cell, 46 chromosomes arranged in 23 pairs or 2n = 46
Significance of mitosis The two chromosomes in each pair have the same structural features and are referred to as homologous chromosomes
Significance of mitosis Gametes contain only one set of unpaired chromosomes, or haploid number of chromosomes (n)
Uncontrolled Mitosis in Living Things Cancer Cancerous cell - tumour
Application of mitosis Cloning
Application of mitosis Tissue culture
Human genetics = 46 chromosome (2n)23 pairs of chromosome
Stages of Meiosis Meiosis I Prophase I Metaphase I Anaphase I Telophase I
Prophase I The homologous chromosomes come together to form bivalentsthrough synapsis process
Prophase I Each bivalent is visible under the microscope as a four-part structure called a tetrad
Prophase I A tetrad consists of two homologous chromosomes, each made up of two sister chromatids
Prophase I Non-sister chromatids exchange segments of DNA in a process known as crossing over. This results a new combination of genes on a chromosome
Prophase I The points a at which segments of chromatids cross over are called chiasmata.
Metaphase I Chromosomes are lined up side by side as tetrads on the metaphase plate. The chromosomes are still in homologous pairs
Metaphase I One chromosome of each pair is attached to the spindle fibre from one pole while its homologue is attached to the fibre from the opposite pole
Metaphase I The centromere does not divide
Anaphase I The spindle fibres pull the homologous chromosomes away from one another and move them to the opposite poles of the cell. Each chromosome still consist of two sister chromatids which move as a single unit.
Anaphase I Although the cell started with four chromosomes, only two chromosomes (each with two sister chromatids) move towards each pole.
Telophase I The chromosomes arrive at the poles. Each pole now has a haploid nucleus because it contains only one set of chromosomes
Telophase I The spindle fibres disappear. The nuclear membrane reappears to surround each set of chromosomes. The nucleolus then reappears in each nucleus
Prophase II The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re-form in each daughter cell
Metaphase II The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.
Metaphase II Each sister chromatid is attached to the spindle fibres at the centromere
Anaphase II The centromeres of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes. The chromosomes move towards the opposite poles of the cell.
Telophase II Finally, the nucleoli and nuclear membranes re-form. The spindle fibres break down. Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.
Telophase II These haploid cells will develop into gametes.
Significance of Meiosis Crossing over in bivalent
Produces new combinations of genes in both chromosomes
Significance of Meiosis (2) Reduction and fusion of gametes
Meiosis produces haploid gametes
In sexual reproduction, a male gamete fertilizes a haploid female gamete to produce a normal diploid zygote
Significance in Mitosis (3) Independent (random assortment)
During metaphase I, homologous pairs of chromosomes align at the equator
It is by chance which “way round” each pair lies, before these homologous pairs of chromosomes separate into two different daughter cells.
Appreciating the movement of chromosomes during mitosis and meiosis
Mutation Mutation is a change in structure, arrangement or quantity of the DNA in the chromosome May be caused by: Mistakes in the replication of DNA Damage to the DNA by radioactive and carcinogenic substance Disruption to the orderly movement of chromosomes during cell division
In Mitosis If the functions of these genes are disrupted due to mutation, cancers may form. Somatic mutations are not transmitted to the offspring, but may cause body cells to malfunction Cancers are caused by somatic mutation
In Meiosis Meiosis involves an orderly movement and reduction (in meiosis I) of a diploid cell to two haploid cells that subsequently divide (in meiosis II) to form four haploid gametes Since these are gametes, so any mistakes – caused by disorderly movement of chromosomes during meiosis --- are inherited by the offspring.
Example: non-disjunction or improper segregation (separation) of chromosome During anaphase I, certain homologous chromosomes fail to segregate, resulting in the production of gametes with either an extra chromosome (n+1) or a missing chromosome (n-1) If this abnormal gametes unites with a normal gamete, an abnormal zygote will be produced.
Down’s syndrome or mongolism 3 copies of chromosomes number 21, instead of the normal 2 chromosomes This means a down syndrome patient has (2n+1 = 47) 47 chromosomes instead of the normal (2n=46) chromosomes