form 4 biology chap5 pt2

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  • 1. LEARNING OUTCOMES• To state the necessity of trait inheritance in offspring for continuation of life,• To state the necessity to maintain diploid chromosomal number from generation to generation,• To state the necessity for production of haploid gametes in sexual reproduction,• To state the significance of meiosis,• To identify the type of cell that undergoes meiosis,• To explain the process of meiosis,• To arrange the various stages of meiosis in the correct order,• To compare and contrast meiosis I & meiosis II,• To compare and contrast meiosis & mitosis.
  • 2. M E IO S IS• A division of the nucleus to produce 4 daughter cells each containing half the chromosome number of the parent nucleus.• A type of cell division - occurs in reproduction organs to produce 4 daughter cells called gametes.• The process of nuclear division that reduces the number of chromosomes in new cells to half the number of chromosomes in the parent cell.• Each daughter cell receives 1 set of chromosome from each pair of homologous chromosomes – haploid cell (n)
  • 3. MEIOSIS• Genetic material in the gametes differs from the parent cell & each other• Animals : testes in males – sperm ovaries in females – ovum• Plants : anthers – pollen (male gametes) ovaries – ovules (egg cells)• Consists of Meiosis I and Meiosis II
  • 4. THE SIGNIFICANCE OF MEIOSIS• To allow trait inheritance in offspring – the transmission of traits• To maintain diploid number in each generation - the organisms are always diploid• To ensure the production of haploid gametes in sexual reproduction• To produce genetic variation among offsprings
  • 5. M E IO S IS• Meiosis I : the homologous chromosomes are separated into two haploid daughter cells• Meiosis II : the sister chromatids of each daughter cell are separated.• Meiosis II = mitosis in a haploid cell• All chromosomes are replicated during interphase  sister chromatids joined at centromere. The chromosomes do not replicate again between Meiosis I & Meiosis II
  • 6. MEIOSIS I PROPHASE I• Centrioles move to opposite poles.• Chromosomes thicken & condense• The homologous chromosomes form bivalent through synapsis• Crossing over happened : non-sister chromatids exchange segments of DNA  new combination of genes on a chromosome• Chiasmata – the points at which segments of chromatids cross over• Nucleus membrane & nucleolus disappear• Spindle fibres are formed
  • 7. M E TA P H A S E I• The paired chromosomes are lined up at the equator of the cell• One chromosome of each pair faces each pole of the cell• the chromosomes attached to the spindle fibres at their centromere• The centromere does not divide
  • 8. ANAPHASE I• The paired chromosomes separate from one another & move to opposite poles• The spindle fibres pulled one chromosome of each pair to each pole
  • 9. TE L O P H A S E I• The chromosomes arrive at the poles.• Each pole has a haploid daughter nucleus (contain one set)• The spindle fibres disappear, the nuclear membrane & nucleolus reappears in each nucleus.• Cytokinesis occurs• No interphase, no replication of chromosomes
  • 10. MEIOSIS II PROPHASE II• Centriole duplicates & move to opposite pole• Nuclear membrane disappear, spindle fibres re- form• The chromosomes move to the equator of the spindle.
  • 11. M E T A P H A S E II• Chromosomes lined up at the equator of the cell facing opposite poles• Each sister chromatid is attached to the spindle fibres at the centromere
  • 12. ANAPHASE II• The sister chromatids together split• Chromatids separate• Spindle fibres pull each chromatid to opposite poles• Centromere are divided
  • 13. T E L O P H A S E II• Chromatids reach the poles & become new chromosomes• Nuclear membrane & nucleolus form again at each chromosome• Chromosomes become extended & not visible• Cytokinesis occurs & 4 haploid daughter cell are formed.
  • 14. T H E C O M P A R IS O N B E T W E E N M E IO S IS I & M E IO S IS II MEIOSIS I SIMILARITIES MEIOSIS II •Consist of 4 stages : P,M,A,T •Involve division of nucleus & cytokinesis DIFFERENCES Occur Synapsis Not occur Yes Cross over No MI-paired homologous Metaphase MII – each chromosomechromosomes line up at with sister chromatids the equator line up at the equatorAI – paired homologous Anaphase AII – the sisterchromosomes separate chromatids separate & & move to opposite move to opposite poles poles 2 haploid cells are At the end 4 haploid cells are formed formed
  • 15. T H E C O M P A R IS O N B E T W E E N M IT O S IS & M E IO S IS MEIOSIS SIMILARITIES MITOSIS •Division of cells •The chromosomes replicates only once DIFFERENCES In reproductive organ Place occur In somatic cell Parent cell divides twice Number of divisions Parent cell divides once Four haploid daughter cells Number of daughter cells Two diploid daughter cells Occurs during prophase I Synapsis of homologous Not occurs chromosomes Occurs twice Number of cytokinesis Occurs once Occurs during prophase I Crossing over of chromatids of Not occurs homologous chromosomesHalf of number of chromosomes Number of chromosomes in Genetically identical to the parent of the parent cell (haploid) daughter cells cellGenetically non-identical to the Genetic composition of Genetically identical to the parent parent cell & each other daughter cells cellOccurs once during interphase DNA replication Occurs during interphase before before meiosis I mitosis begins