MITOSIS AND MEIOSIS

1 EJC H2 Biology T2W10 Mitosis & Meiosis Cell Division – Mitosis & Meiosis 1. Overview of Topic The first single cellular organisms appeared about 3.5 billion years ago, roughly about a billion years after Earth was formed. It took a longer time for more complex life forms to evolve, with the first multicellular organisms only appearing about 600-800 million years ago. As life on earth progressed from single-cellularity to multi-cellularity, there was a need for the cell cycle to be more systemic and precise. Nuclear division is a stage of the cell cycle. Following nuclear division, the genetic information stored in the form of DNA, on chromosomes, is appropriately passed on to daughter cells from the parent cells. In mitosis, the daughter nuclei would possess the same type and number of chromosomes as the parent nuc lei. Meiosis occurs only in the sexually reproducing or ganisms. In meiosis, the daughter nuclei (haploid) end up with half the number of chromosom es as the parent cell (diploid). The daughter chromosomes may also have different allelic combinations due to allelic exchanges between the maternal and paternal chr omosomes. Mitosis and Meiosis bring about genetic stability and genetic variation in the organisms. In this chapter, we will be examining these nuclear division processes. 2. Learning Outcomes a. Describe the events that occur during the mitotic cell cycle and the main stages of mitosis (including the behaviour of chromosomes, nuclear envelope, cell membrane and centrioles). b. Explain the significance of the mitotic cell cycle (including growth, repair and asexual reproduction) and the need to regulate it tightly . (Knowledge that dysregulation of checkpoints of cell division can result in uncontrolled cell division and cancer is required, but detail of the mechanism is not required.) c. Describe the events that occur during the meiotic cell cycle and the main stages of meiosis (including the behaviour of chromosomes, nuclear envelope, cell membrane and centrioles). (Names of the main stages are expected, but no the sub-divisions of prophase.) d. Explain the significance of the meiotic cell cycle (including how meiosis and random fertilisation can lead to variation). 3. References Campbell, N.A. and Reece, J.B. (2008). Biology, 9th edition. Pearson. Jones & Jones. (1997). Advanced Biology. Hoh Yin Kiong (2003). Longman A-Level Course in Biology, Vol. 1. Longman Green, Stout and Soper (1990). Biological Sciences, Vol 1. Cambridge Raven, Johnson, Losos, Mason & Singer (2008). Biology, 8th edition. McGraw-Hill.

2 EJC H2 Biology T2W10 Mitosis & Meiosis Contents 1. Overview of Topic ................................ ................................ ................................ ...... 1 2. Learning Outcomes ................................ ................................ ................................ .... 1 3. References ................................ ................................ ................................ ................. 1 4. Introduction ................................ ................................ ................................ ............... 3 5. Structure of Chromosomes ................................ ................................ ........................ 3 6. Diploidy & Haploidy ................................ ................................ ................................ ... 5 7. Homologous chromosomes ................................ ................................ ....................... 7 8. Factors influencing cell division ................................ ................................ ................. 9 9. The Cell Cycle ................................ ................................ ................................ ........... 10 10. Interphase ................................ ................................ ................................ ........... 11 11. Mitosis ................................ ................................ ................................ ................ 12 a. Early Prophase (Usually the longest phase of mitosis) ................................ ......... 12 b. Late Prophase ................................ ................................ ................................ ...... 12 c. Metaphase ................................ ................................ ................................ ........... 13 d. Anaphase (shortest phase in mitosis) ................................ ................................ .. 13 e. Telophase (2 nuclei forming in one cell) ................................ .............................. 14 12. Cytokinesis (Cytoplasmic division - not part of mitosis) ................................ ....... 16 13. Significance of mitosis ................................ ................................ .......................... 18 14. Meiosis – ‘Reduction division’ ................................ ................................ .................... 19 a. Interphase (not part of meiosis) ................................ ................................ .......... 19 b. Prophase I (early) ................................ ................................ ................................ . 20 c. Prophase I (late) ................................ ................................ ................................ ... 20 d. Metaphase I ................................ ................................ ................................ ......... 22 e. Anaphase I ................................ ................................ ................................ ........... 22 f. Telophase I ................................ ................................ ................................ ........... 23 g. Cytokinesis ................................ ................................ ................................ ........... 23 h. Meiosis II ................................ ................................ ................................ .............. 23 i. Cytokinesis after meiosis II ................................ ................................ ................... 24 15. Significance of Meiosis ................................ ................................ ......................... 26

3 EJC H2 Biology T2W10 Mitosis & Meiosis 4. Introduction The modern cell theory states that all new cells are derived from other cells. Many cells in a multi -cellular organism undergo a well -defined sequence of stages culminating in the division and formation of new cells. The processes involved are collectively grouped under the term cell division. There are some types of cells that are continuously dividing throughout the lifetime of the organism (e.g. bone marrow cells, epithelial cells of the skin, etc.) whilst others stop dividing upon reaching maturity (e.g. neurones, muscle cells, etc.). Cell division occurs in 2 main steps:  Nuclear division &  Cytokinesis (cytoplasmic division) 1) Nuclear Division (= division of the nucleus): - There are 2 types of nuclear division: Mitosis & Meiosis 2) Cytokinesis (= division of the cytoplasm) - Cytokinesis is the division of the cytoplasm to form 2 separate daughter cells immediately after mitosis, meiosis I or meiosis II. Please note that mitosis and meiosis are nuclear divisions and only occur in eukaryotic cells and not prokaryotic cells. 5. Structure of Chromosomes Chromosomes carry the hereditary material (DNA) in cells. The hereditary material is passed on to the next generation through cell division. The structure of the eukaryotic chromosome: o DNA (deoxyribonucleic acid) is the double-stranded, helical molecule found within the nucleus of each cell. DNA carries the genetic information that encodes the proteins necessary for cells to reproduce and to perform their functions. o When a cell is not dividing, chromosomes exist in the