3. Meiosis

CELL DIVISION: MEIOSIS Lecture Book 4 Pg 23

LEARNING OBJECTIVES: PAGE 23 2 (j) Explain the significance of the meiotic cell cycle (reduction division prior to fertilisation and cells not genetically identical) and that meiosis and random fertilisation can lead to variation (detailed description of the behaviour of chromosomes during meiosis is not required. Information about the stages and associated behaviour of the nuclear envelope, cell surface membrane and centrioles is not required.)

MEIOSIS DEFINITION ▰ Reduction division that produces 4 daughter nuclei are not genetically identical ▰ Each have half the number of chromosomes as that of the parent nucleus ▰ Has 2 consecutive cycles of nuclear division – MEIOSIS I & MEIOSIS II PAGE 23

MITOSIS vs MEIOSIS PAGE 23

MITOSIS vs MEIOSIS • Mitosis may occur in almost any living cell (somatic cells) • Meiosis is restricted to germ-line cells that form haploid gametes (i.e. the eggs and sperm). PAGE 23 Gametes undergo meiosis Germ - line cells undergo meiosis to form gametes [Gametocytes/ Gametogonia ]c

REVISION: PLOIDY ▰ Defined by the number of sets of chromosomes ▰ Diploid = 2 sets of homologous chromosomes ▰ Haploid = 1 set ▰ TIP: Count number of chromosomes by counting the no. of centromeres RECAP

MEIOSIS phases PAGES 24 – 28 FYI only

IMPORTANCE OF MEIOSIS PAGE 29

WHY DO WE NEED MEIOSIS? Importance of Meiosis Genetic Variation in Gametes Crossing Over @ Prophase I Independent Assortment @ Metaphase I & II Haploid Gametes Sexual Reproduction to restore diploidy

I) SEXUAL REPRODUCTION ▰ Meiosis + Cytokinesis produce haploid gametes (n) ▰ Fusion of gametes during fertilisation restores diploid condition (2n) ▰ Ensure chromosome number remains the same and prevents doubling PAGE 29