2022 HCI Cell and Nuclear Division Notes

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 1 Cell and Nuclear Division There are two different types of cell cycles: mitotic and meiotic. Cell cycles are tightly regulated at various checkpoints. The mitotic cell cycle is necessary for growth and repair while the meiotic cell cycle is necessary to generate gametes. Meiosis gives rise to genetic variation between gametes through crossing over of homologous chromosomes and the independent assortment of bivalents. Learning Outcomes Candidates should be able to: 2(n) describe the events that occur during the mitotic cell cycle and the main stages of mitosis (including the behaviour of chromosomes, nuclear envelope, cell surface membrane and centrioles). 2(o) 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 details of the mechanism are not required) 2(s) describe the events that occur during the meiotic cell cycle and the main stages of meiosis (including the behaviour of chromosomes, nuclear envelope, cell surface membrane and centrioles) (names of the main stages are expected, but not the sub-divisions of prophase). 2(t) explain the significance of the meiotic cell cycle (including how meiosis and random fertilisation can lead to variation) Use the knowledge gained in this section in new situations or to solve related problems. Learning Experiences 2.5.4 Different Stages of Mitosis The behaviour of chromosomes at the different stages of mitosis can be observed through the preparation of microscopy slides of squash root tip cells. The laboratory procedures can be found on OPAL. 2.5.5 Different Stages of Meiosis The behavio ur of chromosomes at the different stages of meiosis can be observed through the preparation of microscopy slides of squashed anther cells.

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 2 Lecture Outline Page 1. Cell and Nuclear Division 5 1.1 Chromosome number 6 1.2 Ploidy Level 7 1.3 Human Life Cycle 8 2. Important terms and definitions 9 3. The Cell Cycle 13 3.1 Interphase 13 4. Nuclear Division – Mitosis 4.1 Stages of Mitosis 15 4.2 Significance of Stages in Mitosis 18 4.3 Cytoplasmic Division – Cytokinesis 19 4.4 Significance of Mitosis 21 5. Nuclear Division – Meiosis 5.1 Stages of Meiosis I 23 5.2 Cytokinesis I 27 5.3 Stages of Meiosis II 28 5.4 Cytokinesis II 30 5.5 Significance of Stages in Meiosis 32 5.6 Significance of Meiosis 33 6. Comparing Mitosis and Meiosis 35 Websites S/N QR Code What’s so interesting? 1. Mitosis 3D Animation 2. Meiosis Crossing Over and Variability 3D Animation

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 3

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 4

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 5 1. Cell and Nuclear Division The basic unit of structure and function in living organisms is the cell. Cell division hence enables the continuity of life based on the production of cells. In unicellular organisms, the cell division of one cell reproduces an entire organism. This is known as asexual reproduction. In multicellular organisms, cell division enables sexually reproducing organisms to grow and develop from a single cell – the fertilised egg or zygote. After an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear. (Fig.1) Reproduction An amoeba, a single-celled eukaryote, is dividing into two cells. Each cell will be an individual organism. Growth and development This micrograph shows a sand dollar embryo shortly after the fertilised egg divided, forming two cells. Tissue renewal These dividing bone marrow cells (arrow) will give rise to new blood cells. Cell division consists of: 1. Nuclear division ● The process of separating nuclear DNA to daughter cells. ● There are two types of nuclear division: (i) Mitosis which leads to the production of two identical cells (ii) Meiosis which leads to the production of gametes e.g. sperm and egg 2. Cytoplasmic division (cytokinesis) ● The process of separating cytoplasm and organelles to daughter cells. Fig. 1: Cell division results in the production of cells.

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 6 1.1 Chromosome Number The chromosome number is the number of chromosomes in each cell of an organism. ● Different species of plants and animals have different chromosome number. ● There is no relationship between the size of an organism and the number of chromosomes it possesses, as shown in Fig. 2. A karyogram (Fig. 3) shows all the chromosomes in a cell (in this case, a human male’s somatic cell). The chromosomes are sorted according to their sizes. We can observe that: ● there are two sets of chromosomes (maternal and paternal sets) ● chromosomes arranged in homologous pairs ● there are 22 pairs of autosomes and 1 pair of sex chromosomes Fig. 3: Karyogram of a human male. In human females, the two sex chromosomes are alike and known as X chromosomes. In males, there is one X chromosome and one Y chromosome. Fig. 2: Chromosome number in organisms. Number of chromosomes is not a measure of the quantity of genetic material. Genes on autosomes 1 to 22 are described as autosomal. Genes on the sex chromosomes are described as sex-linked.

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 7 1.2 Ploidy Level Ploidy refers to the number of sets of chromosomes within the nucleus of a cell. Haploid cells, such as gametes/sex cells, have only one set of chromosomes, abbreviated as n. Diploid cells have two sets of chromosomes in the nucleus. Human beings, most animals and many plants are diploid organisms sinc e all the cells in the body are diploid except for their gametes. In human, the common representation is: 2n = 46, where: ● n is the number of chromosomes in a set (23 for humans) ● The number written before n indicates the ploidy level i.e. the number of sets. For humans, there are 2 sets of chromosomes where one set comes from the mother, the other set from the father. Organisms with more than 2 sets of chromosomes are termed polyploid. For example, triploid = 3n and tetraploid = 4n. Exercise Fill in the table below: genetic representation ploidy level number of sets of chromosomes number of chromosomes per set (n) total n = 6 2n = 4 4n = 8 2n = 42 2n = 38 3n = 81

HWA CHONG INSTITUTION / 2022-2023 H2 BIOLOGY / Core Idea 2 / Cell And Nuclear Division 8 1.3 Human Life Cycle The human life cycle begins when the haploid sperm from the father fuses with the haploid ovum from the mother. The union of gametes results in fusion of the nuclei called fertilisation. The resulting zygote (or fertilised egg) is now diploid because it contains two sets of chromosomes, one set from the father and the other set from the mother (Fig. 4)