YIJC [H2] CI2.3 Cell Division and the Cell Cycle (N)(Tr)

2023 JC1 BIOLOGY LECTURE NOTES CORE IDEA 2: GENETICS AND INHERITANCE TOPIC 2.1: Cell Division and the Cell Cycle Learning Outcomes: You should be able to: (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) (o) (modified) explain the significance of the mitotic cell cycle (including growth, repair and asexual reproduction) (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) (t) explain the significance of the meiotic cell cycle (including how meiosis and random fertilisation can lead to variation) References: Campbell, N. A., Reece, J. B., et al. (2018). Biology A Global Approach (Eleventh Edition) • Chapter 12 Mitosis, p284 – 299 • Chapter 13 Sexual Life Cycles and Meiosis, p304 - 317 Note: Some of these references (including previous editions) are available in our library. You may wish to borrow them or visit the links to supplement your reading when necessary. Contents 1 INTRODUCTION ................................ ................................ ................................ ..................... 2 2 CHROMOSOMES IN A CELL ................................ ................................ ................................ . 3 3 PHASES OF THE MITOTIC CELL CYCLE ................................ ................................ ............. 4 3.1 INTERPHASE ................................ ................................ ................................ ............... 5 3.2 MITOSIS ................................ ................................ ................................ ........................ 6 3.3 Cytokinesis ................................ ................................ ................................ .................. 9 4 SIGNIFICANCE OF MITOSIS ................................ ................................ ................................ 12 5 PHASES OF THE MEIOTIC CELL CYCLE ................................ ................................ ........... 14 5.1 MEIOSIS ................................ ................................ ................................ ....................... 14 5.1.2 MEIOSIS I ................................ ................................ ................................ ............. 16 5.1.3 MEIOSIS II ................................ ................................ ................................ ............ 21 5.2 SIGNIFICANCE OF MEIOSIS ................................ ................................ ..................... 25 H2

2 A Bridge to O Level Biology (6093 syllabus), You should be able to: (a) state the importance of mitosis in growth, repair and asexual reproduction (b) explain the need for the production of genetically identical cells (c) identify, with the aid of diagrams, the main stages of mitosis (d) state what is meant by homologous pairs of chromosomes (e) identify, with the aid of diagrams, the main stages of meiosis (names of the sub -divisions of prophase are not required) (f) define the terms haploid and diploid, and explain the need for a reduction division process prior to fertilisation in sexual reproduction (g) state how meiosis and fertilisation can lead to variation Note that there are no cell division in 5078 syllabus (Science (Biology)) 1 INTRODUCTION The cell cycle comprises interphase, nuclear division and cytokinesis. There are two types of nuclear division: mitosis and meiosis. • A cell cycle that involves mitosis will give rise to genetically identical cells and this is important for growth, repair and the asexual reproduction of organisms. • A cell cycle that involves meiosis occurs in the reproductive organs of organisms and is important for sexual reproduction. Meiosis results in gametes having half the amount of genetic material present in somatic cells. Fig. 1.1: Types of nuclear and cell division There are some links in the SLS lesson to help you review and revise your knowledge on the following: • Cell Theory

3 2 CHROMOSOMES IN A CELL Homologous Chromosomes In a diploid species, each type of chromosome is found in a pair and are called homologous chromosomes. Homologous chromosomes are similar to each other as they • are nearly identical in size; • have the same banding pattern and same centromere location; • have the same gene loci i.e. a gene found on one homolog will have a corresponding gene at the same position on the next homolog. Fig. 2.1: Homologous chromosome pairs Fig. 2.2: Homologous chromosome pair Homologous chromosomes are not identical to each other as they • may have different alleles for the same gene; • one chromosome originates from the male parent, while the other chromosome originates from the female parent. • • There are some links in the SLS lesson to help you review and revise your knowledge on the following: • Karyotype and Homologous chromosomes

4 3 PHASES OF THE MITOTIC CELL CYCLE Cells do not divide continuously but undergo a regular cycle division separated by periods of cell growth. This is known as the cell cycle and has three stages. Fig. 3.1: A broad overview of the phases in the Cell Cycle The cell cycle is divided into 3 stages, namely • Interphase, which is sequentially further divided into 3 phases o Gap 1 (G1) phase o Synthesis (S) phase o Gap 2 (G2) phase • Mitotic phase, which is sequentially further divided into o Prophase o Metaphase o Anaphase o Telophase • Cytokinesis The duration of these cell cycle phases varies considerably in different kinds of cells. Learning Outcome 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)

5 3.1 INTERPHASE Interphase occurs during most of the cell cycle, and is sometimes known as the resting phase, because no division takes place. However, interphase is marked by a period of intense chemical activity and sub-divided into three parts: • First growth (G1) phase is when the proteins from each cell organelles are synthesised are produced. • Synthesis (S) phase is when the DNA is replicated. • Second growth (G2) phase is when organelles grow and divide, and energy stores are increased. In summary, G 1 and G 2 are important phases to allow time for cell growth , duplication of organelles, storing of energy and synthesis of proteins required for cell division . G1 and G2 phases also provide time for the cell to monitor the internal and external environment to ensure that conditions are suitable and preparations are complete before the cell commits itself to the DNA synthesis and mitosis. Fig. 3.1.1: Phases of the Cell Cycle During interphase in eukaryotic cells, the centrosome duplicates. The centrosome is the major microtubule- organising centre (MTOC) which comprised a pair of centrioles. Centrosomes are not present in plant cells because plant cells do not have centrioles. Fig. 3.1.2: Animal cell with centrosome

6 3.2 MITOSIS Definition Nuclear division followed by cytokinesis to form 2 genetically identical daughter cells with the same number of chromosomes and amount of DNA as the parent cell. Stages of Mitosis The mitotic division is divided into 4 phases – (a) Prophase, (b) Metaphase, (c) Anaphase, (d) Telophase after which cytoplasmic division known as cytokinesis will occur. Fig. 3.2.1: Stages of Mitos