VJC H2 Bio Chpt 08 Mitotic Cell Cycle Notes 2024

1 VICTORIA JUNIOR COLLEGE BIOLOGY DEPARTMENT YEAR ONE LECTURE 2024 (H2 9744 & H1 8876) CORE IDEA 2: GENETICS AND INHERITANCE Mitotic Cell Cycle Learning Outcomes: Candidates 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 membrane and centrioles). (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 detail of the mechanism is not required.) Note: LO (f) – (g) in H1 syllabus Contents: Section Page Learning Outcome 1. The Mitotic Cell Cycle 2-9 LO (n) (f) in H1 8876 (a) Introduction to the cell cycle 2 (b) Organisation of genetic materials during cell cycle 3 (c) Centrosomes, centrioles, and spindle fibres 4 (d) Process of mitosis 6 (e) Process of cytokinesis 9 2. Importance of Mitosis and Regulation of the Mitotic Cell Cycle 10-12 LO (o) (g) in H1 8876 (a) Significance of Mitosis 10 (b) Cell cycle checkpoints 11 Need a song to get in beat with mitosis? (overview) https://www.youtube.com/watch?v=pOsAbTi9tHw https://www.youtube.com/watch?v=f7Dmhfo7XXA

2 1. THE MITOTIC CELL CYCLE 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). (a) Introduction to the cell cycle • Recall t he Cell Theory states that the cell is a fundamental unit of structure, function and organisation in all living organisms and new cells are formed from other existing cells. • The cell cycle is an ordered sequence of events in cell division Fig. 1. Stages of the cell cycle. Source: https://openstax.org/books/biology/pages/10-2-the-cell-cycle 1. Mitosis is the last p art of the cell cycle. In fact, the mitotic (M) phase , which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. 2. Mitotic cell division comes after a much longer stage called interphase, which often accounts for about 90% of the cycle. Interphase consists of three main stages: G1, S and G2 phases. (While G stands for gap, you can use G1 and G2 mnemonically as growth phases, when the cell synthesizes a variety of proteins; For e.g., in G1 the synthesised proteins are required for DNA replication and in G2, microtubule proteins are required during mitosis.) G1 phase • Cell synthesises organelles such as mitochondria and ribosomes. • Builds up a large store of energy. • Manufactures proteins such as histones, ribosomal proteins and tubulin (a subunit of spindle fibres). S phase • DNA replication/ Synthesis occurs. • A microtubule-organizing structure called the centrosome is also duplicated. G2 phase • Cell continues to store energy and manufacture proteins and organelles. • Only cells with the right conditions can proceed to the M phase to ensure that damaged or incomplete DNA is not passed on to the daughter cells. M phase • Mitosis (nuclear division) • Cytokinesis Overview: Cell cycle (& Cancer) Scan me

3 (b) Organisation of genetic materials during cell cycle • During interphase, the genetic material of the cell are in the form of chromatin. • In the first stage of mitosis, the chromatin condenses – The chromatin fibres become densely coiled, forming chromosomes which are much shorter and thicker, and are visible under the light microscope. • Since DNA replication has already occurred in the S phase, each mitotic chromosome is already a duplicated structure, consisting of two sister chromatids. • The two sister chromatids are identical DNA molecules attached to each other at the centromere. • During mitosis, the two sister chromatids separate and move into the two daughter nuclei. • Once they are separated, they are considered individual chromosomes. • Thus, each daughter cell receives an identical set of chromosomes as the parent cell. Fig. 2. Chromatin condensation into chromosomes Fig. 3. Light micrograph of lung cell and representative drawing at G2 phase

4 (c) Centrosomes, centrioles and spindle fibres • Centrosomes are nonmembranous organelles found only in animal cells . They function as the microtubule organizing center (MTOC) , to organize the cell’s microtubule throughout the cell cycle. • In animal cells, each centrosome is composed of two centrioles, which are positioned perpendicular to each other. • Centrioles are microtubule- based cylinders of defined length and diameter. They are found in animal cells but missing in most plant cells. • Each centriole is approx. 500nm long and 200nm in diameter. Each is composed of 9 triplets of microtubules. Fig. 4a: A pair of centrioles lying perpendicular to each other (http://www.basic.northwestern.edu/g- buehler/eyes.htm) Fig. 4b. Electron micrograph showing a cross section of a centriole with its array of nine triplets of microtubules. (https://www.biology-pages.info/C/Centrioles.html) • At the beginning of nuclear division, long protein fibres (microtubules) extend from the centrioles. They form the spindle fibres. Spindle fibres are long, hollow tubes about 24-25nm in diameter. They are made up of protein subunits called tubulin. Fig. 5. Arrangement of tubulin in a spindle fibre • Some of the spindle fibres attach to the kinetochores of the chromosomes are referred to as kinetochore microtubules. Shortening of these fibres by removal of tubulin subunits separates the chromatids and pull them to opposite poles.

5 • Spindle fibres that do not attach to kinetochores are referred to as nonkinetochore microtubules. They interact with nonkinetochore microtubules from the opposite pole of the cell. In a dividing animal cell, these microtubules are responsible for elongating the whole cell during anaphase. Fig. 6. Types of microtubules involved in mitosis • Colchicine is a chemical commonly used to prevent formation of spindle fibre in actively dividing cells. As a result, sister chromatids remain attached in the metaphase plate. This allows the observation of the number and structure of chromosomes, as well as karyotyping.

6 (d) Process of mitosis • Mitosis can be divided into four phases, namely prophase, metaphase, anaphase and telophase. Fig. 7. A series of micrographs of a dividing animal cell. (i) Prophase • Genetic material becomes visible under light microscope as chromosomes due to condensation of chromatin. • Each chromosome consists of two identical sister chromatids, which are joined at the centromere. • The nucleolus disappears. • In animal cells, centrosomes move to opposite ends of the cell. Short microtubules develop from each pair of centrioles in the centrosomes. This forms a star -shaped structure called an aster. • Phosphorylation of various proteins on inner s urface of the nuclear envelope causes the nuclear envelope to disintegrate into small membrane vesicles. • Microtubules extend from the centrioles to form spindle fibres. The two ends of the spindle are known as its poles. • A specialised protein structure, kinetochore assembles at the centromere of the chromosome.

7 • Some of the spindle fibres (microtubules) extend to attach specifically to the kinetochores becoming 'kinetochore microtubules". • Nonkinetochore / polar microtubules are spindle fibres that extend from one pole to the opposite pole. (ii) Metaphase • Metaphase is the longest stage of mitosis, often lasting about 20 minutes. • The centrosomes are now at opposite poles of the cell. • The chromosomes align on the metaphase plate / equator, an imaginary plane that is equidistant between the spindle's two poles