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The Cell Cycle: Mitosis and Meiosis, Lecture notes of Health sciences
A comprehensive overview of the cell cycle, focusing on the processes of mitosis and meiosis. It covers the key stages of the cell cycle, including interphase (g1, s, and g2 phases) and the mitotic phase (prophase, metaphase, anaphase, and telophase). The document also delves into the distinct features of meiosis, which is the process of cell division that produces gametes (sex cells) with half the normal chromosome number. It explains the two meiotic divisions, meiosis i and meiosis ii, and the unique events that occur during these stages, such as chromosome pairing, crossing over, and the separation of homologous chromosomes. The information provided in this document can be valuable for students studying cell biology, genetics, and developmental biology, as it offers a detailed understanding of the fundamental processes that govern cell division and the generation of genetic diversity.
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2023/2024
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Name of the School
Course Code: Course Name:
Topic: The cell cycle
Faculty Name: Program Name:
Prerequisite/Recapitulations
MHC CLASS 1 & 2
Introduction
- (^) Many damaged, dead, and worn out cells can be replaced by growth and division of other similar cells.
- (^) Most body cells have 46 chromosomes and divide by mitosis, a process that results in two new genetically identical daughter cells.
- (^) The only exception to this is the formation of gametes (sex cells), i.e. ova and spermatozoa, which takes place by meiosis.
- (^) The period between two cell divisions is known as the cell cycle, which has two phases that can be seen on light microscopy: mitosis (M phase) and interphase.
Mitosis: This is a continuous process involving four distinct stages seen by light microscopy. PMAT P rophase M etaphase A naphase T elophase
Prophase During this stage the replicated chromatin becomes tightly coiled and easier to see under the microscope. Each of the original 46 chromosomes (called a chromatid at this stage) is paired with its copy in a double chromosome unit. The two chromatids are joined to each other at the centromere. The mitotic apparatus appears; this consists of two centrioles separated by the mitotic spindle , which is formed from microtubules. The centrioles migrate, one to each end of the cell, and the nuclear envelope disappears.
- (^) Telophase
- (^) The mitotic spindle disappears, the chromosomes uncoil and the nuclear envelope reforms.
- (^) Following telophase, cytokinesis occurs: the cytoplasm, intracellular organelles and plasma membrane split forming two identical daughter cells.
- (^) The organelles of the daughter cells are incomplete at the end of cell division but they develop during interphase.
- (^) The frequency with which cell division occurs varies with different types of cell.
Meiosis
- (^) Meiosis produces gametes. On fertilization, when the male gamete (sperm cell) and the female gamete (ovum) unite, they produce zygote which is a diploid cell, because each gamete was haploid.
- (^) Unlike mitosis, meiosis involves two distinct cell divisions rather than one.
- (^) Additionally, meiosis produces four daughter cells, not two, all different from the parent cells and from each other.
- (^) This is the basis of genetic diversity and the uniqueness of each human individual.
- (^) Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell.
First meiotic division:
- (^) Before entering meiosis I , a cell must first go through interphase. As in mitosis, the cell grows during G1 phase , copies all of its chromosomes during S phase , and prepares for division during G2 phase.
- (^) During prophase I , differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length.
- (^) Then the cross over occurs so that the homologues exchange part of their DNA.
- (^) The cross over helped along by a protein structure called the synaptonemal complex that holds the homologues together.
- (^) The chromosomes would actually be positioned one on top of the other—as in the image below—throughout crossing over; they're only shown side-by-side in the image above so that it's easier to see the exchange of genetic material.
- (^) You can see crossovers under a microscope as chiasmata , cross-shaped structures where homologues are linked together.
- (^) The spots where crossovers happen are more or less random, leading to the formation of new, "remixed" chromosomes with unique combinations of alleles.
- (^) After crossing over, the spindle begins to capture chromosomes and move them towards the center of the cell ( metaphase plate ).
- (^) Each pair of chromosomes separates and one travels to each end of the cell, guided by a spindle as in mitosis, and the cytoplasm divides, producing two genetically unique diploid daughter cells.
Finally, in telophase I , the chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes decondense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II. Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells.
Meiosis II: Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I. The cells that enter meiosis II are the ones made in meiosis I. In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes. During prophase II , chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes.