Mitosis and Meiosis

Mitosis and Meiosis Study Guide

Introduction

  • Organisms begin life as a single cell that grows and differentiates into various tissues, organs, and organ systems.

  • Cells replicate once they have reached optimal growth conditions.

  • The cell cycle encompasses the process wherein a cell replicates its genetic material and divides into two identical cells.

  • Mitosis is the primary mechanism of cellular replication.

Keywords

  • Chromosome: A cellular structure composed of one DNA molecule and associated proteins.

  • Sister chromatids: Two identical copies of a single chromosome, formed after DNA replication.

  • Centromere: The region on each sister chromatid where they are closely attached, stabilized by proteins binding to the centromeric DNA.

  • Kinetochore: A protein complex associated with the centromere where microtubules attach during cell division.

  • Centrosome: A cellular structure in animal cells that acts as a microtubule-organizing center, essential during cell division.

  • Diploid: A cell that contains two sets of chromosomes (2n), one set inherited from each parent.

  • Haploid: A cell containing only one set of chromosomes (n).

The Cell Cycle

  • The cell cycle begins with a newly formed cell and ends with the replication of that cell, consisting of three main parts:

    1. Interphase

    • G1 Phase (First Gap)

    • S Phase (Synthesis)

    • G2 Phase (Second Gap)

    1. Mitosis

    2. Cytokinesis

Interphase

  • Before cell division can occur, the cell must replicate its genetic material and prepare necessary cellular machinery.

  • Interphase is composed of three distinct subphases:

    1. G1 Phase

    • The cell grows and carries out most of its functions, producing important molecules necessary for metabolism (e.g., specific proteins).

    • G0 Checkpoint: If conditions are not met, the cell cycle is halted.

    1. S Phase

    • Chromosomes are duplicated, resulting in identical sister chromatids connected at the centromere.

    • G0 Checkpoint: Ensures no mistakes occurred during DNA replication.

    1. G2 Phase (Final phase)

    • The cell synthesizes molecules and structures required for mitosis, leading into the mitotic phase.

Mitosis

  • Mitosis is the process of nuclear replication and division in eukaryotic cells, predominantly occurring in somatic cells, such as muscle and skin cells.

  • Prokaryotic cells undergo binary fission instead of mitosis for replication, which involves the division of the cell after replicating genetic material.

  • Mitosis typically occupies 10% of the cell cycle duration, with an average total cycle length ranging from 10 to 30 hours, yielding two identical daughter cells.

Phases of Mitosis

  1. Prophase

    • Sister chromatids condense into distinct chromosomes, appearing as two identical sister chromatids joined at the centromere.

    • The mitotic spindle begins to form.

    • Centrosomes start to move apart, and microtubules lengthen, forming asters.

    • The nuclear membrane begins to dissolve.

  2. Prometaphase

    • The nuclear membrane fully disintegrates.

    • Microtubules invade the nuclear region.

    • Each chromatid develops a kinetochore, allowing attachment to microtubules (kinetochore microtubules) that manipulate chromosome positioning.

  3. Metaphase

    • Centrosomes are at opposite poles, with chromosomes aligned at the metaphase plate.

    • Kinetochores of sister chromatids attach to kinetochore microtubules from opposing poles.

  4. Anaphase

    • The shortest mitotic stage lasting only a few minutes.

    • Cohesin proteins are cleaved, allowing sister chromatids to separate and form individual chromosomes.

    • Daughter chromosomes move towards opposite poles, driven by shortening kinetochore microtubules, achieving equal distribution by the end of anaphase.

  5. Telophase and Cytokinesis

    • Telophase

      • Nuclear membranes reassemble around each sorted set of chromosomes.

      • Chromosomes decondense back into their chromatin form.

      • Mitosis concludes at this stage.

    • Cytokinesis

      • The cytoplasm divides, typically initiated by the formation of a cleavage furrow in animal cells.

      • In plant cells, a cell plate forms to separate the two new cells.

Stages of Meiosis

  • Meiosis encompasses several key stages that facilitate the formation of gametes (sex cells), also referred to as gametogenesis.

    • Meiosis results in the combination of genes from different parents to create offspring with unique genetic combinations.

    • Eukaryotes possess paired homologous chromosomes (diploid, 2n), whereas gametes are haploid (n) and contain only one chromosome of each homologous pair.

Meiosis Overview

  • Meiosis results in:

    • The production of haploid, recombinant daughter nuclei, known as reduction division.

    • The ability for fertilization with another gamete (e.g., sperm with egg), restoring the diploid state.

    • The generation of new genetic combinations.

Phases of Meiosis

  1. Prophase I

    • Centrosome relocation, spindle formation, and breakdown of the nuclear envelope occur.

    • Chromosomes condense as homologs pair, leading to crossing over.

    • Microtubules connect from opposite poles to kinetochores, binding at each homolog's centromere.

  2. Metaphase I

    • Homologous chromosomes align at the metaphase plate, with pairs facing opposite poles.

    • Each chromatid attaches to kinetochore microtubules extending from one pole.

  3. Anaphase I

    • Cohesin proteins are broken down, allowing for the separation of homologs.

    • Homologs are guided toward opposite poles by the spindle apparatus.

  4. Telophase I and Cytokinesis I

    • Each half of the cell achieves a haploid set of duplicated chromosomes, each consisting of two sister chromatids.

    • Cytokinesis results in two haploid daughter cells.

  5. Prophase II

    • Sister chromatids condense; spindle fibers begin to form; the nuclear membrane fragments.

  6. Prometaphase II

    • The nuclear membrane completely fragments.

    • Sister chromatids attach to spindle fibers via kinetochore microtubules.

  7. Metaphase II

    • Chromosomes arrange themselves along the metaphase plate with kinetochores of sister chromatids attached to microtubules extending from opposite poles.

  8. Anaphase II

    • Cohesins at the centromeres of sister chromatids are cleaved, allowing them to separate into individual chromosomes.

    • Chromatids migrate towards opposite poles.

  9. Telophase II and Cytokinesis II

    • Nuclei reform and chromosomes condense.

    • The meiotic division of the original parent cell results in four genetically distinct haploid daughter cells.