CELL_BIOLOGY-CLASS_8[1]

Page 1: Title Page

• CELL BIOLOGY-CLASS 8

• TOPICS COVERED: Meiosis

• PRESENTATION BY: B. Madhuri Venkatesh

Page 2: Introduction to Meiosis

• Definition: Meiosis is a unique type of cell division that produces gametes with half the number of chromosomes.

• Process Overview:

  • Meiosis consists of two successive cell divisions:

    • Meiosis I: Reductional division, reduces chromosome number from diploid (2n) to haploid (n).

    • Meiosis II: Equational division, separates chromatids into individual chromosomes.

• Similarity to Mitosis: Steps in meiosis are similar to mitosis, sharing the same names, but they differ significantly in function and outcome.

Page 3: Significance of Meiosis

• Essential for Sexual Reproduction:

  • Meiosis is necessary for the production of gametes in eukaryotes.

• Cell Cycle:

  • Begins with one diploid cell (2n) containing two copies of each chromosome (one from each parent).

  • Produces four haploid cells (n), each containing one copy of each chromosome.

• Germ Cell Production: Meiosis is the division type that results in the formation of germ cells.

Page 4: History of Meiosis

• Discovery:

  • First described by Oscar Hertwig in sea urchin eggs (1876).

  • Further described by Edouard Van Beneden in Ascaris worms' eggs (1883).

• Term Origin: The term meiosis was coined by J.B. Farmer and J.B. Moore in 1905.

Page 5: Phases of Meiosis I

• Interphase: Similar in both mitosis and meiosis; meiosis follows with two divisions.

• Prophase I: Chromosomes form and pair up as tetrads.

• Metaphase I: Tetrads align on the equatorial plate.

• Anaphase I: Homologous chromosomes separate to opposite poles.

• Telophase I: Two daughter cells are formed, immediately proceeding to meiosis II without interphase.

Page 6: Meiosis II Overview

• Similarity to Mitosis: Meiosis II resembles mitosis; sister chromatids are separated.

• Outcome: Results in four daughter cells, each with a haploid number of chromosomes.

Page 7: Phases of Meiosis

• Phases:

  • Interphase: Preceding meiosis I.

  • Meiosis I

    • Prophase I: Tetrads form.

    • Metaphase I: Tetrads align.

    • Anaphase I: Homologous pairs separate.

    • Telophase I: Two daughter cells form.

  • Meiosis II

    • Prophase II: Chromosomes condense.

    • Metaphase II: End of pairing.

    • Anaphase II: Sister chromatids separate.

    • Telophase II: Four haploid cells form.

Page 8: Prophase I

• DNA Exchange: Homologous recombination occurs, allowing for chromosomal crossover.

• Bivalents/Tetrads: Paired chromosomes during this stage are termed bivalents or tetrads.

• Process of Pairing: Known as synopsis, where non-sister chromatids may cross-over at chiasmata.

Page 9: Leptotene

• Stage Characteristics:

  • First stage of prophase I.

  • Chromosomes condense into strands within the nucleus, sister chromatids remain tightly bound and indistinguishable.

Page 10: Zygotene

• Stage Overview:

  • Chromosomes align to form homologous pairs (bivalents).

  • Also known as zygonema, meaning "paired threads" in Greek.

Page 11: Pachytene

• Synapsis: Homologous chromosomes become associated more closely.

• Tetrad Formation: Homologous pairs consist of four chromatids and may undergo crossing over.

• Chromosome Condensation: Continues through this stage.

Page 12: Diplotene

• Chromosome Behavior:

  • Homologous chromosomes begin to separate slightly.

  • Chromosomes uncoil slightly, allowing for some transcription of DNA.

Page 13: Diakinesis

• Final Preparations:

  • Chromosomes condense further; visible tetrads with distinguishable chromatids.

  • Chiasmata terminalize, making them clearly observable.

Page 14: Crossing Over

• Definition: Segment exchange between chromatids of homologous chromosomes.

• Relevance: Important for genetic variation. Takes place during prophase I of meiosis.

Page 15: Genetic Recombination

• Process: Homologous chromosomes in tetrads exchange genes.

• Outcome: Leads to genetic recombination in offspring.

Page 16: Importance of Crossing Over

• Significance:

  • Facilitates genetic variation in offspring.

  • Aids in gene arrangement and chromosome mapping.

  • Essential for selecting advantageous recombinations.

Page 17: Metaphase I

• Alignment: Tetrads align randomly across the equatorial plane.

• Spindle Attachment: Spindle fibers attach to both sides of the tetrads.

Page 18: Anaphase I

• Separation Process:

  • Bivalent pairs separate; sister chromatids remain together.

  • Chromosomes move to opposite poles.

Page 19: Telophase I

• Completion:

  • Homologous chromosomes complete migration to poles.

  • Nuclear envelope reforms; cytokinesis follows.

Page 20: Meiosis II Phases

• Division Overview: Meiosis II functions similarly to mitotic division.

  • Phases:

    • Prophase II

    • Metaphase II

    • Anaphase II

    • Telophase II

Page 21: Prophase II

• Initiating Meiosis II:

  • No further chromosome replication; nuclear envelope disassembles.

  • Spindle apparatus assembles, centrioles duplicate.

Page 22: Metaphase II

• Chromosome Arrangement:

  • Chromosomes align on the metaphase plate; centromeres line up.

  • Absence of nuclear membrane is noted.

Page 23: Anaphase II

• Separation of Chromatids: Centromeres divide; individual chromatids move to poles.

  • Separated chromatids are recognized as individual chromosomes.

Page 24: Telophase II

• Finalization of Division:

  • Nuclear envelope reforms around new sets of chromosomes.

  • Nucleolus reappears; chromosomes elongate and decondense.

Page 25: Importance of Meiosis

• Gamete Production:

  • Maintains species’ diploid number across generations.

  • Source of genetic variation via crossing over.

Page 26: Differences Between Meiosis and Mitosis

• Meiosis:

  • Occurs in reproductive cells, halved chromosome number, four daughter cells, genetic exchange occurs.

• Mitosis:

  • Occurs in body cells, maintains chromosome number, two daughter cells, no genetic exchange.