Detailed Notes on Meiosis and Genetic Variation

Meiosis Overview

• Definition of Meiosis: A specialized form of cell division that reduces the chromosome number by half, resulting in four genetically distinct daughter cells.

• Purpose: Essential for sexual reproduction, produces gametes (sperm and eggs).

Key Concepts

• Heredity: Transmission of traits from parents to offspring.

• Variation: Differences in traits observed among offspring.

• Genetics: The scientific study of heredity and variation.

Chromosomes and Genes

• Inheritance of Genes: Offspring acquire genes from parents through chromosomes.

• Genes: Units of heredity made up of DNA, passed via gametes.

  • Normal human somatic cells (non-gametes/reproductive) contain 46 chromosomes (2n).

  • Gametes are haploid, containing one set of chromosomes (n = 23).

  • Genes are always on the locust

Types of Reproduction

• Asexual Reproduction: Single individual produces offspring clones through mitosis.

• Sexual Reproduction: Involves two parents contributing genetic material, resulting in offspring with unique gene combinations.

Human Chromosome Structure

• Karyotype: Arrangement of chromosomes in pairs by size for analysis.

• Homologous Chromosomes: Pairs of chromosomes (one from each parent) that carry the same genes.

• Sex Chromosomes: XX for females and XY for males. Remaining chromosomes are autosomes.

• Diploid vs. Haploid: Diploid cells (2n) have two sets of chromosomes (46 total). Haploid cells (n) have one set (23).

  • Mom and dad chromatids are non sister

  • All somatic cells are diploid

  • gametes have single set of chromsomes (23) sets , making them essential for sexual reproduction as they combine during fertilization to restore the diploid state.

  • unfertilized egg alwys x chromosome

  • sperm cell can be x or y

  • Zygote cell produces somatic cells by mitosis

  • only cells that are not produces my mitosis are gaemetes, formed by specialized cells called germ cells

Meiosis Process

• Meiosis I and II: Two consecutive division processes.

• 

  • Meiosis I reduces chromosome number; homologous chromosomes separate.

  • Meiosis II resembles mitosis; sister chromatids separate.

  • Meisosis results in 4 daughter cells

  • Mitosis results in two daugher cells

• Stages of Meiosis I:

  • Chromosomes duplicate during interphase

    

    • Prophase I: Chromosomes pair up, crossing over occurs between nonsister chromatids.

      • sister chromatids are held together by protein called cohesins

      • zipper like structure called the synaptonemal complex holds the homologs together tightly

      • prophase 2 , spindle appears, membrane dissapears

    • Metaphase I: Homologous pairs align at the metaphase plate.

    • Anaphase I: Homologous chromosomes are pulled to opposite poles.

      • sister chromatids stay together

      • anaphase 2 is when sister chromatids seperate and are pulled towards opposite poles, resulting in the formation of individual chromosomes that will eventually become gametes in the final stages of meiosis.

    • Telophase I: Two haploid cells form, each with duplicated chromosomes.

      • telophase 2 - cells split again so theres 4

  • Animal cells have a cleavage furrow, plant cells form a cell plate

• Independent Assortment: Chromosome pairs orient randomly during Metaphase I, leading to varied gametes.

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I, generating recombinant chromosomes.

• Random Fertilization: The variety of sperm and egg combinations increases genetic diversity in offspring.

Important Comparisons

• Mitosis vs. Meiosis:

  • Mitosis results in two genetically identical cells.

  • Meiosis results in four genetically distinct cells, reduces the chromosome number by half.

• Mitosis: Involves one division cycle, occurs in both diploid and haploid cells.

• Meiosis: Involves two division cycles, occurs only in diploid cells, includes synapsis and crossing over.

Conclusion

• Significance in Evolution: Genetic variation from sexual reproduction contributes to adaptability and evolution, emphasizing the role of meiosis in biodiversity.