Meiosis and Sexual Reproduction Flashcards

Meiosis

• Meiosis is a process that produces haploid cells from a diploid cell through two rounds of division (Meiosis I and Meiosis II).
• Meiosis I: separates homologous chromosomes.
• Meiosis II: separates sister chromatids.

Key Concepts

• Diploid (2n): Two sets of chromosomes. In humans, 2n = 46.
• Haploid (n): One set of chromosomes. Human gametes have 23 chromosomes.
• Bivalent (Tetrad): Homologous pairs form a bivalent during Prophase I.
• Crossing Over: Exchange of DNA segments between non-sister chromatids, increasing genetic variation. Occurs during Prophase I.
• Chiasma: X-shaped regions where arms of chromosomes remain adhered during crossing over.

Stages of Meiosis I

• Prophase I:
  • Homologous chromosomes pair up, forming bivalents, crossing over occurs, nuclear envelope breaks down
• Prometaphase I:
  • Spindle apparatus completes, chromatids attach to kinetochore microtubules from one pole
• Metaphase I:
  • Bivalents align along the metaphase plate
• Anaphase I:
  • Homologous chromosomes separate and move to opposite poles, sister chromatids remain connected
• Telophase I & Cytokinesis:
  • Sister chromatids reach poles, nuclear membranes reform, cell divides into two haploid cells

Stages of Meiosis II

• Sorting events are similar to mitosis.
• Sister chromatids are separated in Anaphase II.
• Prophase II: Spindle apparatus forms, chromosomes move toward metaphase plate
• Metaphase II: Sister chromatids align at the metaphase plate, attached to microtubules from opposite poles.
• Anaphase II: Sister chromatids separate and move to opposite poles
• Telophase II & Cytokinesis: Nuclei form, chromosomes decondense, resulting in four genetically distinct haploid daughter cells.

Comparison of Meiosis and Mitosis

• Meiosis I Unique Events:
  • Synapsis and crossing over in Prophase I.
  • Alignment of homologous pairs at the metaphase plate.
  • Separation of homologs during anaphase I.

Sexual Reproduction

• Two haploid gametes unite during fertilization to form a diploid zygote.
• Sexual Life Cycle: Alternation between haploid and diploid stages.

Life Cycle Types

• Diploid-Dominant: Most animal species; diploid adult stage with haploid gametes.
• Haploid-Dominant: Many fungi and some protists; multicellular organism is haploid.
• Alternation of Generations: Plants and some algae alternate between multicellular diploid sporophyte and multicellular haploid gametophyte.

Variation in Chromosome Number and Sets

• Nondisjunction:
  • Homologous chromosomes or sister chromatids fail to separate during meiosis.
  • Results in gametes with extra or missing chromosomes.
• Aneuploidy:
  • Offspring have an abnormal number of a particular chromosome.
  • Monosomic: one copy =(2n-1).
  • Trisomic: three copies =(2n+1).
• Aneuploidy in Humans:
  • Can result in conditions like Down Syndrome (Trisomy 21).
  • Aneuploidy of sex chromosomes (e.g., Klinefelter syndrome, Turner syndrome) is often less severe than autosomal aneuploidy.
• Polyploidy:
  • Organism has more than two complete sets of chromosomes.
  • Triploidy (3n) and Tetraploidy (4n).

Changes in Chromosome Structure

• Deletion: Removes a chromosomal segment.
• Duplication: Repeats a segment.
• Inversion: Reverses orientation of a segment within a chromosome.
• Translocation: Moves a segment from one chromosome to another.
• Disorders:
  • Cri du chat syndrome: deletion in chromosome 5.
  • Certain cancers (e.g., chronic myelogenous leukemia) caused by translocations.