Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction

• Living organisms are characterized by their ability to reproduce their own kind.

• Genetics (study of heredity) involves the inheritance of genes between generations.

Inheritance of Genes

• Genes are units of heredity composed of segments of DNA.

• Genes are passed to the next generation through gametes (sperm and eggs).

• Each gamete contains one set of chromosomes.

• Humans inherit one set of chromosomes from each parent, resulting in 23 pairs of chromosomes (46 total).

Chromosomes

• Chromosomes in a pair are called homologous chromosomes or homologs.

• Homologous chromosomes carry genes that control the same inherited traits.

• The sex chromosomes in humans are designated as X and Y; the other 22 pairs are autosomes.

Diploid Numbers of Various Species

• Chimpanzee: 2n = 48

• Human: 2n = 46

• Domestic dog: 2n = 78

• Chicken: 2n = 78

• Myrmecia pilosula (ant): 2n = 2

• Ophioglossum reticulatum (fern): 2n = 1200

• Aulacantha (protozoan): 2n = 1600

Meiosis Overview

• Meiosis converts diploid cells (2n) to haploid gametes (n).

• It consists of two rounds of cell division: meiosis I and meiosis II.

• Meiosis results in four daughter cells, each with half as many chromosomes as the parent cell.

Stages of Meiosis

• Meiosis I:

  • Homologous chromosomes separate during this reduction division.

• Meiosis II:

  • Sister chromatids separate during this equational division.

Meiosis I Phases

1. Prophase I:

   • Chromosomes condense and homologous chromosomes pair (synapsis).

   • Crossing over occurs, exchanging genetic material between nonsister chromatids.

2. Metaphase I:

   • Tetrads align at the metaphase plate; microtubules attach to kinetochores.

3. Anaphase I:

   • Homologous chromosomes separate and move to opposite poles.

4. Telophase I and Cytokinesis:

   • Two haploid daughter cells form, each with chromosomes consisting of sister chromatids still attached.

Meiosis II Phases

1. Prophase II:

   • Spindle apparatus forms, and chromosomes move toward the metaphase plate.

2. Metaphase II:

   • Sister chromatids are aligned at the metaphase plate.

3. Anaphase II:

   • Sister chromatids separate and move to opposite poles.

4. Telophase II and Cytokinesis:

   • Four haploid daughter cells with unreplicated chromosomes are formed, each genetically distinct.

Genetic Variation

• Genetic diversity originates from mutations in DNA, creating new allele forms.

• Variations arise through:

  1. Independent assortment of chromosomes:

  • Each homologous pair orients randomly during metaphase I, leading to various combinations.

  • For humans, where n = 23, there are over 8 million possible combinations due to this process (2^{23}).

  1. Crossing over:

  • Occurs in prophase I; segments of nonsister chromatids exchange genetic material, creating recombinant chromosomes.

  • It can occur multiple times per chromosome pair.

  1. Random fertilization:

  • Any sperm can fertilize any egg, resulting in a unique zygote from the combination.

• Each zygote has a unique genetic blueprints, contributing to population diversity.

• Sexual reproduction is significant as it introduces genetic variability through the reshuffling of alleles.

Meiosis Overview

Meiosis converts diploid cells (2n) to haploid gametes (n), consisting of two rounds of cell division: meiosis I and meiosis II. It results in four daughter cells, each with half as many chromosomes as the parent cell.

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate during this reduction division.

  • Prophase I: Chromosomes condense and homologous chromosomes pair (synapsis). Crossing over occurs, exchanging genetic material between nonsister chromatids.

  • Metaphase I: Tetrads align at the metaphase plate; microtubules attach to kinetochores.

  • Anaphase I: Homologous chromosomes separate and move to opposite poles.

  • Telophase I and Cytokinesis: Two haploid daughter cells form, each with chromosomes consisting of sister chromatids still attached.

• Meiosis II: Sister chromatids separate during this equational division.

  • Prophase II: Spindle apparatus forms, and chromosomes move toward the metaphase plate.

  • Metaphase II: Sister chromatids are aligned at the metaphase plate.

  • Anaphase II: Sister chromatids separate and move to opposite poles.

  • Telophase II and Cytokinesis: Four haploid daughter cells with unreplicated chromosomes are formed, each genetically distinct.

Comparison of Meiosis and Mitosis

• Meiosis differs from mitosis in that it produces haploid gametes, whereas mitosis produces diploid somatic cells.

• Meiosis involves two rounds of cell division, resulting in four non-identical daughter cells, while mitosis consists of one division, resulting in two identical daughter cells.

• Genetic recombination occurs during meiosis through crossing over, which does not happen in mitosis.

Conceptual - What Is Meiosis Doing and Why?

Meiosis serves the primary purpose of generating genetic diversity in sexually reproducing organisms. It reduces chromosome number and ensures that each gamete contains a combination of genes from both parents, contributing to variability in the offspring.

Crossing Over

• When: Occurs during Prophase I of meiosis.

• How: Nonsister chromatids of homologous chromosomes exchange segments of genetic material.

• What Occurs: This exchange creates recombinant chromosomes, which have new combinations of alleles.

Generation of Genetic Variation in Sexual Life Cycles

Genetic variation is generated through several mechanisms during meiosis:

• Independent Assortment: Each homologous pair orients randomly during Metaphase I, leading to various combinations of chromosomes in gametes (for humans, this allows for over 8 million combinations due to 2^{23}).

• Crossing Over: As mentioned, occurs in Prophase I, resulting in recombinant chromosomes with new allele configurations.

• Random Fertilization: Any sperm can fertilize any egg, resulting in a unique zygote, further enhancing genetic diversity within a population.