Meiosis and the Sexual Life Cycle

Big Idea: Meiosis in Sexual Reproduction

Meiosis is a specialized type of cell division that produces gametes (sperm and eggs) in sexually reproducing organisms. It ensures genetic diversity and maintains the stability of chromosome number across generations.

• Meiosis produces cells with half the chromosome number of the parent cell.

• Occurs only in specialized cells (germ-line cells).

• Alternates with fertilization in the sexual life cycle.

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Comparison of Asexual and Sexual Reproduction

Asexual and sexual reproduction differ in how genetic material is inherited and how offspring are produced.

• Asexual reproduction: One parent; offspring are genetically identical (clones).

• Sexual reproduction: Two parents; offspring inherit unique combinations of genes.

• Inheritance: Traits are passed from parents to offspring via chromosomes.

Inheritance of Genes and Chromosomes

Genes are the units of heredity, made of DNA, and are located on chromosomes. Chromosomes are passed to offspring through gametes.

• Somatic cells: All body cells except gametes; diploid (2n).

• Gametes: Sperm and eggs; haploid (n).

• Locus: Specific location of a gene on a chromosome.

Chromosome Sets in Human Cells

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).

• Homologous chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Autosomes: Non-sex chromosomes.

• Sex chromosomes: X and Y; determine biological sex.

• Diploid (2n): Two sets of chromosomes (somatic cells).

• Haploid (n): One set of chromosomes (gametes).

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Human Life Cycle: Alternation of Meiosis and Fertilization

The human life cycle alternates between meiosis and fertilization to maintain chromosome number and generate genetic diversity.

• Fertilization: Union of haploid gametes forms a diploid zygote.

• Zygote: First cell of a new organism; diploid.

• Meiosis: Produces haploid gametes from diploid cells.

• Mitosis: Produces somatic cells for growth and development.

Variety of Sexual Life Cycles

Sexual life cycles differ among animals, plants, fungi, and protists in the timing of meiosis and fertilization.

• Animals: Gametes are the only haploid cells; fertilization produces diploid zygote.

• Plants and some algae: Alternation of generations; both diploid (sporophyte) and haploid (gametophyte) multicellular stages.

• Fungi and some protists: Haploid unicellular or multicellular stage predominates.

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Stages of Meiosis

Meiosis consists of two consecutive divisions: Meiosis I and Meiosis II, resulting in four haploid daughter cells.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

• Phases: Prophase, Metaphase, Anaphase, Telophase (in both divisions).

• Synapsis: Homologs pair and crossing over occurs in Prophase I.

• Chiasmata: Sites of crossing over.

Comparison of Mitosis and Meiosis

Mitosis and meiosis are both forms of cell division, but they differ in their outcomes and processes.

• Mitosis: Produces two genetically identical diploid cells.

• Meiosis: Produces four genetically distinct haploid cells.

• Three events unique to meiosis: synapsis and crossing over, alignment of homologous pairs, separation of homologs.

Genetic Variation in Sexual Life Cycles

Sexual reproduction generates genetic variation, which is essential for evolution.

• Mutations: Original source of genetic diversity.

• Independent assortment: Random orientation of homologous pairs during meiosis.

• Crossing over: Exchange of genetic material between homologous chromosomes.

• Random fertilization: Any sperm can fertilize any egg, increasing variation.

Independent Assortment

Each pair of chromosomes sorts independently, creating many possible combinations.

For humans: possible combinations.

Crossing Over

Produces recombinant chromosomes, combining DNA from both parents.

Random Fertilization

Fusion of gametes results in about 70 trillion possible diploid combinations.

Key Terms and Definitions

• Gene: Unit of heredity; segment of DNA.

• Chromosome: DNA molecule with associated proteins.

• Homologous chromosomes: Chromosome pairs with same genes.

• Allele: Variant form of a gene.

• Gamete: Haploid reproductive cell (sperm or egg).

• Zygote: Diploid cell formed by fertilization.

• Synapsis: Pairing of homologous chromosomes.

• Chiasma: Site of crossing over.

Summary Table: Meiosis vs. Mitosis

Equations and Formulas

• Possible gamete combinations from independent assortment:

• For humans:

Examples and Applications

• Hydra: Asexual reproduction by budding.

• Redwoods: Clonal growth.

• Human gametes: Sperm and egg formation via meiosis.

Additional info:

• Meiosis is essential for maintaining chromosome number and generating genetic diversity in sexually reproducing organisms.

• Genetic variation is the foundation for evolution and adaptation.