Chapter 13: Meiosis and Sexual Life Cycle

Big Idea

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid cells (gametes) in sexually reproducing organisms. This process is fundamental for genetic diversity and the continuity of species.

Concept 13.1: Offspring Acquire Genes from Parents by Inheriting Chromosomes

Inheritance and Heredity

• Inheritance (Heredity): The transmission of traits from one generation to the next.

• Genetics: The scientific study of heredity and variation among individuals.

• Offspring inherit genes, the units of heredity, which are segments of DNA located on chromosomes.

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

• Humans have 46 chromosomes in somatic cells (all body cells except gametes and their precursors).

• A gene’s specific location on a chromosome is called its locus.

Comparison of Asexual and Sexual Reproduction

• Asexual reproduction: A single individual passes all its genes to offspring without gamete fusion, producing genetically identical clones.

• Sexual reproduction: Two parents contribute genes, resulting in offspring with unique genetic combinations.

Mechanisms of Inheritance

• Meiosis and fertilization are the biological mechanisms that account for the resemblance between offspring and their parents.

Concept 13.2: Fertilization and Meiosis Alternate in Sexual Life Cycles

Life Cycles and Chromosome Sets

• Life cycle: The sequence of stages in the reproductive history of an organism.

• Human somatic cells have 23 pairs of chromosomes (46 total), arranged as homologous pairs.

• Homologous chromosomes (homologs): Chromosomes of the same length, centromere position, and gene content, one inherited from each parent.

• Autosomes: Non-sex chromosomes (22 pairs in humans).

• Sex chromosomes: X and Y chromosomes determine biological sex (XX = female, XY = male).

Diploid and Haploid Cells

• Diploid cell (2n): Contains two sets of chromosomes (one from each parent); in humans, 2n = 46.

• Haploid cell (n): Contains one set of chromosomes; in humans, n = 23 (gametes).

• After DNA replication, each chromosome consists of two sister chromatids.

Human Life Cycle and Chromosome Behavior

• Fertilization: Union of gametes (sperm and egg) to form a diploid zygote.

• The zygote divides by mitosis to produce somatic cells and develop into an adult.

• Gametes are produced only by meiosis in the ovaries and testes.

• Meiosis reduces chromosome number by half; fertilization restores diploid number.

Variety of Sexual Life Cycles

• All sexually reproducing organisms alternate meiosis and fertilization, but the timing varies:

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

• Plants and some algae: Exhibit alternation of generations with multicellular diploid (sporophyte) and haploid (gametophyte) stages.

• Fungi and some protists: Only the zygote is diploid; meiosis produces haploid cells that divide by mitosis.

Concept 13.3: Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid

Overview of Meiosis

• Meiosis consists of two consecutive divisions: meiosis I and meiosis II.

• Results in four haploid daughter cells, each genetically distinct.

• Preceded by chromosome replication during interphase.

Phases of Meiosis

• Each division (meiosis I and II) has four phases: prophase, metaphase, anaphase, telophase, followed by cytokinesis.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Key Events Unique to Meiosis

• Synapsis and crossing over: Homologous chromosomes physically connect and exchange genetic material during prophase I.

• Alignment of homologous pairs: Homologs align at the metaphase plate during metaphase I.

• Separation of homologs: Homologous chromosomes are separated during anaphase I.

Comparison of Mitosis and Meiosis

• Mitosis: Produces two genetically identical diploid cells; conserves chromosome number.

• Meiosis: Produces four genetically unique haploid cells; reduces chromosome number by half.

Concept 13.4: Genetic Variation Produced in Sexual Life Cycles Contributes to Evolution

Sources of Genetic Variation

• Mutations: The original source of genetic diversity; create new alleles.

• Independent assortment: Homologous chromosomes orient randomly at metaphase I, leading to many possible combinations in gametes.

• Crossing over: Exchange of genetic material between nonsister chromatids during prophase I, producing recombinant chromosomes.

• Random fertilization: Any sperm can fuse with any egg, further increasing genetic variation.

Mathematical Representation of Independent Assortment

• The number of possible chromosome combinations due to independent assortment is , where n is the haploid number.

• For humans (n = 23): possible combinations.

Significance for Evolution

• Genetic variation produced by meiosis and fertilization is essential for evolution by natural selection.

• Each zygote is genetically unique, contributing to the diversity of populations.