Section 7: The Cell Cycle, Mitosis, and Meiosis

Key Roles of Cell Division

Cell division is a fundamental biological process that enables organisms to grow, repair tissues, and reproduce. It ensures the transmission of genetic material from one generation to the next.

• Genetically Identical Daughter Cells: Most cell division, such as mitosis, results in daughter cells that are genetically identical to the parent cell.

• Alternation of Phases: The cell cycle alternates between the mitotic phase (cell division) and interphase (cell growth and DNA replication).

Variations on a Theme: Sexual and Asexual Reproduction

Offspring Acquire Genes from Parents by Inheriting Chromosomes (13.1)

Inheritance is the process by which offspring receive genetic information from their parents through chromosomes. This ensures continuity of traits across generations.

• Genes: Units of heredity composed of DNA, located on chromosomes.

• Gametes: Reproductive cells (sperm and egg) that carry genes from one generation to the next.

• Fertilization: The union of male and female gametes, resulting in offspring with genes from both parents.

• Somatic Cells: Body cells that contain a characteristic number of chromosomes (e.g., 46 in humans, arranged in 23 pairs).

Example: In humans, each parent contributes 23 chromosomes via their gametes, resulting in a zygote with 46 chromosomes.

Comparison of Asexual and Sexual Reproduction

Organisms can reproduce either asexually or sexually, each with distinct genetic outcomes.

• Asexual Reproduction: A single parent produces offspring that are genetically identical to itself (clones).

• Sexual Reproduction: Two parents produce offspring with unique combinations of genes, leading to genetic diversity.

Example: Bacteria reproduce asexually by binary fission, while humans reproduce sexually.

Fertilization and Meiosis Alternate in Sexual Life Cycles (13.2)

Sexual Life Cycle

The sexual life cycle involves the alternation of fertilization and meiosis, ensuring genetic diversity and the maintenance of chromosome number across generations.

• Life Cycle: The sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.

• Chromosome Count: Gametes are haploid (n), while somatic cells are diploid (2n).

• Karyotype: The display of chromosome pairs, arranged by size and shape.

• Homologous Chromosomes: Chromosome pairs matched in length, centromere position, and staining pattern; carry the same genes but may have different alleles.

Example: Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females, XY for males).

Variety of Sexual Life Cycles

Different organisms exhibit variations in their sexual life cycles, but all feature alternation of fertilization and meiosis.

• Animals (e.g., humans): Meiosis occurs in germ cells to produce gametes; fertilization restores diploid state.

• Plants and Some Algae: Alternation of generations with both diploid (sporophyte) and haploid (gametophyte) multicellular stages.

• Fungi and Some Protists: The only diploid stage is the single-celled zygote; meiosis produces haploid cells that divide by mitosis.

Additional info: In plants, spores produced by meiosis develop into gametophytes, which produce gametes by mitosis.

Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid (13.3)

Overview of Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes from diploid precursor cells.

• Two Consecutive Divisions: Meiosis I (homologous chromosomes separate) and Meiosis II (sister chromatids separate).

• Result: Four haploid daughter cells, each genetically distinct from the parent cell and each other.

Equation:

Example: In humans, meiosis produces gametes with 23 chromosomes from cells with 46 chromosomes.

Stages of Meiosis

Meiosis consists of several stages, each with specific chromosomal events.

• Prophase I: Homologous chromosomes pair and exchange genetic material (crossing over).

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

• Anaphase I: Homologous chromosomes separate.

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

Additional info: Crossing over during prophase I increases genetic variation by producing recombinant chromosomes.

Comparison of Mitosis and Meiosis

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

Additional info: Unique events in meiosis I include synapsis and crossing over, alignment of homologous pairs, and separation of homologues.

Genetic Variation Produced in Sexual Life Cycles Contributes to Evolution (13.4)

Sources of Genetic Variation Among Offspring

Sexual reproduction generates genetic diversity, which is essential for evolution and adaptation.

• Independent Assortment: Homologous chromosomes are randomly distributed to gametes, resulting in numerous possible combinations.

• Equation:

possible combinations, where n is the haploid number (e.g., = 8.4 million in humans)

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I creates recombinant chromosomes.

• Random Fertilization: The fusion of gametes from two parents further increases genetic variation.

• Equation:

Number of possible zygotes = (number of possible gametes from one parent) × (number from the other parent) possible zygotes

Additional info: Genetic variation is the raw material for evolution, allowing populations to adapt to changing environments.