Meiosis and Sexual Life Cycles

Variations on a Theme

Offspring typically resemble their parents more than unrelated individuals, but variation exists among siblings. Heredity is the transmission of traits from one generation to the next, while variation refers to differences in appearance among offspring. Genetics is the scientific study of heredity and variation.

• Heredity: The passing of traits from parents to offspring.

• Variation: Differences in traits among individuals.

• Genetics: The study of heredity and variation.

• Example: Family members share some features but are not identical.

Inheritance of Genes

Genes are the units of heredity, made up of DNA segments. They are passed to the next generation via gametes (sperm and eggs). Most DNA is packaged into chromosomes. Humans have 46 chromosomes in somatic cells, and a gene's specific position on a chromosome is called its locus.

• Gene: Segment of DNA coding for a trait.

• Chromosome: Structure carrying genetic material.

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

• Somatic cells: All body cells except gametes.

Comparison of Asexual and Sexual Reproduction

Asexual reproduction involves a single parent passing all its genes to offspring without gamete fusion, producing genetically identical clones. Sexual reproduction involves two parents, resulting in offspring with unique gene combinations.

• Asexual reproduction: Offspring are clones of the parent.

• Sexual reproduction: Offspring inherit a mix of genes from both parents.

• Example: Hydra reproduces by budding; redwoods can form clones from a single parent tree.

Fertilization and Meiosis in Sexual Life Cycles

Sets of Chromosomes in Human Cells

Human somatic cells have 23 pairs of chromosomes. A karyotype is an ordered display of chromosome pairs. Homologous chromosomes have the same length, centromere position, and staining pattern, and carry genes for the same traits. Sex chromosomes (X and Y) determine sex; autosomes are the other 22 pairs.

• Homologous chromosomes: Chromosome pairs with similar structure and gene content.

• Sex chromosomes: X and Y chromosomes determining sex.

• Autosomes: Non-sex chromosomes.

• Diploid cell (2n): Two sets of chromosomes; for humans, 2n = 46.

Chromosome Replication and Chromatid Structure

After DNA synthesis, each chromosome is replicated, forming two identical sister chromatids. Homologous pairs consist of one chromosome from each parent. Nonsister chromatids are maternal and paternal chromatids in a homologous pair.

• Sister chromatids: Identical copies of a chromosome.

• Nonsister chromatids: Chromatids from homologous chromosomes.

Haploid and Diploid Cells

Gametes contain a single set of chromosomes and are haploid (n). Somatic cells are diploid (2n). Fertilization unites gametes, forming a zygote with two sets of chromosomes.

• Haploid cell (n): One set of chromosomes; for humans, n = 23.

• Zygote: Fertilized egg with two sets of chromosomes.

The Variety of Sexual Life Cycles

Meiosis and fertilization alternate in all sexually reproducing organisms. The timing of these events varies among animals, plants, fungi, and protists.

• Animal life cycle: Gametes are the only haploid cells; fertilization forms a diploid zygote.

• Plant and algae life cycle: Alternation of generations with both diploid (sporophyte) and haploid (gametophyte) multicellular stages.

• Fungi and protist life cycle: Only the zygote is diploid; most of the life cycle is haploid.

Meiosis: Reduction of Chromosome Sets

Overview of Meiosis

Meiosis is preceded by chromosome replication and consists of two consecutive divisions: meiosis I and meiosis II. Four haploid daughter cells are produced, each genetically distinct.

• Meiosis I: Homologous chromosomes are separated.

• Meiosis II: Sister chromatids are separated.

Stages of Meiosis I

Meiosis I includes prophase I, metaphase I, anaphase I, and telophase I with cytokinesis. Key events include pairing of homologs, crossing over, and separation of homologous chromosomes.

• Prophase I: Homologs pair and crossing over occurs at chiasmata.

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

• Anaphase I: Homologs separate; sister chromatids remain together.

• Telophase I and Cytokinesis: Two haploid cells form.

Stages of Meiosis II

Meiosis II resembles mitosis, with prophase II, metaphase II, anaphase II, and telophase II with cytokinesis. Sister chromatids are separated, resulting in four haploid cells.

• Prophase II: Spindle apparatus forms; chromosomes move to metaphase plate.

• Metaphase II: Chromatids align at metaphase plate.

• Anaphase II: Sister chromatids separate.

• Telophase II and Cytokinesis: Four genetically distinct haploid cells form.

Crossing Over and Synapsis During Prophase I

During prophase I, sister chromatids are held together by cohesins. Nonsister chromatids break at matching points, and the synaptonemal complex forms, facilitating crossing over. DNA breaks are repaired, joining DNA from one chromatid to another.

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

• Synaptonemal complex: Structure holding homologs together.

Comparison of Mitosis and Meiosis

Mitosis produces two genetically identical cells, conserving chromosome number. Meiosis produces four genetically distinct cells, reducing chromosome number from diploid to haploid. Three events are unique to meiosis I: synapsis and crossing over, alignment of homologs at the metaphase plate, and separation of homologs.

• Mitosis: Conserves chromosome number; produces identical cells.

• Meiosis: Reduces chromosome number; produces genetically diverse cells.

• Unique events in meiosis I: Synapsis/crossing over, homolog alignment, homolog separation.

Genetic Variation and Evolution

Genetic Variation in Sexual Life Cycles

Mutations are the original source of genetic diversity, creating different alleles. Sexual reproduction reshuffles alleles, producing genetic variation. Three mechanisms contribute: independent assortment, crossing over, and random fertilization.

• Mutation: Change in DNA sequence.

• Allele: Variant form of a gene.

• Independent assortment: Random orientation of homologs during meiosis.

• Crossing over: Exchange of genetic material between homologs.

• Random fertilization: Any sperm can fuse with any egg.

Independent Assortment of Chromosomes

Homologous pairs orient randomly at metaphase I, sorting maternal and paternal homologs independently. The number of possible combinations is , where n is the haploid number. For humans, n = 23, so there are over 8 million possible combinations.

• Formula: possible combinations.

• Example: Humans have combinations.

Crossing Over

Crossing over produces recombinant chromosomes, combining DNA from both parents. This process increases genetic variation, with one to three crossover events per chromosome in humans.

• Recombinant chromosome: Chromosome with mixed parental DNA.

Random Fertilization

Random fertilization further increases genetic variation. The fusion of two gametes, each with 8.4 million possible chromosome combinations, results in a zygote with about 70 trillion possible combinations. Crossing over adds even more variation.

• Genetic identity: Each zygote is genetically unique.

Evolutionary Significance of Genetic Variation

Genetic variation within populations is essential for evolution. Natural selection accumulates genetic variations favored by the environment. Mutations and sexual reproduction mix and match alleles, increasing diversity. Some asexual organisms, like bdelloid rotifers, increase diversity by incorporating foreign DNA.

• Natural selection: Favors beneficial genetic variations.

• Example: Bdelloid rotifer reproduces asexually but increases diversity by acquiring foreign DNA.