Ch. 13 Meiosis and Sexual Life Cycles – Study Notes

Chapter 13: Meiosis and Sexual Life Cycles

Key Questions and Concepts

• How many pairs of chromosomes are in a cell?

• What are the different types of alternation of generations?

• What happens in each step of meiosis I and meiosis II?

• What allows for genetic variation in meiosis?

• What happens in meiosis I that does not happen in meiosis II?

• What are the major differences between mitosis and meiosis?

Inheritance and Chromosomes

Genes, Heredity, and Genetic Variation

Inheritance is the process by which offspring acquire genes from their parents. The study of heredity and inherited variation is called genetics.

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

• Genetic variation: Differences among individuals in the composition of their genes or DNA sequences.

• Genes: Units of heredity made up of DNA, passed to the next generation via reproductive cells called gametes (sperm and eggs).

• Somatic cells: All body cells except gametes; in humans, each somatic cell has 46 chromosomes.

• Locus: The specific location of a gene on a chromosome.

• Clone: An organism that is genetically identical to its parent, produced by asexual reproduction.

Fertilization and Meiosis in Sexual Life Cycles

Chromosome Behavior and Human Life Cycle

The behavior of chromosomes during meiosis and fertilization is central to sexual life cycles. Humans and many other organisms alternate between diploid and haploid stages.

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

• Homologous chromosomes (homologs): Chromosome pairs with the same length, centromere position, and staining pattern, carrying genes for the same traits.

• Karyotype: An ordered display of the pairs of chromosomes from a cell.

Table: Types of Human Chromosomes

Type	Number of Pairs	Description
Autosomes	22	Non-sex chromosomes
Sex Chromosomes	1	Determine biological sex (XX or XY)

Diploid and Haploid Cells

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

• Haploid cell (n): Contains a single set of chromosomes. In humans, n = 23. Gametes are haploid.

• During fertilization, two haploid gametes (sperm and egg) unite to form a zygote, restoring the diploid number.

Sexual Life Cycles and Alternation of Generations

Types of Sexual Life Cycles

All sexually reproducing organisms alternate between meiosis and fertilization, but the timing and prominence of haploid and diploid stages vary.

• Animal life cycle: Diploid multicellular organism (most of the life cycle is diploid).

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

• Fungi and some protists: Most of the life cycle is haploid; only the zygote is diploid.

Additional info: Alternation of generations is a key feature in the life cycles of plants and some algae, where both haploid and diploid stages are multicellular.

Meiosis: Reducing Chromosome Number

Overview of Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half, producing four haploid cells from one diploid cell. It consists of two consecutive divisions: meiosis I and meiosis II.

• Preceded by chromosome replication (S phase of interphase).

• Results in four genetically distinct haploid cells.

Phases of Meiosis I

• Prophase I: Homologous chromosomes pair up and exchange genetic material through crossing over at structures called chiasmata.

• Metaphase I: Homologous pairs align at the metaphase plate; microtubules attach to each homolog.

• Anaphase I: Homologous chromosomes separate and move toward opposite poles; sister chromatids remain attached.

• Telophase I and Cytokinesis: Two haploid cells form, each with duplicated chromosomes.

Phases of Meiosis II

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

• Metaphase II: Chromosomes align at the metaphase plate; due to crossing over, sister chromatids are no longer genetically identical.

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

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

Mechanisms of Genetic Variation

Sources of Genetic Variation

Genetic variation is essential for evolution and is generated during sexual reproduction by several mechanisms:

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

• Crossing over: Exchange of genetic material between non-sister chromatids during prophase I creates new combinations of alleles.

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

Table: Mechanisms of Genetic Variation

Mechanism	Description	Result
Independent Assortment	Random orientation of homologous pairs at metaphase I	Many possible gamete combinations
Crossing Over	Exchange of DNA between homologous chromosomes	Recombinant chromosomes
Random Fertilization	Any sperm can fertilize any egg	Unique zygotes

For humans (n = 23), independent assortment alone allows for $2^{23}$ (over 8 million) possible combinations of chromosomes in gametes. The fusion of two gametes can produce a zygote with about 70 trillion possible diploid combinations.

Comparison of Mitosis and Meiosis

Key Differences

• Mitosis produces two genetically identical diploid cells; meiosis produces four genetically unique haploid cells.

• Mitosis conserves chromosome number; meiosis reduces it by half.

• Unique events in meiosis (all in meiosis I):

  • Synapsis and crossing over

  • Alignment of homologous pairs at the metaphase plate

  • Separation of homologs (not sister chromatids) during anaphase I

Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Chromosome Number in Daughter Cells	Diploid (2n)	Haploid (n)
Genetic Identity	Identical	Unique
Role	Growth, repair, asexual reproduction	Sexual reproduction

Summary

• Meiosis and fertilization are central to sexual life cycles, ensuring genetic diversity and stable chromosome numbers across generations.

• Genetic variation arises from independent assortment, crossing over, and random fertilization.

• Meiosis differs from mitosis in its reduction of chromosome number and generation of genetic diversity.