Chapter 13: Meiosis and Sexual Life Cycles

Introduction

This chapter explores the mechanisms of meiosis and its role in sexual life cycles, emphasizing how genetic variation arises and is inherited. Understanding these processes is fundamental to genetics and evolutionary biology.

Key Concepts and Questions

• 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?

Offspring Acquire Genes from Parents by Inheriting Chromosomes

Genetics and Heredity

Genetics is the study of heredity and inherited variation. Heredity refers to the transmission of traits from parents to offspring. Variation is the difference in traits among individuals.

• Genes are units of heredity, made of DNA, and are passed to the next generation via reproductive cells called gametes (sperm and egg).

• Most DNA is packaged into chromosomes.

• Humans have 46 chromosomes in the nuclei of their somatic cells (body cells).

• A gene’s specific position along a chromosome is called its locus.

• Sexual reproduction produces offspring with genetic variation, while asexual reproduction creates genetically identical clones.

Fertilization and Meiosis Alternate in Sexual Life Cycles

Chromosome Behavior in Life Cycles

The behavior of chromosomes during meiosis and fertilization is central to the human life cycle and other sexual life cycles. These processes ensure the transmission and variation of genetic material.

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

• Homologous chromosomes (homologs) are pairs that have the same length, centromere position, and staining pattern, and carry genes for the same inherited characters.

• A karyotype is an ordered display of the chromosome pairs of a cell.

Diploid and Haploid Cells

• A diploid cell (2n) has two sets of chromosomes. In humans, 2n = 46.

• A haploid cell (n) has one set of chromosomes. In humans, n = 23.

• Gametes (sperm and egg) are haploid and produced by meiosis.

• Fertilization restores the diploid number, forming a zygote.

Sexual Life Cycle Overview

• Meiosis reduces chromosome number from diploid to haploid.

• Fertilization restores diploid number.

• Alternation of meiosis and fertilization is common to all sexually reproducing organisms.

• Some plants and algae exhibit alternation of generations, with both diploid (sporophyte) and haploid (gametophyte) multicellular stages.

Meiosis: Reducing Chromosome Number

Overview of Meiosis

Meiosis consists of two consecutive cell divisions: meiosis I and meiosis II. It results in four haploid daughter cells, each genetically distinct from the parent cell and from each other.

• Meiosis is preceded by chromosome replication.

• Each replicated chromosome consists of two identical sister chromatids.

Phases of Meiosis I

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

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

• Anaphase I: Homologous chromosomes separate; sister chromatids remain attached.

• Telophase I and Cytokinesis: Each cell has a haploid set of duplicated chromosomes; cytokinesis forms two daughter cells.

Phases of Meiosis II

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

• Metaphase II: Chromosomes align; sister chromatids are no longer genetically identical due to crossing over.

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

• Telophase II and Cytokinesis: Chromosomes decondense; nuclei form; cytokinesis produces four haploid cells.

Unique Events in Meiosis

• Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information.

• Alignment of homologous pairs at the metaphase plate (meiosis I).

• Separation of homologs during anaphase I.

Comparison of Mitosis and Meiosis

Key Differences

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

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

• In mitosis, cohesins are cleaved at the end of metaphase; in meiosis, cohesins are cleaved along chromosome arms in anaphase I and at centromeres in anaphase II.

Genetic Variation and Evolution

Sources of Genetic Variation

• Mutations are the original source of genetic diversity, creating different versions of genes called alleles.

• Shuffling of alleles during sexual reproduction increases genetic variation.

Mechanisms Contributing to Genetic Variation

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

• Crossing over: Produces recombinant chromosomes, combining DNA from both parents.

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

Equation for possible chromosome combinations due to independent assortment:

(where n is the haploid number; for humans, possible combinations)

Fusion of two gametes results in a zygote with about 70 trillion possible diploid combinations.

Table: Comparison of Mitosis and Meiosis

Table: Mechanisms of Genetic Variation

Summary

• Meiosis and fertilization are central to sexual life cycles and genetic variation.

• Meiosis reduces chromosome number and introduces genetic diversity through independent assortment, crossing over, and random fertilization.

• Genetic variation is essential for evolution and adaptation in populations.