BIO152

Chapter 13: Meiosis

Key Concepts

• Offspring acquire genes from parents by inheriting chromosomes.

• Fertilization and meiosis alternate in sexual life cycles.

• Meiosis reduces the number of chromosome sets from diploid to haploid.

• Genetic variation produced in sexual life cycles contributes to evolution.

Overview of Meiosis

• Living organisms reproduce their own kind.

• Genetics is the study of heredity and variation.

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

Concept 13.1: Offspring Acquire Genes from Parents by Inheriting Chromosomes

• Children inherit genes, not specific physical traits.

• Genes: Units of heredity made of DNA; passed to the next generation via gametes (sperm and eggs).

  • Specific chromosomal location known as a locus.

  • Most DNA is organized into chromosomes.

Page 2: Comparison of Asexual and Sexual Reproduction

Asexual Reproduction

• One individual passes genes to offspring without gamete fusion.

• Produces clones (genetically identical individuals).

• Advantages: No need for a mate, energy conservation, genetically stable traits.

• Disadvantages: Lack of genetic diversity may not be effective against environmental changes.

Sexual Reproduction

• Involves two parents, resulting in unique genetic combinations.

• Produces varied offspring increasing genetic diversity.

Describing Chromosomes

• After DNA synthesis, each chromosome has been replicated, consisting of two identical sister chromatids.

Page 3: Sets of Chromosomes in Human Cells

Gametes and Chromosome Number

• Gametes (sperm or egg): contain a single set of chromosomes (haploid, n).

• Human haploid number is 23 (n=23), comprising 22 autosomes and one sex chromosome.

• Unfertilized eggs contain X sex chromosome; sperm can carry X or Y.

Variety of Sexual Life Cycles

• Only haploid cells are gametes, produced by meiosis without further cell division before fertilization.

• Gametes fuse to form a diploid zygote, which undergoes mitosis to develop a multicellular organism.

Plant and Algal Life Cycles

• Exhibit alternation of generations: includes both diploid (sporophyte) and haploid (gametophyte) stages.

• Spores produced by meiosis grow into gametophytes that make gametes through mitosis.

Page 4: Third Type of Sexual Life Cycle

Haploid Organisms

• Haploid cells grow by mitosis into multicellular organisms.

• Each haploid adult produces gametes through mitosis.

Genetic Variation and Importance in Meiosis

• Meiosis causes genetic variation through halving/doubling of chromosomes, thus aiding evolution.

Overview of Meiosis

• Preceded by chromosome replication.

• Involves two rounds of cell division (meiosis I and II), resulting in four daughter cells with half the chromosomes of the parent cell.

Page 5: Detailed Meiosis Process

Overview of Meiosis Process

• Meiosis begins after chromosomes replicate during interphase.

• Synapsis and Crossing Over: Prophase I

  • Sister chromatids are held together; homologous chromosomes pair up forming tetrads.

  • The synaptonemal complex holds homologs together.

  • Crossing over occurs, exchanging DNA segments between non-sister chromatids.

Key Components of Meiosis

• Mitosis vs. Meiosis: Mitosis conserves chromosome number, while meiosis reduces it.

Page 6: Genetic Variation Produced in Sexual Life Cycles

Mechanisms Contributing to Genetic Variation

1. Independent Assortment of Chromosomes

   • Each gamete can sort chromosomes independently, leading to over 8 million combinations in humans.

2. Crossing Over

   • Produces recombinant chromosomes, increasing genetic diversity.

3. Random Fertilization

   • Any sperm can combine with any ovum, adding to unique genetic combinations across zygotes.

Abnormal Chromosome Number: Meiotic Nondisjunction

• Nondisjunction leads to gametes with abnormal chromosome counts, which may result in conditions like Down syndrome (Trisomy 21).

Page 7: Mendelian Genetics and Chromosomal Functions

Understanding Genetic Inheritance Patterns

• Mendelian inheritance is based on chromosomal behavior during meiosis.

• Sex-linked genes show unique inheritance patterns due to their localization on sex chromosomes.

Chromosomal Theory of Inheritance

• States that genes are located on chromosomes and undergo segregation and independent assortment.

Page 8: Examples of Genetic Disorders from Chromosomal Alterations

Abnormal Chromosome Number and Structure

• Large-scale alterations can lead to spontaneous abortions or genetic disorders.

Sex Chromosome Aneuploidy Examples

1. XXX females: typically healthy.

2. Klinefelter syndrome (XXY): results from an extra X chromosome in males.

3. Turner syndrome (X0): viable monosomy resulting in sterile females.

By summarizing the essential concepts and details from the various pages, these notes will serve as an effective study resource for Meiosis, Genetics, and Natural Selection.'