Chapter 27: Reproductive System – Introduction and Meiosis

Reproductive System: Introduction and Meiosis

Learning Objectives

• Distinguish between sexual and asexual reproduction.

• Define somatic vs. gametic cells and their chromosome counts (2n=46, n=23).

• Explain why gametes must be haploid for normal chromosome number after fertilization.

• State the purpose of meiosis: produce haploid gametes and generate genetic variation.

• Contrast mitosis and meiosis by divisions, products, and genetic similarity/difference.

• Identify autosomes (22 pairs) vs. sex chromosomes (XX or XY) and define "homologous".

• Use correct terms: homologous chromosomes, sister chromatids, tetrad, dyad, allele.

• Describe crossing over (prophase I) and its role in increasing variation.

• Identify what separates in meiosis I (homologous pairs) vs. meiosis II (sister chromatids).

• Predict ploidy/chromosome numbers at the end of meiosis I and II.

• Identify where meiosis occurs (testes/ovaries) and name the processes (spermatogenesis/oogenesis).

Sexual vs. Asexual Reproduction

Definitions and Key Differences

• Asexual reproduction (Mitosis):

  • Process of duplicating cells as clones.

  • Genetic material from diploid (2n) mother cell is the same as daughter cell (2n).

  • Occurs in all somatic cells (body cells: liver, muscle, skin, etc.).

• Sexual reproduction (Meiosis):

  • Combination of two haploid cells (n) to create a genetically diverse/different daughter cell (2n).

  • Occurs in gametes (sperm and egg).

Purpose of Sexual Reproduction

Genetic Variation

• Sexual reproduction increases variation of genetic traits among offspring of the same parents.

• Gametes (haploid cells, n) are sex cells that fuse at fertilization to form a one-celled zygote (2n).

• This process ensures genetic diversity in populations.

Chromosome Numbers and Types

Human Chromosome Sets

• Humans have 23 pairs of chromosomes = 46 total chromosomes.

• Of the 23 pairs:

  • 22 pairs are autosomes (homologous chromosomes).

  • 1 pair are sex chromosomes (XX for female, XY for male).

Terminology

Term	Definition
Somatic cells	Body cells (skin, muscle, nerve, etc.), diploid (2n = 46 chromosomes)
Gametic cells	Reproductive cells (sperm, egg), haploid (n = 23 chromosomes)
Diploid (2n)	Two sets of chromosomes (one from each parent), found in somatic cells
Haploid (n)	One set of chromosomes, found in gametes
Homologous chromosomes	Chromosome pairs with the same genes (one from each parent)
Sister chromatids	Identical copies of a chromosome, joined at the centromere
Tetrad	Pair of homologous chromosomes (4 chromatids) during meiosis I
Dyad	Pair of sister chromatids (after homologous chromosomes separate)
Allele	Different versions of a gene

Meiosis: Process and Goals

Overview

• Meiosis produces four non-identical daughter cells (gametes), each with half the DNA (haploid, n).

• Creates genetic diversity through crossing over and independent assortment.

• Occurs in testes (spermatogenesis) and ovaries (oogenesis).

• Ensures only one of each chromosome and one of each allele is present in gametes.

Why Must Gametes Be Haploid?

• If gametes were not haploid, fertilization would double the chromosome number each generation.

• Haploid gametes (n) + Haploid gametes (n) = Diploid zygote (2n).

• Prevents genetic overload and maintains species-specific chromosome number.

S-Phase and Chromosome Duplication

• Before both mitosis and meiosis, DNA is duplicated during the S-phase of interphase.

• Each chromosome consists of two sister chromatids after duplication.

Stages of Meiosis

• Meiosis I:

  • Separates homologous pairs.

  • Crossing over occurs in Prophase I, creating genetic diversity.

  • Results in two haploid cells, each with duplicated chromosomes (dyads).

• Meiosis II:

  • Separates sister chromatids.

  • Results in four genetically unique haploid gametes.

Comparison Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Genetic Identity	Identical	Genetically different
Chromosome Number	Diploid (2n)	Haploid (n)
Crossing Over	No	Yes (Prophase I)
Function	Growth, repair	Gamete production

Key Phases of Meiosis

Prophase I

• Homologous chromosomes pair up and form tetrads.

• Crossing over occurs, exchanging genetic material between non-sister chromatids.

• Increases genetic variation among offspring.

Metaphase I

• Tetrads align at the metaphase plate.

• Each homologous chromosome faces opposite poles.

Anaphase I

• Homologous chromosomes separate and move to opposite poles.

• Sister chromatids remain attached.

• Cells become haploid (n), but chromosomes are still duplicated (dyads).

Telophase I

• Nuclei reform; each daughter cell has one chromosome from each homologous pair (still as sister chromatids).

Meiosis II

• Similar to mitosis, but starts with haploid cells.

• Sister chromatids separate during anaphase II.

• Results in four haploid gametes, each genetically unique.

Summary of Differences: Mitosis vs. Meiosis

• Mitosis:

  • One division, two identical diploid cells.

  • Used for growth and repair.

• Meiosis:

  • Two divisions, four non-identical haploid cells.

  • Used for gamete production (sperm and eggs).

  • Includes crossing over and independent assortment for genetic diversity.

Specialized Gametogenesis

• Spermatogenesis: Production of sperm in testes.

• Oogenesis: Production of eggs in ovaries.

• Meiosis occurs at different points in life for males and females.

Key Equations

• Chromosome number after meiosis:

  • $2n \rightarrow n$ (diploid to haploid)

• Fertilization restores diploid number:

  • $n + n = 2n$

Practice Questions

1. Meiosis involves two divisions and produces four nonidentical daughter nuclei.

2. One major difference between meiosis I and meiosis II is that crossing-over occurs in prophase I but not in prophase of meiosis II.

Additional info: These notes cover the introductory concepts of the reproductive system, focusing on the cellular and genetic mechanisms underlying sexual reproduction, including detailed comparisons between mitosis and meiosis, and the importance of genetic diversity in populations.