6.2 Meiosis and Sexual Reproduction

Meiosis is the cell division process that creates sex cells. It halves the chromosome count, allowing two gametes to combine during fertilization. This process is crucial for sexual reproduction and genetic diversity.

Meiosis involves two rounds of division: meiosis I and II. These steps include chromosome pairing, crossing over, and independent assortment, which shuffle genes and create unique combinations in offspring.

Meiosis Overview

Process and Purpose of Meiosis

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• Meiosis is a type of cell division that produces gametes with half the number of chromosomes as the parent cell
• Occurs in reproductive cells of sexually reproducing organisms
• Consists of two rounds of cell division, meiosis I and meiosis II, which results in four haploid daughter cells

Haploid and Diploid Cells

• Gametes, reproductive cells such as sperm and egg cells, are haploid cells containing only one set of chromosomes (n)
• Diploid cells, such as somatic cells, contain two sets of chromosomes (2n), one from each parent
• When two haploid gametes fuse during fertilization, the resulting zygote is diploid, restoring the chromosome number

Genetic Variation

• Meiosis introduces genetic variation in the resulting gametes through independent assortment and crossing over
• Independent assortment randomly distributes maternal and paternal homologs to daughter cells
• Crossing over, the exchange of genetic material between homologous chromosomes, creates new combinations of alleles on the chromosomes

Meiosis I

Homologous Chromosomes and Synapsis

• Homologous chromosomes are pairs of chromosomes, one maternal and one paternal, that carry the same genes but may have different alleles
• During prophase I, homologous chromosomes align and undergo synapsis, forming a tetrad or bivalent
• Synapsis involves the formation of the synaptonemal complex, which holds the homologous chromosomes together and facilitates crossing over

Crossing Over

• Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes
• Occurs during prophase I when the homologous chromosomes are closely aligned
• Results in new combinations of alleles on the chromosomes, increasing genetic diversity in the gametes

Meiosis II

Comparison to Meiosis I

• Meiosis II is similar to mitosis, as it involves the separation of sister chromatids
• Unlike meiosis I, there is no pairing of homologous chromosomes or crossing over in meiosis II
• Meiosis II results in the production of four haploid daughter cells, each with half the number of chromosomes as the parent cell

Importance of Meiosis I and II

• Meiosis I is a reductional division that separates homologous chromosomes and reduces the chromosome number by half
• Meiosis II is an equational division that separates sister chromatids, resulting in four haploid daughter cells
• Both meiosis I and II are necessary to produce genetically diverse gametes with the correct number of chromosomes

Independent Assortment

• Independent assortment is the random distribution of maternal and paternal homologs to daughter cells during meiosis I
• Occurs during metaphase I when homologous pairs align independently on the metaphase plate
• Results in a variety of possible combinations of maternal and paternal chromosomes in the gametes, contributing to genetic diversity