Meiosis: Cell Division and Reproduction

• Meiosis is a specialized cell division, halving chromosome number and creating four unique haploid cells, differing genetically from the parent
• This process is essential for sexual reproduction in eukaryotes

Purpose of Meiosis

• Gamete production (sperm and egg in animals, spores in plants/fungi) is the primary purpose of meiosis, which is essential for sexual reproduction
• Fertilization involves the fusion of two gametes, restoring the original diploid chromosome number in the zygote
• Genetic diversity in offspring is achieved through independent assortment and crossing over during meiosis

Phases of Meiosis

• Two successive nuclear divisions characterize Meiosis: Meiosis I and Meiosis II
• Prophase, metaphase, anaphase, and telophase are the stages in each division

Meiosis I

• Meiosis I separates homologous chromosomes (each with two sister chromatids), reducing chromosome number from diploid to haploid

Prophase I

• Leptotene, zygotene, pachytene, diplotene, and diakinesis are the stages of Prophase I, the longest phase of meiosis
• During leptotene, chromosomes condense and become visible
• Synapsis occurs in zygotene, pairing homologous chromosomes and forming a synaptonemal complex
• Complete synapsis characterizes pachytene; crossing over/genetic recombination occurs
• Non-sister chromatids exchange genetic material during crossing over, increasing genetic variation
• The synaptonemal complex disassembles during diplotene, and homologous chromosomes separate but remain attached at chiasmata
• Chiasmata indicate crossing over points
• Chromosomes fully condense, the nuclear envelope breaks down, and the meiotic spindle forms during diakinesis, the final stage of prophase I

Metaphase I

• Homologous chromosome pairs (tetrads) align at the metaphase plate
• Spindle fibers from opposite poles attach to each chromosome

Anaphase I

• Homologous chromosomes separate and move to opposite poles
• Sister chromatids stay connected at the centromere

Telophase I and Cytokinesis

• Chromosomes arrive at opposite poles, and the cell divides into two haploid daughter cells
• Each daughter cell has one chromosome from each homologous pair
• Cytokinesis occurs with telophase I, dividing the cytoplasm to form two cells

Meiosis II

• Meiosis II separates sister chromatids like mitosis, dividing further without changing chromosome number

Prophase II

• Chromosomes condense, and the nuclear envelope breaks down (if it reformed in telophase I)
• Spindle fibers form in each of the two haploid cells

Metaphase II

• Sister chromatids align at the metaphase plate in each cell
• Spindle fibers from opposite poles attach to sister chromatid kinetochores

Anaphase II

• Sister chromatids separate and move to opposite poles
• Separated sister chromatids become individual chromosomes

Telophase II and Cytokinesis

• Chromosomes arrive at opposite poles, and the nuclear envelope reforms
• Cytokinesis divides the cytoplasm in each cell, resulting in four unique haploid daughter cells

Genetic Variation

• Crossing over and independent assortment are the two main mechanisms of meiosis that generate genetic variation

Crossing Over

• Homologous chromosomes exchange genetic information during prophase I
• Recombinant chromosomes result from new allele combinations on the same chromosome

Independent Assortment

• During metaphase I, homologous pairs align randomly at the metaphase plate
• Each pair's orientation is independent, creating different chromosome combinations in gametes
• 2^n is the number of possible chromosome combinations, where n equals the number of chromosome pairs

Meiosis vs Mitosis

• Although both are forms of cell division, meiosis and mitosis differ in key aspects
• Mitosis produces two diploid daughter cells genetically identical to the parent cell
• Meiosis produces four haploid daughter cells with unique genetic compositions
• Mitosis involves one nuclear division, while meiosis involves two
• Meiosis involves synapsis and crossing over, while mitosis does not
• Mitosis is for growth, repair, and asexual reproduction
• Meiosis is for sexual reproduction

Errors in Meiosis

• Errors in meiosis can change chromosome number/structure, lead to genetic disorders
• Nondisjunction (chromosomes or sister chromatids fail to separate properly) causes aneuploidy
• Aneuploidy means cells have an abnormal chromosome number
• Trisomy (extra chromosome) and monosomy (missing chromosome) are examples of aneuploidy
• Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY) are common human aneuploidies

Significance of Meiosis

• Meiosis is essential for gamete (sperm and egg) production
• Meiosis maintains constant chromosome number across generations
• Meiosis generates genetic variation through crossing over and independent assortment
• Resulting genetic variation is crucial for evolution, providing raw material for natural selection