9 1 The Basics of Meiosis 9 1 The Basics of Meiosis cont Animals and plants practice sexual reproduction with parents passing chromosomes to their offspring The chromosomes of each parent exist in pairs called homologues or homologous chromosomes Because each child receives unique combinations of chromosomes from the parents each child differs from the parents The pairs are identical in size shape construction and the genes they contain Meiosis is the process of cell division that contributes to sexual reproduction and the resulting variation in the offspring However the homologues within an organism may contain different versions or alleles of those genes 9 1 The Basics of Meiosis cont 9 1 The Basics of Meiosis cont In humans there are 23 pairs of homologous chromosomes There are 22 pairs of autosomes The sex chromosomes comprise the last pair Males have an X and a smaller Y chromosome Females have two equal X chromosomes 9 1 The Basics of Meiosis cont With 23 pairs of chromosomes humans have a total of 46 chromosomes This total number of chromosomes is called the diploid number The number of chromosome pairs an organism has is its haploid number The Human Life Cycle The life cycle of a sexually reproducing organism includes all the reproductive events from one generation to the next The human life cycle involves two types of cell division Mitosis Meiosis 1 The Human Life Cycle cont The Human Life Cycle cont Mitosis is involved in the growth of a child and repair of tissues In males meiosis is part of sperm production spermatogenesis Meiosis is a special form of cell division associated with sexual reproduction In females meiosis is part of egg production oogenesis Meiosis produces special reproductive cells called gametes eggs and sperm in humans Gametes have only one chromosome of each chromosome pair e g 23 instead of 46 During fertilization an egg and sperm unite to form a zygote restoring the chromosomes to 46 total The Human Life Cycle cont Overview of Meiosis Meiosis involves two cellular divisions Meiosis I Meiosis II These two cellular divisions produce four daughter cells each with one chromosome of each pair Overview of Meiosis cont During meiosis I the homologous chromosomes pair up in synapsis to form a tetrad sister chromatids of two homologous chromosomes Later in meiosis I the homologous pairs separate to provide one member of each homologous pair to each daughter cell After meiosis I the daughter cells have half the number of chromosomes but each chromosome consists of two sister chromatids dyads Overview of Meiosis cont During meiosis II the sister chromatids dyads are separated The two divisions of meiosis insure that the gametes produced by an individual have unique combinations of homologous chromosomes 2 Overview of Meiosis cont Crossing Over While a tetrad is formed during the synapsis of meiosis I the homologues may exchange genetic material by crossing over Crossing over recombines the alleles of the homologous chromosomes creating new combinations and increasing the genetic variability of the gametes Crossing Over cont The Importance of Meiosis One reason meiosis is important is that it maintains the same number of chromosomes in each new generation Another reason meiosis is important is that it contributes new combinations of alleles to each new generation Crossing over can produce different alleles on the sister chromatids of homologous chromosomes The Importance of Meiosis Meiosis produces gametes with many different combinations of homologous chromosomes Fertilization to form a zygote produces one of 70 trillion possible combinations of chromosomes 9 2 The Phases of Meiosis The same four stages of mitosis occur during meiosis Prophase Metaphase Anaphase Telophase The four stages occur twice once during meiosis I and again during meiosis II 3 The First Division Meiosis I Significant events occur during prophase I of meiosis The spindle appears between the separating centromeres The homologous chromosomes undergo synapsis as they condense Crossing over between homologous chromosomes may occur At metaphase I the tetrads attached to the spindle are aligned at the spindle equator The First Division Meiosis I cont The First Division Meiosis I cont The homologous chromosomes are separated during anaphase I Anaphase I is the point at which the diploid cell becomes haploid During telophase I the nuclear envelope reforms around the now haploid nuclei The Second Division Meiosis II The events that occur during meiosis II are essentially the same as those that occur during mitosis As prophase II begins a spindle appears and the nuclear envelope and nucleolus disappear During metaphase II the dyads line up at the spindle equator The Second Division Meiosis II The Second Division Meiosis II cont cont The sister chromatids of the dyad separate during anaphase II and begin migrating towards the poles The spindle disappears and the nuclear envelope reforms in telophase II Cytokinesis occurs to complete the meiosis In summary a single diploid cell undergoes meiosis to produce four haploid gametes 4 9 3 Meiosis Compared to Mitosis Meiosis requires two nuclear divisions while mitosis requires only one Meiosis produces four daughter nuclei while mitosis produces only two Both meiosis and mitosis are typically followed by cytokinesis 9 3 Meiosis Compared to Mitosis cont The daughter cells of meiosis are genetically dissimilar from the parent cell The daughter cells of mitosis are genetically identical to the parent cell 9 3 Meiosis Compared to Mitosis cont After meiosis the daughter nuclei are haploid and contain half the chromosome number of the parent cell Following mitosis the daughter cells have the same chromosome number as the parent cell Occurrence Meiosis occurs only at specific times during the life cycle of sexually reproducing organisms Meiosis in humans occurs only in the testes and ovaries Mitosis is common and occurs throughout the body Process cont Process cont 5 9 4 Abnormal Chromosome Inheritance Humans can occasionally be born with an abnormal number of chromosomes This results because meiosis did not occur properly in one of the parents and a gamete has an abnormal number of chromosomes This error in meiosis is called nondisjunction 9 4 Abnormal Chromosome Inheritance cont There are two causes of nondisjunction Both members of a homologous pair go into the same daughter cell in meiosis I Both sister chromatids go into the same daughter cell in