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• Meiosis is a type of cell division.

Cell Types

• Somatic cells are non-reproductive.
• Germ cells are reproductive and produce gametes.
• Male gametes are sperm.
• Female gametes are ova or eggs.

Ploidy

• Ploidy refers to the number of chromosomes in a chromosome set for a species.
• Haploid (n) cells have a single set of chromosomes, such as human gametes.
• Diploid (2n) cells have two sets of chromosomes, such as human somatic cells.
• Polyploidy refers to cells with more than two sets of chromosomes.
• Humans are diploid with a ploidy of 23, 23 chromosomes in gametes, and 46 in somatic cells.
• Bread wheat is hexaploid with a ploidy of 7, 21 chromosomes in gametes, and 42 in somatic cells.

Sexual Reproduction

• Meiosis and mitosis are involved in sexual reproduction.
• In sexual reproduction, somatic cells have 38 chromosomes.
• Gametes have 19 chromosomes.
• Fertilization occurs when gametes fuse.
• Sperm bearing an X chromosome will produce females (XX).
• Sperm bearing a Y chromosome will produce males (XY).
• In mitosis, genetic material is not reduced, so sperm and ovum would both be diploid (2n).
• If mitosis were used for reproduction, organisms would become increasingly polyploid.
• Sperm (2n) + Ovum (2n) would create a Zygote (4n).

Chromosomes

• Sister chromatids are two identical DNA molecules held together by a centromere.
• Homologous chromosomes are pairs of similar chromosomes inherited from both parents.

Basic Meiosis Process

• Mitosis and meiosis are preceded by interphase in which DNA is replicated during the S phase.
• In meiosis I, homologous chromosomes are separated, reducing the chromosome number by half in each daughter cell.
• In meiosis II, sister chromatids are separated, and will no longer be identical due to crossing over in meiosis I.

Stages of Meiosis I

• Prophase I (P-I): Chromosomes condense, the nuclear membrane dissolves, and homologous chromosomes form tetrads. Synapsis and crossing over occurs.
• Metaphase I (M-I): Spindle fibers connect to bivalents at centromeres and align them in the middle of the cell during prometaphase.
• Random or independent assortment occurs.
• Anaphase I (A-I): Spindle fibers split the tetrad and move the chromosomes to opposite poles of the cell.
• Telophase I (T-I): Chromosomes de-condense, the nuclear membrane may reform, and cells divide (cytokinesis) to form two haploid daughter cells.

Stages of Meiosis II

• Prophase II (P-II): Chromosomes condense, the nuclear membrane dissolves, and centrosomes move to opposite poles similar to mitosis. However, the orientation is perpendicular to what came before.
• Metaphase II (M-II): Spindle fibers from centrosomes attach to chromosomes at the centromere and align them along the cell equator.
• Anaphase II (A-II): Spindle fibers contract and separate the sister chromatids, now daughter chromosomes, moving them to opposite poles.
• Telophase II (T-II): Chromosomes decondense, the nuclear membrane reforms, and cells divide (cytokinesis) to form four haploid daughter cells.

Benefits of Sexual Production

• Genetic variation in offspring is a benefit of sexual reproduction.
• The major sources of genetic variation include crossing over, random assortment of chromosomes, and random fusion of gametes from different parents.

Crossing Over

• In Prophase I, homologs connect in synapsis and pair together, forming a tetrad (or bivalent).
• These homologous chromsomes are held together at points called chiasmata.
• Crossing over of genetic material can occur at chiasmata between non-sister chromatids, which have the same genes, but different alleles.
• Recombinant chromatids consist of a combination of DNA from homologous chromosomes.
• After all crossing over, all four haploid cells are genetically distinct; sister chromatids are no longer identical.
• Crossing over creates new allelic combinations in haploid cells, which increases the genetic diversity of offspring.

Random Assortment

• During Metaphase I, the orientation of homologous chromosomes toward the poles is random and each tetrad is oriented independently.
• Humans have 46 chromosomes (n=23), therefore they can produce 8,388,608 different gametes and even more with crossing over.

Random Fertilization

• Fusion of two haploid gametes produces a diploid zygote.
• Sperm and egg are randomly chosen.

Gametogenesis

• Gametogenesis is when diploid precursor cells undergo meiotic division to become haploid gametes (sex cells).
• In males, this is spermatogenesis which produces spermatozoa (sperm).
• In females, this is oogenesis which produces ova (eggs).
• The process occurs in gonads with multiple mitotic divisions, two meiotic divisions, and differentiation by which haploid cells produce functional gametes.

Spermatogenesis in the Seminiferous Tubules of the Testes

• Spermatogenesis happens in the testes, beginning at puberty.
• The resulting spermatids from meiosis become sperm.
• Mature sperm have a head with a nucleus, little cytoplasm, a midsection with mitochondria, and a flagellum for movement.

Oogenesis

• Oogenesis occurs in the ovaries.
• It starts before birth, where oogonium reproduces by mitosis, and then stops during prophase I.
• At birth, females have all the oocytes they will ever have.
• After puberty, meiosis I continues for one oocyte each month, ending at menopause.
• Oogenesis is unequal, the cells receiving less cyptoplasm/nutrients become polar bodies and eventually degenerate.
• The unequal division creates a cell to support the zygote.
• The final stages of meiosis II are only completed if fertilization occurs.

Spermatogenesis vs Oogenesis

• Spermatogenesis that occurs entirely in the testes has an equal division of cells and is involved in gamete production as it produces four sperm cells smaller than spermatocytes beginning at puberty.
• Oogenesis that occurs mostly in the ovaries is an unequal division of cytoplasm, not involved in gemete production, and ends in menopause as it begins in the fetus.

Twins

• Fraternal twins come from two sperm fertilizing two eggs and have different paternal/maternal DNA.
• Identical twins form when one sperm fertilizes one egg, which then divides and are the same paternal/maternal DNA.
• Rare twins include semi-identical and conjoined.

Meiosis Outcomes

• Genetic reduction produces daughter cells with half the number of chromosomes versus the parent cell.
• Genetic recombination produces daughter cells with different combinations of alleles.

Sexual Reproduction

• A reason for meiosis is that sexually reproducing organisms are diploid, meaning they have two copies of chromosomes.
• To reproduce, these organisms make haploid gametes with one copy of each chromosome.
• Fertilization of two haploid gametes (egg + sperm) created a diploid that grows via mitosis.
• If chromosomes were not halved via gametes, chromosome numbers would double each generation, leading to polyploidy.

Comparing Mitosis vs Meiosis.

• Mitosis has one division, while meiosis has two.
• Mitosis has no independent assortment, synapsis, or crossing over.
• Meiosis has independent assortment in metaphase 1, has synapsis to form bivalents, and also consists of crossing over in prophase 1.
• Mitosis has two cells with the ploidy as diploid which produces body cells consisting of identical genetics.
• Meiosis has four cells with the ploidy as haploid that is used to separate the genetics into sex cells used in the crossing over phase.