Chapter 10: Meiosis and Sexual Reproduction (Outline)  Reduction in Chromosome Number Homologous Pairs  Genetic Recombination Crossing-Over Independent Assortment Fertilization  Phases of Meiosis Meiosis I Meiosis II  Meiosis Compared to Mitosis  Human Life Cycle

Meiosis: Halving the Chromosome Number  Special type of cell division  Used only for sexual reproduction  Halves the chromosome number prior to fertilization Parents diploid (2n) Meiosis produces haploid gametes (1n) Gametes fuse in fertilization to form diploid zygote Becomes the next diploid generation  If gametes were not haploid the number of chromosomes would double itself in each generation

Homologous Pairs of Chromosomes  In diploid body cells, chromosomes occur in pairs  Diploid cells have two of each type  Human cells have 46 chromosomes in 23 homologous pairs  Homologous Chromosomes Paired chromosomes in somatic cells Similar in size, shape and position of their centromeres Carry information about the same genetic traits (not always the same information) When stained, they show similar banding patterns

Homologous Chromosomes

Homologous Pairs of Chromosomes  Homologous chromosomes have genes controlling the same trait at the same position Each gene occurs in duplicate, why?  The variants that exist for a gene are called alleles  An individual may have: Identical alleles for a specific gene on both homologs (homozygous for the trait), or A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

Overview of Meiosis  Meiosis requires 2 nuclear divisions and produces 4 haploid daughter cells  Cells are diploid at beginning of meiosis  Pairs of chromosomes are called homologues  Meiosis I Homologues line up side by side at equator- synapsis Synapsis results in a bivalent When pairs separate, each daughter cell receives one member of the pair Cells are now haploid

Overview of Meiosis (cont.)  Meiosis II No replication of DNA occurs in this division, why? Centromeres divide & sister chromatids migrate to opposite poles to become individual chromosomes Each of the four daughter cells produced has the haploid chromosome number and each chromosome is composed of one chromatid  In plants, daughter cells are haploid spores that germinate to haploid generation; gametes produced by mitosis  In animals, daughter cells are gametes (i.e. sperm or eggs)

Meiosis Overview

Genetic Variation  Meiosis helps ensure genetic recombination  In a changing environment, asexual reproduction might be disadvantageous  Sexual reproduction might give offspring better chance of survival  Meiosis brings about genetic variation in two key ways: Crossing-over Independent assortment

Crossing-Over  Exchange of genetic material between nonsister chromatids of a bivalent during meiosis I At synapsis, a nucleoprotein lattice appears between homologues  Holds homologues together and aligns DNA of nonsister chromatids  Allows crossing-over to occur Homologues are held together by chiasmata Homologues then separate and are distributed to different daughter cells

Synapsis and crossing over

Independent Assortment  Independent assortment: When homologues align at the metaphase plate:  They separate in a random manner  The maternal or paternal homologue may be oriented toward either pole of mother cell Causes random mixing of blocks of alleles into gametes

Fertilization  Gametes produced by one person are genetically different those produced by another person  When gametes fuse at fertilization: Chromosomes donated by the parents are combined In humans, (2 23 ) 2 = 70,368,744,000,000 chromosomally different zygotes are possible  If crossing-over occurs only once (4 23 ) 2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible  Remember, crossing-over can occur several times in each chromosome!

Significance of Genetic Variation  Asexual reproduction produces genetically identical clones  Sexual reproduction produces genetic variety  Asexual reproduction is advantageous when environment is stable  However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous

Phases of Meiosis I  Prophase I Each chromosome is internally duplicated (consists of two identical sister chromatids) Homologous chromosomes (maternal homologue and paternal homologue) align side by side (synapsis) Synapsis results in association of four chromatids (a tetrad) Paired homologous chromosomes exchange genetic material (crossing-over)

Phases of Meiosis I  Metaphase I Homologous pairs (bivalents) or tetrads arranged onto the metaphase plate independently The centrioles are at opposite poles of the cell Spindle fibers from one pole of the cell attach to one duplicated chromosome of each pair (seen as sister chromatids)

Phases of Meiosis I  Anaphase I Synapsis breaks up Homologous chromosomes separate from one another and move towards opposite poles Each pole randomly receives a maternal or paternal chromosome from each homologous pair Each is still an internally duplicate chromosome with two chromatids

Phases of Meiosis I  Telophase I Daughter cells have one internally duplicate chromosome from each homologous pair One (internally duplicate) chromosome of each type (1n, haploid) Nuclear envelope may reorganize, and cytokinesis may take place  Interkinesis Similar to mitotic interphase but shorter No replication of DNA, why?

Meiosis I

Phases of Meiosis II: Similar to Mitosis  Prophase II – Chromosomes condense  Metaphase II – chromosomes align at metaphase plate  Anaphase II Centromere dissolves Sister chromatids separate and move to opposite poles (daughter chromosome)  Telophase II and Cytokinesis II Four haploid cells All genetically unique

Meiosis II

Overview of Meiosis I & II

Contrasting Mitosis and Meiosis  Several fundamental differences between the two processes include: Meiosis requires two nuclear divisions, but mitosis requires one nuclear division Meiosis produces four daughter cells, but mitosis results in two daughter cells following cytokinesis In meiosis, daughter cells are haploid, whereas mitosis preserves chromosome number In meiosis, daughter cells are genetically different from parent and each other, but mitosis results in daughter cells that are genetically identical to parent and to each other

Meiosis vs. Mitosis  Occurrence Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms In humans, meiosis occurs in reproductive organs and produces gametes Mitosis is more common since it occurs in all tissues during growth & repair  Process Meiosis I compared to Mitosis Meiosis II compared to Mitosis

Meiosis I Compared to Mitosis

Meiosis II Compared to Mitosis

Life Cycle Basics: Plants  Life cycle – reproductive events that occur from one generation to the next similar generation  Haploid multicellular gametophyte alternate with diploid multicellular sporophyte  Mosses are haploid most of their life cycle  In fungi and most algae, only the zygote is diploid  In plants, algae and fungi, gametes are produced by haploid individuals

Life Cycle Basics: Animals  In animals, somatic cells are diploid and multiply by mitosis – the only haploid cells produced are gametes  Gametes develop when germ line cells undergo meiosis Gametogenesis is the formation of gametes Spermatogenesis (male gametogenesis) forms four haploid sperm cells for each cell that enters meiosis Oogenesis (female gametogenesis) forms one egg cell (ovum) for every cell that enters meiosis, plus polar bodies

The Human Life Cycle  A sperm and egg fuse at fertilization  Results in a zygote Undergoes mitosis  Results in multicellular embryo  As a result of mitosis, each somatic cell in body Has same number of chromosomes as zygote Has genetic makeup determined when zygote was formed

Oogenesis in Humans  Ovaries contain oogonia that produce primary oocytes during fetal development  Primary oocyte continue to develop at onset of puberty and divide through meiosis I into two cells  One of these cells (secondary oocyte) receive most of the cytoplasm; the other polar body may divide or disintegrate  Secondary oocyte begins meiosis II but stops at metaphase II  Then leaves the ovary and enters the oviduct If sperm enters, meiosis II continues & another polar body forms

Spermatogenesis in Humans  Spermatogenesis takes place within the testes  Stem cells within the testes (spermatogonia) become primary spermatocytes; undergo spermatogenesis  Meiosis produces haploid secondary spermatocytes (meiosis I) and haploid spermatids (meiosis II)  Four spermatids are produced from the original primary spermatocyte – each differentiates into a mature sperm (spermatozoa)

Gametogenesis in Mammals