Meiosis and Sexual Reproduction

 Students know and understand the characteristics and structure of living things, the processes of life, and how living things interact with each other and their environment.  Benchmark 3.10: Cell reproduction/division has various processes and purposes (mitosis, meiosis, binary fission)  Assessment objective 3.10a: Compare and contrast the purposes and processes of mitosis, meiosis, and binary fission.

 Gamete  Zygote  Diploid  Haploid  Homologous chromosomes  Meiosis  Crossing-over  Independent assortment  Life cycle  Sperm  Ovum Video time: 67:48min.

 Reproduction is the process of producing offspring.  Organisms are reproduced by two or sometimes just one parent.  This is known as sexual and asexual reproduction. Comparing mitosis and meiosis (29:07)

 In asexual reproduction, there is only one parent.  The offspring is genetically identical to its parent.  Types include: ◦ Binary fission ◦ Fragmentation ◦ Budding ◦ Parthenogenesis

Binary Fission of an paramecium Fragmentation of a Hydra Budding Yeast Parthenogenesis Dolly the Sheep

Sexual reproduction takes place with two parents where the offspring is genetically different from the parents. This generally takes place in eukaryotic cells.

 Each parent produces a gamete (reproductive cell) that together form a zygote during fertilization.  Because the offspring have both parent’s genetic material, it is not exactly like either parent but a mixture of the two genetic packages.

 Gametes have 23 chromosomes.  Oogenesis is the creation of an ovum (egg cell). It is the female process of gametogenesis.ovum gametogenesis  Human females only produce one functional egg in the ovaries.

 Spermatogenesis is the process by which male spermatogonia develop into mature spermatozoa. Spermatozoa are the mature male gametes in many sexually reproducing organisms.spermatogonia spermatozoagametes

 Germ cells are specialized for sexual reproduction and only they can produce gametes.  Somatic cells are body cells that do not participate in sexual reproduction.  What would be the advantage of sexual reproduction as compared to asexual reproduction?

 Each chromosome has 1000’s of genes that determine an organism’s development and function.  In humans, each cell has two copies of 23 chromosomes for a total of 46.  When fertilization happens, 2 cells combine to form a zygote, that has 46 chromosomes also.  How can they be the same?  The gametes that form a zygote have only one copy of each chromosome or one set of 23. This keeps the chromosome number at a constant 46.

 Diploid Cells (1:04) have two sets of chromosomes. Diploid Cells (1:04  Haploid Cells (1:03) have one set of chromosomes. Gametes are haploid cells. Haploid Cells (1:03)  The symbol n represents the number of chromosomes in one set. Thus, humans are n = 23.

 Each diploid cell has pairs of chromosomes made up of two homologous chromosomes.  Homologous chromosomes are similar in size, shape and genetic makeup.  In humans, 23 chromosomes come from Mom and 23 come from Pops.  These chromosomes can carry different forms of genes, depending on how they go together.

 Sex chromosomes determine the sex of the offspring.  Males have one XY chromosome and females have an XX chromosome.  Autosomes are chromosomes that do NOT determine the sex of an individual.

A meiosis I, B meiosis II, C interphase, D prophase I, E metaphase I, F anaphase I, G telophase I, H prophase II, I metaphase II, J anaphase II, K telophase II, L four haploid cells

 Prophase I: ◦ Chromosomes condense ◦ Nuclear envelope breaks down ◦ Homologous chromosomes pair. ◦ Chromatids exchange genetic information in a process called crossing over. Prophase 1 (1:08)

 Pairs of homologous chromosomes move to the equator of the cell. Metaphase 1 (0:17)

 Pairs of homologous chromosomes separate and the spindle fibers pull the chromosomes of each pair to opposite poles of the cell. The chromatids remain joined at their centromeres. The genetic material recombines. Anaphase 1 (0:24)

 Chromosomes gather at the poles. The cytoplasm divides. Telophase 1 (0:12)

 The two cells from Meiosis I now enter the prophase II phase of Meiosis II.  New spindles form around the chromosomes. Prophase II (0:29)

 Chromosomes line up at the equator. Metaphase II (0:13)

Centromeres divide, and chromatids move to opposite poles. Anaphase II (0:13)

 A nuclear envelope forms around each set of chromosomes. The cells divide. Telophase II (0:13)

 Mitosis makes new cells that are used during growth, development, repair, and asexual reproduction.  Meiosis makes cells that enable an organism to reproduce sexually and are found only in reproductive structures.  The main difference is that in mitosis, the genetic material is not rearranged. Comparing Mitosis and Meiosis (8:02)

 Three key components of genetic variation are:  Crossing over  Independent assortment  Random fertilization Chromosomal Basis of Life (1:57)

 During Prophase I, homologous chromosomes line up next to each other. One arm of a chromatid crosses over the other arm of a chromatid, chromosomes break at the crossover and reform with the new piece from the other chromosome. Increasing Genetic Variation: Crossing Over (2:19)

 During metaphase, chromosomes line up at the center of the cell.  As this is random, any two pairs of chromosomes can line up in either of two equally probable ways.  Since there are 23 pairs of chromosomes in humans that separate independently, there are 2 23 or 8 million possible gene combinations in gametes from a single cell.  Fertilization is a random process that adds to genetic variation.  A zygote is formed by the random joining of two gametes.  Because fertilization of an egg by a sperm is random, the number of possible outcomes is squared.  In humans, the possibility is 2 23 x 2 23 or 70 trillion combinations—that’s a lot of variation!!

 In diploid life cycles, meiosis in germ cells of a multicellular diploid organism results in the formation of haploid gametes.  Males produce gametes called sperm.  Females produce gametes called eggs, or ova. Plural = ovum Three Basic Patterns of Sexual Life Cycles (2:19)

 This life cycle happens in most fungi and some protists.  In haploid life cycles, meiosis in a diploid zygote results in the formation of the first cell of a multicellular haploid individual.

 Plants and most multicellular protists have a life cycle that alternates between a haploid phase and a diploid phase called alternation of generations.  Sporophytes are the multicellular diploid phase in the life cycle of plants.  A spore forms a multicellular gametophyte. Moss Reproduction (1:23)