Unit Three Cell Proliferation and Genetics Meiosis History

Unit Three “Cell Proliferation and Genetics” “Meiosis”

History of Meiosis u In the mid 1880’s after Walter Fleming discovered chromosomes, Belgian cytologist Pierre-Joseph van Beneden found that different types of cells from the same organisms had two different numbers of chromosomes u He further documented the “Gametes” (sperm and egg) each had half the number of chromosomes of the somatic cells (non-sex cells)

History of Meiosis u In 1887, van Beneden proposed an egg and sperm fuse to produce a single cell called the “Zygote” u The Zygote contains two copies of each chromosome u The fusion of the gametes is called “Fertilization” or “Syngamy” u It was clear to van Beneden and his contemporaries some mechanism must be involved in reducing the number of chromosomes by half in preparation for fertilization

Meiosis u Meiosis is the process by which the Diploid number of chromosomes (2 n) is reduced to the Haploid number of chromosomes (n) in preparation for fertilization u Because Meiosis is the halving of chromosomes, it is sometimes referred to as “Reduction Division” u Meiosis is a process that occurs only in species that participate in “Sexual Reproduction” u Species that reproduce asexually via cloning, fission etc. participate in

Meiosis u Meiosis produces viable sperm and egg that are ready to participate in fertilization u Upon fertilization, the single cell that is the result of the sperm and egg combining, begins to divide mitotically to produce the entire fully functioning eukaryotic organism u Both somatic cells and primordial gamete cells are diploid, however, the sperm and egg cells are haploid

Meiosis u Meiosis is divided into: – Meiosis I u Duplicated chromatin (2 sets of 46 chomosomes) from interphase is divided between two daughter cells – Meiosis II u Each daughter cell divides further to reduce the diploid # of 46 to the haploid # of 23

Meiosis I and II

How Meiosis differs from Mitosis u PMAT occurs twice instead of once u In conjunction with the above statement, the resulting 4 cells in Meiosis have a Haploid number of chromosomes, whereas the resulting 2 cells in Mitosis have a Diploid number of chromosomes u To encourage variation in individuals of a species, two mechanisms have evolved which are unique to Meiosis: – “Crossing Over” – “Independent Assortment”

Synapsis & Crossing Over u During Metaphase I, two similar sized chromosomes with variations of the same gene line up next to each other forming a “Tetrad” u The tetrad is made of two sets of “Sister Chromatids” u When the tetrad has formed during Metaphase I, equal segments of two “Nonsister Chromatids” are exchanged amongst each other

Synapsis & Crossing Over

Independent Assortment u Occurs during Meiosis I u Sister chromatids are pulled to either end of the cell in a random fashion u This situation results in additionally variation in the genetic code each egg or sperm cell contains

Independent Assortment

Consequences of Meiosis and Sexual Reproduction u Genetic diversity is favored due to an ever changing environment with new abiotic variables coming to the forefront constantly u The best mode of ensuring diversity is via sexual reproduction as the result of Meiosis u It should be noted, changes in the genetic code are not always beneficial; they can be harmful too

Consequences of Meiosis and Sexual Reproduction u Main benefit to sexual reproduction is variation between offspring which allows for better chance of survival of species u Main cost is the energy requirement of entire process: – Nucleic acid and cellular division – Search for a suitable mate – Physical act of intercourse

Sex Determination within Sexual Reproduction u Sex of offspring is determined in a variety of ways – humans: 23 rd pair/46 th chomosome – Reptiles: temperature of nest – Insects: number of chromosomes – haploid or diploid

Meiotic Abnormalities u Errors during Meiosis I and II result in an incorrect number of chromosomes - Nondisjunction u Turner’s Syndrome, Klinefelter’s Syndrome, Metafemales, Metamales, Trisomy 21/Down Syndrome etc.