Meiosis vs. Mitosis Mitosis: period of nuclear division in which two daughter cells are formed, each identical to the parent cell

Meiosis vs. Mitosis Meiosis: type of division in which four cells are formed, each with half the number of chromosomes as the parent cell

Terms to Know Body cells (diploid, 2n) contain two of each kind of chromosome. chromosomes are paired, one set from each parent Paired chromosomes are called homologous chromosomes Human body cell: 46 chromosomes/23 pairs

Terms to Know Gamete (haploid, n) Zygote Also called a “sex cell” Contains one of each kind of chromosome (or half the number of a body cells) Human sex cell (sperm or egg): 23 chromosomes Zygote the result of the fertilization of an egg cell with a sperm cell forming a diploid cell

Meiosis fertilization is the process in which two haploid cells come together to form a unique diploid This is the premise of sexual reproduction

Meiosis Interphase: stage in which chromosomes duplicate themselves (same as mitosis) Meiosis is then separated into two division stages Meiosis I and Meiosis II

Phases of Meiosis http://player.discoveryeducation.com/index.cfm?guidAssetId=9965DAF6-2344-4A14-8A87-085F0139FB50&blnFromSearch=1&productcode=US

Meiosis I Prophase I Chromosomes coil and spindle forms Each pair of homologous chromosomes and their sister chromatids come together to form a four-part tetrad joined together by a centromere

Meiosis I Prophase I During this phase, an exchange of genetic material may occur between two non-sister chromatids (crossing over) Crossing over allows for more variation in sexual reproduction

Meiosis I Metaphase I Chromosomes become attached to spindle Tetrads are pulled to the mid-line of the cell

Meiosis I Anaphase I Homologous chromosomes, along with their sister chromatids, separate

Meiosis I Telophase I Spindle breaks down Chromosomes uncoil Cytoplasm divides to form two new haploid cells * Each cell has half the genetic information as the original parent cell, BUT the chromosomes are still doubled so another division must take place

Interkinesis Period of rest that may, or may not, occur between Meiosis I and Meiosis II

Meiosis II Prophase II A spindle forms and the chromosomes in each cell attach Sister chromatids are pulled to the center of the cell

Meiosis II Metaphase II Sister chromatids line up on the mid-line of the cells

Meiosis II Anaphase II The centromere breaks allowing the sister chromatids to migrate to opposite ends the cells

Meiosis II Telophase II Nuclei reform Spindles break down Cytoplasm splits

End of Meiosis Meiosis produces FOUR haploid cells, each containing one chromosome from each homologous pair

Meiosis genetic recombination: the reassortment of chromosomes and the genetic information they carry, providing a major source of genetic variation

How much variation? “n”: The number of homologous pairs in a cell Human body cell contains 46 chromosomes (23 homologous pairs) 2n=46 Human sperm cell contains 23 chromosomes (one representative from each homologous pair) n=23 Human egg cell (same as sperm) n=23

WOW!!! Possible different kinds of egg or sperm cells created by a person = 2²³ or 8.4 million Fertilization of one egg by one sperm results in 2²³ x 2²³ or 70 trillion possible zygote combinations * This does not take into consideration possible crossovers that occur during prophase I

End Result---We are ALL truly unique

Does meiosis ever go wrong? YES! Nondisjunction occurs when chromosomes do not divide evenly during an anaphase stage of meiosis Result: a sex cell with an extra chromosome, or missing a chromosome

What’s the big deal? In humans, zygotes that form from gametes with an abnormal number of chromosomes will often spontaneously abort. Several disorders found in humans are a result of nondisjuntion Down Syndrome (Trisomy 21) Turner Syndrome (Monosomy X) Klinefelter’s Syndrome (XXY) Edwards Syndrome (Trisomy 18)

Are extra chromosomes EVER a good thing? YES! Polyploidy: A state in which a cell has more than two sets of chromosomes Many plants (wheat, bananas, apples, strawberries, carnations) and some animals (goldfish, salamanders) Plants, in particular, benefit greatly from extra chromosomal information