Meiosis

Diploid (2n)  Haploid (n) Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n)  Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die.

Meiosis Meiosis is Two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes.

Meiosis: As in mitosis, if a cell wants to make a duplicate of itself, it first must copy its DNA (part of a chromosome). Results in reproductive cells (sperm, eggs, etc). Part of each parent is carried to the four new cells. Meiosis has evolved to solve a problem. The problem is this: some organisms have sex with other organisms – that is, they combine their genes together (creating genetic diversity). Why would this be a problem?

Mitosis: Meiosis: Explain the randomness in meiosis End of first class Each resulting cell still has chromosomes from mom & dad

MITOSIS MEIOSIS Diploid Diploid 1 somatic cell 2n 2n 2 2n 2n 3 2n 2n 4 gamete precursor somatic cell 2n 2n duplication 2 2n 2n 3 2n 2n 4 2n 2n Figure: 10-01 Title: Meiosis compared to mitosis. Caption: 1. Both mitosis and meiosis begin with diploid cells, meaning cells that contain paired sets of chromosomes. The two members of each pair are homologous, meaning the same in shape and function. Two sets of homologous chromosomes are shown in both the mitosis and meiosis figures.The larger chromosome pairs in each cell represents one homologous pair, while the smaller chromosome pairs represent the other homologous pair. One member of each homologous pair (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other member of the pair (in blue) comes from the father of this person. 2. In both mitosis and meiosis, the chromosomes duplicate. Each chromosome is now composed of two sister chromatids. 3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, meanwhile, homologous chromosomes -- not sister chromatids -- line up on opposite sides of the metaphase plate. 4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. 5 In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, in the first of two cell divisions, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on oppositesides of the plate. 6. The sister chromatids of each chromosome then separate. 7. The cells divide again, yielding four haploid cells. division diploid haploid 5 2n 2n 1n 1n 6 division 7 1n 1n 1n 1n

Vocabulary Diploid Haploid Germ cell Somatic cell Interphase Prophase Metaphase Anaphase Telophase Cytokinesis

Meiosis Interphase Meiosis is preceded by interphase. The chromosomes have not yet condensed. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Interphase The chromosomes have replicated, and the chromatin begins to condense. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Prophase I The chromosomes are completely condensed. In meiosis (unlike mitosis), the homologous chromosomes pair with one another http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

During Prophase I “Crossing Over” occurs. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction) During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over.

Crossing Over creates variation (diversity) in the offspring’s traits. Tetrad nonsister chromatids chiasmata: site of crossing over variation

Meiosis Metaphase I The nuclear membrane dissolves and the homologous chromosomes attach to the spindle fibers. They are preparing to go to opposite poles. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Anaphase I The chromosomes move to opposite ends of the cell. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Telophase I & Cytokinesis The cell begins to divide into two daughter cells. It is important to understand that each daughter cell can get any combination of maternal and paternal chromosomes. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Prophase II The cell has divided into two daughter cells. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Metaphase II As in Meiosis I, the chromosomes line up on the spindle fibers. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Anaphase II The two cells each begin to divide. As in Meiosis I, the chromosomes move to opposite ends of each cell. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Telophase II & Cytokinesis With the formation of four cells, meiosis is over. Each of these prospective germ cells carries half the number of chromosomes of somatic cells. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Sexual vs. Asexual Reproduction

Compare the results of uniform or diverse offspring from sexual or asexual reproduction Uniform offspring Diverse offspring Sexual reproduction Asexual reproduction

Sexual Reproduction A type of reproduction in which the genetic materials from two different cells combine, producing an offspring The cells that combine are called sex cells Female – egg Male – sperm Fertilization: an egg cell and a sperm cell join together A new cell is formed and is called a zygote

Advantages: Sexual Reproduction Diverse offspring: genetic variation among offspring Half of the DNA comes from mom Half of the DNA comes from dad Due to genetic variation, individuals within a population have slight differences Plants – resist diseases Traits can develop to resist harsh environments that allows an organism survive

Advantages: Sexual Reproduction Selective Breeding Used to develop many types of plants and animals that have desirable traits Agriculture/Farming: better plants, larger animals Desirable pets

Disadvantages: Sexual Reproduction Time and Energy Organisms have to grow and develop until they are old enough to produce sex cells Search and find a mate Searching can expose individuals to predators, diseases, or harsh environmental conditions Fertilization cannot take place during pregnancy, which can last as long as 2 years for some mammals.