Asexual Reproduction Sexual Reproduction Binary Fission is a form of asexual reproduction Production of offspring by a single parent some eukaryotes reproduce asexually They do this by mitosis Sexual Reproduction Multicellular organisms usually reproduce sexually. Cells from two parents join to form a NEW individual offspring produced sexually are genetically different from either parent meiosis

Gregor Mendel 1822 Austrian monk University of Vienna In charge of the Garden

What Gregor Mendel Knew… Each organism must inherit a single copy of every gene from each of its “parents” Each of the organisms gametes must contain just one set genes When gametes are formed, there must be a process that separates the 2 sets of genes so each gamete gets one set

Meiosis Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

Karyotype A photograph of a person's chromosomes, arranged according to size

Chromosome Number Homologous chromosomes Fruit fly has 8 chromosomes Chromosome that has a corresponding chromosome from the opposite-sex parent Similar in size, shape, and genetic material Fruit fly has 8 chromosomes 4 from mom 4 from dad

Diploid Di= two sets Cell that contains both sets of homologus chromosomes Cell contains 2 complete sets of chromosome 2 complete sets of genes Number of chromosomes in diploid cell represented by 2N For Drosophilia (fruit fly) 2N=8 Mendel said: Each adult cell contains two copies of each gene

Haploid Means “one set” Refers to cells that contain only one set of chromosomes Gametes (sex cells) Represented by N Drosophilia fruit fly N=4

How are haploid (N) gametes made from diploid (2N) cells?

Meiosis 2 distinct stages Meiosis I Meiosis II A diploid cell enters here Meiosis II At the end of this, the diploid cell that entered meiosis has become 4 haploid cells When sex cells form, the chromosomes are copied once. The nucleus then divides twice (meiosis I and meiosis II). The four cells that result have half as many chromosomes as a normal body cell.

Meiosis I Before meiosis 1, each chromosome is replicated (interphase of the cell cycle). Tetrads line up like the sister chromatids did in mitosis What happened in mitosis? PMAT Tetrad STRUCTURE MADE WHEN EACH CHROMOSOME PAIRS UP WITH ITS HOMOLOGOUS CHROMOSOME 4 CHROMATIDS IN A TETRAD

Prophase I Homologous chromosomes pair up (SYNAPSIS) , makes a tetrad As they pair up in tetrads, chromosomes exchange portions of their chromatids in the process …. CROSSING OVER - First way genetic variation in gametes is achieved

Crossing Over Part of a chromatid on one chromosome breaks off and switches position with the matching portion on a chromatid of the homologous chromosome increases the # of genetic combinations in offspring

Metaphase I Homologous pairs line up along the center of the cell in a random fashion= independent assortment Another form of genetic variation When the cell divides, each daughter cell will receive a mix of chromosomes that differ than that of the original cell

Anaphase I spindle fibers separate the paired homologous chromosomes and pull them toward opposite poles of the cell. Telophase I Cell divides to produce two daughter cells each with a complete set of chromosomes

Now what do we have? 2 new daughter cells Are they identical to the parents? No Let’s say the parent started with 8 chromosomes Each daughter cell has 8 chromosomes but they are different because of crossing- over Each daughter cell has a set of chromosomes and alleles different from each other and different from the parent diploid cell moving on to Meiosis II

Prophase II Meiosis I resulted in 2 “seemingly” diploid cells Remember they are genetically different b/c of crossing over in prophase I We still need to cut this number in half to reach our goal of 4 haploid cells Meiosis II doesn’t replicate the chromosomes again Does not form tetrads because the homologs were already seperated during meiosis I

Metaphase II Anaphase II Telophase II Chromosomes line up in middle Sister chromatids separate and move to opposite poles Telophase II Meiosis II results in 4 haploid (N) daughter cells

Gamete Formation Male Haploid gametes produced by meiosis are called spermatozoa Spermatogenesis begins at puberty and continues throughout one’s life Under hormone and environmental control Female Haploid gamete produced by meiosis is called an oocyte Cell divisions at the end of meiosis one and two are uneven so one cell gets most of the cytoplasm (the EGG) and the other three are called polar bodies (don’t participate in reproduction) IMPORTANT! Female gametes are stuck in Prophase 1 until puberty Complete Meiosis 1 every month and the secondary is released from ovary Female gametes only reach and complete meiosis 2 if they are fertilized