Meiosis A process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes to form gametes, or sex cells Two distinct divisions, called meiosis I and meiosis II
Lesson Grabber How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
Chromosomes Chromosome- threadlike structure of DNA and protein that contains genetic information Gene- sequence of DNA that codes for a protein and therefore determines a trait; a true factor that is passed from parent to offspring
Homologous Chromosomes A body cell in an adult fruit fly has eight chromosomes, as shown in the figure. Four chromosomes come from its male parent; Four come from its female parent These two sets of chromosomes are called: Homologous pair - two of the same size and shape chromosomes where one set comes from the female parent and one set comes from the male parent
Diploid Cells Diploid - meaning two sets, a cell that contains both sets (one from mom, one from dad) of homologous chromosomes, with 2 sets of genes - sometimes represented by the symbol 2N - for the fruit fly, the diploid number is 8, which can be written as 2N = 8 - N represents the single set of chromosomes in a sperm or egg cell
Haploid Cells Haploid - meaning one set, a cell that contains only a single set of chromosomes, and therefore a single set of genes - Gametes (sex cells sperm, egg) are haploid - For fruit fly gametes, the haploid number is 4, so N=4 Diploid = 8 (2N) Haploid = 4 (N=4)
Gametes to Zygotes Haploid cells produced by meiosis II are gametes: male animals gametes sperm plants pollen contain haploid sperm cells female animals gametes eggs plants - egg cell contain haploid cells Fertilization the fusion of male and female gametes generates new combinations of alleles in a zygote Zygote undergoes cell division by mitosis and eventually forms a new organism
Fertilization
Stages of Meiosis
Interphase I - Before meiosis I begins, cell undergoes interphase - Chromosome replication - Each replicated chromosome consists of two identical chromatids joined at the center by a centromere.
Meiosis I Begins Prophase I - Chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.
Prophase I - homologous chromosomes pair up and form tetrads, they undergo a process called crossingover (chromatids of the homologous chromosomes cross over one another) - crossed sections of the chromatids are exchanged (new combinations of genes)
Metaphase I - Spindle forms and attaches to each homologous chromosomes - Paired homologous chromosomes line up across the center of the cell.
Anaphase I - Spindle fibers pull each homologous chromosome pair toward opposite ends of the cell - Separated homologous chromosomes cluster at opposite ends of the cell.
Telophase I and Cytokinesis - Nuclear membrane forms around each cluster of chromosomes - Cytokinesis follows Telophase I, forming two new cells.
Meiosis I Ends - Results in two cells (daughter cells) each of which has four chromatids Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell The two cells produced by meiosis I have sets of chromosomes that are different from each other and from the diploid cell that entered meiosis I due to crossing over
Meiosis II Begins - Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II NO INTERPHASE
Prophase II - Chromosomes each consisting of two chromatids become visible.
Metaphase II - Chromosomes line up in the center of each cell
Anaphase II - Paired chromatids separate
Telophase II, and Cytokinesis - Each of the four daughter cells produced in meiosis II receives two chromatids These four daughter cells now contain the haploid number (N) just two chromosomes each
Meiosis v. Mitosis Meiosis Two divisions Homologous chromosomes line up and move to separate daughter cells. Reduces chromosome number by half Produces 4 genetically different haploid cells. Early step in sexual reproduction. Mitosis One Division Two sets of genetic material separate, each daughter cell receives one complete set of chromosomes Doesn t change chromosome number Makes 2 identical diploid cells Form of asexual reproduction