Meiosis PDF - Cell Division

CELL DIVISION (Cell Reproduction) Huiam Salih Alaraby 1  Some organisms look and act exactly like their parent. Others share many similar traits, but they are definitely unique individuals. Some species have two parents, whereas others have just one. How an organism reprod...

CELL DIVISION (Cell Reproduction) Huiam Salih Alaraby 1  Some organisms look and act exactly like their parent. Others share many similar traits, but they are definitely unique individuals. Some species have two parents, whereas others have just one. How an organism reproduces determines the amount of similarity the organism will have to its parent. Asexual reproduction produces an identical individual, whereas sexual reproduction produces a similar, but unique, individual. Raiyed M. Adeel 02/10/2018  ASEXUAL REPRODUCTION Are there male and female bacteria? How could you tell? Remember, bacteria have just one chromosome; they do not have an X or Y chromosome. So they probably have a very simplified form of reproduction. Asexual reproduction, the simplest and most primitive method of reproduction, produces a clone, an organism that is genetically identical to its parent. Raiyed M. Adeel 02/10/2018 SEXUAL REPRODUCTION AND MEIOSIS Why do you look similar to your parents, but not identical? First, it is because you have two parents. Second, it is because of sexual reproduction. Whereas asexual reproduction produces genetically identical clones, sexual reproduction produces genetically diverse individuals. As both parents contribute half of the new organism’s genetic material, the offspring will have traits of both parents, but will not be exactly like either parent. Raiyed M. Adeel 02/10/2018 Germ cells: the cells that provide the continuity of life from generation to the next. Raiyed M. Adeel 02/10/2018 Huiam Salih Alaraby 6  Organisms that reproduce sexually by joining gametes, a process known as fertilization, must have a mechanism to produce haploid gametes. This mechanism is meiosis, a type of cell division that reduce the number of chromosomes. Meiosis involves two nuclear and cell divisions without an interphase in between, starting with one diploid cell and generating four haploid cells. Each division, named meiosis I and meiosis II, has four stages: prophase, metaphase, anaphase, and telophase. Raiyed M. Adeel 02/10/2018 Huiam Salih Alaraby 8  + = Sperm (23) Egg (23) Fertilized Cell (46) Huiam Salih Alaraby 9 Raiyed M. Adeel 02/10/2018  Prior to meiosis, the cell’s DNA is replicated, generating chromosomes with two sister chromatids. A human cell prior to meiosis will have 46 chromosomes, 22 pairs of homologous autosomes, and 1 pair of sex chromosomes. Homologous chromosomes are similar in size, shape, and genetic content. You inherit one chromosome of each pair from your mother and the other one from your father. Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018  PROPHASE I In prophase I, the nuclear envelope breaks down, the chromosomes condense, and the centrioles begin to migrate to opposite poles of the cell, with the spindle fibers growing between them. During this time, the homologous chromosomes form pairs. These homologous chromosomes line up gene- for-gene down their entire length, allowing crossing-over to occur. This is an important step in creating genetic variation and will be discussed later. Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018  Prophase I: Leptotene The chromosomes become visible as they start to condense. Prophase I: Zygotene Homologous chromosomes align directly opposite each other and are held together at several points along their length. Prophase I: Pachytene Each pair of homologous chromosomes becomes tightly coiled around each other. Raiyed M. Adeel 02/10/2018  Prophase I: Diplotene The homologous recombinant chromosomes now begin to separate but remain attached at the points where crossing over has occurred. Prophase I: Diakinesis Separation of the homologous chromosome pairs proceeds as the chromosomes become maximally condensed. The nuclear envelope disappear and so the nucleolus. Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018  Metaphase I In metaphase I, the 23 pairs of homologous chromosomes line up along the equator of the cell. During mitosis, 46 individual chromosomes line up during metaphase. Raiyed M. Adeel 02/10/2018  Anaphase I During anaphase I the spindle fibers shorten, and the homologous chromosome pairs are separated from each other. One chromosome from each pair moves toward one pole, with the other moving toward the other pole, resulting in a cell with 23 chromosomes at one pole and the other 23 at the other pole. Raiyed M. Adeel 02/10/2018  Anaphase I During anaphase I the spindle fibers shorten, and the homologous chromosome pairs are separated from each other. One chromosome from each pair moves toward one pole, with the other moving toward the other pole, resulting in a cell with 23 chromosomes at one pole and the other 23 at the other pole. The sister chromatids remain attached at the centromere. Raiyed M. Adeel 02/10/2018  Telophase I The spindle fiber disassembles and the nucleus reforms. This is quickly followed by cytokinesis and the formation of two haploid cells, each with a unique combination of chromosomes, some from the father and the rest from the mother. After cytokinesis, both cells proceed into a phase known as interkinesis which is just like interphase but the DNA is not copied in between. Then daughter cells immediately enter meiosis II;. Meiosis II is essentially the same as mitosis, separating the sister chromatids from each other. Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018  Prophase II: Once again the nucleus breaks down, and the spindle begins to reform as the centrioles move to opposite sides of the cell. Metaphase II: The spindle fibers align the 23 chromosomes, each made out of two sister chromatids, along the equator of the cell. Anaphase II: The sister chromatids are separated and move to opposite poles of the cell. As the chromatids separate, each is known as a chromosome. Anaphase II results in a cell with 23 chromosomes at each end of the cell; each chromosome contains half as much genetic material as at the start of anaphase II. Telophase II: The nucleus reforms and the spindle fibers break down. Each cell undergoes cytokinesis, producing four haploid cells, each with a unique combination of genes and chromosomes. Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 Raiyed M. Adeel 02/10/2018 THANK YOU