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SEXUAL REPRODUCTION When somatic cells undergo mitosis as part of the cell cycle they are reproducing asexually the daughter cells are exact copies of the parent cell however, human children are not exact copies of their parents and this is because humans undergo sexual reproduction Sexual reproduction involves two parents and leads to the production of genetically distinct offspring HAPLOID & DIPLOID CELLS IN SEXUAL REPRODUCTION Sexual reproduction involves the fusion of a male reproductive cell with a female reproductive cell The reproductive cells are called gametes and the cell that results from this fusion is called a zygote, the process through which gametes are combined to form a zygote is called fertilization In humans the male gamete is the sperm cell and the female gamete is the egg cell HAPLOID & DIPLOID Gametes, which contain single, unpaired chromosomes are said to be haploid (from a Greek word meaning single), the haploid number of chromosomes in an organism is represented by n Cells that contain pairs of chromosomes, which includes all somatic cells, are said to be diploid (from a Greek word meaning double), the diploid number of chromosomes in an organism is represented by 2n Each human gamete is haploid, with n = 23 after fertilization, the zygote cell is diploid with a total of 2n chromosomes, that is n chromosomes from the female parent plus n chromosomes from the male parent the diploid number in humans, is therefore 46 chromosomes notice that n also describes the number of pairs of chromosomes in an organism, when two human gametes combine, 23 pairs of homologous chromosomes are formed MEIOSIS: PRODUCING HAPLOID GAMETES Meiosis is the process through which gametes with a haploid number of chromosomes are produced Meiosis has 2 key outcomes: 1) Genetic Reduction: daughter cells are produced with half the number of chromosomes per cell 2) Genetic Recombination: the products of meiosis have different combinations of alleles, genetic recombination gives rise to offspring that are genetically different from one another and their parents this greatly increases the genetic variation in a population INTERPHASE Cells that divide by meiosis (that is sperm and egg cells) go through the growth and synthesis phases of interphase before dividing This includes the replication of chromosomes and therefore at the beginning of meiosis a cell contains duplicated chromosomes, each of which are made up of a pair of identical sister chromatids held together at the centromere PHASES OF MEIOSIS **Like Mitosis, Meiosis involves specific phases: prophase, metaphase, anaphase and telophase...however, these four phases are repeated twice, once in Meiosis I and the second time in Meiosis II Each pair of chromosomes lines up side by side, this lining up of homologous chromosomes is called synapsis It is during at this point that genetic material can be exchanged between the two homologous chromosomes Throughout this phase the centrosomes move to the poles of the cell and the spindle apparatus forms METAPHASE I The pairs of chromosomes line up along the equator line or metaphase plate and the spindle fibers have attached to the centromere of each homologous chromosome ANAPHASE 1 The homologous chromosomes separate and move to opposite poles of the cell Since the entire chromosome moves, the sister chromatids are in this case held together, so the centromeres do not split as they do in mitosis As a result one chromosome moves to each pole of the cell and the chromosome number is reduced from 4n to 2n (diploid) TELOPHASE I The homologous chromosomes now begin to uncoil and the spindle fibers disappear Cytokinesis takes place, and a nuclear membrane forms around each group of homologous chromosomes and two cells form each of these new cells is now diploid MEIOSIS II The phases of Meiosis II are similar to the phases of Mitosis, the key difference is that the starting cells are diploid and are not replicated again before dividing The chromosomes condense, the centrosomes move to the poles of the cell and the spindle apparatus forms METAPHASE II The spindle fibers guide the chromosomes to the center line of the cell, they then attach to the centromere of each chromosome The centromeres split apart and the sister chromatids separate from each other The spindle fibers shorten and the chromosomes are pulled towards the poles of the cell --------------------------------------------- The chromosomes have reached the opposite poles of the cell, they then start to unwind into chromatin The spindle fibers break down and a nuclear membrane forms around the new set of chromosomes Cytokinesis follows, splitting the cell into two haploid gametes GAMETE FORMATION IN ANIMALS The products of Meiosis are gametes: in humans these gametes are sperm and eggs, the process of sperm formation is called spermatogenesis and the process of egg production is called oogenesis both processes involve meiosis, but they happen in slightly different ways In most male animals, meiosis takes place in the testes • the process of spermatogenesis begins with a diploid cell called a spermatogonium • beginning at puberty, spermatogonia reproduce by mitosis and the resulting cells undergo meiosis to form 4 haploid cells • The cells then develop into mature sperm after Meiosis II • The nucleus and certain molecules required by the cell are organized into a "head" region, the midsection holds many mitochondria, which are an energy source for the cell and finally a long tail-like flagellum develops for locomotion OOGENESIS In most female animals, meiosis takes place in the ovaries Oogenesis starts with a diploid cell called an oogonium Before birth, the oogonia reproduce by mitosis, and they begin meiosis, but they stop at Prophase I Meiosis will continue for one cell each month Beginning at puberty Oogenesis involves an unequal division of cytoplasm, the cell that receives most of the cytoplasm after the first division continues through meiosis I and II to form a viable egg, the egg needs a large quantity of nutrients that will support the zygote after fertilization The other, smaller cell is called a polar body and it will degenerate The final stages of meiosis II are not completed unless fertilization by a sperm cell occurs When meiosis II is completed, the mature egg and another polar body is produced MULTIPLE BIRTHS Fraternal twins are formed from two eggs being fertilized by two sperm cells Identical twins are born when a single zygote divides into two separate bodies in the first few days of development INDEPENDENT ASSORTMENT During metaphase I, chromosomes are arranged in homologous pairs along the metaphase plate (equatorial) In each pair, the chromosome of maternal origin is oriented toward one pole of the cell and the chromosome of paternal origin is oriented toward the other pole of the cell The orientation of each pair of chromosomes is independent of the orientation of the other pairs Depending on how the chromosomes line up, a number of different combinations are possible • The number of genetically distinct gametes that can be produced from a diploid cell is 2n • Therefore each human can produce 223 or 8 388 608 genetically distinct gametes CROSSING OVER While homologous chromosomes are lined up during Prophase I, non sister chromatids may exchange pieces of chromosome, this is known as crossing over Crossing over can occur at several points along the sister chromatids, a section of chromosome that is crossed over may contain hundreds or even thousands of genes As a result individual chromosomes contain some genes of maternal origin and some of paternal origin This once again dramatically increases the genetic diversity of the gametes produced ERRORS DURING MEIOSIS While independent assortment and crossing over provide opportunities for genetic diversity they also provide the potential for chromosomal abnormalities Many of the errors during meiosis produce gametes that will not survive; however, some do and can potentially pass those errors on to the zygote if fertilized Since every cell in the offspring is produced from that one fertilized zygote cell therefore all of the cells in the embryo will contain the chromosomal abnormality There are two types of chromosomal errors that can occur during meiosis: 1) changes in chromosome structure 2) changes to chromosome number ERRORS CAUSED BY CHANGES IN CHROMOSOME STRUCTURE To facilitate crossing over the chemical bonds that hold the DNA together in the chromosome must be broken and reformed, sometimes this does not happen correctly Also, non homologous pairs may cross over, producing chromosomes that contain genes not normally on that chromosome Errors include: 1) Deletion 2) Duplication 3) Inversion 4) Translocation DELETION A piece of chromosome is deleted Example of Genetic Disorder: Cri du Chat (French for " cry of a cat") syndrome is caused by a deletion in chromosome 5 Many children with this syndrome cry with a high pitched, catlike sound, other symptoms include: low birth weight, widely spaced eyes, recessed chin and developmental and cognitive delays There is no cure for this disorder DUPLICATION A section of a chromosome appears two or more times in a row Example of Genetic Disorder: Charcot MarieTooth Disease is caused by duplication of a gene on chromosome 7 The most common symptoms are muscle weakness and loss of some sensation in the lower legs, feet, and hands A high foot arch with constantly flexed toes is often present There is no cure for this disorder INVERSION A Section of the chromosome is inverted Example of Genetic Disorder: FG Syndrome, a form of FG syndrome is caused by the inversion of the X Chromosome This syndrome occurs almost exclusively in males Symptoms include intellectual disabilities of varying degrees, delayed motor development, low muscle tone and broad toes and thumbs There is no cure for this disorder TRANSLOCATION A segment of one chromosome becomes attached to a different chromosome Example of Genetic Disorder: Chronic Myelogenous Leukemia, most cases of CML are are caused by a translocation between chromosome 9 and 22, which results in an abnormal gene this is a cancer of the white blood cells Treatment involves using a drug that stops the increased production of white blood cells that the abnormal gene causes Some disorders are associated with more than one type of error For example: autism can be caused by various duplications, inversions and translocations, it is a complex developmental disorder that is found in 1 in 165 children ERRORS CAUSED BY CHANGES IN CHROMOSOME NUMBER Sometimes homologous chromosomes or sister chromatids do not separate as This is known as nondisjunction This phenomenon occurs in anaphase I or II of meiosis In anaphase I, nondisjunction occurs when homologous chromosome pairs do not separate to opposite poles, instead one entire pair is pulled to the same pole In anaphase II, nondisjunction occurs when sister chromatids do not separate to opposite poles, instead they are pulled toward the same pole Nondisjunction always results in gametes with too few or too many chromosomes as seen above GENETIC DISORDERS ASSOCIATED WITH CHROMOSOME NUMBER Many of the genetic disorders that have been identified to date are due to an individual having an incorrect number of chromosome Many individuals born with this syndrome have an extra chromosome or an extra piece of chromosome 21 The incidence of nondisjunction leading to Down syndrome increases with maternal age, women ages 20 to 24 the chances are 1 in 1490, for women at age 40 the chances are 1 in 106 and for women at age 49 it increases to 1 in 11 One example is Down Syndrome PHYSICAL FEATURES OF BABIES WITH DOWN SYNDROME HOMEWORK