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Introduction to Biology Chromosomes in Human Cells • Somatic Cells Include All Cells in the Human Body Except Sperm and Eggs

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Introduction to Biology Chromosomes in Human Cells • Somatic Cells Include All Cells in the Human Body Except Sperm and Eggs Growth and Reproduction 2 Introduction to Biology Chromosomes in Human Cells • Somatic cells include all cells in the human body except sperm and eggs. • Gametes are human sperm and egg cells. • Each human somatic cell has 23 pairs of chromosomes, 46 total. • Each pair of chromosomes are called homologous chromosomes. • Each homologous chromosome carries a copy of the same genes, either from the father or mother. LE 13-3 Pair of homologous 5 µm chromosomes Centromere Sister chromatids • This is called a karyotype. All 23 pairs of homologous chromosomes are lined up. LE 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Centromere Two nonsister Pair of homologous chromatids in chromosomes a homologous pair (one from each set) • The sex chromosomes are called X and Y o Human females have two X chromosomes. o Human males have one X and one Y chromosome • The 22 pairs of chromosomes that do not determine sex are called autosomes. Inheritance of Genes • A gene is a unit of heredity that carries the information for a specific trait or body function. o A gene is made of a segment of DNA. o Each gene is located on a specific chromosome. o Everyone has two copies of each gene (one on each homologous chromosome). • A cell with a full pair of each chromosome is called diploid. o Diploid is written shorthand as 2n. o All somatic cells are diploid (46 chromosomes). • A cell with only one of each homologous chromosome is called haploid. o Haploid is written shorthand as n. o All gametes are haploid and have 23 total chromosomes. • Gametes are haploid cells, containing only one set of chromosomes • For humans, this means 23 total chromosomes (no pairs) o This includes 22 autosomes and a single sex chromosome o In an unfertilized egg (ovum), the sex chromosome is always X o In a sperm cell, the sex chromosome may be either X or Y Chromosomes and the Human Sex Cycle • At sexual maturity, the ovaries and testes begin producing sperm and eggs through meiosis. o Gametes are the only types of human cells produced by meiosis, rather than mitosis • Meiosis is a form of cell division that results in one set of chromosomes in each gamete instead of two. o The resulting daughter cells are haploid. • When fertilization occurs, the haploid sperm and haploid egg fuse together to form a diploid embryo. Interphase • At the end of interphase, each cell has grown into its full size, produced a full set of organelles, and duplicated its DNA. o The cell is diploid at this point. • The nucleus contains 23 homologous chromosome pairs. • Each chromosome is made of two sister chromatids (copies). Prophase I • The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids. Prophase I • As homologous chromosomes pair up and form tetrads, they undergo a process called crossing- over. • First, the chromatids of the homologous chromosomes overlap each other. • Then, the crossed sections of the chromatids are exchanged. • Crossing-over is important because it produces new combinations of genes in the cell. Metaphase I • As prophase I ends, a spindle forms and attaches to each tetrad. • During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell. Anaphase I • During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell. • When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell. Telophase I and Cytokinesis • During telophase I, a nuclear membrane forms around each cluster of chromosomes. • Cytokinesis follows telophase I, forming two new cells. Summary of Meiosis I • Two new haploid cells have been produced. • Each haploid cell contains one chromosome out of the original pair. • Each chromosome still contains two sister chromatids. Prophase II • As the cells enter prophase II, their chromosomes— each consisting of two chromatids—become visible. • The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I. Metaphase II • During metaphase of meiosis II, chromosomes line up in the center of each cell. Anaphase II • As the cell enters anaphase, the paired chromatids separate. Telophase II and Cytokinesis • The two daughter cells from Meiosis I divide, resulting in four daughter cells, each with two chromatids. • These four daughter cells now contain the haploid number (N)—just two chromosomes each. Summary of Meiosis II • A total of four cells have been produced. • Each cell is haploid and only contains one out of the original pairs of homologous chromosomes. • Each chromosome only contains a single chromatid. A Comparison of Mitosis and Meiosis • Mitosis produces cells that are genetically identical to the parent cell. • Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid). • Meiosis allows crossing over of chromosomes. o This produces cells that are genetically different from the parents and each other. • Three events are unique to meiosis, and all three occur in meiosis l: o Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information o At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes o At anaphase I, it is homologous chromosomes, instead of sister chromatids that separate and are carried to opposite poles of the cell LE 13-9 MITOSIS MEIOSIS Parent cell Chiasma (site of MEIOSIS I (before chromosome replication) crossing over) Propase Prophase I Chromosome Chromosome replication replication Tetrad formed by Duplicated chromosome synapsis of homologous 2n = 6 (two sister chromatids) chromosomes Chromosomes Tetrads Metaphase positioned at the positioned at the Metaphase I metaphase plate metaphase plate Anaphase Sister chromatids Homologues Anaphase I Telophase separate during separate Telophase I anaphase during Haploid anaphase I; n = 3 sister chromatids remain together Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Mitosis Meiosis DNA During During replication interphase interphase Divisions One Two Synapsis and Do not occur Form tetrads in crossing over prophase I Daughter Two diploid, Four haploid, cells, genetic identical to different from parent cell parent cell and composition each other Role in animal Produces cells Produces body for growth and gametes tissue repair Genetic Variation Among Offspring • The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation • Three mechanisms contribute to genetic variation: o Independent assortment of chromosomes o Crossing over o Random fertilization Independent Assortment of Chromosomes • In independent assortment, each pair of chromosomes sorts maternal and paternal homologous chromosomes into daughter cells independently of the other pairs. • Example: o One human sperm cell could contain 15 chromosomes from his father, and 8 from his mother o Another contains 20 from the mother, 3 from the father. LE 13-10 Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Independent Assortment of Chromosomes • The number of combinations possible when chromosomes assort independently into gametes is calculated by 2n, where n is the haploid number • For humans (n = 23): o 223 = 8,388,608 possible combinations! Crossing Over • Crossing over produces new chromosomes with a mixture of genes from each parent. • Instead of a chromosome that is 100% from the person’s father or mother, it might now be 95% from the father, 5% from the mother. LE 13-11 Prophase I Nonsister of meiosis chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any egg. Genetic Diversity • How many possible combinations of genes are there from two parents? • Independent assortment: 223 = 8,388,608 combinations of chromosomes in each sperm or egg cell. • Random assortment: 8.4 million possible sperm combinations + 8.4 million possible egg combinations = 16.8 trillion possible embryos Genetic Diversity • How many possible combinations of genes are there from two parents? • Crossing over o Average of 1,000 genes in each chromosome o At the most, about half of the chromosome can cross over to its homologous partner. o This results in 3.3 novemquardragintillion (1 followed by 150 zeros) gene combinations for each chromosome pair crossing over. Genetic Diversity • How many possible combinations of genes are there from two parents? • Total 3.3 novemquardragintillion possible chromosome combinations x 23 chromosomes x 16.8 trillion possible sperm-egg combinations =1.3 quinquinquagintillion (1 followed by 168 zeros) possible different genetic combinations for two people. .
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