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Center for Academic Services & Advising October 25, 2016 Biology I CSI Worksheet 8 1. Write a short definition of each word: a. Cytokinesis- The division of the cytoplasm to form two separate daughter cells immediately after mitosis b. Chromosome- A long, threadlike DNA molecule and its associated proteins (histones) c. Chromatin- The complex of DNA and proteins that makes up a chromosome in eukaryotic organisms d. Sister Chromatids- Replicated strands of the same chromosome, joined together at the centromere e. Nucleosome- The “beads on a string”; a segment of DNA wound around 8 histones. f. Histone –The small protein that binds to the DNA, contributing to the chromatin structure g. Centromere- The central region that joins two sister chromatids h. Centrosome- The microtubule organizing center present in the cytoplasm i. Homologous Chromosomes- Chromosomes that possess genes coding for the same traits at the same loci. One chromosome is inherited from the father, one from the mother. j. Chiasma- A point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands. k. Non-sister Chromatids- Either of the two copies inherited by either parent (a maternal and paternal copy rather than two identical copies). l. Cohesins- A protein complex that facilitates the separation of sister chromatids during meiosis 2. Draw the phases of meiosis. COLORADO SCHOOL OF MINES Logan Weinman I Core Supplemental Instruction Facilitator | Biology I Center for Academic Services & Advising 3. Explain the three sources of genetic variability. The three sources of genetic variability are independent assortment, crossing over, and random fertilization. Independent assortment is the random organization of homologous chromosomes during metaphase I of meiosis. Crossing over is the recombination of genetic information from the criss-crossing of sister chromatids on homologous chromosomes. Essentially, the chromosomes trade segments of themselves. Random fertilization means that out of the many sperm cells and eggs produced, it is completely random which two combine to form a zygote. 4. Explain why a monohybrid cross illustrates Mendel’s Law of Segregation COLORADO SCHOOL OF MINES Logan Weinman I Core Supplemental Instruction Facilitator | Biology I Center for Academic Services & Advising A monohybrid cross involves alleles of a single gene; Mendel’s Law of Segregation states that alleles for a single gene divide evenly between gametes, so that (in the above example) a heterozygous man produces 50% gametes with the dominant allele and 50% gametes with the recessive allele. The Punnet squares that we use for a monohybrid cross assume the Law of Segregation. 5. Explain why a dihybrid cross illustrates Mendel’s Law of Independent Assortment A dihybrid cross considers two genes, each with two alleles, and illustrates the Law of Independent Assortment by assuming that each gene acts independently of the other. In the example of a cross between a pea plant with yellow, round seeds and a plant with green, wrinkled seeds, the color gene and the texture gene act independently of one another. 6. Distinguish between haploid and diploid and where would you find each of these cells Haploid: one set of chromosomes Diploid: two sets of chromosomes Gametes are haploid, somatic cells are diploid 7. Explain how genetic variation would be affected if maternal chromosomes always lined up together on one side of the metaphase plate during meiosis I and paternal chromosomes always lined up on the other side. If maternal chromosomes always lined up on one side and paternal on the other there would never be any independent assortment. Independent assortment allows for 223 different combinations of chromosomes. Without this, the amount of genetic variation in offspring is significantly decreased. This would limit the success of sexual reproduction. COLORADO SCHOOL OF MINES Logan Weinman I Core Supplemental Instruction Facilitator | Biology I Center for Academic Services & Advising 8. If a cell has a diploid number of 50, how many chromosomes are present in the nucleus at the beginning of meiosis? How many chromosomes are present in each resulting nucleus at the end of meiosis? d. Beginning 25; End 50 9. Crossing over occurs _____. d. Between chromatids of homologous chromosomes 10. Which of the following statements helps to explain why sexual reproduction is favored in many groups? d. Sexually reproducing parents can produce offspring that are unique and lack deleterious alleles. 11. The cells that produce sperm in humans contain 46 chromosomes. If one of these cells undergoes meiosis to form sperm cells, and chromosomal nondisjunction occurs in chromosome 22 during meiosis I, what is the chromosome number in each of the resulting sperm? b. 24, 24, 22, and 22 12. Phase Number of Sets Diploid or Haploid of Chromosomes Cells? End of S phase 2 Diploid End of Meiosis I 2 Haploid End of Meiosis II 1 Haploid COLORADO SCHOOL OF MINES Logan Weinman I Core Supplemental Instruction Facilitator | Biology I