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April 30, 2017
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
COLORADO SCHOOL OF MINES
Logan Weinman I Core Supplemental Instruction Facilitator | Biology I
Center for Academic Services & Advising
2. Draw the phases of meiosis.
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.
COLORADO SCHOOL OF MINES
Logan Weinman I Core Supplemental Instruction Facilitator | Biology I
Center for Academic Services & Advising
4. Explain why a monohybrid cross illustrates Mendel’s Law of Segregation
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