Grade 11A
Name:______________________________ Class: _________________
Active reading 11A – Biological basics of inheritance
Topic 3: Meiosis
How do gametes form?
Recall that gametes are haploid cells. Haploid gametes form from diploid cells
through the process of meiosis. Meiosis is a form of cell division that produces
daughter cells with half as many chromosomes as the parent cell. During meiosis,
a diploid cell goes through two divisions to form four haploid cells. There are two
main parts of meiosis: meiosis I and meiosis II. Each part is, in turn, made up of
several stages. The figure below and on the next page show the stages of meiosis.
Grade 11A
Name:______________________________ Class: _________________
How are meiosis and mitosis different?
It can be easy to confuse mitosis and meiosis. The table below shows how they
are different.
Process
Mitosis
Description
Produces two genetically
identical diploid cells
Meiosis
Produces four genetically
different haploid cells
Function
Makes new cells for
growth, development,
repair, and asexual
reproduction
Makes sex cells (gametes)
for sexual reproduction
Grade 11A
Name:______________________________ Class: _________________
Sexual reproduction produces much more genetic variation than asexual
reproduction. Genetic variation is important to a species. It can allow members of
the species to survive changes in their environment.
Three processes that contribute to genetic variation during sexual reproduction
are crossing over, independent assortment, and random fertilization.
Crossing over occurs during prophase I, when homologous chromosomes form
pairs. During crossing over an arm of one chromatid crosses over the same arm
on another chromatid. The chromatids break at the point of the crossover. The
chromatid re-forms, it contains a piece of the other chromatid.
During metaphase I, chromosomes line up randomly along the equator. This
random distribution of homologous chromosomes during meiosis is called
independent assortment. This process produces gametes with different genetic
formation, as shown below.
Grade 11A
Name:______________________________ Class: _________________
Fertilization is a random process. It can increase genetic variation. For example,
because of independent assortment, there are more than 8 million possible
chromosome combinations for a human female gamete. There are also more than
8 million possible chromosome combinations for a human male gamete.
Therefore there are more than (8 million)2 = 64 trillion possible chromosome
combinations in each human zygote.
Key words
Meiosis
Variation
Gamete
Crossing over
Haploid
Diploid
Independent assortment
Grade 11A
Name:______________________________ Class: _________________
A. Write one or more sentences to answer the following questions
1. How many times does a diploid cell divide during meiosis?
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2. What is the function of meiosis?
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3. Compare the differences between what happens during anaphase I and
what happens during anaphase II?
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4. What are the three processes that contribute to genetic variation during
sexual reproduction?
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5. Why is sexual reproduction helpful to a species?
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6. Give evidence supporting the fact that random fertilization contributes to
variation?
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Grade 11A
Name:______________________________ Class: _________________
B. Fill in the blank spaces in the table below.
Stage of meiosis
Description
Chromosomes condense, homologous chromosomes pair up,
and crossing over occurs.
Cytokinesis occur, and two new cells form
Pairs of sister chromatids move to the equators of the two
cells.
Cytokinesis occurs, and four new cells form.
C. Give two differences between meiosis and mitosis.
Types of cell division
Meiosis
Mitosis
D. Circle the letter of the phrase that best answers the question.
1. Which of the following steps would NOT lead to variation of genetic
material?
A)
Crossing over of non-sister chromatids
B)
Crossing over of sister chromatids
C)
The alignment of the chromosomes during metaphase I
D)
The combination of sperm and egg genes.
Grade 11A
Name:______________________________ Class: _________________
2. During which stage of meiosis does crossing-over occur?
A)
prophase I
B)
anaphase I
C)
telophase I
D)
prophase II
3. What is the importance of crossing-over?
A)
It provides extra genetic material for the daughter cells.
B)
It increases the likelihood that daughter cells contain different
genetic material.
C)
It produces the proteins that are associated with DNA in
chromosomes.
D)
It increases chromosome condensation.
E. Make a flowchart describing the three processes that contribute to
genetic variation.
Grade 11A
Name:______________________________ Class: _________________
F. With a partner talk about ways that genetic variation might help a species
survive changes in the environment. Remember that an organism’s genes
affect its traits.