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BIO 122 Study Guide - Quiz Guide: Prophase, Genetic Variation, Mitosis

Biology - A&S
Course Code
BIO 122
Andrew Koob
Study Guide

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Genes, Chromosomes, and Sexual Reproduction
Meiosis ensures the transmission of traits from one generation to the next. At the same
time, it is a key process that introduces genetic variation into the traits that offspring
inherit from their parents. In this tutorial, you will explore the genetic context of meiosis.
Meiosis terminology
Interactions among chromosomes
This diagram shows a diploid nucleus (2n=8) in which chromosome replication has
occurred in preparation for mitosis (top) and meiosis (bottom). The nucleus at top right is
now in prophase of mitosis; the nucleus at bottom right is now in prophase I of meiosis.
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Animal life cycles
In the life cycle of an organism, meiosis is paired with the process of fertilization.
Understanding the life cycle of an organism is the key to understanding how sexual
reproduction ensures the inheritance of traits from both parents and also introduces
genetic variation.
Meiosis creates gametes (eggs and sperm) with only a single chromosome set (haploid or
n) from parental cells with two chromosome sets (diploid or 2n). During fertilization, the haploid
sperm (n) and egg (n) fuse, producing a diploid zygote (2n). The cells of the zygote then divide
by mitosis (which does not change the ploidy level) to produce an adult organism (still 2n) of the
next generation.
In sexual life cycles, meiosis and fertilization keep the number of chromosomes constant from
generation to generation.
The Mechanism
Meiosis is mechanistically similar to mitosis in many ways, although it involves two
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sequential nuclear and cellular divisions rather than one. The two stages of meiosis are
Meiosis I, which consists of prophase I, metaphase I, anaphase I, and telophase I
(followed by cytokinesis)
Meiosis II, which consists of prophase II, metaphase II, anaphase II, and telophase
II (followed by cytokinesis)
The stages of meiosis
Meiosis involves two sequential cellular divisions. In meiosis I, homologous chromosomes pair
and then separate. Thus, although the parent cell is diploid (containing two chromosome sets,
one maternal and one paternal), each of the two daughter cells is haploid (containing only a
single chromosome set). In meiosis II, the sister chromatids separate. The four daughter cells that
result are haploid.
Crossing over
Crossing over plays a critical role in increasing the genetic variation among offspring of
sexual reproduction. It is important to understand how crossing over occurs and its
consequences in meiosis. Look carefully at the diagrams depicting different stages in
meiosis in a cell where 2n = 6. Assume that the red chromosomes are of maternal origin
and the blue chromosomes are of paternal origin.
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