Notes From Reading
CHAPTER 12:M EIOSIS PGS .257-279)
- Meiosis is a type of nuclear division. It results in cells that have half as many chromosomes as
the parent cell, and in animals it occurs prior to the formation of eggs and sperm. When an egg
and a sperm cell combine to form an offspring, the original number of chromosomes is restored.
- Each cell produced by meiosis receives a different combination of chromosomes. Because genes
are located on chromosomes, each cell produced by meiosis receives a different complement of
genes. Meiosis leads to offspring that are genetically distinct from each other and from their
- The leading hypothesis to explain meiosis is that genetically variable offspring are more likely to
thrive in changing environments.
- If mistakes occur during meiosis, the resulting egg and sperm cells may contain the wrong
number of chromosomes. Offspring with an incorrect number of chromosomes usually do not
- During sexual reproduction, a sperm and an egg unite to form a new individual, a process called
- Meiosis is nuclear division that precedes the formation of gametes (egg and sperm) and results
in a halving of chromosome number.
12.1 How Does Meiosis Occur?
- Each organism has a characteristic number of chromosomes.
- The karyotype is the number and types of chromosomes present in an organism.
- Sex chromosomes determine the sex of the individual; all other chromosomes are autosomes.
- Chromosomes of the same type are called homologous chromosomes, or homologs.
- Chromosomes carry genes. A gene is a section of DNA that influences one or more hereditary
traits in an individual. Different versions of a specific gene are called alleles.
- Homologs carry the same genes in the same locations, but each one may contain different
- Organisms whose cells contain just one of each type of chromosome are called haploid. Those
whose cells contain two versions of each type of chromosome are termed diploid. Organisms
with three or more versions of each type of chromosome are called polyploid.
- The haploid number n indicates the number of distinct types of chromosomes present in an
organism’s cells. A cell’s ploidy (n, 2n, etc.) indicates the number of each type of chromosome
An Overview of Meiosis
- Meiosis reduces chromosome number by half. In diploid organisms, the products of meiosis are
haploid. Notes From Reading
CHAPTER 12:M EIOSIS (PGS.257-279)
- Just before meiosis begins, each chromosome in the diploid (2n) parent cell is replicated. When
replication is complete, each chromosome consists of two identical sister chromatids attached
at the centromere.
- When two haploid gametes fuse during fertilization, a full complement of chromosomes is
restored. The cell that results from fertilization is diploid and is called a zygote.
- In this way, each diploid individual receives both a haploid chromosome set from its mother and
a haploid set from its father. Homologs are therefore referred to as being either maternal
chromosomes, from the mother, or paternal chromosomes, from the father.
- To put these events in a larger context, the changes in chromosome number during a diploid
organism’s life cycle are shown in Figure 12.3. Note how the ploidy of the organism changes as a
result of meiosis and fertilization.
The Phases of Meiosis I
- Meiosis I is a continuous process with five distinct phases. These phases are as follows:
(1) Early prophase
(2) Late prophase
- Early Prophase I: The homolog pairs come together in a pairing process called synapsis. The
structure that results from synapsis is called a tetrad, consisting of two homologs. The
chromatids of the homologs are called non-sister chromatids.
- Late Prophase I: These non-sister chromatids begin to separate. Exchange or crossing over
between homologous non-sister chromatids occurs where chiasmata are formed during this
- Metaphase I: The pairs of homologs line up at the metaphase plate.
- Anaphase I: The paired homologs separate and begin to migrate to opposite ends of the cell.
- Telophase I: The homologs finish migrating to the poles of the cell. Then the cell divides in the
process of cytokinesis.
- The end result of meiosis I is that one chromosome of each homologous pair is distributed to a
different daughter cell.
The Phases of Meiosis II
- At the start of meiosis II, each daughter cell is haploid; but each replicated chromosome is still
composed of two identical sister chromatids.
- Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II.
- Prophase II: The spindle apparatus forms and attaches to each side of the chromosomes, one
spindle fiber to each sister chromatid.
- Metaphase II: Replicated chromosomes, consisting of two sister chromatids, line up at the
metaphase plate. Notes From Reading
CHAPTER 12:M EIOSIS(PGS.257-279)
- Anaphase II: Sister chromatids separate. The unreplicated chromosomes that result begin
moving to opposite sides of the cell.
- Telophase II: Chromosomes finish moving to opposite sides of the cell. A nuclear envelope forms
around each haploid set of chromosomes.
- Meiosis II results in four haploid cells, each with one chromosome of each type. Thus, one
diploid cell with replicated chromosomes gives rise to four haploid cells with unreplicated
Comparison of Meiosis and Mitosis
Feature Mitosis Meiosis
Number of cell divisions - One - Two
Number of chromosomes in Same Half
daughter cells, compared with
Synapsis of homologs No Yes
Number of crossing-over None One more per pair of
events homologous chromosomes
Makeup of chromosomes in Identical Different – only one of each
daughter cells chromosome type present,