Meiosis
INTRODUCTION
This week, we will discuss meiosis, a second form of cellular (nuclear)
division. Although mitosis and meiosis have many similarities, significant
differences exist between the two processes. For instance, mitosis alone is
required for asexual reproduction while meiosis is essential for sexual
reproduction. Recall that mitosis produces two genetically identical daughter
cells from one parent cell. The process of meiosis produces four daughter
cells, which are genetically different from the parent cell. Look for additional
similarities and differences between the two processes as you study meiosis.
The purpose of meiosis is to produce sex cells, or gametes. The male
gamete is the sperm while the female gamete is the egg, or ovum (ova-pl.).
Sex cell production occurs in the sex organs, or gonads. The male gonad
is the testis (testes-pl) while the female gonad is the ovary. Thus, meiosis can
occur only in an ovary or testis, whereas mitosis occurs throughout the body,
including in the gonads.
Gametes are unique cells in that they contain only one chromosome from
each homologous pair. Recall that body cells, or somatic cells, contain two
chromosomes of each type.
Scientists refer to an organism's "normal" number of chromosomes as
its diploid number (2n). For humans, the diploid number is 46. (2n = 46) The
diploid number is eight for a fruit fly; in other words, the fly has eight
chromosomes in each of its somatic cells.
The number of chromosomes in a gamete is referred to as the haploid
number (n). For humans, the haploid number is 23. (n = 23) Each human
sperm or ovum contains 23 chromosomes, one of each type. What is the
haploid number of a fruit fly? (Click here for answer)
Meiosis is often referred to as reduction division because it reduces
the number of chromosomes per cell. This reduction is very important. When
a haploid sperm fertilizes a haploid ovum, the resulting zygote is diploid. Thus
the new organism (the zygote) contains its species "normal" number of
chromosomes.
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STAGES OF MEIOSIS
Meiosis actually involves two rounds of cell division, referred to as
meiosis I and meiosis II. As you read through the stages of meiosis, refer to
the diagrams. Notice that homologs are represented with like colors. For this
organism, note that the diploid number = 4.
MEIOSIS I
INTERPHASE
Interphase
Each molecule of DNA (chromosome) replicates in response to a
hormonal stimulus.
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PROPHASE I
Normal prophase events occur. The nuclear membrane breaks down,
the nucleolus disappears, spindle fibers form, centrioles begin to migrate
toward the poles of the cell.
The DNA coils into chromosomes and one can see that tetrads have
formed. A tetrad is a pair of replicated chromosomes in close association with
one another. Tetrads only form in meiosis I. Remember that in mitosis, the
homologous chromosomes basically "ignored" each other. In meiosis, homologs
come together to form tetrads.
Sometimes while the homologs are in tetrad form, crossover occurs. (See
figure in your text.) In this process, matching chromosomes exchange homologous
pieces of DNA. The pieces of DNA simply break off and switch places!
Crossover is important because it creates new gene combinations which can be
inherited by the offspring.
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METAPHASE I
The tetrads align on the cell’s “equator” (the equatorial plane). Note
that the “equator” runs between homologs.
(NOTE: This isn’t really that great of picture of METAPHASE I because the tetrads
are already coming apart! In reality, the two duplicated yellow chromosomes should be right
next to each other – attached, actually. The same is true for the two pink chromosomes.)
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ANAPHASE I
The homologs are pulled apart from each other, and travel toward opposite
poles of the cell.
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TELOPHASE I
Cytokinesis occurs. Two cells are formed. Note that these two cells
are haploid (n). Reduction in chromosome number has occurred!!
Other typical telophase events occur. The spindle disappears and the
nuclear membrane reforms. Sometimes the DNA returns to chromatin form,
but sometimes it remains in visible (condensed) chromosome form.
This is the end of meiosis I. Notice that while both daughter cells are
haploid (n), the chromosomes in each cell are still in duplicated form, that is
they are twice their normal size. For this reason, these two daughter cells
cannot become gametes – they have too much DNA per cell. In order to
restore the chromosomes to their “normal” size, we must continue to Meiosis
II . . .
MEIOSIS II
PROPHASE II
Note that the cells do NOT return to interphase. Normal prophase
events occur in both of the cells.
Notice that there are no homologous pairs present in either cell.
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METAPHASE II
The replicated chromosomes line up on the equator. This stage
resembles metaphase in mitosis, except that each chromosome does NOT have
a matching homolog in the same cell.
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ANAPHASE II
Each replicated chromosome splits in half at the centromere. The
chromatids (now “normal-sized” chromosomes) migrate toward opposite poles.
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TELOPHASE II
Cytokinesis and normal telophase events occur. Four haploid
daughter cells exist which can mature into gametes.
Cytokinesis and normal telophase events occur. Four haploid daughter cells
exist which can mature into gametes.
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ADDITIONAL NOTES:
1. Meiosis does not OCCUR in the gametes; it PRODUCES gametes.
2. Sperm production is called spermatogenesis while egg production is
called
oogenesis. The chromosomes behave the same way in
spermatogenesis and oogenesis; however, other differences exist
between the two processes.
2. In spermatogenesis, four sperm cells are produced as a result of one
cell (spermatogonium) undergoing meiosis.
4. In oogenesis, the cell undergoing meiosis also produces four daughter
cells; however, only one of these cells receives the great majority of
the cytoplasm. This cell matures to become the ovum. The other three
daughter cells, called polar bodies, simply die and are never fertilized.
Oogenesis and spermatogenesis will be explored in more detail later in
this module.