WEB TUTORIAL 8.1
Chromosome Separation
in Meiosis: Disjunction
OVERVIEW
Normal chromosome disjunction during meiosis produces haploid gametes (or cells) from
a diploid parent cell. If chromosomes or chromatids fail to separate during meiosis I or II
(respectively), abnormal gametes are produced. This tutorial describes how chromosome
nondisjunction occurs during meiosis and discusses how zygotes formed from these
gametes are affected by these errors.
TEXTBOOK REFERENCES
Variation in the Number of Chromosomes Results from Nondisjunction (p. 188)
Normal Disjunction in Meiosis
In meiosis, the chromosomes separate properly during the two divisions to produce normal haploid (n) cells from a diploid (2n) parent cell.
Before meiosis begins, the chromosomes are duplicated, resulting in two chromatids per
chromosome.
During the first meiotic division, homologous chromosomes synapse, giving rise to
tetrads consisting of four chromatids each. The components of the tetrad then separate
into two dyads, a process called disjunction. Each dyad consists of two sister chromatids
joined at a common centromere. Meiosis I is a reductional division.
The second meiotic division is similar to the division in mitosis in that each dyad spits into
two monads. Again, the normal process of chromosome separation is called disjunction.
By the end of meiosis II, four gametes are produced, each containing the proper number
of monads. Meiosis II is an equational division.
The products of meiosis are the haploid gametes. When gametes fuse with other gametes
during fertilization, the normal diploid number of chromosomes is restored.
Nondisjunction in Meiosis I
When chromosomes fail to separate to opposite poles of the cell during meiosis, the error
is called nondisjunction. Nondisjunction can occur in the first or second meiotic division.
Here we show nondisjunction in meiosis I for the human X chromosome during oogenesis. After meiosis I, one cell has one too many chromosomes and the other cell has one too
few.
If the resulting gametes are fertilized, the otherwise diploid zygotes will have one chromosome too many or one too few. Both display aneuploidy, a condition in which a cell
lacks an exact multiple of a haploid set.
Nondisjunction in Meiosis II
Nondisjunction in meiosis II also results in either too many or too few chromosomes in the
gametes.
After fertilization, the resulting zygotes may be aneuploid.
Cells lacking one of a pair of homologous chromosomes are monosomic (2n - 1). Cells with
an extra chromosome (three rather than a normal pair) are trisomic (2n + 1). Both monosomy and trisomy are aneuploid conditions.
Note that nondisjunction in meiosis I results in four abnormal meiotic products (and
zygotes). Nondisjunction in meiosis II results in half normal and half abnormal gametes.
Here is a summary of aneuploidy of the X chromosome in humans. In a male with an extra
X chromosome, the individual has Klinefelter syndrome. These males may have large
breasts and small testes and may be mentally retarded. Males do not develop if they lack
an X chromosome; this condition is lethal.
A female with an extra X chromosome is considered “47,XXX.” Most females with this
condition go through their lives without being aware of it and are essentially normal.
Females with just one chromosome have Turner syndrome and are sterile due to underdeveloped ovaries.
CONCLUSION
Disjunction is the separation of chromosomes during meiosis. Gametes formed from meiosis fuse during fertilization, and the resulting zygote inherits a complete haploid set of
chromosomes from each parent. However, if chromosomes fail to separate properly during meiosis, the resulting gametes may contain too many or too few chromosomes.
Fertilization of these gametes may produce aneuploid zygotes, which contain a chromosome number that is not an exact multiple of the haploid cell. Nondisjunction during
meiosis I produces two gametes that contain two copies of a particular chromosome and
two gametes that are deficient for that chromosome. Nondisjunction during meiosis II produces two normal gametes, one gamete with two copies of a particular chromosome, and
one gamete deficient for that chromosome. Normal fertilization of the abnormal gametes
results in trisomic or monosomic zygotes.
YOU
•
SHOULD NOW BE ABLE TO
Define aneuploidy.
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•
•
Predict the gametes that result if nondisjunction occurs during meiosis I and II of
oogenesis.
Describe the possible outcomes of fertilization if nondisjunction occurs during
oogenesis.
Describe the chromosomal anomalies that characterize Klinefelter syndrome and
Turner syndrome.
KEY TERMS
aneuploid
disjunction
Klinefelter syndrome
metacentric
monosomic
nondisjunction
telocentric