PREPARATION FOR WEEK 6 EMBRYOLOGY
REVIEW OF MEIOSIS AND NON-DISJUNCTION
LEARNING OBJECTIVES
By completing this pre-week preparation and further learning during the week, the learner should be able to:
1. Describe the significance of gametes.
2. Describe the process of meiosis.
3. Describe the significance of meiotic non-disjunction.
1. INTRODUCTION
Meiosis is a special type of cell division, restricted to the sex cell (gamete) lineage in which the number of chromosomes in the gamete
nucleus is reduced from 46 to 23. Each of the resultant haploid cells (female and male gametes) contains a single member of every
chromosome pair from the parent nucleus. Meiosis therefore ensures that when male (sperm) and female (ovum) sex cells fuse at
fertilization, the diploid complement of chromosomes is maintained from generation to generation.
The process of meiosis also creates new combinations of genes in the chromosomes through physical exchange of chromosomal
regions (crossing over). This, along with the independent assortment of chromosomes during the cell divisions of meiosis, increases
genetic diversity in the resultant gametes.
2. OVERVIEW OF MEIOSIS
Diploid cells contain pairs of chromosomes, each member of which carries the same set of genes. The paired members are called
homologous chromosomes or homologs. One member of each pair is inherited from the mother while the other is from the father. Diploid
cells in humans contain 23 homologous chromosome pairs. For purposes of a simple diagram, Figure 1 (A and B) illustrates the 2
homologous pairs in a cell from a hypothetical organism that has just 2 pairs of chromosomes in its diploid nuclei.
Replication of deoxyribonucleic acid (DNA) occurs in advance of meiotic cell division. Each chromosome duplicates itself to form two
identical copies during replication (Figure 1, Stage C). The identical copies are attached at a region known as the centromere. Each copy
is known as a chromatid. At this stage, each chromosome now consists of 2 chromatids known as sister chromatids.
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Figure 1. Diagramatic illustration of meiosis from Moore and Persaud, The Developing Human. Clinically oriented Embryology, 7th
edition.
Meiotic cell division consists of two stages (meiosis I and II) which result in the production of four gamete cells. The
distinctive characteristics of meiosis include a reduction in the number of chromosomes by half and mixing of parental
chromosomes through swapping of regions between homologous chromosomes (Figure 1, stages D and E). In the first
meiotic cell division or meiosis I, homologs are separated from each other (Figure 1, stages E-H). This reduces the
number of chromosomes in our hypothetical cell from 4 to 2, although each chromosome still consists of 2 chromatids.
This is followed by meiosis II in which sister chromatids are separated from each other, a process which is similar to
mitosis (Figure 1, stages H-K). Each of the gametes that result now contains only 2 single stranded chromosomes.
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2.1 MORE ABOUT MEIOSIS I
Following DNA replication in advance of meiosis I, the chromatin (a complex of DNA and protein) of each chromosome condenses
during prophase I (Figure 2). Each chromosome now consists of sister chromatids, joined at a central point known as the centromere.
Between prophase I and metaphase I, the pairs of homologous chromosome come together to form tetrads. Within the tetrad, any pair of
chromatid arms can overlap and exchange in a process called crossing-over or recombination. Recombination is a process that breaks,
recombines and rejoins sections of DNA to produce new combinations of genes. In metaphase I, the homologous pairs of chromosomes
align on either side of the equatorial plate. Then, in anaphase I, the homologous pairs, each with two sister chromatids, separate from
each other and move toward opposite poles of the cell. Finally the cell divides in a process called cytokinesis during telophase I. Each of
the two daughter cells produced thereby is haploid and has only one member of each homologous pair, and thus half the total number
of chromosomes of the original cell.
Figure 2. Stages of the first meiotic division
2.2 MORE ABOUT MEIOSIS II
Meiosis II is similar to mitotic cell division and involves further division of each of the haploid cells produced in meiosis I. The chromosomes
align along the equatorial plate in each cell during metaphase II. This is followed by separation of the sister chromatids at the centromeres
in anaphase II (Figure 3). The separated single-stranded chromosomes move toward each pole of the cell. Finally, the chromosomes
are enclosed in nuclear membranes during telophase II, followed by cytokinesis. At the conclusion of the meiotic process, there are four
haploid daughter cells that go on to develop into either sperm or egg cells.
Figure 3. Stages of the second meiotic division
See this YouTube video for a summary of meiosis
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2.3 MEIOSIS AND CYTOKINESIS I
During cytokinesis, the cytoplasm of the dividing cell is distributed into the new cells. The division of cytoplasm during the development
of sperm cells is even whereas it is very uneven in the development of egg cells or ova (Figure 4). During the cell divisions of oogenesis,
most of the cytoplasm is allotted to only one of the daughter cells resulting in one functional egg and several “polar bodies” (Figure 4).
This unequal division ensures the single egg will have a larger volume of cytoplasm to support the metabolism of early development. The
polar bodies consist only of a haploid nucleus and cell membrane with very little cytoplasm. They soon degenerate.
Figure 4. Normal gametogenesis from Moore and Persaud, The Developing Human. Clinically Oriented Embryology, 7th edition
2.4 NON-DISJUNCTION IN MEIOSIS
Non-disjunction refers to the failure of chromosomes to distribute normally during cell division. Non-disjunction during gametogenesis
affects about 2-3% of sperms and 20-60% of oocytes. As a result, some cells will contain one or more extra chromosomes while others
will be deficient. At fertilization, the fusion of a sperm or an egg cell which contains an abnormal complement of chromosomes with
a normal gamete will result in an early embryo with an abnormal number of chromosomes in its cells. Typically such embryos do not
survive. However, abnormal numbers affecting certain of the chromosomes can remain viable, leading to the development of syndromes
such as Down’s syndrome, Turner’s syndrome, and others.
If non-disjunction occurs in meiosis I, two of the four cells that result by the end of the meiotic process will have an extra chromosome
each while the other two cells will have one less chromosome. The union of the cell with extra chromosome with a normal cell will result
in trisomy while the fertilization between a normal cell and a cell with one less chromosome will result in monosomy.
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The occurrence of non-disjunction in meiosis II will result in one of the four cells resulting from the meiotic process with an extra
chromosome, one cell with one less chromosome, and the other two cells will have normal chromosome complement. Just like in nondisjunction in meiosis I, the union of the cell with extra chromosome with a normal cell will result in trisomy while the fertilization between
a normal cell and a cell with one less chromosome will result in monosomy.
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SOME USEFUL DEFINITIONS
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Chromosome: A structure within the nucleus of animal cells which contain a linear thread of DNA and associated molecules.
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Chromatid: One of the paired chromosome strands joined at a centromere following duplication of a chromosome in a dividing cell.
Each goes to a different pole of the dividing cell and each becomes a chromosome of one of the two daughter cells.
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Aneuploidy: Deviation from an exact multiple of the normal haploid number of chromosomes:
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Down’s syndrome: Also called trisomy 21, is a genetic disorder characterised by the presence of extra genetic material from
chromosome 21.
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Klinefelter’s syndrome: Results from a congenital abnormality of the sex chromosomes and is characterised by the presence of
extra X chromosomes in addition to a Y chromosome. The extra X chromosome causes abnormal development of the testicles,
which leads to underproduction of testosterone.
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Turner’s syndrome: is characterized by partial or complete loss (monosomy) of one of the sex chromosomes in female embryos.
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Polyploidy: More than 2 full sets of chromosomes (e.g. 3N = triploid; 4N = tetraploid).
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Hypoploidy: Less than one full set of chromosomes.
Year 1 Foundations Curriculum, Faculty of Medicine, University of Toronto