What is Nondisjunction in Meiosis?
All of your body cells, except eggs and sperm cells, have two sets of 23 chromosomes, for a total of 46 chromosomes. Having two sets of chromosomes is called diploid and designated as 2n. Eggs and sperm cells are called gametes, and they have only one set of 23 chromosomes, which is called a haploid, or n. When two haploid gametes (one egg and one sperm cell) fuse during fertilization, a diploid cell is produced. This diploid cell has one set of chromosomes from the egg and one set of chromosomes from the sperm cell. This diploid cell will divide and develop into a baby.
Meiosis is the process of creating haploid gametes from a diploid cell. If everything goes smoothing during meiosis, chromosomes will be separated and distributed evenly to produce four haploid gametes. However, sometimes chromosomes do not separate properly. This is called nondisjunction and results in gametes with either too many or too few chromosomes. In humans, nondisjunction becomes more common the older one gets.
Production of Normal Gametes
To get ready for meiosis, a diploid germ cell must first undergo DNA replication. This process duplicates every strand of DNA so that there are two copies of every chromosome. The two copies are called sister chromatids and are attached to each other. Once DNA replication is complete, two rounds of chromosomal divisions take place, one each in meiosis I and meiosis II. During meiosis I, homologous chromosomes pair up, line up, and then are split apart.
Homologous chromosomes are those that are the same size and carry the same genes. You get one from your mom and one from your dad. In human diploid cells, there are 23 pairs of homologous chromosomes. After meiosis I, two cells are present, each with 23 different chromosomes that are still in duplicate as sister chromatids.
Both the cells produced in meiosis I undergo meiosis II. During this round, chromosomes line up and sister chromatids are pulled apart into two sets. Now, a total of four haploid cells are present. The pulling apart of either homologous chromosomes or sister chromatids is called disjunction and occurs during anaphase of meiosis I or meiosis II, respectively.
Nondisjunction Produces Abnormal Gametes
Sometimes during anaphase, chromosomes will fail to separate properly. Remember, this is called nondisjunction. This can happen either during meiosis I or meiosis II.
If nondisjunction occurs during anaphase I of meiosis I, this means that at least one pair of homologous chromosomes did not separate. The end result is two cells that have an extra copy of one chromosome and two cells that are missing that chromosome. In humans, n + 1 designates a cell with 23 chromosomes plus an extra copy of one for a total of 24 chromosomes. n - 1 designates a cell missing a chromosome for a total of only 22 chromosomes in humans.
If nondisjunction occurs during anaphase II of meiosis II, it means that at least one pair of sister chromatids did not separate. In this scenario, two cells will have the normal haploid number of chromosomes. Additionally, one cell will have an extra chromosome (n + 1) and one will be missing a chromosome (n - 1).
Fertilization with Abnormal Gametes
If one of these abnormal gametes undergoes fertilization, then a baby with an abnormal number of chromosomes in its cells could be born. Trisomy is the condition of having 3 copies of one chromosome type. It is designated as 2n + 1 because the cell has the normal two sets of each 23 types of chromosomes plus an extra copy of one chromosome. Monosomy is the condition of having only 1 copy of a chromosome and is designated as 2n - 1.
Syndromes Caused by Nondisjunction
Most of the time when fertilization involves gametes with an abnormal number of chromosomes, the resulting cell will not survive to produce a new individual. However, there are some cases in which individuals do survive. These individuals often have some set of symptoms or a syndrome caused by having too many or too few chromosomes. This table shows the most common human disorders caused by nondisjunction:
|Syndrome||Chromosomes Involved||Total Chromosome Number||Signs and Symptoms|
|Down syndrome||Trisomy 21||47||Individuals have three copies of chromosome 21 instead of the normal two. This results in mental retardation, short stature, flattened facial features.|
|Patau syndrome||Trisomy 13||47||Brain and eye defects, circulatory system defects, do not survive more than a few months after birth.|
|Edward's syndrome||Trisomy 18||47||Every organ affected, life span is a few days to a few months.|
|Klinefelter syndrome||Two X chromosomes in males||47||Normal males have one X and one Y chromosome (XY), but Klinefelter males have two X chromosomes (XXY). Symptoms include reduced genitals, inability to reproduce, enhanced feminine body features.|
|XYY syndrome||Two Y chromosomes in males||47||Males with XYY instead of XY, taller than average, slightly below average intelligence.|
|Trisomy X||Three X chromosomes in females||47||Normal females have two X chromosomes (XX), but those with trisomy X have three (XXX). Usually healthy and fertile, cannot be distinguished from a normal XX female.|
|Turner's syndrome||Monosomy X in females||45||Females with only one X chromosome, do not mature sexually, cannot reproduce, short stature. The only case of nonfatal monosomy in humans.|
Down syndrome, which you may have heard of before, is caused by trisomy, but is not usually fatal. However, as you can see, several of the syndromes can significantly shorten lifespan, including Patau syndrome and Edward's syndrome. Other syndromes have varying effects but typically result in noticeable abnormalities, including Klinefelter syndrome, XYY syndrome, and Turner's syndrome, whereas Trisomy X often goes unnoticed.
Let's review. Meiosis is a process that takes one diploid cell and divides it into four haploid cells called gametes. Gametes are egg and sperm cells that fuse during fertilization to produce a new individual. In humans, gametes have 23 chromosomes. Sometimes either homologous chromosomes or sister chromatids fail to separate properly during meiosis. This is called nondisjunction, and it produces gametes with an abnormal number of chromosomes. If these gametes undergo fertilization, new individuals with abnormal numbers of chromosomes in their cells may be produced. These individuals will usually have some set of symptoms.
After you have finished, you should be able to:
- State the difference between diploid and haploid cells
- Recall the number of chromosomes in a gamete and what happens during meiosis
- Explain what nondisjunction is and recall some of the syndromes that result from it
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Nondisjunction in Meiosis: Crossword Puzzle
In this activity, you'll check your knowledge regarding the process of nondisjunction during meiosis.
Complete the crossword by filling in a word that fits each of the given clues. For this activity, you'll need a printer to reproduce the following page. With a pencil and an eraser, neatly write your answers in the boxes provided.
2. The cell division of a diploid cell into __________ haploid cells transpires during meiosis.
5. In genetics, nondisjunction refers to the failure of __________ pairs to separate properly during meiosis.
8. In general, __________ have two sets of 23 chromosomes, for a total of 46 chromosomes.
10. A __________ diploid cell is produced from two haploid cells.
1. __________ is a genetic condition where a single chromosome type occurs in three instances.
3. A reproductive cell, having only half of a complete set of chromosomes.
4. A syndrome that results in mild to moderate intellectual disability, short stature, and flattened facial features.
6. Diploid cells are produced when an egg cell __________ with a sperm cell during fertilization.
7. A syndrome caused by a chromosomal abnormality that leads to multiple and complex organ defects.
9. In the replication process, every strand of the __________ is duplicated so that there are two copies of every chromosome.
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