Key Difference – Nondisjunction in Meiosis 1 vs 2
Cell division is a vital process in multicellular organisms as well as in unicellular organisms. There are two major cell division processes known as mitosis and meiosis. Genetically identical diploid cells are produced by mitosis and gametes (haploid) with half chromosome sets are produced by meiosis. During the cell division processes, homologous chromosomes and sister chromatids are separated without errors to produce daughter cells with the same number of chromosomes or half the number of the chromosomes. It is referred to as chromosomal disjunction. Although cell division is almost a perfect process, errors can happen during the chromosomal disjunction at a very small error rate. These errors are known as nondisjunction errors. Nondisjunction is the inability or the failure of homologous chromosomes or sister chromatids to separate correctly during the cell division in mitosis and meiosis. Nondisjunction can occur during meiosis I and meiosis II, resulting in abnormal chromosomes number of gametes. The key difference between nondisjunction in meiosis 1 and 2 is that during meiosis 1, homologous chromosomes fail to separate while in meiosis II sister chromatids fail to separate.
CONTENTS
1. Overview and Key Difference
2. What is Nondisjunction in Meiosis 1
3. What is Nondisjunction in Meiosis 2
4. Side by Side Comparison – Nondisjunction in Meiosis 1 vs 2
5. Summary
What is Nondisjunction in Meiosis 1?
Meiosis is a process which produces gametes (eggs and sperms) from diploid cells for reproduction. Gametes contain 23 chromosomes (n). Meiosis occurs via two major stages called meiosis I and meiosis II. Meiosis I consists of five major stages named prophase I, metaphase I, anaphase I, telophase I and cytokinesis. During the anaphase I, homologous chromosomes separate from each other and move towards the two poles. Sometimes, homologous chromosomes show a failure to separate properly. If this happens, gametes will be produced with extra or missing chromosomes. Hence, the total number of chromosomes in gametes will differ from the normal number. Once these gametes are fertilized, they produce abnormal chromosome numbers, which is referred to as aneuploidy.
Figure 01: Nondisjunction in meiosis
These unusual chromosome numbers create several syndromes (disease conditions) in offspring. For example, trisomy of chromosome 21 results in Down syndrome babies. Down syndrome is a result of a gamete containing n+1 chromosomes. When this gamete is fertilized, it creates a zygote which contains 2n+1 number of chromosomes (total of 47 chromosomes). Another example is Turner’s syndrome. It occurs due to nondisjunction of sex chromosome (monosomy XO). It results in n-1 number chromosomes in gametes and after fertilization, the offspring will bear 2n-1 number of chromosomes (total of 45 chromosomes).
Figure 02: Trisomy due to nondisjunction in meiosis I
What is Nondisjunction in Meiosis 2?
Meiosis II is the second consecutive division of meiosis which resembles mitosis. During meiosis II, four gametes are produced from two cells. It is followed by several distinct phases named prophase II, metaphase II, anaphase II, telophase II and cytokinesis. Chromosomes line up at the middle of the cell (metaphase plate) and attach to spindle fibers from their centromeres during the metaphase II. They become ready to divide into two sets and proceed into anaphase II. During the anaphase II, sister chromatids split evenly and are pulled towards poles by microtubules. This step will ensure the correct number of chromosomes in gametes. Sometimes sister chromatids fail to separate properly at this stage due to several reasons such as wrong alignment and attachment at the metaphase plate etc. It is known as nondisjunction in meiosis II. Due to this failure, gametes will be produced with abnormal numbers of chromosomes (n+1 or n-1).
Nondisjunction in meiosis I or II results in gametes with abnormal chromosomal numbers and produce babies with various syndromes such as Down’s syndrome (trisomy 21), Patau syndrome (trisomy 13), Edward’s syndrome (trisomy 18), Klinefelter syndrome (47, XXY males), Trisomy X (47, XXX females), Monosomy X (Turner’s syndrome), etc.
Figure 03: Nondisjunction in meiosis II
What is the difference between Nondisjunction in Meiosis 1 and 2?
Nondisjunction in Meiosis 1 vs 2 |
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| Failure of homologous chromosomes to separate towards the poles during anaphase 1 is known as nondisjunction in meiosis 1. | Failure of sister chromatids to separate towards the poles during anaphase 2 in meiosis is known as nondisjunction in meiosis 2. |
| Chromosomes vs Sister Chromatids | |
| Homologous chromosomes fail to separate properly in meiosis I. | Sister chromatids fail to separate properly in meiosis II. |
Summary – Nondisjunction in Meiosis 1 vs 2
Nondisjunction is a process that creates gametes with an abnormal number of chromosomes. It occurs due to the failure of homologous chromosomes to separate during anaphase I or the failure of sister chromatids to separate during anaphase II in meiosis. Thus, the main difference between nondisjunction in meiosis 1 and 2 is nondisjunction in meiosis 1 occurs in homologous chromosomes while nondisjunction in meiosis II occurs in sister chromatids. Once these gametes are fertilized, aneuploidy individuals may result in several syndromes such as Down’s syndrome, Klinefelter syndrome, Turner’s syndrome, etc.
Reference:
1.”Nondisjunction.” Wikipedia. Wikimedia Foundation, 28 Mar. 2017. Web. 20 May 2017. <https://en.wikipedia.org/wiki/Nondisjunction>.
2. Griffiths, Anthony JF. “Aneuploidy.” An Introduction to Genetic Analysis. 7th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 20 May 2017
Image Courtesy:
1. “Nondisjunction Diagrams” By Tweety207 – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Trisomy due to nondisjunction in maternal meiosis 1” By Wpeissner – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “Meiosis II Non Disjunction” By Dracocephalus~(based on copyright claims). (Public Domain) via Commons Wikimedia