
The nondisjunction in meiosis may result in an extra copy of chromosome in a cell. During which phase the above phenomenon may occur?
a) Prophase-I, Prophase-II
b) Metaphase-I, Anaphase-II
c) Anaphase-I, Anaphase-II
d) Anaphase-I, Telophase-II
Answer
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Hint: Meiosis is generally regarded as the reduction division because it leads to decrease in chromosome number to half in daughter cells i.eThe resulting \[4\] daughter cells are haploid non-identical. Unlike mitosis, meiosis is divided into Meiosis-I and Meiosis-II. Both these stages have prophase, metaphase, anaphase and telophase. Reduction division occurs in meiosis because there are two divisions but only single replication during s-phase. This generally occurs in germ cells.
Complete step by step answer:
- Prophase-I, Prophase-II: Although these stages are slightly different (explained later in the additional information section) but mainly, during prophase stages, the chromatin material condenses, centriole start moving towards opposite poles, by the end Golgi bodies, endoplasmic reticulum, nucleolus, nuclear membrane disappear.
Hence, option (a) is incorrect.
- Metaphase-I, Anaphase-II: During Metaphase-I, all the bivalent tetrad lies in the equatorial plate, spindle attaches to kinetochore of each chromosome, but the alignment of all the pair is independent of each other, so independent assortment occurs in metaphase-I stage.
Hence, option (b) is incorrect.
- Anaphase-I, Anaphase-II: Separation of homologous chromosomes is known as the disjunction of chromosomes. In anaphase-I the pairs of homologous chromosomes separate and segregation of characters occurs and actual haploidy occurs here. In anaphase-II the sister chromatid separates. If due to some reason, chromosomes do not separate (nondisjunction), this leads to polyploidy.
Hence, option (c) is correct.
- Anaphase-I, Telophase-II: In telophase-II, the nucleolus and nuclear membrane reappears, and all \[4\] daughter cells are formed each having a different genotype.
Hence, option (d) is incorrect.
Additional Information: The prophase-I stage is slightly more complicated and long lived than prophase-II.
Prophase-I is divided into \[5\]smaller stages namely, leptotene, zygotene, pachytene, diplotene, diakinesis.
- Leptotene: The chromatin material condenses and chromosome material appears.
- Zygotene: Pairing of homologous chromosomes (synapsis) occurs, and a bivalent dyad of synaptonemal complex is formed.
- Pachytene: chromosome divides to form sister chromatid and a bivalent tetrad is formed. Crossing over occurs between the non-sister chromatids of homologous chromosomes by the action of recombinase enzyme and recombination nodules are formed.
- Diplotene: The synaptonemal complex dissolves except at the place of crossing over, where a x-shaped structure, ‘chiasmata’ is formed.
- Diakinesis: The terminalization of chiasmata occurs here, a meiotic spindle is assembled and by the end the nuclear membrane and other organelles disappear.
Therefore, the correct answer is (c).
Note: Metaphase stage is the best stage to study the number and morphology of chromosomes and the anaphase stage is the best stage to study the shape of chromosomes. The short-lived phase between M-I and M-II is known as ‘interkinesis’, during which the centriole doubles again for the second division.
Complete step by step answer:
- Prophase-I, Prophase-II: Although these stages are slightly different (explained later in the additional information section) but mainly, during prophase stages, the chromatin material condenses, centriole start moving towards opposite poles, by the end Golgi bodies, endoplasmic reticulum, nucleolus, nuclear membrane disappear.
Hence, option (a) is incorrect.
- Metaphase-I, Anaphase-II: During Metaphase-I, all the bivalent tetrad lies in the equatorial plate, spindle attaches to kinetochore of each chromosome, but the alignment of all the pair is independent of each other, so independent assortment occurs in metaphase-I stage.
Hence, option (b) is incorrect.
- Anaphase-I, Anaphase-II: Separation of homologous chromosomes is known as the disjunction of chromosomes. In anaphase-I the pairs of homologous chromosomes separate and segregation of characters occurs and actual haploidy occurs here. In anaphase-II the sister chromatid separates. If due to some reason, chromosomes do not separate (nondisjunction), this leads to polyploidy.
Hence, option (c) is correct.
- Anaphase-I, Telophase-II: In telophase-II, the nucleolus and nuclear membrane reappears, and all \[4\] daughter cells are formed each having a different genotype.
Hence, option (d) is incorrect.
Additional Information: The prophase-I stage is slightly more complicated and long lived than prophase-II.
Prophase-I is divided into \[5\]smaller stages namely, leptotene, zygotene, pachytene, diplotene, diakinesis.
- Leptotene: The chromatin material condenses and chromosome material appears.
- Zygotene: Pairing of homologous chromosomes (synapsis) occurs, and a bivalent dyad of synaptonemal complex is formed.
- Pachytene: chromosome divides to form sister chromatid and a bivalent tetrad is formed. Crossing over occurs between the non-sister chromatids of homologous chromosomes by the action of recombinase enzyme and recombination nodules are formed.
- Diplotene: The synaptonemal complex dissolves except at the place of crossing over, where a x-shaped structure, ‘chiasmata’ is formed.
- Diakinesis: The terminalization of chiasmata occurs here, a meiotic spindle is assembled and by the end the nuclear membrane and other organelles disappear.
Therefore, the correct answer is (c).
Note: Metaphase stage is the best stage to study the number and morphology of chromosomes and the anaphase stage is the best stage to study the shape of chromosomes. The short-lived phase between M-I and M-II is known as ‘interkinesis’, during which the centriole doubles again for the second division.
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