This set of Tricky Cytogenetics Questions and Answers focuses on “Meiosis Prophase 1 : Zygotene”.
1. Which of the following will you expect to see in zygotene stage?
a) Condensed chromosome
b) Thin chromatid
c) Bead on a string model
d) O shaped chromosome
Explanation: The zygotene chromosomes are more condensed than that in leptotene stage. The ball and seen mainly in leptotene stage while the O shaped structure is in diplotene. The thin chromatins are seen in interphase or pre-leptptene.
2. Which of the following events is seen in zygotene phase?
d) Equatorial ring formation
Explanation: The main process taking place in zygotene is synapsis i.e. lengthwise pairing of homologous chromosome. Recombination follows this pairing and occurs in the pachytene stage.
3. If the pairing starts at terminal and proceeds towards centre it is known as ________________
Explanation: In pro-terminal lengthwise pairing the pairing begins at the terminal and then it proceeds towards the centre. The reverse case is known as procentric.
4. If there is an improper attachment of spindle fiber, which protein sends the negative signal?
Explanation: Mad2 is responsible for signalling improper binding of the spindle fiber to the centromere. This negative signal prevents apoptosis. Mdm2 is linked to p53 while Mcm activates ORC. Mek is a part of monomeric G protein signaling.
5. Pairing of chromosome is initiated by hydrodynamic force.
Explanation: Due to vibrations in the cytosol the chromosomes pair. This pairing is mainly due to hydrodynatic force. Another theory for pairing is via electrostatic force.
6. Which of the pairing is not feasible?
a) Pairing between homologous chromosomes
b) Pairing between homologous parts of non-homologous chromosome
c) Pairing between 4 chromosomes following translocation
d) Pairing between more than 2 chromosomes
Explanation: Translocation may result in the chromosome that is partially homologous to two different chromosomes, this result in pairing of 4 chromosomes. Also homologous segments in non-homologous chromosomes can par. But pairing with more than 2 chromosomes in a normal instance is not seen.
7. Which of the following will not be seen in zygotene?
a) Synthesis of Histones
b) Synthesis of DNA
c) Formation of synaptonemal complex
d) Equatorial positioning of chromosomes
Explanation: Unlike mitosis in meiosis there is both DNA synthesis and histone synthesis for protein compaction in meiosis stage. However, equatorial positioning is seen in metaphase only.
8. What is the percentage of DNA synthesized during zygotene?
Explanation: Very less amount of DNA is replicated during zygotene while most are replicated in S phase. This unreplicated in DNA in chromosomes helps to hold the chromosomes together after zygotene.
9. What is the size of DNA that is left to be replicated in zygotene?
a) 100 bp
c) 104 bp
d) 104 kb
Explanation: The amount of DNA left unreplicated is very less about 104 bp. Proper replication of this ensures zygotene progression.
10. There is a direct correlation between this unreplicated DNA and crossing over sites.
Explanation: The carrying over of the phenomenon of DNA replication to the next stage of meiosis suggests that these sites are probably the locations where crossing over will occur.
11. The formation of synaptonemal complex is ______________ DNA replication in zygotene.
d) Unrelated to
Explanation: The replication of DNA in zygotene is mandatory for a synaptonemal complex formation to occur. These regions are also the first to get paired.
12. Which of the following is true?
a) Zygotene is the phase preceding diplotene
b) DNA replicated in zygotene is incorporated in chromosome 5-6 days later
c) Synaptonemal complex formed is longer than the physical length of chromosomes itself
d) The pairing of chromosomes forces the DNA to replicate
Explanation: DNA replicated in zygotene phase is not immediately incorporated in the chromosome but it takes 5-6 days. While zygotene is followed by pachytene and the synaptonemal complex is shorter than the chromosome itself. The pairing follows replication and not vice versa.
Sanfoundry Global Education & Learning Series – Cytogenetics.
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