Question
The fruit fly has 8 chromosomes $(2 n)$ in each cell. During interphase of mitosis, if the number of chromosomes at $G_1$-phase is 8 , what would be the number of chromosomes after S-phase? [NEET 2021]
(a) 8
(b) 16
(c) 4
(d) 32
Answer/Explanation
Ans. (a)
During S phase or synthetic phase of interphase, replication of DNA and synthesis of histone protein, centromere and centrioles occur, but the number of chromosomes remains same from beginning till the end of $S$ phase.
Hence, number of chromosome will remain 8 after the $S$ phase in fruitfly.
Question
In a mitotic cycle, the correct sequence of phases is [NEET (Oct.) 2020]
(a)S,G $, \mathrm{G}_2, \mathrm{M}$
(b) $\mathrm{G}_1, \mathrm{~S}, \mathrm{G}_2, \mathrm{M}$
(c) $M, G_1, G_2, S$
(d) $G_1, G_2, S, M$
Answer/Explanation
Ans. (b)
In a mitotic cycle, the correct sequence of phases is $G_1, S, G_2, M$. The first three phases, i.e. $G_1, S_1 G_2$ occurring during interphase whereas the M-phase is the period of actual cell division. The major event occurring in each phase is tabulated below
Question
Match the following (Columns) events that occur in their respective phases of cell cycle and select the correct option from the codes given below. [NEET (Oct.) 2020]
Codes
A B C D
(a) 2 3 4 1
(b) 3 4 1 2
(c) 4 1 2 3
(d) 1 2 3 4
Answer/Explanation
Ans. (d)
Question
After karyogamy followed by meiosis, spores are produced exogenously in [NEET 2018]
(a) Agaricus
(b) Alternaria
(c) Neurospora
(d) Saccharomyces
Answer/Explanation
Ans. (a)
Agaricus Meiospores are produced exogenously after karyogarmy and meiosis. It belongs to Basidiomycetes. Aftemaria belongs to the Deuteromycetes class of fungi. The fungi of this class lack sexual reproduction. Therefore, sexual spores are not formed. Neurospora and Saccharomyces belong to Ascomycetes class of fungi. They produce ascopores as meiospores. Their ascospores are produced endogenously.
Question
Anaphase Promoting Complex $(\mathrm{APC})$ is a protein degradation machinery necessary for proper mitosis of animals cells. If APC is defective in a human cells, which of the following is expected to occur? [NEET 2017]
(a)Chromosomes will not condense
(b) Chromosomes will be fragmented
(c) Chromosomes will not segregate
(d) Recombination of chromosome arms will occur
Answer/Explanation
Ans. (c)
If anaphase promoting complex is defective in a human cell, the chromosomes will not segregate during anaphase of mitosis. APC triggers the transition from metaphase to anaphase by tagging specific proteins for degradation.
Concept Enhancer Anaphase stage of mitosis is characterised by two events
(a) Splitting of centromeres and segregation of chromosomes.
(b) Movement of chromatids towards the opposite poles.
Question
Which of the following options gives the correct sequences of events during mitosis ? [NEET 2017]
(a) Condensation $\rightarrow$ nuclear membrane disassembly $\rightarrow$ crossing over $\rightarrow$ segregation $\rightarrow$ telophase
(b) Condensation $\rightarrow$ nuclear membrane disassembly $\rightarrow$ arrangement at equator $\rightarrow$ centromere division $\rightarrow$ segregation $\rightarrow$ telophase
(c) Condensation $\rightarrow$ crossing over $\rightarrow$ nuclear membrane disassembly $\rightarrow$ segregation $\rightarrow$ telophase
(d) Condensation $\rightarrow$ arrangement at equator $\rightarrow$ centromere division $\rightarrow$ segregation $\rightarrow$ telophase
Answer/Explanation
Ans. (b)
During mitosis following events occurs as follows
Condensation of chromosomal material, which takes place at an early prophase stage. During late prophase nuclear membrane disintegrates. Then chromosomes get arranged at equator in the metaphase stage. After that splitting of centromere and segregation of chromosomes occur in the anaphase stage. In telophase stage chromosomes move to opposite poles of the cell. It is last stage of mitosis.
Question
Which of the following is not a characteristic feature during mitosis in somatic cells? [NEET 2016, Phase I]
(a) Disappearance of nucleolus
(b) Chromosome movement
(c) Synapsis
(d) Spindle fibres
Answer/Explanation
Ans. (c)
Synapsis is pairing of homologous chromosomes. It occurs during zygotene stage of meiosis. The homologous chromosomes come closer leading to cross over in the next stage called pachytene. These
are not observed during mitosis.
Question
Spindle fibres attach on to [NEET 2016, Phase I]
(a) kinetochore of the chromosome
(b) centromere of the chromosome
(c) kinetosome of the chromosome
(d) telomere of the chromosome
Answer/Explanation
Ans. (a)
Spindle fibres attach to kinetochores of chromosomes during cell division. They help the chromosomes/chromatids to get separated to the two daughter cells, towards opposite poles.
Question
The complex formed by a pair of synapsed homologous chromosomes is called [NEET 2013]
(a) equatorial plate
(b) kinetochore
(c) bivalent
(d) axoneme
Answer/Explanation
Ans. (c)
The process of pairing of homologous chromosomes is called synapsis. Each pair of synapsed homologous chromosome called bivalent.
Question
A stage in cell division is shown in the figure. Select the answer which gives correct identification of the stage with its characteristic mentioned. [NEET 2013]
(a) Telophase – Nuclear envelope reforms, Golgi complex reforms
(b) Late anaphase – Chromosomes move away from equatorial plate, Golgi complex not present
(c) Cytokinesis – Cell plate formed, mitochondria distributed between two daughter cells
(d) Telophase – Endoplasmic reticulum and nucleolus not reformed yet
Answer/Explanation
Ans. (a)
Telophase is reverse of prophase. The chromosome that have reached their respective poles decondense, i.e., nuclear envelop reforms, Golgi complex reforms, etc. In late anaphase centromeres split and chromatid separate and chromatid move to opposite poles. Cytokinesis is process in which cell itself is divided two daughter cells.
Question
Select the correct option with respect to mitosis. [CBSE AIPMT 2011]
(a) Chromatids start moving towards opposite poles in telophase
(b) Golgi complex and endoplasmic reticulum are still visible at the end of prophase
(c) Chromosomes move to the spindle equator and get aligned along equatorial plate in metaphase
(d) Chromatids separate but remains in the centre of the cell in anaphase
Answer/Explanation
Ans. (c)
In metaphase of mitosis, spindle fibres attach to kinetochore of chromosomes. Chromosomes are moved to spindle equator and get aligned along metaphasic plate through spindle fibres to both pole.
Question
During mitosis ER and nucleolus begin to disappear at [CBSE AIPMT 2010]
(a) late prophase
(b) early metaphase
(c) late metaphase
(d) early prophase
Answer/Explanation
Ans. (d)
In mitosis, prophase is the longest phase of karyokinesis. In early prophase, nuclear membranes, nucleolus start disintegrating. Cells cytoskeleton, Golgi complex, ER, etc. disappear.
Question
Which stages of cell division do the following figures $A$ and $B$ represent respectively? [CBSE AIPMT 2010]
(a) Metaphase – Telophase
(b) Telophase – Metaphase
(c) Late anaphase – Prophase
(d) Prophase – Anaphase
Answer/Explanation
Ans. (c)
In the given figures
A. Late anaphase is characterised by following events
(i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles.
B. Prophase is characterised by centriole separation.
Question
Centromere is required for [CBSE AIPMT 2005]
(a) movement of chromosomes towards poles
(b) cytoplasmic cleavage
(c) crossing over
(d) transcription
Answer/Explanation
Ans. (a)
The arms of chromosome are known as chromatids. These arms are held together at a point called the centromere (or primary constriction). Centromere occurs any where along the length of
chromosome. During cell division spindle fibres are attached to centromere and help in the movement of chromosomes towards the poles.
Question
If you are provided with root tips of onion in your class and are asked to count the chromosomes which of the following stages can you most conveniently look into? [CBSE AIPMT 2004]
(a) Metaphase
(b) Telophase
(c) Anaphase
(d) Prophase
Answer/Explanation
Ans. (a)
At metaphase, the chromosomes are clearly visible as composed of two closely associated halves (chromatids) and the chromosomes have undergone maximum contraction so, these can be counted conveniently. In metaphase chromosome align in the equator of the cell before being separated into each of the two daughter cells.
Question
Which one of the following precedes reformation of the nuclear envelope during M-phase of the cell cycle? [CBSE AIPMT 2004]
(a) Decondensation from chromosomes and reassembly of the nuclear lamina
(b) Transcription from chromosomes and reassembly of the nuclear lamina
(c) Formation of the contractile ring and formation of the phragmoplast
(d) Formation of the contractile ring and transcription from chromosomes
Answer/Explanation
Ans. (a)
At telophase stage, nuclear membrane vesicles associate with the surface of individual chromosomes and fuse to reform the nuclear membranes, which partially enclose clusters of chromosomes before coalescing to reform the complete nuclear envelope. During this process the nuclear pores reassemble and the dephosphorylated reassociate to form the nuclear lamina. One of the lamina proteins (lamina-B) remains with the nuclear membrane fragments throughout mitosis and may help nuclear reassembly. After the nucleus reforms, the chromosome decondense and RNA synthesis resumes, causing the nucleolus to reappear.
Question
Mitotic spindle is mainly composed of which protein? [CBSE AIPMT 2002]
(a) Actin
(b) Myosin
(c) Actomyosin
(d) Myoglobin
Answer/Explanation
Ans. (a)
Spindles formed during mitosis and meiosis are nothing but microtubule complex. Microtubules are made up of small units of tubulin which has amino acid composition similar to actin.
Question
Best material for the study of mitosis in laboratory is[CBSE AIPMT 2002]
(a) anther
(b) root tip
(c) leaf tip
(d) ovary
Answer/Explanation
Ans. (b)
Root tips have active meristematic zone where cells divide mitotically leading to increase in the length of the roots. This is the best site for the study of mitosis, e.g. onion root tips.
Question
35 During cell division, the spindle fibres attach to the chromosome at a region called [CBSE AIPMT 2000]
(a) Chromocentre
(b) kinetochore
(c) centriole
(d) chromomere
Answer/Explanation
Ans. (b)
During late prophase, specialised structures called kinetochores develop on either surface of the centromere. Chromosomal fibres get attached to kinetochore. Chromomeres are beaded structures
on the chromosomes which are found
particularly at the prophase-I (particularly at leptotene) of meiosis-l. Chromocentre is developed due to the fusion of centromeric regions of all the chromosomes of a cell. Centrosome is found in animal cells (absent in higher plant cells). It is found near the nucleus. Each centrosome is made up of two centrioles and each centriole is a cylindrical structure with a diameter of 1500-1800 $\AA$ and is made up of nine fibrils.
Question
How many mitotic divisions are needed for a single cell to make 128 cells? [CBSE AIPMT 1997]
(a) 7
(b) 14
(c) 28
(d) 64
Answer/Explanation
Ans. (a)
A single mitotic division results in the production of two cells from single cell.
Question
Which one of the following structures will not be common to mitotic cells of higher plants? [CBSE AIPMT 1997]
(a) Cell plate
(b) Centriole
(c) Centromere
(d) Spindle fibres
Answer/Explanation
Ans. (b)
Centrosome is found in animals, Euglena, Nitella, some fungi and members of dinoflagellate. It is found near the nucleus. Centriole is not common to mitotic cell of higher plants. Main function of centrosome is at the time of cell division when the two centrioles separate and move on two poles. Aster and spindle are formed from it which
pulled by the spindle to opposite poles of the cell. Thus, the centromere splits, freeing the sister chromatids from each other. Other options can be explained as:
In metaphase I, the homologous pair of chromosomes align on either side of the equatorial plate.
During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells.
During telophase II, the two groups of chromosome once again get enclosed by nuclear envelope.
Question
The point, at which polytene chromosomes appear to be attached together, is called [CBSE AIPMT 1995]
(a)centriole
(b) centromere
(c)chromomere
(d) chromocentre
Answer/Explanation
Ans. (d)
Polytene chromosomes are infact formed by pairing of two somatic homologous chromosomes which undergo repeated endomitosis, forming a number of etrande. These etrande remain attached to a common large chromocentre of all polytene chromosomes and are rich in heterochromatin.
Question
Best stage to observe shape, size and number of chromosomes is [CBSE AIPMT 1994]
(a) interphase
(b) metaphase
(c) prophase
(d) telophase
Answer/Explanation
Ans. (b)
Metaphase can be characterised by the chromosomes that are least coiled which show maximum condensation and are shortest in length. It is the best stage to study the structure, size and number of chromosome in a cell. Idiogram/karyotype of chromosomes is prepared at metaphase.
Question
In salivary gland chromosomes/polytene chromosomes pairing is [CBSE AIPMT 1993]
(a)absent
(b) occasional
(c) formed between non-homologous chromosomes
(d) formed between homologous chromosomes
Answer/Explanation
Ans. (d)
Polytene chromosomes/salivary gland chromosomes was reported by Balbiani (1881) from cells of salivary glands of Chironomus larva (insect of Diptera groupl. The polytene chromosomes become giant due to the endoduplication, i.e., repeated replication of chromatids without their separation and cytokinesis. In fact, each polytene chromosome is formed by pairing of two somatic homologous chromosomes which undergo repeated endomitosis to form numerous strands attached to a common large chromocentre.
Question
Number of chromatids at metaphase is [CBSE AIPMT 1992]
(a) two each in mitosis and meiosis
(b) two in mitosis and one in meiosis
(c) two in mitosis and four in meiosis
(d) one in mitosis and two in meiosis
Answer/Explanation
Ans. (a)
In metaphase, chromosomes are thick, shortest least coiled and minimum in size. Each chromosome has its both chromatids attached at centromere, oriented at the equator of spindle apparatus.
In meiotic metaphase, each chromosome with two chromatids in a bivalent is connected to the spindle pole of its side by a kinetochore microtubule instead of two as in metaphase of mitosis.
Question
Mitotic anaphase differs from metaphase in possessing [CBSE AIPMT 1991]
(a) same number of chromosomes and same number of chromatids
(b) half number of chromosomes and half number of chromatids
(c) half number of chromosomes and same number of chromatids
(d) same number of chromosomes and half number of chromatids
Answer/Explanation
Ans. (d)
Mitotic metaphase is the best stage to observe the structure, size and number of chromosomes in a cell. Centromeres of all chromosomes lie closely at equator and their arms in different directions towards poles.
Chromosomes are shortest in metaphase but thickest in anaphase. In anaphase, centromere of each chromosome divides so that each sister chromatid now has its own centromere. Thus, mitotic anaphase differs from metaphase in possessing same number of chromosomes and half number of chromatids.
Question
A bivalent consists of [CBSE AIPMT 1989]
(a) two chromatids and one centromere
(b) two chromatids and two centromeres
(c) four chromatids and two centromeres
(d) four chromatids and four centromeres
Answer/Explanation
Ans. (c)
Each pair of homologous chromosome carrying one maternal and one paternal chromosome of similar type is called bivalent. Each chromosome has two sister chromatids and a centromere.
Thus, bivalents possesses four chromatids, two centromeres. This bivalent with four chromatids is called pachytene tetrad (quadrivalent).