Q: The fruit fly has 8 chromosomes (2 n) in each cell. During interphase of Mitosis if the number of chromosomes at G1 phase is 8 , what would be the number of chromosomes after S phase ?

During mitotic cell cycle if the chromosome number in G1 phase is 8 , it remains same till Metaphase. As during S phase it is the DNA content that gets replicated (doubled), but the chromosome number remains the same. Hence, even after S phase same chromosome number 8 is maintained in the cell of fruit fly.

Question 1

Spindle fibres attach on to:

Accepted Answer

Kinetochore of chromosomes facilitates the attachment of spindle fibre (chromosomal fibre) and the poles.

Question 2

Which of the following is not a characteristic feature during mitosis in somatic cells?

Accepted Answer

Synapsis is pairing of homologous chromosomes which occurs during meiosis but it is absent in mitosis.

Question 3

The complex formed by a pair of synapsed homologous chromosomes is called:

Accepted Answer

Homologous chromosome are paired and synapsed during zygotene stage of meiosis-I and known as bivalent.

Question 4

Which of the following options gives the correct sequence of events during mitosis?

Accepted Answer

The correct sequence of events during mitosis would be as follows (i) Condensation of DNA so that chromosomes become visible occurs during early to mid-prophase. (ii) Nuclear membrane disassembly begins at late prophase or transition to metaphase. (iii) Arrangement of chromosomes at equator occurs during metaphase, called congression. (iv) Centromere division or splitting occurs during anaphase forming daughter chromosomes.(v) Segregation also occurs during anaphase as daughter chromosomes separate and move to opposite poles. (vi) Telophase leads to formation of two daughter nuclei.

Question 5

Anaphase promoting complex (APC) is a protein degradation machinery necessary for proper mitosis of animal cells. If APC  is defective in a human cell, which of the following is expected to occur?

Accepted Answer

Anaphase Promoting Complex (APC) is a protein necessary for separation of daughter chromosomes during anaphase. If APC is defective then the chromosomes will fail to segregate during anaphase.

Question 6

Why is mitosis called as equational division?

Accepted Answer

Mitosis is the process of cell division wherein the chromosomes replicate and gets equally distributed into two daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell, i.e., diploid. Hence, mitosis is known as equational division.

Question 7

How does cytokinesis in plant cells differ from that in animal cells?

Accepted Answer

Cytokinesis in plant cellsCytokinesis is animal cellsThe division of the cytoplasm takes place by cell plate formation.The division of the cytoplasm takes place by cell furrow method.Cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls.Furrow starts at the periphery and then moves inward, dividing the cell into two parts.

Question 8

Analyse the events during every stage of the cell cycle and notice how the following two parameters change:(i) Number of chromosomes (N) per cell (ii) Amount of DNA content (C) per cell

Accepted Answer

During meiosis, the number of chromosomes and the amount of DNA in a cell change.(i) Number of chromosomes (N) per cellDuring anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.(ii) Amount of DNA content (C) per cellDuring anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present. During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.

Question 9

The fruit fly has 8 chromosomes (2 n) in each cell. During interphase of Mitosis if the number of chromosomes at G1​ phase is 8 , what would be the number of chromosomes after S phase ?

Accepted Answer

During mitotic cell cycle if the chromosome number in G1​ phase is 8 , it remains same till Metaphase. As during S phase it is the DNA content that gets replicated (doubled), but the chromosome number remains the same. Hence, even after S phase same chromosome number 8 is maintained in the cell of fruit fly.

Question 10

Select the correct option with respect to mitosis.

Accepted Answer

The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. The key features of metaphase are, (1) Spindle fibres attach to kinetochores of chromosomes (2) Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.

Question 11

Which of the phases of mitosis is the longest?

Accepted Answer

Prophase is the longest phase of mitosis. This phase separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells.

Question 12

Synaptonemal complex occurs when associated with which of the following?

Accepted Answer

Meiosis is characterized by pairing of homologous chromosomes, exchange of segments between homologous chromosomes and separation of homologous chromosomes. The pairing of homologous chromosomes occurs during zygotene substage of prophase I which lead to formation of bivalent. The two homologous chromosomes of a bivalent are held together by nucleoprotein core. The whole structure is called synaptonemal or synaptonemal complex.

Question 13

Which one of the following precedes reformation of the nuclear envelope during M phase of the cell cycle?

Accepted Answer

M phase is known as mitotic phase and cell division occurs in this phase. The reformation of the nuclear envelope during M-phase of the cell cycle takes place during telophase. The nuclear membrane which disappears during prophase reappears in the telophase. But before it is reformed the nucleus gets reorganised by decondensation from chromosomes and reassemble of the nuclear lamina.

Question 14

Which of the following options gives the correct sequence of events during mitosis ?

Accepted Answer

Mitosis is divided into four phases : prophase, metaphase, anaphase and telophase. During prophase the indistinct and interwined DNA molecules condenses to form elongated chromosomes. The nuclear membrane disintegrates during prometaphase. During metaphase, the chromosomes align themselves at the equatorial plate. During anaphase, centromere of each chromosome divides into two so that each chromosome comes to have its own centromere. Chromatids move towards opposite poles along the path of their chromosome fibres. Finally during telophase, two chromosome groups reorganise to form two nuclei. Nuclear envelope reappears, Golgi complex and endoplasmic reticulum are reformed.Option (c) also gives the correct sequence of events but it misses step II (nuclear membrane disassembly). Hence, is ruled out as best appropriate answer is option (a).

Question 15

Mitotic anaphase differs from metaphase in possessing

Accepted Answer

Mitotic anaphase differs from metaphase in possessing same number of chromosomes and half number of chromatids. During anaphase of mitosis, chromosomes divide at the point of centromere or kinetochore and thus two sister chromatids are formed which are called as chromosomes. While during metaphase, chromosomes become maximally distinct due to further contraction and thus size of chromosomes is measured at mitotic metaphase.

Question 16

Anaphase Promoting Complex (APC) is a protein degradation machinery necessary for proper mitosis of animal cell. If APC is defective in a human cell, which of the following is expected to occur?

Accepted Answer

During anaphase in mitosis, sister chromatids segregate at opposite poles. Therefore, a defective APC will affect chromosome segregation.

Question 17

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?

Accepted Answer

Metaphase is the best time to count and study the number and morphology of chromosomes. The distinctly visible chromosomes arrange themselves at the equatorial or metaphasic plate. The centromeres lie at the equatorial plate while the limbs are placed variously according to their size and spiral arrangement. At prophase the chromosomes appear thin and filamentous, forming a network. So they are not very clearly visible. At telophase the chromosomes uncoil and lengthen and therefore are not clearly seen. Anaphase also shows chromosomes distinctly and they can be counted. But during anaphase chromatids separate and start moving towards opposite pole. So for counting chromosomes metaphase is the best stage.

Question 18

Colchicine is an inhibitory chemical, which:

Accepted Answer

Colchicine is an alkaloid derived from theautumn crocus, Colchicum autumnale. It inhibits spindleformation in cells during mitosis so that chromosomes cannot separate during anaphase, thus inducing multiplesets of chromosomes. Colchicine is used in genetics,cytology, and plant breeding research and also in cancertherapy to inhibit cell division.

Question 19

During cell division in apical meristem, the nuclear membrane appears in :

Accepted Answer

In apical meristems mitotic divisions occur at a rapid rate. In late telophase of mitosis, a nuclear membrane appears on the outside from either pieces of nuclear envelope or endoplasmic reticulum. The telophase may last as long as the prophase.

Question 20

Microtubule is involved in the :

Accepted Answer

Microtubules are unbranched hollow submicroscopic tubules of protein tubulin which develop on specific nucleating regions and can undergo quick growth or dissolution at their ends by assembly or disassembly of monomers. Microtubules form spindle during cell division. Centrioles help in cell division by forming spindle poles or microtubules. In animal cells,microfilament collect in the middle region of the cell below the cell membrane. They induce the cell membrane to invaginate.In plant cells, cell plate is formed to separate the two daughter cells. Some of the spindle fibres called interzonal microtubules are deposited around phragmoplast.Vesicles from Golgi apparatus are deposited and coalesce on the phragmoplast to form a cell plate.

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Solution for Class 11, Biology, Cell Cycle and Cell Division

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