22 Questions with Answers and Explanations on Cell Reproduction for Biology Students.
1. Meiosis I is reductional division. Meiosis II is equational division due to
(a) pairing of homologous chromosomes
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(b) crossing over
(c) separation of chromatids
(d) disjunction of homologous chromosomes.
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Answer and Explanation:
1. (c): August Weismann in 1887 predicted that the number of chromosomes must be reduced by one half during gamete formation. The two divisions of meiosis are called the first and the second nieiotic divisions.
In meiosis I, the number of chromosomes are reduced from diploid to haploid condition, whereas in meiosis II, the two chromatids of each chromosomes separate from each other and go to separate daughter cells, as a result the number of chromosomes remains the same as produced by meiosis – I.
2. Segregation of Mendelian factor (Aa) occurs during
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(a) diplotene
(b) anaphase I
(c) zygotene/pachytene
(d) anaphase II.
Answer and Explanation:
2. (b): Segregation of Mendelian factor (Aa) occurs during Anaphase-I. The paired homologous chromosomes I separate in meiosis-I so that each gamate receives one chromosomes of each homologous pair. During Anaphase- I chromosome divides at the point of centromere or kinetochore and thus two sister chromatids are formed, which are called as chromosomes.
3. Mitotic anaphase differs from metaphase in possessing
(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 and Explanation:
3. (d): 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.
4. In meiosis, the daughter cells differ from parent cell as well as amongst themselves due to
(a) segregation, independent assortment and crossing over
(b) segregation and crossing over
(c) independent assortment and crossing over
(d) segregation and independent assortment.
Answer and Explanation:
4. (a): In meiosis, the daughter cells differ from parent cell as well as amongst themselves due to segregation, independent assortment and crossing over. Daughter cells inhibit variations. Meiosis leads to recombinations or new combinations of genes or characters as a result of crossing over. Due to these recombinations, variations are created, which have role in process of evolution.
5. Number of chromatids at metaphase is
(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 mitosfs and two in meiosis.
Answer and Explanation:
5. (a): Number of chromatids at metaphase is two each in mitosis and meiosis. Chromatid is a half chromosome during duplication in early prophase and metaphase of mitosis and between diplotene and the second metaphase of meiosis. After these stages chromatids are called a daughter chromosome.
6. Meiosis II performs
(a) separation of sex chromosomes
(b) synthesis of DNA and centromere
(c) separation of homologous chromosomes
(d) separation of chromatids.
Answer and Explanation:
6. (d): Meiosis II is shorter than the typical mitotic division because of the shortening of prophase of this division. The division maintains the number of chromosomes produce at the end of reduction division. Hence, it is called homotypic or equational division, though it is similar to mitosis.
The main function of homotypic division or meiosis II is to separate the chromatids of univalent chromosomes which differ from each other in their linkage groups due to crossing over.
7. In a somatic cell cycle, DNA synthesis takes place in
(a) Gi phase
(b) prophase of mitosis
(c) S-phase
(d) G2 phase.
Answer and Explanation:
7. (c): Interphase is the stage between two successive cell divisions. During interphase, chromosomes are decondensed and are distributed throughout the nucleus. It is the largest period in the cell cycle and is divided into three phase – G, S and G1.
During G, phase the cell grows and synthesis of tRNA, mRNA, ribosomes, enzymes and proteins necessary for DNA synthesis occurs. During S phase replication of DNA takes place. The nucleotides get assembled and DNA molecules are synthesized. During G2 phase organelles like centrioles are doubled and mitochondria, chloroplasts etc. divide.
8. Which of the following represents the best stage to view the shape, size and number of chromosomes?
(a) prophase
(b) metaphase
(c) interphase
(d) telophase.
Answer and Explanation:
8. (b): Metaphase is the best time to count and study the number and morphology of chromosomes. The distinctly visible chromosome, arrange themselves at the equatorial or metaphasic plate. The centromeres lie at the equational plate while the arms 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.
9. Which statement best explains the evolutionary advantage of meiosis?
(a) meiosis is necessary for sexual reproduction
(b) genetic recombinations are possible from generation to generation
(c) meiosis alternates with mitosis from generation to generation
(d) the same genetic system is passed on from generation to generation.
Answer and Explanation:
9. (b): Meiosis involves exchange of genes between homologous chromosomes. So the gametes produced are genetically different from each other. Offsprings produced by the fusion of gametes therefore also show recombinations or genetic variations. These variations in the offsprings make organisms more adaptable to the environment and these have a definite role in evolution.
10. When paternal and maternal chromosomes change their materials with each other in cell division this event is called
(a) bivalent-forming
(b) dyad-forming
(c) synapsis
(d) crossing-over.
Answer and Explanation:
10. (d): Crossing-over is responsible for inducing variability. It involves an exchange of equal segments of non-sister chromatids belonging to two different but homologous chromosomes. Crossing over takes place at four stranded stage. Only two of the four chromatids take part in crossing over.
The other two are called non crossovers. Zygotene is characterized by pairing of homologous chromosomes which is called synapsis. The first meiotic division which is completed at first telophase may be followed by cytokinesis giving rise to a dyad.
11. Which typical stage is known for DNA replication?
(a) S-phase
(b) G2-phase
(c) metaphase
(d) G,-phase.
Answer and Explanation:
11. (a): Refer answer 7.
12. How many mitotic divisions are needed for a single cell to make 128 cells?
(a) 28
(b) 32
(c) 7
(d) 14.
Answer and Explanation:
12. (c): Mitosis is an equational division where after division each cell produces two daughter cells therefore after 7 divisions one cell will give 128 cells in case of mitosis.
13. During cell division in apical meristem, the nuclear membrane appears in
(a) telophase
(b) cytokinesis
(c) metaphase
(d) anaphase.
Answer and Explanation:
13. (a): 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.
14. Microtubule is involved in the
(a) muscle contraction
(b) membrane architecture
(c) cell division
(d) DNA recognition.
Answer and Explanation:
14. (c): 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, microfilaments 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.
15. Spindle fibre unites with which structure of chromosomes?
(a) chromocentre
(b) chromomere
(c) kinetochore
(d) centriole.
Answer and Explanation:
15. (c): Spindle is microtubular apparatus that appears in many eukaryotic cells at the beginning of nuclear division and is responsible for the ordered separation of the chromosomes, chromosomes being attached to the spindle fibres by their centromeres. Two types of spindle fibres can be distinguished as the interpolar fibre, which stretches continuously from pole to pole of the spindle; the kinetochore fibre, which stretches from the pole to the centromere (kinetochore) of an individual chromosome.
The mechanism by which the chromosomes move and the spindle fibres contract remains unclear. Cells of animals and lower plants possess centrioles, which act as organizer regions for spindle microtubule formation, but centrioles are absent from the cells of higher plants.
16. In which state of cell cycle, DNA replication occurs
(a) G1-phase
(b) S-phase
(c) G2-phase
(d) M-phase.
Answer and Explanation:
16. (b): Refer answer 7.
17. Mitotic spindle is mainly composed of which protein?
(a) actin
(b) myosin
(c) actomyosin
(d) myoglobin.
Answer and Explanation:
17. (c): A spindle of fine fibres begins to develop during prophase. It consists of microtubules which are made of protein called tubulin and certain other associated proteins. These delicate fibres radiate from the centriole and constitute aster.
This option was not given in the entrance paper. As actin and myosin are involved as contractile machinery in many nonmuscle cells so it can be considered as the correct answer. Myoglobin is present in muscles which can bind to oxygen.
18. Best material for the study of mitosis in a laboratory is
(a) anther
(b) root tip
(c) leaf tip
(d) ovary.
Answer and Explanation:
18. (b): Mitosis occurs both in somatic cells as well as in germ cells of the gonads. In plants mitosis occurs in the meristematic cells of root tip or shoot tip. These cells divide at a faster rate. So the root tip shows active cell division and is used in the laboratory to study mitosis. For studying meiosis young anthers are used.
19. In the somatic cell cycle
(a) in G phase DNA content is double the amount of DNA present in the original cell
(b) DNA replication takes place in S-phase
(c) a short interphase is followed by a long mitotic phase
(d) G2 phase follows mitotic phase.
Answer and Explanation:
19. (b): Refer answer 7.
20. 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?
(a) metaphase
(b) telophase
(c) anaphase
(d) prophase
Answer and Explanation:
20. (a): Metaphase is the best time to count and study the number and morphology of chromosomes. The distinctly visible chromosome arrange themselves at the equatorial or metaphasic plate. The centromeres lie at the equational plate while the limit are placed variously according to their size and spiral arrangement. At prophase the chromosome 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 metaphase is the best stage.
21. Which one of the following precedes re-formation of the nuclear envelope during M phase of the cell cycle?
(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 and Explanation:
21. (c): M-phase or mitotic phase is the actual division phase and formation of contractile ring and formation of phragmoplast precedes reformation of nuclear envelope. Contractile ring is belt-like bundle of actin and myosin that appears during cell division immediately below the plasma membrane.
Contraction of this ring leads to the separation of the two daughter cells. Phragmoplast is the region of plant cell cytoplasm that becomes evident in the latter stages of mitosis.
It forms from the residual microtubules of the polar mitotic spindle and appears to function in transporting materials to the new cell plate forming between the daughter cells. Once the cell plate is complete, the phragmoplast is divided and gradually disappears, the cell plate finally becoming transformed into the middle lamella lying between the new cell walls.
22. At what stage of the cell cycle are histone proteins synthesized in a eukaryotic cell?
(a) during G-2 stage of prophase
(b) during S-phase
(c) during entire prophase
(d) during telophase.
Answer and Explanation:
22. (b): During S phase or synthetic phase the replication of DNA takes place. For replication of DNA histone proteins are required so they are also synthesized during this phase. It takes about 30%-50% of the total cell cycle.
Prophase and telophase are stages involved in mitosis or meiosis. During G2 phase division of centrioles, mitochondria and chloroplasts occurs.