Past Entrance Papers

Hint

(a) During \(S\) phase or synthesis phase of interphase replication of DNA and synthesis of histone protein, centromere and centrioles occur. During the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm
of the cell.

Hint

(c) During DNA replication, the unwinding of strands leaves a single strand vulnerable. In the eukaryotic cell cycle, chromosome duplication occurs during ‘S phase'(the phase of DNA synthesis) and chromosome segregation occurs during the ‘M phase'(the mitosis phase).
During the \(G_{2}\) phase, extra protein is often synthesised, and the organelles multiply until there are enough for two cells. Other cell materials such as lipids for the membrane may also be produced.
The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter \(G_{0}\) temporarily until an external signal triggers the onset of \(G_{1}\). Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in \(G_{0}\) permanently.
\(G_{1}\) phase corresponds to the interval between mitosis and initiation of DNA replication. During \(G_{1}\) phase the cell is metabolically active and continuously grows but does not replicate its DNA.

Hint

(a) During the 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 the same from the beginning till the end of the S phase.

Hence, the number of chromosomes will remain at 8 after the S phase in the fruitfly.

Hint

(c) Diakinesis is the final stage of meiotic prophase 1. In this stage, the two homologous chromosomes do not separate completely but remain attached together at one or more
points as indicated by \(X^{\prime}\) arrangement known as chiasmata. The
displacement of chiasmata is termed as terminalisation of chiasmata which is completed in the diakinesis phase.

Hint

(c) During anaphase II, each pair of chromosomes are separated into two identical, independent chromosomes. The chromosomes are separated by a structure called the mitotic spindle made up of many long proteins called microtubules, which are attached to a chromosome at one end and to the pole of a cell at the other end. The sister chromatids are separated simultaneously at their centromeres. The separated chromosomes are then 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 chromosomes once again get enclosed by the nuclear envelope.

Hint

(b) During \(G_{1}\) phase the cell is metabolically active and continuously grows but does not replicate its DNA. DNA synthesis takes place in \(\mathrm{S}\) phase. Nuclear division occurs during Karyokinesis. Reorganization of all cell components takes place in M-Phase.

In some organisms, karyokinesis is not followed by cytokinesis as a result of which, the multinucleate condition arises leading to the formation of the syncytium.

Hint

(d) Some dividing cells exit the cell cycle and enter vegetative inactive stage, called quiescent stage \(\left(\mathrm{G}_{0}\right)\). This process occurs at the end of the M-phase and the beginning of the \(G_{1}\) phase.

Human cancers have an apparent low growth fraction, the bulk of cells presumed to being out of cycle in a \(\mathrm{G}_{0}\) quiescent state due to the inability in the past to distinguish \(\mathrm{G}_{0}\) from \(\mathrm{G}_{1}\) cells. Thus, human cancers are blocked in transition in \(\mathrm{G}_{1}\) and are not predominantly in \(\mathrm{a} \mathrm{} \mathrm{G}_{0}\) or quiescent differentiated state.

Hint

(d) During the metaphase stage of the cell cycle, spindle fibres originating from the centrosomes attaches to the kinetochore of chromosomes.
The kinetochore is a disc-shaped structure at the surface of the centromere through which the sister chromatids are held together. During metaphase, the chromosomes arrange themselves at the equator on metaphasic plate. Due to this arrangement, the attachment of spindle fibres to the kinetochore is clearly visible.

Hint

(b) In a mitotic cycle, the correct sequence of phases is \(G_{1}, S, G_{2}, M\). The first three phases, i.e. \(G_{1}, S_{1} G_{2}\) occur during the interphase whereas the M-phase is the period of actual cell division. The major event occurring in each phase is tabulated below

\(\begin{array}{ll}
\hline \text { Phases } & {\text { Activities }} \\
\hline G_{1} \text {-phase } & \begin{array}{l}
\text { Cell becomes metabolically } \\
\text { active, enzymes and proteins } \\
\text { required for replication are } \\
\text { synthesised. }
\end{array} \\
\hline \text { S-phase } & \begin{array}{l}
\text { Synthesis or replication of DNA } \\
\text { occurs so that amount of DNA } \\
\text { per cell gets doubled. }
\end{array} \\
\hline G_{2} \text {-phase } & \begin{array}{l}
\text { Proteins required for mitosis are } \\
\text { synthesised while the growth } \\
\text { of cell continues. }
\end{array} \\
\hline \text { M-phase } & \begin{array}{l}
\text { Cell divides to form daughter } \\
\text { cells. }
\end{array} \\
\hline
\end{array}\)

Hint

(b) During the zygotene stage of meiosis-1, chromosomes start pairing together
and this process of association is called synapsis. Such paired chromosomes
are called homologous chromosomes. 

Hint

(b) Dissolution of the synaptonemal complex occurs during the diplotene stage of prophase-I of meiosis-I. The prophase of meiosis-l is long and complex. It is comprised of Ieptotene, zygotene, pachytene,diplotene and diakinesis. During diplotene, in most places, the synaptonemal complex dissolves.

Hint

(a) Zygotene \(\rightarrow\) Synapsis
Pachytene \(\rightarrow\) Crossing over
Diplotene \(\rightarrow\) Chiasmata formation
Diakinesis \(\rightarrow\) Terminalisation

Hint

(c) Meiotic division of secondary oocyte is completed after the entry of sperm in secondary oocyte which lead to the formation of a large ovum and a tiny \(2^{\text {nd }}\) polar body.

Hint

(d) \(G_{0}\) phase is the stage in which the cells exit the cell cycle. It is the resting or quiescent phase in which the cells do not divide. It is the permanent state for some cells, e.g., neurons.

Hint

(d) Cell cycle is the complete series of events from one cell division to the next. The correct sequence of phases of cell cycle is \(\mathrm{G}_{1} \rightarrow \mathrm{S} \rightarrow \mathrm{G}_{2} \rightarrow \mathrm{M}\)

Hint

(c) Meiosis I is reductional division thus the resultant cell just after meiosis I will have half the number of chromosomes as compared to diploid parent cells. Therefore if the parent cell has \(4 \mathrm{C}\) amount of DNA the daughter cell will have \(2 \mathrm{C}\) amount of DNA and each gamete will have \(1 C\) amount of DNA.

Hint

(c) Crossing over is the exchange of genes which takes place between non-sister chromatids of homologous chromosomes at the pachytene stage of meiosis I resulting in new allelic combinations in the daughter cells. Crossing over is essential for the normal segregation of chromosomes during meiosis. Crossing over also accounts for genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical.

Hint

(a) Agaricus Meiospores are produced exogenously after karyogamy and meiosis. It belongs to Basidiomycetes. Alternaria 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 the Ascomycetes class of fungi. They produce ascopores as meiospores. Their ascospores are produced endogenously.

Hint

(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.

Hint

(a) During mitosis following events occurs as follows
Condensation of chromosomal material, which takes place at an early prophase stage. During the late prophase, the nuclear membrane disintegrates. Then chromosomes get arranged at the 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 the last stage of mitosis.

Hint

(a) In the cycle of cell division, interphase is the longest phase consisting of \(G_{1}, S\), \(G_{2}\)-phases. In this phase, cell prepares itself for cell division. In \(S\) or synthetic phase DNA duplication (synthesis) takes place.

Hint

(b) If the cell has stalled DNA replication fork, it implies that it has crossed \(\mathrm{CG}_{1}\) or \(\mathrm{G}_{1}\) cyclin cell cycle checkpoint and has entered S-phase of the cell cycle, where it is preparing for chromosome replication. Afterward it will enter \(\mathrm{G}_{2}\) phase and will soon approach the second checkpoint called mitotic cyclin (CM) which lies between \(\mathrm{G}_{2}\) and \(M\)-phase).

Hint

(c) Synapsis is the pairing of homologous chromosomes. It occurs during the 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.

Hint

(a) Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position at the centre of the cell.

Hint

(d) Crossing over is a process of exchange of genetic material or chromatid segments between two homologous chromosomes. It is initiated during the pachytene stage of meiosis.

Hint

(a) Various phases of meiosis and their characteristic features are
Pachytene – Crossing-over takes place
Metaphase-I – Chromosomes align at the equatorial plate
Diakinesis – Terminalisation of chiasmata
Zygotene – Pairing of homologous chromosomes

Hint

(b) The correct sequence of events of meiosis are Synapsis in zygotene \(\longrightarrow\) Crossing over in pachytene \(\longrightarrow\) Terminalisation of chaismata in diplotene \(\longrightarrow\)
Disapperance of the nucleolus in diakinesis.

Hint

(a) In diploid cells (somatic cells) during \(\mathrm{G}_{1}\) phase, DNA content is \(2 \mathrm{C}\) and chromosome number is \(2 n\) whereas in haploid cells (gamete) during \(\mathrm{G}_{1}\) phase, DNA content is \(1 \mathrm{C}\) and chromosome number is \(n\). S phase is marked by replication of DNA and the amount of DNA per cell is doubled i.e., it becomes \(4 \mathrm{C}\) in somatic cells, whereas chromosome number remains same i.e., \(2 n\). Thus, a somatic cell which has just completed S phase, will have 4C DNA content but \(2 n\) chromosome number, while the gamete cell has \(1 \mathrm{C}\) DNA content and \(n\) chromosome number.

Hint

(d) Synapsis aligns homologous chromosomes – Zygotene

Synthesis of RNA phase and protein \(\quad-\mathrm{G}_{2}\) 

Action of enzyme recombinase – Pachytene

Centromeres do not separate but chromatids move towards opposite poles – Anaphase I 

Hint

(c) In M-phase, both \(4 \mathrm{C}\) and \(2 \mathrm{C}\) of DNA are present in different stages. S phase marks the period during which the amount of DNA in the cell doubles. Thus, at G2 phase the amount of DNA is 4C. These doubled cellular contents prepare the cell to sustain further mitotic division.

Hint

(a) In ‘ \(\mathrm{S}\) ‘ phase of cell cycle the DNA content doubles, i.e., \(1 \mathrm{C}\) to \(2 \mathrm{C}\) for haploid cells and \(2 \mathrm{C}\) to \(4 \mathrm{C}\) for diploid cells. However, the number of chromosomes does not increase.

Hint

(a) Pachytene is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Nodules contain multienzyme complex called recombinase. Recombinase is made of endonuclease, exonuclease, unwindase, R-protein, etc.

Hint

(a) During the zygotene stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.

Hint

(c) The figure given in the question shows a stage of mitotic cell division called as telophase stage. The individual chromosomes are no longer seen and chromatin material tends to collect in a mass at the two poles. This is the stage that shows the following key events:
– Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
– Nuclear envelope assembles around the chromosome clusters.
– Nucleolus, Golgi complex, and ER reform.

Hint

(a) In diploid organisms, specialised cells called meiocytes (gamete mother cell)
undergo meiosis. Conidia and gemmules are asexual reproductive structures found in Penicillium and sponge respectively. Megaspores are female gametes in plants which
undergo sexual reproduction.

Hint

(a) During zygotene or zygonema of meiotic prophase I the chromosomes become shorter and thicker. The homologous chromosomes come to lie side-by-side in pairs. This pairing of homologous chromosomes is known as synapsis or syndesis. A pair of homologous chromosomes lying together is called a bivalent.

Hint

(a) In metaphase, chromosomes consisting of two sister chromatids get arranged at the equator. Discontinuous fibres radiate out from two spindle poles and get connected to the disc-shaped structure at the surface of the centromere called kinetochores. These are known as chromosome fibres or tractile fibrils. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome.

Hint

(b) In metaphase, chromosomes consisting of two sister chromatids get arranged at the equator. Discontinuous fibres radiate out from two spindle poles and get connected to the disc-shaped structure at the surface of the centromere called kinetochores. These are known as chromosome fibres or tractile fibrils. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome.

Hint

(c) During gamete formation, the enzyme recombinase participates during the pachytene stage of prophase I. This stage is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.

Hint

(a) The given figure shows crossing over i.e., exchange of segments between two homologous chromosomes. Crossing over is characteristic of meiosis and occurs during the pachytene stage of prophase I.

Hint

(c) During anaphase I, from each tetrad two chromatids of a chromosome move as a unit (dyad) to one pole of a spindle, and the remaining two chromatids of its homologue migrate to the opposite pole. Thus, the homologous chromosomes of each pair, rather than the chromatids of a chromosome, are separated. As a result, half of the chromosomes, which appear in the early prophase, go to each pole. Thus the paternal and maternal chromosomes of each homologous pair segregate during anaphase I independently of the other chromosomes.

Hint

(c) 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 the spindle equator and get aligned along metaphase plate through spindle fibres to both poles.

Hint

(c) The key feature of metaphase is the attachment of spindle fibres to kinetochores of chromosomes. Kinetochores are disc-shaped structures at the surface of the centromeres. These structures serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position.

Hint

(d) During the early prophase of mitosis, the nucleus and cell become spheroid. Viscocity and refractivity of cytoplasm increases. DNA molecules condense to form a shortened chromosome. The endoplasmic reticulum and nucleolus start disappearing.

Hint

(c) In the given figures
A. Late anaphase is characterised by the following events
(i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles.
B. Prophase is characterised by centriole separation.

Hint

(c) In zygotene of prophase-l, homologous chromosomes pair up. This process is called synapsis. One chromosome of the pair is from the male parent and one from the female parent.

Hint

(b) In the cell cycle there are two main phases-interphase and the mitotic phase. Interphase is divided into 3 stage \(G_{1}, S\) and \(G_{2} . G_{1}\) is first growth phase. \(S\) is synthetic phase and \(G_{2}\) is second growth phase.

Hint

(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 \(\)\mathrm{G}_{2}\(\) phase division of centrioles, mitochondria, and chloroplasts occurs.

Hint

(a) The arms of chromosomes are known as chromatids. These arms are held together at a point called the centromere (or primary constriction). Centromere occurs anywhere along the length of chromosome. During cell division spindle fibres are attached to
centromere and help in the movement of chromosomes towards the poles.

Hint

(b): 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 phases \(-G_{1}, S\) and \(G_{2}\). During \(G_{1}\) phase the cell grows and synthesis of \(t[latex] RNA, [latex]m\) RNA, 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 \(G_{2}\) phase organelles like centrioles are doubled and mitochondria, chloroplasts, etc. divide.

Hint

(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 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 the opposite pole. So for counting metaphase is the best stage.

Hint

(a) M phase is known as mitotic phase and cell division occurs in this phase. The reformation of the nuclear envelope during the 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 reassembly of the nuclear lamina.

Hint

(b) The process of crossing over takes place in the pachytene stage of prophase-I of meiosis-I. In this process, some genes of two non-sister chromatids of a bivalent are exchanged.

Hint

(d) A chromosome has one centromere, may have many chromomeres, two chromatids; but four telomeres(two each at the opposite ends of each chromatid).

Hint

(a) Spindles formed during mitosis and meiosis are nothing but microtubule complex. Microtubules are made up of small units of tubulin which have amino acid composition similar to actin.

Hint

(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 the root tip or shoot tip. These cells divide at a faster rate. So the root tip shows active cell division and are used in the laboratory to study mitosis. For studying meiosis young anthers are used.

Hint

(b) During the late prophase, specialised structures called kinetochores develop
on either surface of the centromere. Chromosomal fibres get attached to the kinetochore.
Chromomeres are beaded structures on the chromosomes which are found particularly at the prophase-I (particularly at leptotene) of meiosis-I. 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.

Hint

(b) 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 \(-\mathrm{G}_{1} \mathrm{~S}\) and \(\mathrm{G}_{2}\). During \(\mathrm{G}_{1}\) 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 \(\mathrm{G}_{2}\) phase organelles like centrioles are doubled and mitochondria, chloroplasts, etc. divide.

Hint

(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, microfilament collects 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 the phragmoplast. Vesicles from the Golgi apparatus are deposited and coalesce on the phragmoplast to form a cell plate.

Hint

(a) In apical meristems mitotic divisions occur at a rapid rate. In the 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.

Hint

(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.
\(
\stackrel{1}{\longrightarrow} 2 \stackrel{2}{\longrightarrow} 4 \stackrel{3}{\longrightarrow} 8 \stackrel{4}{\longrightarrow} 16 \stackrel{5}{\longrightarrow} 32 \stackrel{6}{\longrightarrow} 64 \stackrel{7}{\longrightarrow} 128
\)

Hint

(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 the 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 helps in the movement of chromatids. They form basal body, cilia, flagella, etc. Centriole is rich in tubulin and ATPase. Centrioles replicate in \(G_{2}\)-phase of interphase of cell cycle but do not initiate cell division.

Hint

(c) In pachytene stage of prophase-I of meiosis, there is breakage and reunion of chromatids, which results in an exchange of segments between non-sister chromatids of a bivalent, known as crossing over. It leads to the recombination of linked genes/alleles
and is a major source of continuous type of genetic variations in sexually reproducing organisms.

Hint

(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 the 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.

Hint

(a) 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 \(-\mathrm{G}_{1} \mathrm{~S}\) and \(\mathrm{G}_{2}\). During \(\mathrm{G}_{1}\) 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 \(\mathrm{G}_{2}\) phase organelles like centrioles are doubled and mitochondria, chloroplasts, etc. divide.

Hint

(d) Polytene chromosomes are in fact formed by pairing of two somatic homologous chromosomes which undergo repeated endomitosis, forming a number of strands. These strands remain attached to a common large chromocentre of all polytene chromosomes and are rich in heterochromatin.

Hint

(b) Lampbrush chromosomes were reported by W Flemming (1882) and described by Ruckert (1892) from nuclei of yolk-rich primary oocytes of newts and frogs (amphibians). These are also found in the spermatocytes of many animals. These are found in the permanent diplotene stage of meiosis and do not undergo cell cycle. Each such chromosome has a double main axis made up of DNA and histones. The chromosomes are coiled and held at many places forming cross-like structure called chiasmata. Loops arising laterally has uncoiled DNA which helps in rapid transcription and yolk synthesis.

Hint

(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.

Hint

(d) Recombination takes place in meiosis but still, Meiosis maintains the chromosome number constant. It produces haploid gametes by reducing the chromosome number to half. Crossing over produces new combination of linked genes and is major source of genetic variation. Also, the distribution of bivalents which is at random in metaphase-I provides the secondary source of genetic variation in the organisms and is essential for speciation and evolution.

Hint

(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 \(-\mathrm{G}_{1} \mathrm{~S}\) and \(\mathrm{G}_{2}\). During \(\mathrm{G}_{1}\) phase the cell grows and synthesis of tRNA, mRNA, ribosomes, enzymes, and proteins necessary for DNA synthesis occurs. During the S phase replication of DNA takes place. The nucleotides get assembled and DNA molecules are synthesized. During \(\mathrm{G}_{2}\) phase organelles like centrioles are doubled and mitochondria, chloroplasts, etc. divide.

Hint

(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.

Hint

(d) Polytene chromosomes/salivary gland chromosomes were reported by Balbiani (1881) from cells of salivary glands of Chironomus larva (insect of Diptera group). 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.

Hint

(d) Meiosis II is shorter than the typical mitotic division because of the shortening of the prophase of this division. The division maintains the number of chromosomes produces 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.

Hint

(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 daughter chromosomes.

Hint

(d) Mitotic anaphase differs from metaphase in possessing the same number of chromosomes and half the 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 chromosomes. While during metaphase, chromosomes become maximally distinct due to further contraction, and thus the size of chromosomes is measured at mitotic metaphase.

Hint

(a) In meiosis, the daughter cells differ from the 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 a role in process of evolution.

Hint

(b) Segregation of Mendelian factor (Aa) occurs during Anaphase I. The paired homologous chromosomes 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 chromosomes.

Hint

(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, and two centromeres. This bivalent with four chromatids is called pachytene tetrad (quadrivalent).

Hint

(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 meiotic 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.

Hint

(b) In bacteria DNA replication occurs in cytoplasm prior to fission. Prokaryotes due to their primitive nature do not show well-marked S-phase.

Hint

(c) \(\frac{175}{35} \mathrm{~min}=5\) divisions Number of cells formed \(=(2)^{5} \times 10^{5}=32 \times 10^{5}\)

Hint

(b) Colchicine inhibits spindle formation, due to which chromatids are unable to separate during anaphase which results in doubling of chromosomes. So, if a diploid cell is treated with colchicine, there is doubling of chromosomes and it becomes tetraploid. It is an alkaloid obtained from Colchicum autumnale.

Hint

(b) Colchicine is a mitotic poison which inhibits the formation of spindle fibres.

Hint

(b) Genetic mapping help in studying the architecture of the chromosomes and of the entire genome.

Genetic mapping – also called linkage mapping can offer firm evidence that a disease transmitted from parent to child is linked to one or more genes. Mapping also provides clues about which chromosome contains the gene and precisely where the gene lies on that chromosome.