c) G1 S G2 M
- (a) G 1
- (b) S
- (c) G 2
- (d) G 0
(d) G 0
d) G 0 phase
The correct answer is (i) a and b. Undifferentiated cells are those that have not yet specialized and retain the ability to divide and differentiate into various cell types. Stem cells in animals are undifferentiated cells that can self-renew and differentiate into specialized cell types. Meristematic cells in plants are undifferentiated cells located in growing regions such as root tips and shoot apices that continuously divide to produce new cells for growth. In contrast, red blood cells (RBCs) are highly specialized cells that have lost their nucleus and organelles in mammals and are differentiated for the specific function of oxygen transport. Mesophyll cells are also differentiated cells specialized for photosynthesis. Therefore, only stem cells and meristematic cells qualify as undifferentiated cells.
c) Chromosomes will not segregate
- (a) Mirabilis
- (b) Orchid
- (c) Moringa
- (d) Oryza
(b) Orchid
The correct answer is a) Amount of DNA doubles in each cell. During the S phase (synthesis phase) of the cell cycle, DNA replication occurs. Each chromosome, which initially consists of a single chromatid, is replicated to form two identical sister chromatids joined at the centromere. Since the cell contains multiple chromosomes, and each chromosome is replicated, the total amount of DNA in the cell doubles. However, the chromosome number remains the same because sister chromatids are still counted as one chromosome until they separate during anaphase. The DNA content increases from 2n to 4n (where n represents the haploid number), while the chromosome number remains at 2n. This doubling of DNA is essential for ensuring that each daughter cell receives the correct amount of genetic material during cell division.
b) Kinases & Cyclin
d) movement of chromosome towards the pole
- (a) 8
- (b) 16
- (c) 32
- (d) 64
(a) 16
c) two homologous chromosomes
a) Direct cell division
b) Pachytene
The correct answer is (III) Shortening of microtubules. During anaphase, the separation and movement of chromosomes to opposite poles of the cell is primarily facilitated by the shortening of microtubules that form the spindle apparatus. The spindle microtubules attached to the kinetochores of chromosomes shorten, pulling the separated chromatids (now individual chromosomes) toward opposite poles. This shortening of microtubules is the primary mechanism responsible for chromosome movement during anaphase. The phragmoplast is a structure that forms during telophase in plant cells and is not involved in chromosome movement during anaphase. The thickening of chromosomes does not cause their movement, and while asters (star-shaped microtubule arrays) are present in animal cells, their shortening is not the primary cause of chromosome separation during anaphase.
a) Anaphase
- (a) Singer & Nicolson
- (b) Prevost & Dumans
- (c) Schleider & Schwann
- (d) Boveri
(b) Prevost & Dumans
b) Synapsis
a) RBC – platelets, tracheids & vessels
c) Higher plants
c) Inter kinesis
Mitosis is a fundamental process of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typically for growth, repair, and asexual reproduction. One significant aspect of mitosis is that it produces an exact copy of the parent cell, meaning the daughter cells are genetically identical to the parent cell. This genetic stability is crucial as it ensures that all cells in an organism carry the same genetic information, which is vital for proper functioning and development. Secondly, mitosis plays a critical role in the repair of tissues; damaged or worn-out cells must be replaced by identical new cells through mitotic division, maintaining the integrity and function of organs and tissues. Lastly, mitosis is essential for regeneration, a process observed in many organisms, such as the regeneration of arms in starfish, where lost body parts are regrown through the proliferation of cells via mitosis. It also contributes to the growth of multicellular organisms from a single zygote.
- (a) G 0
- (b) G 1
- (c) S
- (d) G 2
(c) S
II. Find out the true and false statements from the following and on that basis find the correct answer.
Mitosis and meiosis are two distinct types of cell division with significant differences. Mitosis involves one division of the nucleus, whereas meiosis involves two successive divisions. In mitosis, the number of chromosomes in daughter cells remains the same as the parent cell (2n), while in meiosis, the chromosome number is halved in the daughter cells (n), producing haploid cells. During mitosis, homologous chromosomes do not pair up and line up separately on the metaphase plate, whereas in meiosis I, homologous chromosomes pair up to form bivalents and line up together at the metaphase plate. Chiasmata do not form and crossing over does not occur during mitosis, but both processes occur during meiosis I, leading to genetic recombination. The daughter cells produced by mitosis are genetically identical to the parent cell and to each other, while the daughter cells produced by meiosis are genetically different from the parent cell and from each other due to crossing over and independent assortment. Mitosis produces two daughter cells, while meiosis produces four haploid daughter cells. Mitosis is responsible for growth, repair, and asexual reproduction, whereas meiosis is essential for sexual reproduction and the production of gametes.
The G0 phase, also known as the quiescent stage or G-zero phase, is a state that some cells enter after exiting the G1 phase of the cell cycle. Cells in G0 remain metabolically active and can perform their specialized functions, but they do not undergo proliferation or division. These cells can exist in the G0 phase for extended periods, sometimes for the entire lifespan of the organism. In the G0 phase, cells cease growth and exhibit a reduced rate of RNA and protein synthesis compared to actively dividing cells. However, the G0 phase is not necessarily permanent for all cell types. Some cells, such as mature neurons and skeletal muscle cells, remain permanently in G0 and do not re-enter the cell cycle under normal circumstances. In contrast, many other cells in animals can remain in G0 unless they receive appropriate growth factors or other extracellular signals that stimulate them to re-enter the G1 phase and resume proliferation. It is important to note that G0 cells are not dormant; they continue to carry out metabolic activities and maintain their cellular functions while remaining in a non-dividing state.
3. Pachytene stage
* Chromosome appear as bivalent or tetrads
* 4 chromatids & 2 centromeres are seen
* Synapsis of homologous chromosomes between non-sister chromatids completes except at chiasmata where crossing over occurs
* Recombination (exchange of chromosomal bits is completed by the end) – but chromosomes are linked at the sites of crossing over
* Enzyme – Recombinase mediates the process.
4. Diplotene
* Synaptonemal complex disassembled & dissolves
* Nonsister chromatids of homologous chromosomes get attached where x like shape occur at Crossing over known as chiasmata holding the homologous chromosomes together the homologous chromosomes tend to separate except at chiasmat
* The sub-stage last for days or years depending on the sex & the organism follows Pachytene
* synaptical complex disassembled & dissolves
* The chromosomes are actively transcribed in females as the eggs stores up materials for embryonic development
* Exception In Lamp brush chromosome prominent loops occur.
11th Bio Botany Guide Cell Cycle Additional Important Questions and Answers
I.Choose the correct answer: (1 Marks)
The correct answer is b) False – False – True – True. The nucleolus does not disappear during metaphase; it disappears during prophase when the nuclear envelope breaks down. Microtubules do not arrange to form asters in plant cells; asters are characteristic structures found in animal cells during mitosis. In plant cells, the spindle apparatus is organized differently, and a phragmoplast forms during telophase. In plant cells, the phragmoplast is indeed formed prior to the formation of the cell plate, which is the correct statement. Mitosis is responsible for the regeneration of lost arms in starfish and other organisms through the process of asexual reproduction and regeneration, making this statement true.
The correct answer is b) True – False – True – False. The word protoplasm was indeed coined by Purkinje, making the first statement true. The structure of bacteria was not first observed by Edouard Van Beneden; Van Beneden made significant contributions to understanding cell division and chromosome behavior, but bacterial structure was studied by other scientists, making the second statement false. The centrosome and chromosome theory were proposed by Theodor Boveri, making the third statement true. Cell division in the round worm (Caenorhabditis elegans) was observed by Walther Flemming, but Flemming is more famously known for his observations of mitosis in other organisms; however, the specific attribution to round worms makes this statement false.
The correct answer is a) True – False – False – True. The C value is indeed the amount in picograms of DNA contained within a haploid nucleus, representing the total DNA content of a single set of chromosomes. The nucleolar membrane does not disappear during anaphase; it disappears during prophase when the nuclear envelope breaks down and the nucleolus disintegrates. The arrangement of microtubules to form asters is not a unique feature of plant cells; asters are characteristic structures found in animal cells during mitosis, whereas plant cells lack centrioles and do not form asters. One of the proteins synthesized during the G2 phase is indeed known as maturation promoting factor (MPF), also called cyclin-dependent kinase, which is responsible for driving the cell from G2 phase into mitosis.
The correct match is Diakinesis – Terminalisation of chiasmata occur and nucleolus disappear. During diakinesis, which is the final stage of prophase I in meiosis, the chiasmata move toward the ends of the bivalents in a process called terminalization. Additionally, the nucleolus disappears during diakinesis as the nuclear envelope begins to break down in preparation for metaphase I. The other stages have different characteristics: Zygotene is when synapsis of homologous chromosomes begins to occur, Pachytene is when synapsis is complete and bivalents are fully formed, and Diplotene is when the homologous chromosomes begin to separate slightly while chiasmata are still visible.
Among the given options for the duration of different phases of the cell cycle, the correct match is (III) G2 Phase – 4 Hours. The G2 phase is a relatively short but crucial period of growth and preparation for mitosis, where the cell synthesizes proteins and organelles necessary for cell division. The other options provided, such as S Phase – 12 Hours, G1 Phase – 11 Hours, and M Phase – 2 Hours, are not typically accurate representations of the average duration for these phases in a typical mammalian cell cycle, which can vary but generally sees G1 as the longest phase, followed by S, then G2, and M phase being the shortest.
The correct answer is (IV) Homologous chromosomes appear as bivalent or tetrad – Metaphase II. However, this statement requires clarification. During metaphase II, individual chromosomes (each consisting of two sister chromatids) line up at the metaphase plate, not bivalents. Bivalents or tetrads appear during metaphase I when homologous chromosomes are paired together. The correct statement should be that homologous chromosomes appear as bivalents or tetrads during metaphase I, not metaphase II. Statement (I) is correct for anaphase I, where chromosomes do not divide at the centromere because homologous chromosomes separate as whole units. Statement (II) is correct for anaphase of mitosis, where sister chromatids separate by splitting of the centromere. Statement (III) is correct for anaphase II, where sister chromatids separate by splitting of the centromere.
d) C-A- D- B
a) Assertion and Reason are correct. Reason is explaining assertion
The diagram likely depicts stages of prophase I of meiosis or a similar chromosomal condensation event. A represents the Leptotene stage, which is the first substage of prophase I, characterized by the condensation of chromatin into visible chromosomes. During this stage, B indicates that chromosomes are becoming distinctly visible under a light microscope as they continue to condense and shorten. C describes that paired sister chromatids begin to condense, meaning each chromosome, already duplicated during the S phase, consists of two sister chromatids that are becoming more compact and discernible.
The two primary types of nuclear division are Mitosis and Meiosis. Mitosis is a process of cell division that results in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. It is essential for growth, repair, and asexual reproduction. Meiosis, on the other hand, is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell. Meiosis is crucial for sexual reproduction, as it produces gametes (sperm and egg cells) or spores.
- Maintain chromosome number constant
- Crossing over (exchange of genetic meterial) leads to variations
- Variation – the raw material for Evolution
- Finally, meiosis produces genetic variability by partitioning different combinations of genes into gametes through an independent assortment.
- Responsible for Adaptations of organisms to various environmental stress.
The various stages of Prophase I:
1. Prophase I – Prophase I is of longer duration and it is divided into 5 substages – Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
2. Leptotene – Chromosomes are visible under light microscope. Condensation of chromosomes takes place. Paired sister chromatids begin to condense.
3. Zygotene – Pairing of homologous chromosomes takes place and it is known as synapsis. Chromosome synapsis is made by the formation of synaptonemal complex. The complex formed by the homologous chromosomes are called as bivalent (tetrads).
4. Pachytene – At this stage bivalent chromosomes are clearly visible as tetrads. Bivalent of meiosis I consists of 4 chromatids and 2 centromeres. Synapsis is completed and recombination nodules appear at a site where crossing over takes place between non – sister chromatids of homologous chromosome. Recombination of homologous chromosomes is completed by the end of the stage but the chromosomes are linked at the sites of crossing over. This is mediated by the enzyme recombinase.
5. Diplotene – Synaptonemal complex disassembled and dissolves. The homologous chromosomes remain attached at one or more points where crossing over has taken place. These points of attachment where ‘X’ shaped structures occur at the sites of crossing over is called.
6. Chiasmata: Chiasmata are chromatin structures at sites where recombination has been taken place. They are specialised chromosomal structures that hold the homologous chromosomes together. Sister chromatids remain closely associated whereas the homologous chromosomes tend to separate from each other but are held together by chiasmata. This substage may last for days or years depending on the sex and organism. The chromosomes are very actively transcribed in females as the egg stores up materials for use during embryonic development. In animals, the chromosomes have prominent loops called lampbrush chromosome.
7. Diakinesis – Terminalisation of chiasmata. Spindle fibres assemble. Nuclear envelope breaks down. Homologous chromosomes become short and condensed. Nucleolus disappears.
The correct answer is (IV) Dead or injured neurons can be replaced by stem cell therapy. Most neurons remain in the G0 phase and do not divide because they are highly differentiated cells that have exited the cell cycle. This presents a significant challenge in treating neurological damage and diseases like dementia. While electric shocks cannot activate neurons to divide, and surgical procedures cannot replace dead neurons, stem cell therapy offers a promising approach. Stem cells, which are undifferentiated cells capable of self-renewal and differentiation, can be transplanted into damaged neural tissue where they can differentiate into neurons and potentially replace dead or injured neurons. This therapeutic approach is being actively researched for treating various neurodegenerative diseases and spinal cord injuries, making it the most viable option among the given choices for addressing neuronal loss in conditions like dementia.
c) C-D- B- A
VI.
b) Assertion and Reason are correct but Reason is not explaining Assertion
The correct sequence of events in the cell cycle, as depicted by the options, is a) Cell growth – Interphase – Mitotic phase – Cytokinesis. The cell cycle begins with a period of cell growth, primarily occurring during the G1 phase of interphase. Interphase itself is a preparatory phase comprising G1, S, and G2, where the cell grows, synthesizes DNA, and prepares for division. Following interphase is the Mitotic phase (M phase), which includes karyokinesis (nuclear division). Finally, Cytokinesis is the division of the cytoplasm, which typically follows karyokinesis to complete the formation of two separate daughter cells.
- Deprivation of nutrition
- Lack of growth factors or density-dependent inhibition occur
- Some metabolic changes leads to Go – stage
Mitosis in plants and animals shows several key differences. In plants, centrioles are absent, whereas in animals, centrioles are present and play a crucial role in organizing the spindle apparatus. Asters, which are star-shaped structures radiating from the centrosomes, are not formed during plant mitosis but are clearly visible during animal mitosis. The mechanism of cytokinesis differs significantly between the two: in plants, cell division involves the formation of a cell plate at the equator, which eventually develops into the new cell wall and middle lamella, whereas in animals, cell division occurs through furrowing and cleavage of the cytoplasm, where the cell membrane pinches inward to divide the cell. Additionally, mitosis in plants occurs mainly at meristematic regions such as root tips and shoot apices, where cell division is concentrated, while in animals, mitosis occurs in tissues throughout the body during growth, repair, and maintenance. These differences reflect the structural and functional distinctions between plant and animal cells.
Prophase – Longest Phase
* Chromosomes visible thread like condenses into thick chromosomes
* Initiation of spindle fibres occur
* Nucleolus disappear Nuclear envelope breaks down
* Golgi apparatus & ER not seen
Metaphase
* Sister chromatids attached to spindle fibres by kinetochore of centromere
* Chromosome align on the equatorial plane (metaphase plate)
(spindle assembly check point decide the fate of the cell)
Anaphase
* Centromere split daughter chromatids move to opposite poles
* Shortening of spindle create pull divide centromere & divide chromosome into a chromatids
(APC /C leads to degradation of protein lead to separation of chromatids
Telophase
* Genetic material division completed Nucleolus & Nuclear membrane reform.
* Sister chromatids become thick chromosomes with its own centromere.
c) Assertion is true but Reason is wrong
The correct labeling for the given diagram, which likely illustrates a chromosome, is b) Centromere – Chromatin – Secondary constriction – Kinetochore. The Centromere is the constricted region of a chromosome that holds sister chromatids together and serves as the attachment point for spindle fibers during cell division. Chromatin refers to the complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. A Secondary constriction is a region on a chromosome, distinct from the centromere, often associated with the nucleolus organizer region. The Kinetochore is a protein structure that forms on the centromere of a chromosome during cell division and mediates the attachment of the chromosome to the spindle microtubules.
Cells are primarily arrested in the G1 phase, preventing them from proceeding to the S phase, due to several critical reasons. One significant factor is nutrient deprivation; if essential nutrients are scarce, the cell lacks the necessary resources to synthesize DNA and proteins required for division, thus halting its progression. Another reason is the lack of growth factors or density-dependent inhibition; cells require specific external signals, such as growth factors, to initiate division, and high cell density can also trigger a stop signal. Furthermore, if conditions are unfavorable, cells may undergo metabolic changes and enter into a quiescent state known as the G0 phase. In G0, cells are metabolically active but no longer proliferate, effectively exiting the cell cycle until conditions become favorable again or they receive appropriate stimulatory signals to re-enter the cycle.
Endomitosis is a fascinating process where the replication of chromosomes occurs in the absence of both karyokinesis (nuclear division) and cytokinesis (cytoplasmic division), leading to the formation of numerous copies of chromosomes within a single cell. This results in a polyploid cell, meaning it has multiple sets of chromosomes. During endomitosis, several characteristic events that normally occur in typical mitosis are absent: the chromonema (chromatin threads) do not fully separate to form distinct chromosomes, the nuclear membrane does not rupture, and there is no formation of a spindle apparatus. Consequently, each chromosome can consist of over thousands of synapsed chromatids, leading to giant chromosomes. A classic example of this phenomenon is observed in the salivary gland chromosomes of Drosophila, which are known as polytene chromosomes, where repeated DNA replication without cell division leads to these exceptionally large, multi-stranded chromosomes.
(i) S Phase: The S phase, also known as the synthetic phase, is a crucial part of the interphase of mitosis where significant cellular activities occur. During this phase, the growth of the cell continues steadily. Most importantly, the replication of DNA takes place, ensuring that each chromosome is duplicated, resulting in two identical sister chromatids. Concurrently, histones, which are proteins essential for packaging DNA into nucleosomes, are synthesized and subsequently attach to the newly replicated DNA. Furthermore, the duplication of centrioles occurs in animal cells, which are vital for the formation of the spindle fibers during cell division. As a result of DNA replication, the DNA content of the cell doubles from 2C to 4C, while the chromosome number remains the same. (ii) G2 phase: The G2 phase is the final preparatory stage before mitosis. In this phase, the cell maintains its 4C amount of DNA, having already duplicated its genetic material. Cell growth continues vigorously, and there is a significant synthesis of various organelles, including mitochondria and chloroplasts (in plant cells), to ensure that the daughter cells will have sufficient cellular machinery. Tubulin, the protein subunit of microtubules, is synthesized, and microtubules begin to form, which are essential for the construction of the spindle apparatus. The cell also synthesizes other proteins necessary for mitosis. This phase is critical for ensuring that the cell is fully prepared for nuclear division, which immediately follows. A key regulatory molecule, the Maturation Promoting Factor (MPF), is formed during G2, and its activation triggers the condensation of interphase chromosomes into their mitotic form, initiating entry into the M phase.
d) Assertion is true but Reason not explaining assertion
VII.
The diagram illustrates a bivalent or tetrad structure formed during meiosis, specifically prophase I. A points to Non-sister Chromatids, which are chromatids belonging to homologous chromosomes that are paired up. These are the sites where crossing over, or genetic recombination, typically occurs. B indicates the Centromere, the constricted region on each chromosome that holds the sister chromatids together. C highlights the Chiasma (plural: chiasmata), which is the visible manifestation of crossing over between non-sister chromatids, appearing as an X-shaped structure where homologous chromosomes are still connected. D labels the entire structure as a Bivalent or Tetrad, referring to the pair of homologous chromosomes, each consisting of two sister chromatids, thus forming a group of four chromatids.
Karyokinesis and cytokinesis are two distinct processes that occur during amitosis. Karyokinesis refers to the division of the nucleus, during which the nucleus develops a constriction at its center, becoming dumbbell-shaped. This constriction gradually deepens, eventually dividing the nucleus into two separate nuclei. Cytokinesis, on the other hand, refers to the division of the cytoplasm and the entire cell. During cytokinesis, the plasma membrane develops a constriction along with the nuclear contraction. This constriction deepens centripetally, meaning it progresses inward from the periphery toward the center, ultimately resulting in the complete division of the cell into two daughter cells. While karyokinesis focuses specifically on nuclear division, cytokinesis encompasses the division of all cellular contents, including cytoplasm and organelles, ensuring that each daughter cell receives the necessary components for survival and function.
Anaphase I and Anaphase II are two distinct stages of meiosis with important differences. During Anaphase I, homologous chromosomes separate and move toward opposite poles of the cell. Each chromosome still consists of two sister chromatids joined at the centromere, so the number of chromosomes moving to each pole is half the diploid number. This separation of homologous pairs is the key event that reduces the chromosome number. In contrast, during Anaphase II, sister chromatids separate at the centromere and move toward opposite poles. At this stage, the cell is already haploid, and the separation of sister chromatids produces individual chromosomes. Therefore, the fundamental difference is that Anaphase I involves the separation of homologous chromosomes while sister chromatids remain attached, whereas Anaphase II involves the separation of sister chromatids themselves.
Amitosis is a form of direct cell division, sometimes referred to as incipient cell division, which is characterized by the absence of several key events seen in mitosis and meiosis. In amitosis, there is no spindle formation, and the chromatin material does not undergo the typical condensation into distinct chromosomes. Instead, the nucleus elongates and then constricts in the middle, dividing directly into two parts. This process primarily involves two steps: karyokinesis, which is the division of the nucleus, followed by cytokinesis, the division of the cytoplasm. However, amitosis has significant drawbacks. One major issue is that it often leads to an unequal distribution of chromosomes into the daughter cells. This uneven partitioning of genetic material can result in daughter cells having an abnormal number of chromosomes or an incomplete set of genetic information. Such chromosomal abnormalities can, in turn, lead to various problems, including disruptions in normal cellular metabolism and impaired reproductive capabilities, as the cells may not function correctly or be able to divide further in a healthy manner. This makes amitosis a less precise and potentially detrimental form of cell division compared to mitosis or meiosis.
Exact copy of the parent cell is produced by mitosis (genetically identical).
* Genetic stability – Daughter cells are genetically identical to parent cells.
* Growth – As multicellular organisms grow, the number of cells making up their tissue increases. The new cells must be identical to the existing ones.
* Repair of tissues – Damaged cells must be replaced by identical new cells by mitosis.
* Asexual reproduction – Asexual reproduction results in offspring that are identical to the parent. Example Yeast and Amoeba.
* In flowering plants, structure such as bulbs, corms, tubers, rhizomes and runners are produced by mitotic division. When they separate from the parent, they form a new individual. The production of large numbers of offsprings in a short period of time, is possible only by mitosis. In genetic engineering and biotechnology, tissues are grown by mitosis (i.e. in tissue culture).
* Regeneration – Arms of starfish
Closed mitosis and open mitosis are two distinct patterns of nuclear division observed in different eukaryotic organisms. In closed mitosis, the nuclear envelope remains intact throughout the process, and chromosomes migrate to opposite poles of a spindle that forms within the nucleus. The spindle apparatus functions entirely within the confines of the nuclear membrane, and there is no breakdown of the nuclear envelope. This type of mitosis is characteristic of unicellular eukaryotes such as yeast and slime molds. In contrast, open mitosis involves the breakdown of the nuclear envelope at the beginning of mitosis, allowing the spindle apparatus to form in the cytoplasm and interact directly with chromosomes. After the chromosomes have separated and moved to opposite poles, the nuclear envelope reforms around each set of separated chromosomes to create two new nuclei. This type of mitosis is observed in most higher plants and animals. The key distinction lies in the fate of the nuclear envelope: it persists in closed mitosis but disintegrates and reforms in open mitosis.
Anastral and Amphiastral cell divisions are two distinct types of mitotic divisions, primarily differentiated by the presence or absence of asters and centrioles. Anastral cell division typically occurs in plant cells. In this type, there are no asters or centrioles formed at the poles of the spindle. Instead, only spindle fibres are formed, which organize the chromosomes during cell division. The absence of asters is a characteristic feature. Conversely, Amphiastral cell division is characteristic of animal cells. During this process, asters and centrioles are prominently formed at each pole of the spindle. Asters are radial arrays of microtubules that extend from the centrosomes, which contain the centrioles. These structures play a crucial role in organizing the spindle fibres and ensuring proper chromosome segregation. Therefore, the presence of asters and centrioles is the key distinguishing feature, with plants exhibiting anastral division and animals exhibiting amphiastral division.
At the end of telophase in mitosis in plant cells, several important events occur to complete cytokinesis. A phragmoplast, which is a structure composed of microtubules and other cytoplasmic elements, forms between the two daughter cells at the equatorial region. Within this phragmoplast, a cell plate is formed, which is a precursor to the new cell wall. The cell plate develops from the fusion of vesicles derived from the Golgi apparatus, which contain polysaccharides and other materials necessary for cell wall formation. As the cell plate matures, it gradually develops into a complete cell wall composed of cellulose and other polymers. Simultaneously, the reconstruction of the cell wall takes place, and the middle lamella forms between the two daughter cells. Finally, the two daughter cells are completely separated through the distribution of organelles and macromolecules into the newly formed daughter cells, ensuring that each daughter cell possesses the necessary cellular components for independent survival and function.
Meiosis in plants and animals exhibits several important differences related to where and when the process occurs. In flowering plants, meiosis occurs during microsporogenesis in the anther, where it produces microspores that develop into male gametophytes, and during megasporogenesis in the ovule, where it produces megaspores that develop into female gametophytes. The process is intimately linked to the formation of spores rather than gametes directly. In animals, meiosis takes place in reproductive organs during the production of gametes. In males, the process is called spermatogenesis, which produces four functional haploid sperms from each diploid cell, ensuring efficient gamete production. In females, the process is called oogenesis, which produces one functional haploid egg and three polar bodies from each diploid cell, with unequal distribution of cytoplasm. Additionally, in plants, meiosis is followed by mitotic divisions of spores to produce gametophytes, whereas in animals, meiosis directly produces functional gametes. These differences reflect the distinct reproductive strategies and life cycles of plants and animals.
- The biochemical substances or factors which promote cell cycle acceleration & proliferation is called Mitogen.
- Eg. Gibberellin, Ethylene, Indole Acelic Acid, Kinetin.
- They are also known as Growth promotors.
Endomitosis is a specialized type of nuclear division in which chromosomes replicate multiple times without undergoing nuclear division or cytoplasmic division, resulting in numerous copies of chromosomes within a single cell. During endomitosis, the chromonema do not separate to form individual chromosomes but instead remain closely associated with each other, forming polytene chromosomes. The nuclear membrane does not rupture during this process, so no spindle apparatus forms, distinguishing it from typical mitosis. Endomitosis occurs notably in the salivary glands of Drosophila and other dipteran flies, where it produces giant polytene chromosomes. These polytene chromosomes consist of thousands of intimately associated or synapsed chromatids arranged in parallel, creating a banded appearance under the microscope. The banding pattern reflects the linear arrangement of genes along the chromosome. Polytene chromosomes are valuable in genetic research because their large size and distinct banding patterns allow for direct visualization of chromosomal structure and gene locations. The cells containing polytene chromosomes become highly specialized for secretion, with increased metabolic activity to support the production of large quantities of proteins such as digestive enzymes.