Biology 2 Marks Questions

	Cytokinesis		Karyokinesis
i.	Cytokinesis is the biological process involving the division of a cell’s cytoplasm during mitosis or meiosis.	i.	Karyokinesis is the biological process involving the division of a cell’s nucleus during mitosis or meiosis.
ii.	Stages such as prophase, metaphase, anaphase, and telophase are not present in cytokinesis.	ii.	It is divided into four stages –prophase, metaphase, anaphase, and telophase

Bivalent Bivalent or tetrad is a pair of synapsed homologous chromosomes. They are formed during the zygotene stage of prophase I of meiosis.

Chiasmata Chiasmata is the site where two non-sister chromatids of homologous chromosomes have crossed over. It represents the site of cross-over. It is formed during the diplotene stage of prophase I of meiosis.

Synapsis The pairing of homologous chromosomes is called synapsis. This occurs during the second stage of prophase I or zygotene.

In some insects and lower plants, fertilization is immediately followed by zygotic meiosis, which leads to the production of haploid organisms. This type of life cycle is known as haplontic life cycle.

The phenomenon of polyploidy can be observed in some haploid cells in higher plants in which cell division does not occur. Polyploidy is a state in which cells contain multiple pairs of chromosomes than the basic set. Polyploidy can be artificially induced in plants by applying colichine to cell culture.

All the chromosomes lie at the equator, with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibre from the opposite pole. The plane of alignment of all the chromosomes at the equator during metaphase, is referred to as metaphase plate.

1. Interphase.
2. Kinetochore.
3. G-phase.
4. Mitotic or M-phase.

Cell division, cell reproduction or cell multiplication is the process of formation of new daughter cells from the pre-existing cell or parent cell.It is of three types:

1. Amitosis.
2. Mitosis.
3. Meiosis.

A cell divides when it attains the size and the nucleocytoplasmic ratio disturbs. The DNA duplication also causes a cell to divide.

Colchicine is considered to be the mitotic poison because

1. This alkaloid inhibits the formation of spindle by preventing assembly of microtubules. It is an alkaloid obtained from the corms of Autumn crocus.
2. The chromosomes duplicate, but they remain within the same cell, increasing the number of chromosomes. As a result, the colchicine treated meristematic cells show doubling of chromosome.

1. X-Mitosis. Y-Meiosis.
2. 1. In X, there is no association of homologous chromosomes, but in Y, homologous chromosomes pair up together to form bivalents during prophase-I.
   2. In X, there is no crossing over as there is no formation of chiasmata, but in Y, crossing over occurs at the chiasmata whereby some genes Pontos are swapped between homologous chromosomes.

Metaphase I	Metaphase
The homologous pairs of chromosomes are arranged in two rows near the centre of the spindles. There is no metaphasic plate formed.	Centromeres of all the chromosomes are arranged in a single row at the centre of the spindles
There is no metaphasic plate formed.	A metaphasic plate is formed.

Glycocalyx is significant in the following ways:

1. It protects the cell from loss of water and nutrients.
2. Serves as a protective layer against attack by phagocytosis and viruses.
3. Helps in attachment to a surface.
4. Provides virulence.

1. It is a meiotic division showing synapsis and it is in the zygotene stage of prophase-I of meiosis-I.
2. A-centromere, B-chromatid.

	Animal Cell	Plant Cell
1.	Cell wall absent	Cell wall present
2.	Chloroplast absent	Chloroplast present in green parts
3.	Centriole present	Centriole absent
4.	Vacuoles are smaller	Comparatively vacuoles are larger

Meiosis happens during gametogenesis and as a result gametes have half the number of chromosomes. During fertilization, when gametes fuse together two different sets of chromosomes make a new set. This results in an offspring, who has distinct characters, compared to parents.

1. No, bacterial cells do not have well-defined nucleus instead, the genetic material in them is dispersed in the cytosol, also there is no internal membrane that delimits a nucleus.
2. No, there is no pluricellular bacteria, i.e., made up of more than one cell. All bacterial cells are unicellular prokaryotes.

Chiasmata is X-shaped structure of separated bivalents of recombined homologous chromosomes. Chiasmata formation is necessary for the separation of homologous chromosomes which have undergone the process of crossing over.

All cells are autonomous because of the following facts:

1. Each and every cell converts non-living materials into components of living matter.
2. Each cell builds up its own macromolecules from macromolecules.
3. All cells obtain or manufacture food.
4. A cell always buildsup new structure to replace the worn out ones.

1. Pill and fimbriae are the two surface extensions that are not concerned with motility.
2. Structure that helps in motility of bacterial cell is flagella.

The DNA present in mitochondria and chloroplast is extra chromosomal DNA. It has nothing to do with the nuclear divisions. In mitosis, only nuclear DNA takes part.

Functions of centrioles are:

1. The centrioles play a vital role in cell division by forming spindle fibres, which act as guides for the alignment of the chromosomes as they separate later during the process of cell division.
2. The centrioles are also responsible for formation of basal bodies, cilia and flagella.

1. prophase.
2. Metaphase.
3. Meiosis-II.
4. homologous.

Differences between metaphase of mitosis and metaphase-I of meiosis are

S.No.	Mitotic Metaphase	Meiotic Metaphase
1	The chromosomes at the metaphase are arranged in such a way that the centromeres lie at the metaphase plate and the arms of chromosomes are free.	The chromosomes at the metaphase (meiosis-I) are arranged in such a way that the centromeres of homologous chromosomes lie on either side of the metaphase plate, pointing towards the opposite poles.
2	A chromosome is connected to both the spindle poles due to the presence of two tractile fibrils.	In metaphase-I, a chromosome is connected to only one spindle pole of its side due to the presence of a single tractile fibril.

1. Meiosis is essential for all sexually reproducing organisms.
2. It occurs in reproductive cells, so that gametes formed are haploid or have half the number of chromosomes of those cells which are directly derived from zygote.
3. Two types of gametes fuse during zygote formation. As a result, zygote comes to have double the number of chromosomes contained in gametes. Meiosis maintains a fixed number of chromosomes of a species by reducing it to number by half.

If DNA duplication does not occur than cell might not be able to surpass S-phase of cell-cycle as no chromosome formation will take place, and will not be able to enter M-(mitotic phase) or if in case it enters mitosis the cycle will cease.

In unicellular organisms, mitosis is a method of asexual reproduction. E.g., in bacteria, mitosis results in formation of identical daughter cells. However in multicellular organisms, mitosis helps in growth and repair of cells and tissues. E.g., repair of damaged skin through production of new skin cells.

The cell wall of Gram positive bacteria is single-layered (outer membrane is absent) and contains 70-80% peptidoglycan and little amount of lipids. It's wall also has teichoic acid. The light cell wall of Gram negative bacteria on the other hand is two-layered, outer membrane is present, i.e., thin
layer of peptidoglycan and plasma membrane is present, teichoic acid is absent in wall.

If nuclear membrane fails to disintegrate, the spindle fibres would not be able to reach chromosomes and they would not move towards opposite poles of the cell. In certain protozoan’s, such as Amoeba, the spindle is formed within the nucleus and this is called intra nuclear mitosis or pre-mitosis.

1. Meiosis involves two sequential cycles of nuclear and cell division, called meiosis I and meiosis II, but only a single cycle of DNA replication.
2. Meiosis I is initiated after the chromosomes have replicated to produce identical sister chromatids at S-phase.
3. Meiosis involves pairing of homologous chromosomes and recombination between them.
4. Four haploid cells are formed at the end of meiosis II.

These are the 4 subsequent cell divisions to produce 16 cells and each division takes = 20 min Total time = 20 × 4 = 80 minutes or 1 hr 20 minutes So, 1 cell produces 16 cells in 1 hour 20 minutes and 2 cells produces 32 cells in 1 hour 20 minutes.

In this case, non-sister chromatids can cross over. It is important to remember that crossing over always takes place between two non-sister chromatids and never between two sister chromatids.

S.No.	Column I	S.No.	Column II
A	Karyokinesis	3	Nuclear division
B	Cytokinesis	4	Cytoplasmic division
C	C. Meiosis	1	Meiocytes
D	D. Cell plate	2	Plant cells

Telophase I:

• The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as diad of cells.
• Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus.
• The stage between the two meiotic divisions is called interkinesis and is generally short lived. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.

Unicellular organisms possess the following cellular autonomy:

1. The cell has complete independent existence.
2. All life activities are carried out by the same cell.
3. Division of labour is absent.
4. The cell depends upon its own internal and intrinsic information.

Prokaryotic cells has shorter cell division time than eukaryotic cells. A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours. In bacteria (E.coli) cell cycle is of 20 minutes.

A chromatid consists of centromere and arms. These attributes are required by it to be classified as chromosome. Chromatids separate at the time of division (sister chromatids) and form chromosomes.

Stages during which recombination of genes occurs in meiosis are bidy

1. Crossing over is exchange of genes between the non-sister chromatids i of homologous chromosomes. It occurs at recombination nodules during pachytene stage of prophase-I of meiosis-I and is regulated by recombinase enzyme.
2. Arrangement of bivalents at the equator of the spindle during metaphase-I of meiosis.

1. Stages and type of cell division are
   1. Metaphase-I-meiosis.
   2. Anaphase-I-meiosis.
2. For metaphase.

Mitosis-2n (diploid); Meiosis-n (haploid).

Cell wall is stronger and less impervious than plasma membrane. It tends to provide greater mechanical strength to plants. Since, plants have to withstand vagaries of nature and they cannot run for safety, so they need a stronger structure. Cell wall also prevents entry of undesirable matters in cell and facilitates exchange of materials among cells.

All somatic cells except germ cells of the human body can be used to demonstrate stages in mitosis. Mitotic cell division is seen in diploid somatic cells in animals. It is also called equational division or somatic cell division.

1. Anaphase.
2.  

1. Meiosis ensures the maintenance of a constant chromosome number, characteristic of a species, through genemtions.
2. The crossing over results in variations of genetic characters in the progeny, variation is necessary for survival of species and it is the raw material for evolution.

1. Meiosis is known as equational division because number of chromosomes maintain equal in daughter cells as in the parental cells after division.
2. Cytokinesis is different in plant cells and animal cells. Animal cell divides by cell furrow system, while plant cell divides by cell plate method.

If the centromeres do not divide one of the daughter cell will receive a complete pair of chromosomes and other cell would not get any of them. This may result in trisomy.

Meiosis takes place only in the reproductive cells or germ cells (gametes) of the body which include the cells of testes and ovaries in animals and the male and female reproductive parts of the plant i.e. the Androecium and Gynoecium.

Vacuoles in plant cell contains not only unuseful substances and water, but also contain pigments and toxic molecules. Tonoplast is the covering membrane of the vacuoles.

1. Metaphase I, meiosis.
2. Anaphase I, meiosis.

Cancer is the pathological condition in which the cells lose control over cell division and this result into malformation of the organ in which such cell division occurs.

1. Telophase.
2. Anaphase.
3. Metaphase.
4. Zygotene of Prophase.

1. Meiosis is a specialised kind of cell division that reduces the number of chromosomes to half and forms haploid daughter cells.

• Diakinesis is the last-stage of prophase I.
• It is marked by the terminalisation of chiasmata.

The disadvantages of multicellularity are:

1. Specialised cells often lose the power of division, so that injury is not repaired, e.g., Nerve cells.
2. Regeneration ability of multicellular organisms decrease with specialisation.
3. Specialised cells may lose vital functions in order to carry out specific activity, e.g., RBCs, sieve tube cells.

1. Two divisions take place in meiosis, i.e., meiosis-I and meiosis-II. During meiotic division, chromosomes replicate one time in meiosis II.
2. 2 daughter cells are produced in mitotic division and 4 daughter cells are produced in meiotic division.
3. During mitosis, DNA and chromosomes remain same in daughter cells as in parental, while DNA and chromosome number reduces to half in meiosis in daughter cells.

• Chiasmata are the X-shaped structures formed at the point of crossing over.
• They represent the sites of cross overs and hence exchange of genetic material or recombination.

Cells with 16 chromosomes are produced by meiosis while that with 32 chromosomes are produced by mitosis.

• Cells with 16 chromosomes could have arisen from cells with 32 chromosomes through meiosis.
• Cells with 32 chromosomes could have arisen from cells with 16 chromosomes through fertilization or syngamy.

Prokaryotes do not have nucleus. So, there is no elaborate karyokinesis, as seen in eukaryotes. In prokaryotes the replication of DNA starts the process of cell division. Once genetic material is replicated, it is followed by division of cytoplasm. The process is known as binary fission.

Ribosomes in the cytoplasm of prokaryotic and eukaryotic cells slightly differ. However, prokaryotes have 70S ribosomes, each consisting of a (small) 30S and a (large) SOS subunit whereas, eukaryotes have SOS ribosomes, each consisting of (small) 40S and a bound (large) 60S submit. Both prokaryotic and eukaryotic ribosomes are composed of ribosomal RNA and ribosomal proteins. Their basic functioning is protein synthesis.

The key events of telophase are:

1. Chromosomes cluster at the opposite poles of the spindle and their identity is lost as discrete bodies as they start uncoiling.
2. Nuclear envelope becomes assembled around the chromosome clusters.
3. Nucleolus, endoplasmic reticulum and golgi complex are reformed.

Cytokinesis is the process by which the cytoplasm of a cell divides; it corresponds to the separation of the two daughter nuclei (formed by karyokinesis) into two daughter cells.

• In animal cells, a furrow appears in the cell membrane during late anaphase or early telophase.
• The furrow deepens and when it joins in the centre, the cytoplasm (cell) is divided into two compartments; each compartment with a daughter nucleus, is known as a daughter cell.

If cytokines is does not occur, then multinucleate condition called coenocytes, syncytium is produced, as in Rhizopus and Vaucheria, etc.

Centrioles play an important role in formation of spindle during cell division. They organise the mitotic spindle and thus, help in the completion of cytokinesis. They generate the cell's cytoskeleton and help in the formation of the mitotic spindles. In organisms with flagella and cilia, the position of these organelles is determined by the mother centriole, which becomes the basal body.

1. Plant cells cannot undergo cytokinesis like animal cells because they have a rigid/ inextensible cell wall.

2.  

• In plant cells, cytokinesis starts with the formation of a precursor, called cell plate.
• This extends from the centre of the cell towards the lateral walls and cuts the cell into two halves, each with a daughter nucleus.
• During cytokinesis, the cell organelles like mitochondria, plastids, etc. also get distributed between the daughter cells.

When a cell has 32 chromosomes and undergoes a mitotic division, the chromosome number (n) during metaphase is the same. The number of chromatids gets double during replication. Each chromosome has 2 chromatids. But the number of chromatid will be 64.

Centrioles also serve other purposes apart from its role in spindle formation it formed the basal body of cilia and flagella. They also help in formation of microtubules and sperm tail.

The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.

• Crossing over is defined as the phenomenon of exchange of equivalent segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
• The enzyme involved is recombinase.

Meiosis is reductional division which ensures production of haploid phase in life cycle of sexually reproducing organisms, but fertilization restores the diploid phase, thus, enables conservation of specific chromosome number. E.g., gametes produced by 2n parents are haploid (n), but the zygote is diploid.

The given diagram shows transition from prophase to metaphase.

Ribosomes are found in plant cell at following different sites:

1. Freely in the cytoplasm.
2. Attached to ER in the cytoplasm.
3. Inside mitochondria.
4. Inside plastids (chloroplasts).

Cells usually stop dividing after certain number of divisions. In malignant or cancerous cells this automatic stoppage of cell division does not work. As a result malignant cells keep on dividing endlessly causing the cancerous growth.

In the absence of meiosis the next generation would have double the number of chromosomes after fusion of gametes. This would have resulted in the birth of an altogether new species. The maintenance of characters set would have been possible only through asexual reproduction.