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Compilation of questions on Cell:- the unit of life for class 11.
1. Define Cell. Explain its Structure.
Meaning and Types of Cell- Basic Unit of Life:
Robert Hooke (1665) observed the so called cells for the first time in a thin slice of cork under a very primitive microscope invented by him. He coined the term “cell”. Living cells were seen for the first time by Anton Van Leeuwenhoek (1632-1723), with his improved microscope. Much later (1838-39) cell-theory was proposed by two German biologists separately – viz., M.J. Schleiden for plants and Theodor Schwann for animals. According to them, “Cells are the structural and functional units of living organisms.” Later, Rudolph Virchow (1855) extended the cell theory and suggested that all living cells arise from pre-existing cells (Omnis cellula e cellula).
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Viruses are the most notable exception to the cell theory because they lack internal organisation and protoplasm. Other exceptions include protozoans, fungi and algae because their entire organisation is represented by just one cell.
Size of the Cell:
Usually the cells are microscopic and their size varies between 10 µm and 100 µm. The smallest cells are those of PPLO (Pleuropneumonia like organisms) whose size may vary between 0.1 to 0.4 µm. The largest cell is the egg of ostrich measuring about 15 cm in its outer diameter. The longest animal cell is the nerve cell which may be approximately one metre long, while the longest plant cell is the sclerenchymatous fibre of Boehmeria nevia (about 55 cm long).
The factors governing the size of the cell are:
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(i) The ratio between the volume of the nucleus and that of the cytoplasm.
(ii) The ratio of the cell surface to the cell volume.
(iii) The rate of metabolism.
(iv) The size and the number of chromosomes.
Shape of the Cell:
There is a great variation in the shape of cell. Some cells, e.g., Amoeba, slime moulds and WBCs have a constantly changing shape while others (e.g., neurons, muscle cells, RBCs, etc.) have a characteristic shape. The shape is governed by the plasma membrane and the cell wall, if present.
Numbers of the Cells:
The body of protozoans, bacteria, certain fungi and algae is represented by one cell only. They are called as unicellular or acellular forms. Most of the animals and plants are made up of several cells. They are called multicellular.
On the basis of the nuclear organisation, cells are of two types:
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1. Prokaryotic:
Cells in which mitochondria, chloroplast and nuclear membrane are absent are called prokaryotic cells. For example bacteria, blue green algae (cyanobacteria), and mycoplasma.
2. Eukaryotic:
Cells in which nucleus and membrane bound organelles are present are called eukaryotic cells. They are found in all plants and animals.
2. What is a Cell Wall? What are the Functions of it?
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The presence of a cell wall is a characteristic feature of plant cells. It is always formed by the activity of the protoplasm.
The adjacent cell walls are cemented together by middle lamella composed of calcium or magnesium pectate cell wall is differentiated into three layers, viz.:
1. Primary Cell Wall:
It is the outermost layer of the cell wall present on both the sides of middle lamella. It is usually thin (1-3 µm), and elastic. The chief constituents of the primary cell wall are cellulose and hemicellulose, some amount of pectin and a structural protein extension (rich in proline and hydroxyproline) may also be present. In thin walled cells (like meristematic cells, parenchyma, collenchyma and mesophyll cell) primary cell wall remains as the only layer.
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2. Secondary Cell Wall:
It is much thicker (5-10 µm), rigid and inelastic It is formed only when the growth in the surface area of the primary cell wall ceases. Its position is in between the primary wall and the protoplast. The secondary cell wall may be thickened on account of the deposition of substances like cutin, suberin, lignin and pectin.
3. Tertiary Cell Wall:
It is of rare occurrence (in tracheids of gymnosperms). It is deposited on the inner side of the secondary cell wall. It is relatively richer in xylem (a polymer of pentose sugar D-xylose) than cellulose.
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Functions of Cell Wall:
Functions of cell wall are largely mechanical. It acts like a skeleton of the plant by providing rigidity strength and flexibility. It maintains the shape and structure of the cells and the tissues, and protects the protoplasm against mechanical injuries. By impregnation of cutin and suberin it also reduces loss of water by transpiration. It, being freely permeable, helps in the absorption and transportation of water and solutes in the different parts of the plant.
3. What is Protoplasm? Explain the Nature and Properties of it.
Felix Dujardin (1835) described protoplasm in protozoa. He called it “Sarcode”. The term protoplasm was coined by Johannes E. Purkinje (1839) and Hugo Von Mohl (1846) independently. Huxley called protoplasm as the physical basis of life, for life cannot exist apart from it.
Protoplasm refers to the living substance of the cell and includes all parts of the cell. It is the set of all metabolic functions. The protoplasm can be divided into cytoplasm and nucleus.
Cytoplasm:
It is the part of cell occurring between plasma membrane and nucleus. This term was introduced by Strassburger (1882).
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It is composed of two distinct types of structures, viz.:
(i) A continuous fluid like substance called cytosol.
(ii) A number of organelles which are having definite function.
Physical Nature of Protoplasm:
(i) It is a thick, greyish, viscous jelly-like translucent fluid of colloidal nature.
(ii) The colloid particles exhibit Brownian movement.
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(iii) It shows Tyndall’s effect, i.e., when a beam of strong light is passed through it in a dark room, the path of light appears like a cone.
(iv) It is a reversible colloidal system. It can be watery (Sol) at one time and jelly like at another (Gel). The sol and gel states are reversible.
Chemical Nature of Protoplasm:
Generally the elements of protoplasm are grouped in the following three categories according to their abundance in the protoplasmic matrix:
(i) Major Constituents:
These include Oxygen (62%), Carbon (20%), Hydrogen (10%) and Nitrogen (3%).
(ii) Trace Elements:
These occur in very low quantities or in traces.
The trace elements are- calcium, potassium, phosphorus, sodium, chlorine, magnesium, sulphur, iodine and iron.
(iii) Ultrastructure Elements:
These are required by the cell as co-factors for various metabolic reactions, e.g., copper, cobalt, manganese, zinc, molybdenum, boron, silicon, etc.
Biological Properties of Protoplasm:
(i) Irritability:
Protoplasm shows the ability to respond to stimuli.
(ii) Conductivity:
Protoplasm of neurons is specially adapted to conduct the impulses.
(iii) Metabolism:
Protoplasmic matrix is the seat of various metabolic processes of the cell.
(iv) Growth:
A successful metabolism always results into synthesis of new protoplasm thereby causing growth of the cell.
(v) Reproduction:
Protoplasm is a self-perpetuating substance.
4. Define Plasma Membrane. What are the Functions of it? Explain its Structure.
Cytoplasm of all the living cells is enclosed by a living membrane called as cell membrane, plasma membrane or plasmalemma. The term cell membrane was given by C. Nageli and C. Cramer (1855) while the term plasmalemma was coined by J.Q. Plower (1931).
To explain the structure of plasma membrane several models have been proposed. The most accepted model is fluid mosaic model.
The Fluid Mosaic Model:
Proposed by Singer and Nicholson (1972), this model is now widely accepted. According to this model, there is a continuous bilayer of phospholipid molecules and globular proteins are embedded in it. The membrane is, thus, considered to be a semifluid structure in which lipids as well as intrinsic proteins are able to make movements within the bilayer. The concept of fluidity implies that lipids and proteins are held in their position by non-covalent bonds.
The proteins in the membrane are of two types:
(i) Extrinsic (Peripheral) Proteins:
These are superficially attached to outer and inner surfaces of lipid bilayer. They are soluble and can readily dissociate from the membrane, e.g., spectrin of RBC membrane.
(ii) Intrinsic (Integral) Proteins:
They penetrate partially or even completely through the lipid bilayer, e.g., ATPase, cytochrome oxidase, rhodopsin, etc. These are amphipathic like the phospholipids. Their hydrophilic head protrudes from the surface of the membrane while hydrophobic end is embedded in the membrane. These are capable of lateral diffusion in the lipid bilayer.
Besides lipids and proteins, carbohydrates also occur at the outer surface of the membrane. These are covalently linked to polar heads of phospholipids or proteins forming glycolipids, or glycoproteins. The glycoproteins form the glycocalyx of the animal-cell surface, which is helpful in cell adhesion and cell recognition.
Functions of Plasma Membrane:
1. It forms a limiting boundary of the cell.
2. Being selectively permeable it allows only useful substances to enter the cell and thus maintains the homeostasis of the cell.
3. Through its receptors it helps in binding hormones, drugs, neurotransmitters, growth factors, etc.
4. The glycocalyx of the membrane helps in cell recognition, adhesion and in exchange of materials or information.
5. It helps in bulk transport by phagocytosis, pinocytosis and exocytosis.
5. What is Mitochondria? Explain its Structure and Functions.
Historical Background:
Mitochondria were first observed in flight muscles of insects by Kolliker (1850). W. Fleming (1882) called them as fila. R. Altman (1892) called them bioplast. The term mitochondria were given by Benda. In plant cells, mitochondria of a cell are collectively called as “chondriome”, whereas those of muscle cells are called as sarcosomes.
Shape:
Mitochondria vary in shape but are generally rod shaped, filamentous or granular.
Size:
The average length of mitochondria is between 3-4 µm and the average diameter 0.5-2.0 µm. In the oocytes of Rana pipiens 40 µm long mitochondria have been reported.
Number:
On an average 200-300 mitochondria are present in a cell. But variations are also reported. For example in the algae Micromonas and Microsterias, only one mitochondria is seen in a cell. Their maximum number has been reported in a protozoan Chaos where it is estimated to be approximately 5,00,000.
Ultrastructure:
A mitchondrion is a double membrane bound structure, each membrane being about 60 Å thick. The two membranes are separated by a perimitochondrial space of about 60-80 Å.
The outer membrane is smooth, tightly stretched and elastic. The inner membrane is rough and selectively permeable. It encloses an inner chamber filled with a matrix which contains most of the enzymes of Krebs’ cycle, 70 S ribosomes, two to six circular DNA molecules, divalent cations, e.g., Ca++, Mg++, etc.
The inner membrane is rich in enzymes like succinate dehydrogenase, cytochrome oxidase, ATPase, etc. The side of the inner membrane facing the matrix is called the M-face while the side facing the outer chamber or cytoplasm is called the C-face. The inner membrane is thrown into several fingers like folds projecting into the matrix. These folds are called as crests or cristae. The cavity of the cristae is called the intracristae space and is continuous with the perimitochondrial space.
Attached to M-face of inner membrane are several elementary particles, or oxysomes. Each particle is made up of three parts-viz.; a polyhedral head, a stalk, and a cuboidal base.
These particles are placed at regular intervals of 100 Å. In a mitochondrion their number may vary from 104 – 105.
Functions of Mitochondria:
(i) Pyruvic acid produced during glycolysis enters mitochondria where it is subjected to Krebs cycle and electron transport system to produce ATP by oxidative phosphorylation. Almost total usable energy of a cell is produced by mitochondria. Hence, it is called as the “Powerhouse” of the cell.
(ii) It accumulates certain ions (e.g., Ca++ and Fe+++), ferritin, phospholipids, and bile pigments.
(iii) It stores neutral fats and lipids, vitamin C, vitamin A, carotenoids and carcinogenic hydrocarbons.
(iv) It is involved in the elongation of fatty acids and in the synthesis of lipids.
(v) It has an important role in the synthesis of structural proteins, yolk and glycogen. Mitochondrion is a semiautonomous organelle. It is so, because it contains DNA as well as ribosomes, and is therefore able to synthesize some of the proteins required by it.
Mitochondrion is a symbiotic prokaryote. Altman believed mitochondria (and chloroplast also) to be a prokaryotic organism which had entered the cytoplasm of a eukaryotic cell during early days of evolutionary history. The reasons in favour of this hypothesis are the resemblances between their ribosomes, DNA and structure of membranes.
6. What are Plastids? What are the Types of Plastids? Explain the Structure and Function of Chloroplast.
The term plastid was coined by A.F.W. Schimper (1885). Schimper and Meyer (1883-85) is covered these organelles. These are the second largest organelle of the cell and are scattered in the cytoplasm of all green plants. They are not found in blue-green algae, fungi and bacteria.
The plastids are of three types, viz.:
1. Leucoplasts
2. Chromoplasts
3. Chloroplasts
One form of plastid can change into the other.
1. Leucoplasts:
These are the plastids without any pigment and are chiefly concerned with food storage. These are found in embryonic cells, gametes, meristematic regions, seeds and in underground parts.
They are of the following three types:
(i) Amyloplasts:
These store starch. They are found in tubers, cotyledons and endosperm.
(ii) Elaioplasts:
These store fats and oils; for example, in seeds.
(iii) Proteinoplasts or Aleuronolasts:
They store proteins. These are abundant in cotyledons of pulses.
2. Chromoplasts:
These contain pigments of various colours, e.g., carotenoids and xanthophylls.
3. Chloroplasts:
These are the most important plastids found in almost all plants except parasitic plants.
Shape:
Their shape is usually ovoid, discoid or ellipsoid in higher plants. In lower plants the chloroplasts have very unusual shape and size. In Spirogyra chloroplast is ribbon shaped, in Oedogonium it forms a net-work, in Desmids and Zygnema the chloroplasts are like radiating platelets, in Chlamydomonas it is cup shaped, in Ulothrix girdle shaped and in Anthoceros spindle shaped.
Size:
The size of chloroplast is variable but on an average its diameter ranges between 4-6 µm and the length between 90-100 µm. These are relatively larger in polyploid plants and sciophytes.
Number:
Normally, there are 20-50 chloroplasts in a plant cell but in algae just one chloroplast may be present in a cell.
Ultrastructure:
Chloroplast is a double membrane bound organelle. Each membrane is 60 Å thick. The space between outer and inner membranes is called as periplastidial space (100-300 Å). The inner space is filled with a granular and transparent fluid known as stroma or matrix. The stroma contains fat globules, starch grains, osmiophilic granules, pyrenoids, and enzymes of dark reaction. The matrix also contains RNA, DNA and 70S ribosomes.
In the stroma, a characteristic system of lamellae is present. These are made up of unit membrane bound structures called thylakoids (100-300 Å wide) which are stacked over one another. One such stack is called as a granum. A granum may comprise 50-100 thylakoids. In a chloroplast usually 40-60 grana are found. The grana are interconnected by intergrana lamellae (also called as stroma lamellae or the frets).
On the inner surface of thylakoid membranes particles of 185 Å lengths, 150 Å widths and 100 Å thicknesses are present. These are called “quantasomes”. These were discovered by Park and Biggins (1963). These are the smallest photosynthetic units capable of carrying out photochemical reaction.
Chloroplast is a semiautonomous organelle, due to presence of DNA, RNA and ribosomes. Chloroplast is capable of synthesising some of its proteins required for integrity of thylakoid membranes. The DNA of chloroplast is responsible for cytoplasmic inheritance and dividing ability of chloroplasts. Due to all these facts, chloroplast, like mitochondria, is said to be semiautonomous organelle.
Functions of Chloroplast:
In the presence of light, carbon dioxide and water chloroplasts manufacture organic food for the plants by the process of photosynthesis. The food prepared by plants is then made available to heterotrophs. The entire process of photosynthesis is completed in two steps- viz.; light reaction and dark reaction. The light reaction takes place in grana which trap the solar energy and store it as chemical energy. During dark reaction, which occurs in stroma, this energy is utilised to combine CO2 and water to build carbohydrates.
Another important function of chloroplasts is to manufacture ATP by the process of photophosphorylation.
7. What is Endoplasmic Reticulum? What are the types and functions of it?
Meaning of Endoplasmic Reticulum:
The endoplasmic reticulum is an extensive network of vesicles and tubules in the cytoplasm. It is more concentrated in the inner region of the cytoplasm than in its peripheral region hence the name endoplasmic reticulum. It was discovered by Porter (1945). It is found in most of plant and animal cells, except mature RBCs, prokaryotes and blue green algae.
The endoplasmic reticulum has the typical unit membrane structure having a thickness of 50 – 60 Å. It exists in three main forms in different cells, depending upon their metabolic state.
These forms are as follows:
1. Cisternae (Lamellae):
These are elongated and unbranched tubules arranged in parallel bundles. They may be interconnected with each other. This form of endoplasmic reticulum is characteristic of cells which are actively involved in protein synthesis.
2. Tubules:
These are small, smooth walled and branched structures of different sizes and shapes. These are characteristic of non-secretory cells, e.g., developing spermatids, muscle cells, etc., and are mainly concerned with storage and transport of steroid hormones, cholesterol, glycerides, etc.
3. Vesicles:
They are large, rounded or irregular structures of smooth membrane. They are abundant in synthetically active cells, e.g., Liver cells, pancreatic cells, developing spermatocytes, etc.
Types of Endoplasmic Reticulum:
There are two distinct morphological types of endoplasmic reticulum, viz.:
1. Rough Endoplasmic Reticulum (RER):
It is called as rough or granular because the membranes are covered with ribosomes giving them a rough appearance. Ribosomes are attached to the membranes through their larger subunit (60S) by a specific glycoprotein called as ‘ribophorin’. The RER is more stable and is predominantly found in those cells which are actively engaged in protein synthesis, e.g., the enzyme secreting cells.
2. Smooth Endoplasmic Reticulum (SER):
In this type, the membranes do not bear ribosomes hence appear smooth. They are usually tubular; cisternae are rare. This form of endoplasmic reticulum is less stable. It is characteristic of cells in which synthesis of non-protein substances, phospholipids, glycolipids and steroid hormones takes place, for example in adipose tissue, adrenal cortex, interstitial cells of testis, etc.
When a cell type has abundant SER, it usually has little RER and vice-versa.
Functions of Endoplasmic Reticulum:
1. The endoplasmic reticulum provides mechanical support for the colloidal structure of the cytoplasm.
2. It helps in exchange of materials between nucleus and cytoplasm.
3. It separates the cytoplasm into compartments and maintains the ionic gradients and electrical potential across these compartments.
4. The endoplasmic reticulum may help intracellular circulation of various substances.
5. The RER provides a site for protein synthesis by attaching ribosomes on it.
6. Jones and Fawcett (1966) have shown the presence of drug metabolising and detoxifying enzyme-systems in the endoplasmic reticulum.
7. The SER helps in synthesis and storage of lipids, cholesterol and glycogen.
8. In testis, ovary and adrenal cortex it synthesises steroid hormones.
8. What are Lysosomes? Explain the Ultrastructure and Functions of it.
These are smallest membrane bound organelles. They originate directly from the endoplasmic reticulum or from the Golgi complex. These were discovered by Christian de Duve (1955).
It is universally present in animal cells. It is not found in plant cells and prokaryotes.
Ultrastructure:
These are single unit-membrane bound globular structures filled with enzymes. Their diameter varies between 0.2 to 0.8 µm. The lysosomal membrane is impermeable to substrates of enzymes contained in the lysosome.
The enzymes are kept in an inert condition through electrostatic binding of acid groups in the lipoprotein matrix of membrane.
If the enzymes are released, they can digest the cell itself, hence, the lysosomes are also called as “suicide bags” of the cell. Since most of the lysosomal enzymes function better under acidic conditions, they are collectively termed as “acid hydrolases”.
Functions of Lysosome:
1. Extracellular Digestion:
Lysosomal enzymes are released outside the cell where they digest the substrate.
2. Intracellular Digestion:
It may involve autophagy or heterophagy. During autophagy the lysosomes digest the organelles of their own cell while during heterophagy exogenous materials are broken down. Sometimes both autophagy and heterophagy may occur simultaneously in the same lysosomal vesicle. Such vesicles are called as “ambilysosomes.”
3. Hormone Secretion:
Lysosomes modify the secretory products synthesised by the cell before they are released. For example thyroid hormones are released by hydrolysis of thyroglobulin in the secondary lysosomes. Secretion of prolactin from anterior pituitary is controlled by lysosomes.
4. Fertilisation:
The acrosome of sperm is looked upon as a giant lysosome. It helps in dissolving the egg membrane to facilitate the entry of sperm.
5. Developmental Processes:
Resorption of the tadpole tail and regression of insect larval tissues involves lysosomal acid hydrolases. In mammalian females the involution of uterus and mammary glands immediately after the child-birth involves lysosomes.
6. Malfunctioning of Lysosome:
Malfunctioning of lysosome results in tissue damage and may cause several diseases including some cancers.
9. What is Golgi Complex? Explain the Ultrastructure and Functions of it.
It was discovered by Camillo Golgi (1898) in the nerve cells of barn owl.
In plant cells these are also called dictyosomes.
Ultrastructure:
Electron microscope reveals the presence of three membranous components in it, viz.:
1. Cisternae or Lamellae:
They are flattened; parallel sacs piled one upon the other to form stacks.
The cisternae may be flat but are more usually slightly curved. This gives the whole stack convex and concave faces. There are named them as forming or proximal face and maturing or distal face respectively as new lamellae are formed on the forming face and mature lamellae are lost on the maturing face.
2. The Small Vesicles:
They arise from the cisternae by budding.
3. Tubules:
These are like cisternae, but are highly branched.
Functions of Golgi Complex:
1. General Secretion:
These are involved in extra and intra-cellular secretions.
2. Synthesis of Polysaccharides:
The Golgi complex in the goblet cells of the colon produces mucigen. This secretory material contains a large portion of carbohydrate.
3. Glycosylation:
Addition of carbohydrates to the proteins occurs in the Golgi complex as well as in the rough endoplasmic reticulum as both of them contain the enzyme glycosyl transferase. After completion of glycosylation the glycoprotein is released into the lumen of Golgi cisternae.
4. Sulphation:
Golgi complex takes part in sulphate metabolism. Compounds containing active sulphur are formed in two steps. Sulphate is first activated by ATP then the activated sulphur is transferred to acceptor molecule by sulphotransferases.
5. Plasma Membrane Formation:
Secretory granules originating from the Golgi complex fuse with the plasma membrane. The membrane of the granules becomes incorporated in to the plasma membrane and thus contributes to the renewal of the membrane.
6. Cell-Plate Formation:
Substances like pectin and hemicelluloses, which form the matrix of the cell plate, are contributed by the Golgi complex.
7. Lipid Packaging and Secretion:
Golgi complex provides a membrane for envelopment of lipid, so that it can be released from the cell.
8. Acrosome Formation:
Electron microscopic studies have revealed the derivation of acrosomal membrane from the membranes of Golgi derived vesicles.
9. Lysosome Formation:
Primary lysosomes are formed by the Golgi complex.
10. Neurosecretion:
In many cells, neurosecretory material is synthesised by ribosomes or endoplasmic reticulum, and are packed in Golgi complex.
10. What are Chromosomes? Explain its Structure and Functions.
Meaning of Chromosomes:
All the living organisms have specific characteristics which they transmit to their offspring through successive generations. The characteristics are identified as hereditary traits. These traits are controlled by special units, called as genes, which are borne by the chromosomes. The chromosomes are, thus defined as self-duplicating nuclear filaments having specific organisation and individuality.
Historical Aspects:
W. Fleming (1897) saw deeply stainable thread like material in the nucleus and called it as chromatin. These threads were named chromosomes by Waldeyer (1888). Sutton and Boveri suggested and later proved experimentally that chromosomes were the physical carriers of hereditary characters. Based on this fact they proposed the chromosomal theory of inheritance.
Chromosome Number:
Benden and Boveri (1887) reported that the number of chromosomes is constant for a particular species. The number of chromosomes present in gamete is said to represent one complete set and is called haploid number which is represented by ‘n’. The total number of genes present in a haploid set of chromosomes is known as genome. The somatic cells contain two sets of chromosomes which together represent the diploid number (2n).
Similarly if an individual possesses more than two sets of chromosomes it is said to be polyploid condition (3n, 4n …… and so on). In polyploid individuals the ancestral primitive number is called as base number and is represented as X. For example in the common Triticum aestivum the diploid number (2n) is 42 and haploid number (n) is 21, but its base number (x) is 7 which means that Triticum is a hexaploid (i.e., 2n = 6x).
The minimum number of chromosomes recorded in plants is n = 2 in Haplopappus gracilis (Compositae). The maximum number has been reported from the fern Ophioglossum reticulatum (2n = 1260).
Morphology of Chromosomes:
Size:
The size of chromosomes varies from species to species but is generally specific for a particular species. The chromosome size is generally measured at the mitotic prophase. The chromosomes may be 0.2 to 50 mm in length, for e.g., 3 mm in Drosophila, 5 mm in human beings, 8-12 mm in Zea Mays, 0.25 mm in fungi and 30 mm in Tillium.
Plant chromosomes are usually larger than animal chromosomes and likewise the chromosomes of the monocots are larger than those of the dicots.
Shape:
The shape of the chromosome changes from phase to phase during the continuous process of cell division. During interphase the chromosomes appear as extended fine thread like stainable structures called chromatin threads. However, the shape and structure of the chromosomes can be studied best at the metaphase and anaphase stages of cell division because at these phases the chromosomes contract to the maximum. They
may be rod shaped, J-shaped, L-shaped or V-shaped, depending on position of the primary constriction (centromere) along the length of a chromosome.
Structure:
During interphase, the stained chromosome appears as a thin and coiled filament, composed of chromatin. This filament was named as chromonema by Vejdovsky (1912). Sometimes chromonema and chromatid are used synonymously, but actually these are different. A chromatid refers to one half of the chromosome which is connected at the centromere, while the chromonema represents thread like structures constituting respective chromatids.
Earlier it was thought that chromonema remain embedded in a Paranemic coils amorphous matrix which in turn is covered by a very thin chromosomal sheath or pellicle. However, the electron microscopic studies have not confirmed the presence of matrix and pellicle. The chromonema may be composed of two or more fibres depending on the species.
These fibres remain coiled with each other forming either paranemic or plectonemic coils. In paranemic coiling the coils of the chromonemal fibres are easily separable but in the plectonemic coiling the chromonemal filaments remain so intimately coiled that they cannot be separated easily.
The following parts are distinguished in a condensed chromosome:
1. Primary Constriction:
Each chromosome has a non-stainable region at a specific point along its length. This region is called primary constriction.
2. Centromere:
Within the primary constriction, there is a clear central zone called centromere. This is the point of attachment of the sister chromatids and also the site of attachment of the mitotic spindle fibre. The portion of the chromosome on either side of the centromere is called arm of the chromosome. Functionally the centromere is related to the movement of the chromosomes at anaphase. During this movement, depending upon the relative ratio of the two arms, the chromosomes acquire the shape of I, J, L or V.
The centromere is made up of four very small granules arranged in a square. These granules are called centromeric chromomeres which remain connected to the chromatid fibres. The chromosomes of many organisms contain only one centromere. Such chromosomes are called monocentric, those with two or more centromeres are respectively called dicentric and polycentric.
3. Kinetochore:
The kinetochore is a proteinaceous disc attached to the centromeric chromomeres. Two kinetochores, one in each chromatid, are observed. These are centres of assembly for the microtubules at the metaphase.
4. Secondary Constriction I:
It is also called as nucleolar organiser. The part of the chromosome beyond the secondary constriction is called as satellite or trabant. The chromosomes having a satellite are called as SAT chromosome. SAT stands for “sine acid thymonucleinico”, means absence of thymonucleic acid in this part. It contains genes for synthesis of ribosomal RNAs.
5. Secondary Constriction II:
One or more additional constrictions called secondary constriction II may also be present on the chromosome. Their position is fixed hence these are useful in identifying a chromosome in a set.
6. Telomere:
Tips of the chromosomes containing heterochromatic material or repetitive DNA sequences are called telomeres. Each telomere has definite polarity. It does not allow other chromosomes to stick with it or its union with the broken ends.
According to position of centromere, following types of chromosomes are identified:
(i) Telocentric:
Their centromere is situated at one end. At anaphase, it looks like ‘I’.
(ii) Acrocentric:
Their centromere lies almost near the tip of chromosome so that one arm is exceptionally short and the other is long. At anaphase, these chromosomes look like ‘J’.
(iii) Submetacentric:
In this type of chromosome the centromere lays a little away from the centre, dividing the chromosome into two unequal arms. Such chromosome looks like ‘L’ at the anaphase.
(iv) Metacentric:
In this type of chromosome, the centromere lies in the middle of the chromosome, dividing it into two equal arms. The metacentric chromosome becomes V-shaped during anaphase.
Chemical Composition and Models of Chromosomes:
The major constituents of the chromosomes include DNA, RNA, histone and non-histone proteins and metal ions. It carries the genetic information from one generation to other. The RNA is transcribed by DNA and most of it is transported to the cytoplasm. The histone proteins present in the chromosomes are basic proteins that are composed of basic amino acid such as lysine and arginine.
These remain associated with the DNA and act as repressors of gene activity. The non-histone proteins are mostly acidic and act as enzymes, important among them are DNA polymerase, RNA polymerase and nucleoside triphosphatase. Besides these metallic ions such as Mg++, Ca++, etc. keep them intact and also act as regulators of various enzymes.
Models of Chromosome Structure:
Various models showing the mode of attachment between DNA and proteins in a chromosome have been proposed, among which the nucleosome model is most accepted one.
Nucleosome Model:
A.L. Olins and D. E. Olins (1974) reported the presence of a series of bead like structures in electron micrographs of interphase chromatin fibres. These particles were called as nu (h) bodies. Later Outdet (1975) called them as nucleosome. R.D. Kornberg and Thomas discovered that each spherical unit representing the core is composed of 140 base pairs of DNA and a histone octamer having two molecules of each of four different histones- viz., H2A, H2B, H3 and H4.
The histone H1 is loosely associated with the chromatin. Around the core particle of nucleosome, DNA molecule is wrapped 1.75 times. The complete nucleosome is a flattened particle of 55 Å in height and 110 Å in diameter. The core particle made up of histone octamer is 40 Å high and 80 Å wide. The beaded nucleosomes are interconnected by DNA filaments called linker DNA. Their length may vary from 8 to 114 nucleotide base pairs.
The nucleosomal beads are further coiled to form a super-coiled structure called as solenoid.
Functions of Chromosomes:
The chromosomes are most vital component of the cell. These control almost all cellular activities at physiological, molecular and morphological levels.
Besides these, they perform the following main functions:
1. The chromosomes maintain the identity of species.
2. These determine the sex of species of animals and plants.
3. These act as a vehicle of hereditary characters from one generation to other.
4. With the help of their chemical constituents, the DNA and RNA, they synthesize proteins and enzymes.
5. The lampbrush chromosomes synthesize yolk in oocytes of many vertebrates.
11. What is Nucleus? When was it Discovered? Explain the Structure of it.
Discovery of Nucleus:
Nucleus was discovered by Robert Brown (1833). It regulates overall activities of an individual cell. Although universally present in all kinds of cells, well organised nucleus is generally absent in bacteria and blue-green algae. The nucleus may even be absent in some cells, e.g., mature RBCs and sieve tubes of phloem.
Shape:
The shape of nucleus is variable. The most usual shape of nucleus is spherical, ellipsoid or discoid.
Number:
Generally cells are uninucleate. Some cells may contain two nuclei, e.g., Paramecium, hepatocytes and chondrocytes. In certain cells many nuclei may be present. These are called multinucleate cells for example Opalina (a ciliate), Vaucheria (an algae) and polykaryocytes of bone marrow.
In animals, the multinucleate cells are called as syncytial cells. These are formed due to loss of cell boundaries during the course of development (e.g. epidermis of Ascaris). The multinucleate cells of plants represent a coenocyte which is usually formed due to rapid nuclear division without cytokinesis.
Structure of Nucleus:
The nucleus is made up of the following structures:
1. Nuclear Envelope:
Nucleus is externally surrounded by a nuclear envelope or nuclear membrane or karyotheca. It is composed of an outer and an inner membrane. The two membranes are separated by a perinuclear space. Sometimes the outer nuclear membrane shows structural continuity with the membranes of endoplasmic reticulum.
The inner nuclear membrane is internally supported by a fibrous structure of uniform thickness (300 Å), which is known as the fibrous lamina or internal dense lamella. The nuclear envelope is perforated by many circular or octagonal pores; around their margins the outer and inner nuclear membranes are continuous.
The nuclear pores are plugged by hollow cylindrical structure called as annuli. The pore and annulus together form a pore complex which is an octagonal structure. The nuclear pores are the pathways for the exchange of macromolecules between cytoplasm and nucleus which is facilitated by a specific protein nucleoplasmin.
2. Nucleoplasm:
The nucleus is filled with a transparent, semi-solid jelly like granular and acidophilic ground substance or matrix known as nucleoplasm, karyolymph, karyoplasm or nuclear sap. Chromatin fibre and nucleolus are suspended in it. Main constituents of chromatin fibers are nucleic acids (DNA and RNA) and nucleoproteins (e.g., histones).
Chromatin Fibres in Nucleus:
These can be seen only during interphase. During cell division these threads become thick and are then called as chromosomes. On the basis of their staining properties two types of chromatin materials have been identified, viz., heterochromatin and euchromatin (by Emil Heitz).
(i) Heterochromatin:
These are dark staining and condensed regions of chromatin. It contains relatively small amounts of DNA and large amount of RNA. Therefore, it is supposed to be metabolically and genetically inactive.
Heterochromatin is of two types, viz., constitutive and facultative. Constitutive heterochromatin is found in those regions of chromosomes which are proximal to centromere and are constant, thus, serve as chromosome markers. Constitutive DNA is highly repetitive and it was originally called as satellite DNA. Facultative heterochromatin is represented by sex chromosomes which becomes heterochromatic only at certain stages.
(ii) Euchromatin:
It is the light stained and diffused region of the chromatin. It contains relatively larger amounts of DNA and is genetically as well as metabolically active.
Nucleolus:
Nucleolus was discovered by Fontana (1781). Nucleolus is attached to a specific region of a particular chromosome. This region is called as nucleolar organiser or secondary constriction. The nucleolus is even synthesised at this region. It disappears at late prophase and reappears during telophase. It does not have a membrane of its own.
It is made up of following components:
(i) Pars Amorpha:
Pars amorpha or matrix of the nucleolus is homogeneous, several types of granules and fibrils are scattered in it.
(ii) Pars Chromosoma:
It is represented by the perinucleolar chromatin that surrounds the nucleolus like a discontinuous covering from perinuclear chromatin, thread like intranucleolar chromatin projects into the pars amorpha.
(iii) Pars Granulosa:
It is represented by granules of ribonucleoprotein. These granules are like cytoplasmic ribosomes.
(iv) Pars Fibrosa:
It is represented by 50-80 Å long fibrils of ribonucleoproteins.
Functions of Nucleolus:
Nucleolus is one of the most active sites of RNA synthesis. It is the source of ribosomal RNA. Its main function, therefore, is the formation of ribosomes.
Functions of Nucleus:
The nucleus acts as ‘brain’ of the cell.
It performs the following important functions:
1. It co-ordinates activities of different parts of the cell.
2. The DNA of the nucleus regulates protein synthesis by transcription.
3. It transfers parental characters to offspring through chromosomes.
4. It determines sex of offspring.
5. It regulates growth and reproduction.
12. What is the Differences between Heterochromatin and Euchromatin in Nucleus?
Heterochromatin:
1. It stains dark and appears condensed.
2. Amount of DNA is small.
3. It is late replicating, i.e., replicates at the end of the S-phase.
4. It is affected by temperature, sex, age of parents, etc.
5. It is genetically and metabolically less active.
Euchromatin:
1. It is light staining and of diffused appearance.
2. Contains relatively larger amount of DNA.
3. It replicates during the early stage of the S-phase.
4. It is relatively stable and not influenced by these factors.
5. It contains more genes therefore it is more active.
13. What is Centrosome? Explain the Structure Chemical Composition and Functions of it.
The centrosome was discovered by E. Van Beneden (1875). The term centrosome was given by T. Boveri (1888). It is found in algae (except red algae), fungi, bryophytes, ferns, gymnosperms, protozoans and most of the multicellular animals. It is absent in prokaryotes, diatoms, yeast and higher plants. It is situated close to the nuclear membrane during interphase.
Structure:
Each centrosome contains two centrioles associated in a pair. A pair of centrioles is called as diplosome. They are placed at right angles to each other. There is a clear area around the centrioles which is called centrosphere or halo. Centriole and centrosphere are collectively called as the centrosome.
Each centriole is a cylindrical structure of about 300-500 nm long and 150-180 nm wide. The wall of each cylinder is made up of nine triplet microtubules arranged around a central axis. Each triplet is made up of three microtubules which are designated ‘A’, ‘B’, and ‘C’, the innermost tubule being labelled as ‘A’. All the nine triplets are embedded in an amorphous matrix. Strands of this matrix extend inward from each tubule and join together in the centre to form a hub like structure. The strands, microtubules and the hub give the appearance of a cartwheel in a cross-section.
Chemical Composition:
The microtubules of centrioles are made up of a structural protein tubulin, alongwith lipid molecules.
Functions:
1. The centrioles form the mitotic poles in higher animals.
2. They serve as the centre for the production of new centrioles and basal bodies.
3. Their main function is the generation of cilia and flagella.
4. In spermatozoa, one centriole of a pair gives rise to the tail fibre.
14. What are Cilia and Flagella? Distinguish between them. Explain their Structure and Functions.
Cilia and flagella are hair like structures projecting from the free surface of several plant and animal cells.
There is little difference between cilia and flagella, however, both can be distinguished from each other on the basis of following features:
Flagella:
1. Less in number (1-2).
2. Occur at one end of the cell.
3. Relatively longer (approx. 150 µm).
4. Usually beat independently.
Cilia:
1. More in number (3000 or more).
2. Distributed uniformly all over the cell surface.
3. Shorter (5-10 µm).
4. Beat in a co-ordinated manner.
Structure:
The cilia (or flagella) consist of a basal body from which a shaft arises. The shaft consists of a framework called as axoneme. The axoneme is surrounded by membrane which encloses a matrix into which the other components are embedded. Each axoneme contains eleven fibrils (made up of tubulin proteins) running longitudinally. Of these eleven fibrils, two are situated in the centre and the remaining nine are arranged peripherally. It gives rise to the characteristic 9 + 2 pattern.
Each central fibril is made up of single microtubule whereas each peripheral fibril is made up of two ellipsoid microtubules (doublets) termed subfibril A and B. From subfibril ‘A’ two small arms arise, the outer arm terminates in a hook like bend and the inner remains connected with the subfibril ‘B’ of the adjacent fibril. The arms of all doublets are oriented in a clockwise manner. They are made up of a protein dynein.
The central fibrils are enclosed in a common central sheath which gives out nine radially oriented spokes to each subfibril ‘A’.
The flagellum of bacteria is different from that of eukaryotic cells. It does not show 9 + 2 pattern and is made up of a single fibre. The fibres are cylindrical and composed of a fibrous protein, flagellin.
Functions of Cilia/Flagella:
(i) Help in the locomotion of the cell or the organism.
(ii) Cilia create water current in lower organisms which draw the food in.
(iii) In respiratory tract, ciliary movements help in removal of particulate matter from it.
(iv) In amphibians and mammals, the ova are pushed out of the oviducts by the beating action of cilia.
15. What are Ribosomes? What are the Types and Functions of it.
The ribosomes were first observed in the cells of bean roots by Robinson and Brown (1953), shortly afterwards Palade (1955) observed them in animal cells.
The ribosomes are the smallest organelles of the cell having diameter of 150 – 250 Å. They are found either freely in the cytoplasmic matrix, or remain attached with the membranes of the endoplasmic reticulum and nucleus. They are also found in the matrix of mitochondria and chloroplast. In prokaryotic cells the ribosomes occur freely in the cytoplasm.
Ribosomes are found abundantly in the cells which are actively involved in protein synthesis. In a cell there may be about 20,000-30,000 ribosomes. However, their number changes according to the rate of protein synthesis in the cell.
Types of Ribosomes:
Ribosomes are generally classified in two types on the basis of their sedimentation co-efficient which is expressed in terms of Svedberg units (S).
1. 70S ribosomes:
These are found in prokaryotes, mitochondria and chloroplast. Their sedimentation coefficient is 70 S Dalton. It is made of a large subunit of 50 S and a smaller subunit of 30 S.
2. 80 S Ribosomes:
These occur in the cytoplasm of eukaryotic cells. Their sedimentation constant is 80 S. The larger subunit of these ribosomes is 60 S and the smaller is 40 S.
The two ribosomal subunits usually remain separate. Their association to form complete 80S ribosomes requires Mg++ concentration of 0.001 M. If this concentration is lowered, the subunits dissociate again. If Mg++ concentration is increased ten times (0.01 M), two ribosomes become associated with each other to form a dimer having the sedimentation coefficient of 120S. During protein synthesis many ribosomes becomes aggregated linearly on a common mRNA to form polyribosome or polysomes or ergosomes.
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Functions of Ribosomes:
Ribosomes take part in protein synthesis. They function as templates bringing together different components involved in the synthesis of proteins.
Ribosomes also have a protective function. They protect the mRNA from ribonucleases. They also protect the newly synthesised polypeptide chain passing through the tunnel in ribosomes.
16. What are the Types of Microbodies Found in Cell?
When improved methods of cell fractionation were available, in addition to lysosomes, certain other membrane bound vesicles (0.2 to 1.2 µm diameter) were discovered in the cytoplasm. These appear as granular bodies filled with a dense matrix and bound by a single membrane (60 – 80 Å thick). These were collectively called as microbodies.
On the basis of biochemical studies following types of microbodies are distinguished:
1. Peroxisomes:
These were isolated from animal cells by Rhodin (1954). In plant cells, they were first observed by Tolbert (1969). The term ‘peroxisomes’ was coined by Beaufay and Berther (1963) because they showed significant peroxidative activity. The enzymes present in peroxisomes are catalase and peroxidases (e.g., amino acid oxidase, urate oxidase and α-hydroxy acid oxidase).
Function:
The peroxisomes oxidise a variety of substrates in a two-step reaction. In the first step, substrates like uric acid, amino acids and lactic acid are oxidised by molecular oxygen to form hydrogen peroxide by the specific peroxidase.
2. Glyoxyomes:
These were discovered by Beevers (1961). They occur in the cells of yeasts, Neurospora, germinating and oil rich seeds of many higher plants, e.g., castor (Ricinus communis). Their structure resembles that of peroxisomes except that their crystalloid core consists of dense rods of 6.0 µm diameter.
Function:
Glyoxysomes contain enzymes of a β-oxidation of fatty acid, and for glyoxylate cycle. In the seeds of plants β-oxidation occurs in glyoxysomes alone but in other parts of the plants β-oxidation occurs in glyoxysomes and mitochondria. Acetyl CoA, the end product of β-oxidation is converted through enzymes of glyoxysomes into carbohydrate. It is thus involved in gluconeogenesis. The enzymes found in glyoxysomes are isocitratase, malate synthetase, aconitase, glycolate oxidase and catalase.
3. Spherosomes:
They were discovered by Dangered in 1919 in plants. These are very small (0.5 to 1.0 µm diameter), spherical bodies enclosed by a single unit membrane. They are homologous to lysosomes because they contain acid hydrolases, acid ribonucleases, acid phosphatases and acid esterases like lysosomes. But they differ from lysosomes in the absence of some characteristic enzymes, e.g., lipase and Aryl sulphatases A and C. Their main function is synthesis and storage of lipids.
