Assignment Code: BBCCT-103 / TMA / 2021 -2022

Course Code: BBCCT-103

Assignment Name: Cell Biology

Year: 2021 - 2022

Verification Status: Verified by Professor

Note: Attempt all questions. The marks for each question are indicated against it.

For any question worth 2 marks, the word limit is 50-100 words, for 5 marks question it is 200-250 words; for 10 marks it is 350-400 words.

PART-(A)

1. Fill in the blanks with correct words: [10]

i) The most primitive organism on earth is................

Ans) The most primitive organism on earth is procaryotes.

ii) Bright field microscope forms a bright image against.............................

Ans) Bright field microscope forms a bright image against a darker background.

iii) Sample tubes fixed at angle 14º-40º degree in.......................................

Ans) Sample tubes fixed at angle 14º-40º degree in the Rotor.

iv) Ribosome biogenesis and ribosomal RNA are synthesized in…………

Ans) Ribosome biogenesis and ribosomal RNA are synthesized in the nucleous.

v) Synthesis of membrane lipid take place in ..........................................

Ans) Synthesis of membrane lipid take place in the endomembrane compartment.

vi) Lysosomal hydrolases work best at .......................... pH.

Ans) Lysosomal hydrolases work best at 5 pH.

vii) Chitin is the structural constituent of…………........

Ans) Chitin is the structural constituent of fungi cell walls mainly glucans.

viii) Microtubules are polar structures because...............................

Ans) Microtubules are polar structures because all protofilaments in a microtubule have the same orientation, one end of a microtubule is ringed by α-tubulin, while the opposite end is ringed by β-tubulin.

ix) The diameter of microfilaments is......................

Ans) The diameter of microfilaments is 3-6 nm in diameter.

x) .............................is the major component of ECM.

Ans) Protein Collagen  is the major component of ECM

2. (a) Explain the structural organisation of bacteria. [5]

Ans) Bacteria (singular: bacterium) are prokaryotes, which are single-celled organisms that lack a nucleus and carry DNA that floats freely in a twisted, thread-like mass called the nucleoid or in distinct, circular portions called plasmids. Ribosomes are spherical units in bacterial cells that construct proteins from individual amino acids with the help of information encoded in ribosomal RNA.

Bacterial cells are protected by two layers of protection: an exterior cell wall and an inner cell membrane. Some bacteria, such as mycoplasmas, have no cell walls at all. A third, outermost protective layer termed the capsule may exist in some bacteria. Bacteria commonly have whip-like extensions on their surfaces, either long flagella or short pili, that help them move around and connect to a host.

Q2. (b) Differentiate between gram positive cell wall and gram negative cell wall in bacteria. [5]

Ans) Depending on the staining method, bacteria might be gram-positive or gram-negative. The thickness of the peptidoglycan layer and the presence or absence of the outer lipid membrane are the two essential factors that cause Gram positive and Gram negative organisms to have different visibility properties. This is due to the cell wall's capacity to hold the crystal violet stain used in the Gram staining method, which can then be seen under a light microscope. Gram negative bacteria have a thin peptidoglycan layer and an outside lipid membrane, whereas ram positive bacteria have a thick peptidoglycan layer and an outer lipid membrane. Gram-Positive Bacteria have a single-layered cell wall that is smooth. Gram-Negative Bacteria have a double-layered, wavy cell wall.

When referring to their structure rather than their staining properties, Gram positive bacteria are called monoderms because they lack an outer lipid membrane. Gram negative bacteria have an exterior lipid membrane, which is why they are called diderms when referring to their physical structure.

Following are the important differences between gram-positive and gram-negative bacteria:

1. The cell wall of gram-positive bacteria is composed of thick layers peptidoglycan.

2. The cell wall of gram-negative bacteria is composed of thin layers of peptidoglycan.

3. In the gram staining procedure, gram-positive cells retain the purple coloured stain.

4. In the gram staining procedure, gram-negative cells do not retain the purple coloured stain.

5. Gram-positive bacteria produce exotoxins.

6. Gram-negative bacteria produce endotoxins.

3. (a) Discuss the different types of rotors? [5]

Ans) Rotors are classified according to their type, sample volume capacity, tube count, and maximum speed. Based on the position of tubes relative to the axis of rotation, there are three types of centrifuge rotors.

The following are the details:

1. Fixed-angle rotor: The tubes containing the sample are put in the apertures in the rotor that are at a set angle relative to the rotation axis in a fixed angle rotor. Regardless of rotor speed, the tubes remain in that location during the run. The sample tube reorients to the axis of rotation when the rotor starts to revolve, and particles migrate towards the tube's wall and deposit asymmetrically at the tube's bottom. Before pelleting, the sedimenting particles must travel a short distance. Fixed angle rotors can contain more tubes and tolerate significantly higher gravitational forces than swinging bucket rotors, making them the most extensively utilised rotor type. Differential centrifugation and density gradient centrifugation can both benefit from a fixed-angle rotor.

2. Swinging-bucket rotor: The sample containers (buckets) are linked to the rotor body by hinge pins on a swinging bucket or horizontal rotor. When force is applied, the tubes swing out from vertical to horizontal, and the containers are at a 90o angle relative to the angle of rotation and parallel to the applied centrifugal field. As the centrifuge slows down, the bucket returns to its original upright position. At the tube's bottom, the pelleted material is symmetrically dispersed. Draining the supernatant without disturbing the particle is easier. This rotor is designed to separate huge volumes of samples (up to 12 L) at low speeds. The primary application of a swinging-bucket rotor is rate-zonal centrifugation.

3. Vertical rotor: A vertical rotor is one that is positioned vertically. They're zero-angle rotors, which means the tubes are always vertically aligned in the rotor's body. They were first used in high-speed and ultra-centrifuges in the 1970s. They're ideal for rate zonal centrifugation and isopycnic centrifugation. Because the pellet tends to come off as the liquid is decanted, they are not suitable for pelleting.

Q3. (b) What is Electron microscopy? Explain its role in cell biology. [5]

Ans) The method of electron microscopy (EM) is used to create high-resolution images of biological and non-biological specimens. It's utilised in biomedical research to look into the structure of tissues, cells, organelles, and macromolecular complexes in great detail. The employment of electrons as a source of illuminating radiation leads in the great resolution of EM pictures. To address specific problems, electron microscopy is utilised in concert with a range of supplementary techniques. EM pictures reveal important details on the structural basis of cell function and illness.

The highest-resolution limit of a spectrum of complimentary morphological techniques is electron microscopy. EM is the only technique with adequate resolution to localise proteins to tiny membrane subdomains in the context of the cell when paired with molecular detection methods. The power of EM as a cell-biological tool has increased as a result of recent procedural and technical advancements.

The transmission electron microscope (TEM) is widely used in cell and molecular biology research for histological analysis of cells and tissue, drug research, disease diagnosis, and the study of subcellular organelles like the nucleus, mitochondria, and endoplasmic reticulum, as well as biomolecules.

Because biology is primarily concerned with the study of cells (and their contents), genes, and all creatures, the microscope is essential. Some organisms are so minute that they can only be seen with magnifications of 2000 to 25000, which are only possible with a microscope. The smallest cells can't be seen with the naked eye.

Q4. Describe the structure and function of Golgi Body. [10]

Ans) The Golgi apparatus is a membrane-bound intracellular organelle that plays a crucial role in the trafficking, processing, and sorting of newly generated membrane and secretory proteins and lipids. Golgi membranes have a unique layered structure that allows them to accomplish these functions optimally. The Golgi structure is dynamic but strictly regulated; it disassembles and reassembles rapidly during the cell cycle of mammalian cells, and it is disturbed under stress and pathological situations. Over the last decade, a lot of work has gone into figuring out the molecular mechanisms that control Golgi membrane design and function. To improve our understanding of Golgi structure and function in health and disease, we review the important discoveries in the mechanisms of Golgi structure development, control, and modification in connection to its functions in physiological and pathological circumstances.

Function of Golgi Body

Membranes inside the cell's cytoplasm form a stack of little flat sacs (gel-like fluid). Proteins and lipid (fat) molecules are prepared for utilisation in other parts of the cell by the Golgi complex. A cell organelle is the Golgi complex. Also known as the Golgi apparatus or the Golgi body. The Golgi complex is made up of stacked membrane-enclosed cisternae. It is involved in the N-glycosylation of proteins at their terminals as well as the O-glycosylation of proteins and glycolipids. Protein sorting is the main function of the Golgi body. It collects secretory proteins from the ER and separates them into vesicles that transport them to their eventual destination.

Golgi body mainly involved in the packaging of materials for export:

Explanation:

1. Golgi body mainly involved in the packaging of materials for export from the cell across plasmalemma by process of reverse pinocytosis. The packaging materials differ in plants and animals.

2. In Plants-The parts of the substances of the primary and secondary cell wall etc., are packed.

3. In Animals-The zymogen of pancreatic cells, mucous secretions, compounds of thyroxine etc., are packed.

Q5. Write short notes on the following: [5+5=10]

Q5. (a) Gap Junction

Ans) Gap junctions are unique intercellular connections that connect a variety of different animal cell types. They connect the cytoplasm of two cells directly, allowing numerous chemicals, ions, and electrical impulses to move directly between cells through a regulated gate.

Gap junctions also allow cells to share important metabolites including ATP, sugar phosphates, amino acids, and many coenzymes, allowing them to collaborate metabolically. This is especially crucial in tissues that lack blood arteries, such as the lens and cornea of the eye. Two protein hexamers (or hemichannels) called connexons in vertebrates and innexons in invertebrates make up a gap junction channel. The hemichannel pair bridges the gap between two cells by connecting across the intercellular space. Gap junctions are similar to plasmodesmata, which connect plant cells.

With the exception of fully formed adult skeletal muscle and mobile cell types such as sperm or erythrocytes, gap junctions can be found in practically all tissues of the body. Simpler species, such as sponges and slime moulds, lack gap junctions.

Q5. (b) Actin filament

Ans) Actin filaments (F-actin) are linear polymers of globular actin (G-actin) subunits that appear as microfilaments in the cytoskeleton and as thin filaments in muscle and non-muscle cells' contractile machinery (see contractile bundles). They usually lie beneath the plasma membrane and are built at the cell's periphery from adhesion sites or membrane extension sites. Actin filaments can form a variety of linear bundles, two-dimensional networks, and three-dimensional gels, and actin binding proteins can alter the filaments' unique structure.

Under the plasma membrane, actin filaments form a network that provides mechanical support, controls cell shape, and facilitates movement of the cell surface, allowing cells to migrate, ingest particles, and divide.

PART-(B)           

 Maximum marks: 50

Q6. State whether the following statements are true (T) or false (F). If false, point out the error: [10]

i) The signal sequences comprise of 13-36 amino acids. [ ]

Ans) True.

ii) SRP is a docking protein. [ ]

Ans) True.

iii) N-glycosylation is initiated in the ER and is completed in the ER. [ ]

Ans) False. The process is initiated co-translationally in the ER, but it continues by way of trimming and addition reactions both in the ER and Golgi apparatus.

iv) COPI coated vesicle drives the anterograde vesicle transport. [ ]

Ans) True.

v) ATP is the energy source for nuclear transport. [ ]

Ans) False. The import of nuclear proteins through the pore complex concentrates specific proteins in the nucleus, thereby increasing order in the cell, which must consume energy. The energy is thought to be provided by the hydrolysis of GTP by the monomeric GTPase Ran.

vi) Hsp100 is a chaperone that helps to maintain proteins in an unfolded form. [ ]

Ans) True.

vii) The surface area and volume of a cell increases with increasing cell size. [ ]

Ans) True.

viii) The chromosomes become highly decondensed and relaxed in G2 phase. [ ]

Ans) False. The process of chromosome condensation begins at this stage and continues till metaphase.

ix) Cyclins are activated and degraded during cell cycle [ ]

Ans) True.

x) The p53 protein regulates both cell cycle progression and programmed cell death. [ ]

Ans) True

Q7. Describe the vesicular fusion. [10]

Ans) The fusion of a vesicle with other vesicles or a portion of a cell membrane is known as vesicle fusion. It is the final stage of secretion from secretory vesicles, in which their contents are ejected from the cell by exocytosis. Other target cell compartments, such as a lysosome, can also fuse with vesicles.

Maintaining the diverse properties of the many eukaryotic organelles requires the targeting of different classes of transport vesicles to their specific membrane destinations. Because coat proteins like as clathrin are prevalent in distinct trafficking routes, other coat proteins must identify the direction of transport and eventual destination of a certain vesicle.

The interaction and fusion of the vesicle and target membranes is a multistage process. Membrane recognition is the initial step. Tethering is the following step, which is a loose interaction. The resulting interaction, known as docking, pushes the opposing membranes much closer together. Membrane fusion occurs as a result of docking.

Thus, targeting specificity might be thought of as a series of events that includes:

1. specification of the vesicle delivery site;

2. the recruitment of components capable of initiating vesicle ‘capture’;

3. the formation of a bridge between the vesicle and the target membrane;

4. conformational change that allows the vesicle and target membrane proteins to come close enough to interact;

5. dissociation of the tethering proteins, to free them for another round of transport.

TRAPPs are proteins that are involved in the tethering process (transport protein particles). TRAPP (Mr 1,100,000) is a ten-subunit complex that is required for vesicle trafficking. TRAPP proteins are membrane-associated proteins that are extremely conserved. Surface molecular markers on all forms of transport vesicles aid in distinguishing the vesicles according to their origin and payload. Complementary receptors on the target membrane must also recognise them. SNARES and Ras associated binding (Rab) proteins are thought to be involved in this recognition. Each organelle is thought to have its own SNARE. These SNAREs appear to have two main functions: one is to promote membrane fusion and the other is to help ensure membrane fusion specificity.

Q8. (a) Differentiate between O- glycosylation and N-glycosylation. [5]

Ans) The attachment of an oligosaccharide sugar molecule to a nitrogen atom in the asparagine residue of a protein molecule is known as N glycosylation or N-linked glycosylation. Glycan is another name for this sugar molecule. It is connected to the nitrogen atom of the asparagine residue's amide group. Furthermore, several eukaryotic proteins rely on this linking process for both structure and function. Furthermore, this mechanism occurs in eukaryotes, archaea, and bacteria rather rarely.

The attachment of a sugar molecule to the oxygen atom of serine or threonine residues in a protein molecule is known as O glycosylation or O-linked glycosylation. This is a post-transitional change that occurs after the protein is synthesised. Both eukaryotes and prokaryotes can go through this procedure. N glycosylation occurs in the endoplasmic reticulum, Golgi apparatus, and occasionally in the cytoplasm in eukaryotes, but only in the cytoplasm in prokaryotes.

The key difference between N glycosylation and O glycosylation

Asparagine residues are N glycosylated, whereas the side chains of serine and threonine residues are O glycosylated.

Glycosylation is the regulated addition of a sugar molecule to an organic molecule, such as a protein, by an enzyme. It's a crucial metabolic activity and a tightly regulated secondary protein processing mechanism within cells.

The biochemical processes of N glycosylation and O glycosylation are crucial. The main distinction between N and O glycosylation is that N glycosylation takes place in asparagine residues, whereas O glycosylation takes place in the side chain of serine or threonine residues. N glycosylation is mostly found in eukaryotic creatures and archaea, whereas O glycosylation is only seen in prokaryotic organisms.

Q8. (b) Explain how proteins import to the peroxisomes. [5]

Ans) Peroxisomes are uncommon among single-membrane organelles in that they may directly import both their matrix and membrane proteins from the cytoplasm. Nuclear-encoded peroxisomal matrix proteins are produced on free polyribosomes [3]. Peroxisomal matrix proteins carry particular targeting signals that are detected by receptors that bring them to the membrane translocation site, just as other targeting and translocation systems. The consumption of energy in the form of ATP is required for translocation. Peroxisomal protein transport differs from other translocation mechanisms in several ways. Peroxisomal proteins, for example, can fold, acquire cofactors, and assemble into oligomers in the cytosol prior to import; targeting receptors, on the other hand, bind cargo in the cytosol but imbed into or cross the membrane as part of their targeting cycle.

Nuclear encoding is also found in peroxisomal membrane proteins (PMPs). The method of their import into peroxisomes, on the other hand, is less well understood. PMPs and matrix proteins are known to utilise separate machineries for import. The vast number of proteins necessary for matrix protein import is reflected in the fact that most peroxisome biogenesis mutants have empty membrane structures. Peroxins are the proteins essential for peroxisome biogenesis (PEX). Peroxisomal membrane structures are absent in just two PEX mutants in yeast (and three in mammals).

Q9. (a) Point out the key differences between apoptosis and necrosis. [5]

Ans) Cell death in multicellular organisms is mediated by two mechanisms: apoptosis and necrosis. Apoptosis is a physiological process that occurs naturally, whereas necrosis is a pathological process triggered by external agents such as poisons, trauma, and infections. Necrosis is an unregulated, random process, whereas apoptosis is a highly regulated, timed process. Necrosis is characterised by inflammation and tissue destruction.

Necrosis is a type of cell injury that occurs when cells die uncontrollably as a result of internal or external stimuli such as mechanical injuries, chemical agents, or viruses. Because of the loss of osmotic pressure, the process is usually quick, resulting in cell enlargement (oncosis) and bursting.

The main distinction between apoptosis and necrosis is that apoptosis is a predetermined cell suicide in which the cell actively destroys itself in order to maintain normal body function, whereas necrosis is an unintentional cell death caused by uncontrollable external factors in the cell's external environment.

Q9. (b) Discuss the characteristics of subdivision of Interphase. [5]

Ans) The absence of sight of the chromosome is the most well-known interphase feature. Because nuclear DNA is loosely organised in chromatin fibres, chromosomes are not visible under light microscopy.

Another tool for better visualising some of the aspects of interphase is fluorescence microscopy.

Interphase Subphases: G1

The first stage of interphase is the first gap phase (G1). This name was given because the cell appears inactive under a microscope. However, several important changes are occurring at the biochemical level.

The cell size is growing. The cell is also acquiring proteins and energy needed for synthesizing chromosomal DNA.

G1 Checkpoint

A G1 checkpoint scans the cellular DNA for damage. This check is mediated by a gene called p53 located on chromosome 17. DNA damage elevates the level and activity of the protein products of the p53 gene.

P53 is called the tumor suppressor gene because a mutation in this gene’s DNA is present in many cancer cells.

The G1 checkpoint is an important step because any DNA damage can be repaired before the error is replicated during the S phase. This is in contrast to DNA damage discovered at the G2 checkpoint. G2 checkpoint errors will be in two copies of the DNA because replication has already occurred.

Naturally, this creates more opportunity for an error in repair to occur since two errors must be fixed instead of only one detected at the G1 checkpoint.

S Phase

DNA synthesis makes the S phase the longest subphase of interphase. The cell synthesizes two identical copies of its chromosomes, creating sister chromatids. These chromatids are joined together by a specific DNA sequence called a centromere.

The centrosome is one of several organelles copied during the S phase. Centrosomes each contain a pair of centrioles. Centrioles create the mitotic spindle that organizes chromosome movement during mitosis.

DNA content doubles at the end of the synthesis phase (n → 2n), but because the chromatids remain attached to each other via the centromere, the number of chromosomes stays the same.

G2 Phase

After DNA synthesis is complete, the G2 phase or second gap phase begins. Again, this refers to the fact that chromosomes are not visible under the microscope. This phase is shorter than G1 and is also when cell growth resumes.

Proteins such as microtubules are produced. The cell also replenishes its energy stores in preparation for mitosis. The G2 checkpoint occurs.

G2 Checkpoint

The G2 checkpoint is a ‘quality control’ check for damaged DNA. Damage must be repaired before the cell can leave the G2 phase to enter mitosis. If the damage to the DNA is too severe, the cell will not be permitted to enter mitosis and instead will undergo a programmed cell death called apoptosis.

This checkpoint also searches for non-replicated DNA. If any portion of DNA that has not been copied is found, the cell is switched into a cycle arrest phase. The cell will remain in G2 until all DNA is copied.

Q10. Write short notes on the following: [5+5=10]

Q10. (a) Receptor-mediated selective transport

Ans) Some proteins enter the cell by receptor-mediated endocytosis from the surrounding medium. An LDL particle is a blood carrier of cholesterol esters with a protein on its surface called apoB-100 that acts as a ligand for LDL receptors.

The receptor is separated into vesicles and returned to the plasma membrane in separate vesicles. The LDL particle-carrying vesicle merges with the lysosomes, where cholesterol esters are hydrolyzed into fatty acids and cholesterol; Apo B-100 of LDL is also broken down into amino acids and released into the cytoplasm. The cholesterol released could be integrated into the membrane, used to make hormones, or even retained.

To achieve access, many viruses and poisons take advantage of the host's receptor-mediated endocytosis system.

Q10. (b) Stem cell

Ans) Stem cells are unique human cells that can differentiate into a variety of cell types. This can include everything from muscle cells to brain cells. Stem cell-based therapies, according to researchers, could one day be utilised to cure major disorders including paralysis and Alzheimer's disease.

Stem cells are divided into 2 main forms:

1. Embryonic stem cells:  Today's embryonic stem cells are derived from discarded embryos. These are the results of in vitro fertilisation. They've been given to science. These pluripotent embryonic stem cells are pluripotent stem cells. This means they can transform into a variety of cells.

2. Adult stem cells: Adult stem cells are divided into two categories. The brain, skin, and bone marrow are examples of completely developed tissues. These tissues contain just a modest amount of stem cells. They are more likely to produce only specific cell types. A stem cell from the liver, for example, will only produce more liver cells