31 Years NEET Previous Year Questions: Cell: The Unit of Life - 1

If centromere is situated in the middle of the chromosome, it is called as Metacentric.

• Based on the position of the centromere, chromosomes are categorised as metacentric, submetacentric, acrocentric and telocentric.
• Metacentric have centromere in the middle of the chromosome with two equal arms of the chromosomes.
• Sub-metacentric has sub-terminal centromere with one shorter arm and one longer arm.
• Acrocentric have centromere close to its end forming one extremely short and one very long arm.
• Telocentric have terminal centromere.
• Thus, the correct answer is option A.

Proteins and lipids are formed in the endoplasmic reticulum and some of them are modified to form glycoproteins and glycolipids in the Golgi apparatus.

Enlarged image of Endoplasmic reticulum and golgi apparatus (relation). Only golgi body is given here.

• Lysosomes are formed by the process of packaging in the Golgi apparatus.
• They are membrane-bound and possess hydrolytic enzymes which work optimally in acidic pH.
• Lysosomes bud off from trans face to Golgi bodies.
• Precursor of lysosomal enzymes are synthesized by RER and then send to Golgi bodies for further processing.

So, the correct answer is 'Lysosomes are formed by the process of packaging in the endoplasmic reticulum'

• Nucleolus is naked (devoid of membrane), round or slightly irregular structure which is attached to the chromatin at a specific region called nucleolar organiser region (NOR).
• Nucleolus is a site for rRNA synthesis. Nucleolus disappears when the cell undergoes division and reappears after its completion.

• Mitochondria are miniature biochemical factories where food stuffs or respiratory substrates arc completely oxidised to carbon dioxide and water.
• During aerobic respiration, carbohydrates are completely oxidised in mitochondria. In mitochondria, the Krebs cycle and electron transport chain occur, and ATP is generated. As a result, mitochondria are known as a cell's powerhouse.

• ATP comes out of mitochondria and helps perform various energy-requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement, etc. Because of the formation of ATP, the mitochondria arc called power houses of the cell.

• Mitochondria are considered as semi-autonomous cell organelle due to the following counts: Mitochondria have their own DNA which can replicate independently.
• The mitochondrial DNA produces its own mRNA, tRNA and rRNA. The organelles possess their own ribosomes, called mitoribosomes. Hence they have their own protein synthesising machinery.

The same applies to chloroplast as well.
Hence, option C is correct.

• The microtubules are found in the cytoplasmic matrix of all eukaryotic cells. They also occur in cilia, flagella, centrioles, basal bodies, and mitotic apparatus.
• Spindle fibres form the protein structure which helps in mitosis and meiosis.

• Centrioles are the two cylindrical structures of a cell organelle called centrosome.

• Cilia and flagella are fine hair-like outgrowths of the membrane.
• While peroxisomes are the microbodies.

So, the correct answer is 'Spindle fibers, centrioles, and cilia'.

• Option A: Ribosomes: They are the protein factories. They have the ribosomal RNA and protein but lack its DNA. It does not have the genes of its own but forms the protein for the body.
• Option B: Nucleus: It is the house for the genetic makeup of the organism. It has all the chromosomal DNA so is called nuclear DNA. Here the genes work to make the functioning of the body.
• Option C: Mitochondria: Mitochondria is the semi autonomous organelle. They have the singular stranded DNA.
• Option D: Chloroplast: It is also the semi autonomous organelle having the ss DNA.

Correct Answer is Option A

• A nuclear membrane, also known as the nuclear envelope, nucleolemma or karyotheca, is the double lipid bilayer membrane which surrounds the genetic material and nucleolus in eukaryotic cells.
• Recent developments have shown that nuclear membrane is derived from rough endoplasmic reticulum.
• During cell division, nuclear membrane is disintegrated.
• The nuclear envelope transmembrane proteins are absorbed in the RER.
• Once the division is completed, RER reassembles the nuclear envelope.

• Smooth endoplasmic reticulum is the major site for synthesis of lipids.
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• Rough endoplasmic reticulum contains ribosomes found attached to it, therefore RER synthesise proteins.
• Phospholipids are different from simple lipids as simple lipids are esters of fatty acid with various alcohols while compound or conjugated lipids like phospholipids are esters of fatty acids with alcohol but contain some other substances also like phosphate group in phospholipid.
• The smooth endoplasmic reticulum(SER) packages proteins for transportation, synthesizes membrane phospholipids, and release calcium.

Ribosomes are the protein factory of a cell. These are non-membranous organelle which attaches on endoplasmic reticulum by transcolon.

• Most cells in animal tissues (with the exception of a few terminally differentiated cells such as skeletal muscle cells and blood cells) are in communication with their adjoining cells via gap junctions.
• At the place where gap junction is present, membranes of two adjacent cells are separated by a uniform narrow gap of about 2-4 nm.
• The gap is spanned by channel forming proteins called connexins, which allow inorganic ions, and other small water soluble molecules to pass directly from cytoplasm of one cell to cytoplasm of other cell.

• Protoplasts are cells with the plasma membrane, cytoplasm and nucleus which have their cell wall removed by the action of enzymes.
• Cellulase enzymes digest the cellulose in plant cell walls while Pectinase enzymes break down the pectin holding cells together. Once the cell wall has been removed ,the resulting protoplast is spherical in shape.

• Cellular organelles with membranes are Lysosomes, Golgi apparatus and mitochondria.
• Lysosomes are single membrane-bound organelles which contain acid hydrolases or digestive enzymes that digest excess or worn out organelles, food particles, engulfed viruses or bacteria and macromolecules.
• Golgi apparatus is a single membrane-bound organelle consists of folded and flattened membranes that are involved in secretion and helps in intracellular transport.
• The mitochondria is a double-membraned cell organelle, known as the powerhouse of the cell which is present in all eukaryotic cells.
• So, the correct answer is 'Lysosomes, Golgi apparatus and mitochondria'.

The cell wall is the hard protective covering of all plant, fungal and some bacterial cells. Mycoplasma is a genus of bacteria that lack a cell wall and surrounded by their cell membrane only. 

• Satellite DNA forms the minor peak after centrifugation of DNA.
• These are repetitive DNA sequences that do not code for any protein.
• They show high degree of polymorphism and are heritable from parents to children, this forms the basis of DNA fingerprinting.

Image: Chromosomes showing satellite DNA

• The cytoskeleton is unique to eukaryotic cells. It is a dynamic three-dimensional structure that fills the cytoplasm. This structure acts as both muscle and skeleton, for movement and stability. The long fibers of the cytoskeleton are polymers of subunits. The primary types of fibers comprising the cytoskeleton are microfilaments, microtubules, and intermediate filaments.
• Microfilaments are fine, thread-like protein fibers, 3-6 nm in diameter. They are composed predominantly of a contractile protein called actin.

• Microtubules are cylindrical tubes, 20-25 nm in diameter. They are composed of subunits of the protein tubulin--these subunits are termed alpha and beta. 
• Intermediate filaments are about 10 nm diameter and provide tensile strength for the cell. Lamin is the protein present in intermediary filaments.

So, the correct answer is 'Microfilaments'.

• Vacuoles are non cytoplasmic areas present inside cytoplasm and separated from latter by tonoplast.
• They are believed to be formed by expansion and pinching off from ER.
• There occurs a large central vacuole and many small vacuoles in plant cells.
• They play a major role in osmotic expansion of cell.
   

1. Centriole: There are two main functions of centrioles

• Cell division :The main function of the centriole is to help with cell division in animal cells.  The centrioles help in the formation of the spindle fibres that separate the chromosomes during cell division (mitosis).
• Ciliogenesis :The second function of centrioles that we will focus on is ciliogenesis. Ciliogenesis is simply the formation of cilia and flagella on the surface of cells.  Cilia and flagella help the cell move. 

2. Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts.

3. A crista is a fold in the inner membrane of a mitochondrion.

4. Ribosomes comprise a complex macromolecular machine, found within all living cells, that serves as the site of biological protein synthesis  (translation). Ribosomes link amino acids together in the order specified by messenger RNA(mRNA) molecules.

• In some bacteria, (e.g., Bacillus subtilis) the plasma membrane forms certain invaginations or in foldings called mesosomes in the cytoplasm.
• The mesosomes have various functions, viz., respiratory, secretory etc. Mesosomes are the infolding of the bacterial cell membrane.
• These structures are thought to be analogous to the eukaryotic mitochondria.

• Prokaryotes are the primitive organism which lacks membrane-bounded organelles and nucleus. Prokaryotic cells have some microscopic structures in their cytoplasm. They serve specific purposes for the cell.
• Inclusions are aggregates of specific chemical compounds and often serve as a reservoir of energy or carbon. Common inclusions are poly--hydroxyalkanoate, sulfur globules, cyanophycin globules, and polyphosphate.
• Polyribosomes (or polysomes) also known as ergosomes are a cluster of ribosomes, bound to the mRNA molecule during translation process found in the eukaryotes.

In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens.

RER is frequently observed in the cells actively involved in protein synthesis and secretion. RER is well developed in cells engaged in synthesis of secretory products.

A → (iii),

Thylakoids: Any of the membranous disks of lamellae within plant chloroplasts that are composed of protein and lipid and are the sites of the photochemical reactions of photosynthesis.

B → (iv),

Cristae: Cristae are the sub-compartments of the inner membrane of the mitochondria which throws itself into many folds. These structures are known to increase the surface area that is available, in order to promote the productivity of cellular respiration. 

C → (i),

Cisternae: The flattened membrane disk containing fluid, such as those occurring in the Golgi apparatus and the endoplasmic reticulum

D → (ii),

Chromatin: Chromatin is a substance within a chromosome consisting of DNA and protein.

Lysosomes are enclosed by a single membrane while Mitochondria, Chloroplasts and Nuclei are enclosed by double membrane.