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PART B

  • Radioimmunoassay (RIA) is considered the most sensitive measure of antibody because it can detect very small amounts of a substance in a sample.
  • It uses radioactive isotopes and antibodies to measure the concentration of antigens (substances that trigger an immune response) in a sample.
  • The sensitivity of RIA is due to its ability to detect even picogram levels of antigen, making it more sensitive than other tests like precipitation, agglutination, or radial immunodiffusion.
  • HMG-CoA reductase (3-hydroxy-3-methylglutaryl-CoA reductase) is the key enzyme in cholesterol biosynthesis because it catalyzes the conversion of HMG-CoA to mevalonate.
  • This step is the rate-limiting step in the pathway, meaning it’s the slowest and thus determines the overall rate of cholesterol synthesis.
  • The activity of HMG-CoA reductase is regulated by multiple mechanisms, including feedback inhibition by cholesterol and other Sterols, and control of the synthesis and degradation of the enzyme itself. This makes HMG-CoA reductase a primary control point in cholesterol biosynthesis.
    • regulation of HMG-CoA reductase.
    • 1. Feedback Inhibition: When the levels of cholesterol are high, the enzyme is inhibited, slowing down the production of more cholesterol. This is a form of negative feedback.
    • 2. Gene Regulation: The gene that codes for HMG-CoA reductase can be turned on or off. When cholesterol levels are low, the gene is turned on to produce more of the enzyme and thus more cholesterol. When cholesterol levels are high, the gene is turned off.
    • 3. Degradation: The enzyme can be marked for destruction when cholesterol levels are high. This is done by adding a small protein called ubiquitin to the enzyme, which signals it for degradation.
    • 4. Phosphorylation: The activity of the enzyme can also be regulated by phosphorylation, a process where a phosphate group is added to the enzyme. When the enzyme is phosphorylated, it becomes less active.
  • Koch’s postulates are a set of criteria established by Robert Koch to identify the causative agent of a particular disease.
  • The second postulate states that the microorganism must be isolated from a diseased organism and grown in pure culture.
  • If a microorganism cannot be isolated and grown in a pure culture, it’s impossible to proceed with the other postulates, which include demonstrating that the cultured microorganism causes disease when introduced into a healthy organism and that the same microorganism can be re-isolated from the experimentally infected organism. Therefore, if a microorganism cannot be isolated in pure culture, it makes it impossible to satisfy all of Koch’s postulates.
  • Cell differentiation during animal development does not involve the loss of genetic information because all cells in an organism contain the same DNA.
  • Differentiation occurs when certain genes are turned on or off, which is a process known as gene expression. This is how different types of cells (like skin cells, muscle cells, and nerve cells) are made.
  • Despite their differences, they all contain the same genetic information. The process of differentiation doesn’t remove or alter the genetic information; it just changes how it’s used.
  • The term “loss of developmental potential” refers to the process by which cells become more specialized during development.
  • In the early stages of development, cells are considered to be “totipotent” or “pluripotent,” meaning they have the potential to become any type of cell in the body. As these cells differentiate and become more specialized, they lose this potential. For example, a skin cell can’t change into a nerve cell because it has lost that developmental potential. This is a normal part of development and is necessary for the formation of complex organisms.
  • Epigenetic mechanisms of gene regulation play a crucial role in cell differentiation. Epigenetics refers to changes in gene expression that don’t involve alterations to the underlying DNA sequence.
  • These changes can be caused by factors like DNA methylation and histone modification. For example, when a methyl group is added to DNA (DNA methylation), it can change the activity of a DNA segment without changing the sequence. This can turn genes on or off, influencing the development of specific cell types. So, epigenetic mechanisms are a key part of how cells differentiate during development.
  • Sodium dodecyl sulfate (SDS) is a detergent that binds to proteins and gives them a negative charge.
  • The charge is proportional to the length (and therefore the mass) of the protein, so each protein gets the same amount of charge per unit mass. This uniform charge density allows the proteins to be separated based on size during gel electrophoresis, as smaller proteins will move through the gel faster than larger ones. Without SDS, the proteins might have different charges and shapes, which would affect their movement through the gel and make the results less clear.
  • Many members of the phylum Arthropoda, which includes insects, spiders, and crustaceans, have a distinctive larval stage as part of their life cycle.
  • This is because they undergo a type of development called metamorphosis. During metamorphosis, the organism changes its body form dramatically as it matures from a larva to an adult. This allows the larval and adult stages to exploit different resources or habitats, reducing competition among individuals and increasing the chances of survival for the species.
  • Cytokinins are a type of plant hormone that promote cell division, or cytokinesis, in plant roots and shoots. They are also involved in plant growth and development, including the growth of lateral buds.
  • Coconut milk contains a high concentration of cytokinins, which is why it’s often used in plant tissue culture for promoting growth.
  • Auxin is a plant hormone that plays a crucial role in the growth and development of plants. When auxin diffuses from the apical bud, it inhibits the growth of lateral shoots, a phenomenon known as apical dominance.
  • This is because the high concentration of auxin in the apical bud suppresses the growth of lateral buds, allowing the plant to grow taller and reach more sunlight. This is why it’s referred to as an inhibitory effect.
  • Morphogens are a type of signaling molecule that can trigger different cell responses based on their concentration. They diffuse through tissues and form a concentration gradient.
  • Cells respond to this gradient in a concentration-dependent manner, meaning that high concentrations of a morphogen might trigger one type of cell differentiation, while lower concentrations might trigger a different type. This allows for the complex patterning necessary for proper development of an organism. For example, in the development of an embryo, morphogens help determine the head-to-tail and back-to-belly orientation of the body.
  • The most probable order of the genes on the bacterial chromosome would be (c) A D C B.
  • This is because the frequency of recombination is directly proportional to the distance between genes. The greater the distance between two genes, the higher the chance of recombination occurring between them. In this case, the recombination frequency is highest between A and B, followed by A and C, and then A and D, suggesting that B is furthest from A, C is in the middle, and D is closest to A. Therefore, the order would be A D C B.
  • Psychrophilic microorganisms, which thrive in cold environments, have a high level of unsaturated fatty acids in their cell membranes. This is because unsaturated fatty acids have kinks in their structure due to double bonds, which prevent the fatty acids from packing closely together. This results in a more fluid membrane at lower temperatures, allowing the cell to function properly in cold conditions.
  • Interferon-beta is a type of protein that is produced by cells in response to viral infection. When a cell is infected by a virus, it releases interferon-beta, which signals to neighboring cells to activate their antiviral defenses.
  • This helps to slow or stop the spread of the virus. Bacteria and fungi do not trigger the same response, so interferon-beta is not produced in response to these types of infections.
  • The Sabin vaccine, which is an oral polio vaccine, uses live attenuated strains of all three immunological types of poliovirus. This is because the live attenuated virus can stimulate a strong and long-lasting immune response.
  • The virus is weakened, or “attenuated,” so it doesn’t cause the disease in healthy people, but it’s still alive and can multiply in the body. This allows the immune system to recognize and fight off the virus in the future. It’s like a training exercise for the immune system.

Long-chain acyl-CoA penetrates mitochondria in the presence of (b) carnitine.

  • Carnitine plays a crucial role in the transport of long-chain fatty acids across the inner mitochondrial membrane. The long-chain acyl-CoA cannot directly cross this membrane. Instead, it is converted into acylcarnitine by the enzyme carnitine palmitoyltransferase I (CPT I) located on the outer mitochondrial membrane.
  • The acylcarnitine is then transported across the inner mitochondrial membrane by a translocase. Once inside, the acyl group is transferred back to CoA by carnitine palmitoyltransferase II (CPT II), and carnitine is returned to the cytosol. This allows the fatty acid to be broken down through beta-oxidation
  • DNA Polymerase I has both 5′-3′ and 3′-5′ exonuclease activities. The 5′-3′ exonuclease activity allows it to remove RNA primers used in DNA replication and replace them with DNA.
  • The 3′-5′ exonuclease activity provides a proofreading function, where it can remove mispaired nucleotides, ensuring the accuracy of DNA replication. Other polymerases like Klenow polymerase, DNA polymerase III, and Taq DNA polymerase do not have both of these activities.
  • Brief explanation of the other options:
  • (a) Klenow Polymerase: This is a large fragment of DNA Polymerase I that retains the 5′-3′ polymerase activity and the 3′-5′ exonuclease proofreading activity, but lacks the 5′-3′ exonuclease activity. It is often used in molecular biology applications because of its high fidelity.
  • (b) DNA Polymerase III: This is the main enzyme involved in DNA replication in bacteria. It has 5′-3′ polymerase activity and 3′-5′ exonuclease proofreading activity, but it does not have 5′-3′ exonuclease activity.
  • (d) Taq DNA Polymerase: This is a thermostable DNA polymerase named after the thermophilic bacterium Thermus aquaticus from which it was originally isolated. It is commonly used in PCR (polymerase chain reaction) because it can withstand the high temperatures of the procedure. Taq polymerase has a 5′-3′ polymerase activity, but lacks 3′-5′ exonuclease proofreading activity, which means it is more prone to errors than other polymerases.
  • Lactobacilli can grow in acidic pH because they are acidophilic bacteria, meaning they thrive in acidic environments. This is due to their metabolic processes which produce lactic acid, lowering the pH of their environment. They have adapted to survive and grow in these conditions, unlike many other types of bacteria.

Prokaryotic DNA gyrase is inhibited by (d) nalidixic acid.

  • Nalidixic acid inhibits the function of DNA gyrase in prokaryotes by interfering with the enzyme’s ability to break and rejoin the double-stranded DNA during replication.
  • This prevents the DNA from being unwound, which is a necessary step in DNA replication. As a result, the replication process is halted, inhibiting the growth and multiplication of the bacteria.
  • An explant is the piece of plant tissue that is taken from a plant and used to initiate a culture. It can be any part of the plant like the leaf, stem, flower, seed, or even a single cell.
  • A callus, on the other hand, is a mass of unorganized plant cells that is produced when the explant is cultured in the appropriate medium. These cells are totipotent, meaning they have the ability to grow into any type of plant cell and can eventually be induced to form an entire plant.
  • In the regulation of gene expression, an inducer works by binding to a repressor protein. This binding changes the shape of the repressor protein, preventing it from binding to the operator region of the DNA. When the repressor is unable to bind to the operator, it cannot block the RNA polymerase. This allows the RNA polymerase to move along the DNA and transcribe the gene, leading to gene expression. This process is a key part of the lac operon model in prokaryotes.
  • A prosthetic group is a non-protein molecule that is a permanent part of a functioning protein molecule. In this case, (a) TPP (Thiamine pyrophosphate), (b) FAD+ (Flavin adenine dinucleotide), and (d) Lipoic acid can all be considered prosthetic groups. (c) NAD+ (Nicotinamide adenine dinucleotide), however, is typically considered a coenzyme rather than a prosthetic group because it is not permanently bound to the protein.
  • The overall rate of a reaction is determined by the step with the highest activation energy because this step is the slowest. Activation energy is the energy required for a reaction to occur. The higher the activation energy, the more energy is needed to start the reaction, making the reaction slower. This slowest step is known as the rate-determining step because it limits the rate at which the entire reaction can proceed. Even if other steps in the reaction could occur more quickly, they are essentially “waiting” for the slowest step to complete.
  • Phosphorylation is a process where a phosphate group is added to a molecule, such as an amino acid. This process often occurs on the hydroxyl (-OH) group of certain amino acids.
  • Serine (c) has a hydroxyl group, making it a possible site for phosphorylation. Arginine, cysteine, and phenylalanine do not have a hydroxyl group, so they are less likely to be phosphorylated.
  • Lyases are a class of enzymes that catalyze the breaking of various chemical bonds by means other than hydrolysis (splitting a molecule using water) and oxidation (loss of electrons).
  • They can also form new bonds or add groups to double bonds. In other words, they can remove groups from substrates without the addition or removal of water
  • The intrinsic fluorescence of proteins is primarily due to aromatic amino acids because they contain aromatic rings in their structure. These aromatic rings, specifically in the amino acids tryptophan, tyrosine, and phenylalanine, have the ability to absorb ultraviolet light and then re-emit it as fluorescence.
  • The other options you mentioned – sulphur-containing amino acids, histidine, and proline – do not have this same ability. Sulphur-containing amino acids (like cysteine and methionine) lack the necessary aromatic ring structure. Histidine and proline, while they do contain ring structures, these are not aromatic and thus do not exhibit the same fluorescence properties.
  • The length of one base pair of DNA is approximately 0.34 nanometers (nm).
  • First, convert the length of the DNA from kilobase pairs to base pairs: 3×10^6 kilobase pairs=3×10^9 base pairs
  • Then, multiply the total number of base pairs by the length of one base pair to find the total length in nanometers: 3×10^9 base pairs×0.34 nm/base pair=1.02×10^9 nm
  • Finally, convert the length from nanometers to centimeters (1 nm = 1 x 10^-7 cm): 1.02×10^9 nm×1×10^−7 cm/nm=1.02×10^2 cm So, the total length of human haploid DNA is approximately 102 cm.
  • Therefore, the correct answer is (a) 102.
  • Nektons are marine animals that can swim and move independently of water currents. These include larger and more powerful organisms like fish, squid, and whales. Their size and strength allow them to combat and navigate through water currents, unlike planktons which are small and drift with the currents.
  • The term “centimorgan” is named after geneticist Thomas Hunt Morgan. It is a unit of measure for genetic linkage or recombination frequency. One centimorgan (cM) or one map unit (m.u.) is defined as the distance between gene pairs for which one product of meiosis in 100 is recombinant.
  • This is equivalent to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing over in a single generation. In other words, a distance of 1 cM between two loci means that these loci segregate independently approximately 1% of the time.
  • Secondary structures of proteins, such as alpha-helices and beta-sheets, are primarily maintained by hydrogen bonds. These bonds form between the oxygen atom of the carbonyl group in one amino acid and the hydrogen atom of the amino group in another amino acid.
  • This interaction helps to stabilize the structure. The specific arrangement of these hydrogen bonds leads to the formation of the characteristic shapes of the secondary structures.
  • The H1N1 strain of the influenza A virus is an example of genetic reassortment because it contains genetic components from multiple sources – a swine influenza virus, an avian virus, and a human influenza virus. Genetic reassortment occurs when two different strains of the flu virus infect the same cell and exchange genetic material.
  • This can result in a new strain of the virus, like H1N1, with a mix of characteristics from the original strains. This process is different from antigenic shift or drift, which involve changes in the virus’s surface proteins, and point mutations, which are small changes in the virus’s genetic code.
  • Antigenic shift and antigenic drift are processes that involve changes in the influenza virus’s surface proteins, not its genetic components. Antigenic drift involves small, gradual changes in the genes of influenza viruses that happen over time as the virus replicates. These small genetic changes can accumulate over time and result in viruses that are related but not identical to the original virus.
  • Antigenic shift, on the other hand, is an abrupt, major change to the influenza A virus, resulting in new hemagglutinin and/or neuraminidase proteins. This can lead to a new subtype of virus to which most people have little to no immunity.
  • Point mutations are changes in a single nucleotide in a DNA sequence. While point mutations can lead to changes in an organism’s traits, they do not involve the mixing of genetic material from different organisms, as seen in the H1N1 virus.
  • In the case of the H1N1 virus, it’s the combination of genetic material from different viruses (human, avian, and swine) that led to its creation, which is why it’s an example of genetic reassortment.
  • DNA replication is called semiconservative because each new DNA molecule consists of one strand from the original DNA molecule and one newly synthesized strand. This process was first described by Matthew Meselson and Franklin Stahl in 1958.
  • They found that when DNA replicates, the two strands of the original DNA molecule separate, and each serves as a template for the synthesis of a new, complementary strand. This results in two new DNA molecules, each containing one of the original (or “parental”) strands and one new strand. Hence, the term “semiconservative” replication.
  • Methylation is a type of covalent modification that can occur on both histones and DNA. In DNA methylation, a methyl group (-CH3) is added to the DNA molecule, typically at a cytosine or adenine DNA base.
  • This can change the activity of a DNA segment without changing the sequence. In histone methylation, a methyl group is added to the amino acids of histone proteins. This can affect the structure of the histone and therefore influence gene expression. Phosphorylation, acetylation, and succinylation are also types of covalent modifications, but they do not commonly occur on both histones and DNA.
  • Nucleosomes are the basic unit of DNA packaging in eukaryotes, which include organisms like fungi, plants, and animals. Saccharomyces cerevisiae, commonly known as baker’s yeast, is a type of fungus and therefore a eukaryote.
  • This means it has a well-defined nucleus in its cells where the DNA is packaged into nucleosomes. On the other hand, Escherichia coli and Rickettsia spp. are bacteria, and Influenza is a virus. These are all prokaryotes or non-cellular entities, which do not have a well-defined nucleus and therefore do not package their DNA into nucleosomes.
  • Disinfection is a process that eliminates many or all pathogenic microorganisms, except bacterial spores, on inanimate objects. In health-care settings, objects usually are disinfected by liquid chemicals or wet pasteurization.
  • The main aim of disinfection is to prevent the spread of infections, which is why option (c) “Prevention of infection” is the correct answer. It’s important to note that disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores.
  • (a) Removal of microbes from liquids: This is more accurately described as filtration or sterilization, not disinfection. Disinfection involves the use of physical or chemical methods to kill or deactivate pathogens.
  • (b) Destruction of all microbes on inanimate objects: This is a description of sterilization, not disinfection. Sterilization is a process that destroys or eliminates all forms of microbial life, including spores, which is more intense than disinfection.
  • (d) Inhibition of bacterial growth: This is more related to the concept of bacteriostatic agents, which inhibit the growth of bacteria, rather than disinfection.
  • Disinfectants aim to kill or deactivate pathogens, not just inhibit their growth. So, while disinfection can involve aspects of these other options, they are not the most accurate or complete descriptions of the process. The primary goal of disinfection is to prevent infection by reducing the number of pathogenic microorganisms to a safe level.
  • Eukaryotic genes contain introns, which are non-coding sequences of DNA. These introns are transcribed into mRNA but are then removed during a process called RNA splicing. This process allows for one gene to produce different proteins through a process called alternative splicing. On the other hand, prokaryotic genes do not contain introns; their genes are typically a continuous stretch of DNA that codes for a protein.
  • Introns play several important roles in eukaryotic cells:
    • 1. Alternative Splicing: Introns allow for one gene to produce multiple different proteins. During RNA splicing, different combinations of exons (the coding regions of the gene) can be joined together, leading to different proteins. This increases the diversity of proteins that a single gene can produce.
    • 2. Gene Regulation: Introns can contain sequences that help regulate gene expression. These sequences can be binding sites for proteins that influence how often the gene is transcribed into mRNA.
    • 3. Evolution: Introns can play a role in evolution. Because they do not code for proteins, they can accumulate mutations without affecting the function of the protein. These mutations can then potentially lead to new functions in the future.
  • Agrobacterium is widely used as an effective vector for gene transfer in plants because it naturally infects plants and integrates its DNA into the plant’s genome.
  • This bacterium has a plasmid called the Ti (tumor-inducing) plasmid, which can be manipulated in the lab to carry desired genes.
  • When Agrobacterium infects a plant, it transfers this Ti plasmid (and any genes it carries) into the plant cells. This makes it a useful tool for creating genetically modified plants.
  • Contact dermatitis is an example of cell-mediated hypersensitivity because it involves the immune system’s T cells, a type of white blood cell. When your skin comes into contact with a substance that the body perceives as foreign (like poison ivy or certain soaps), the immune system launches a defense, causing skin inflammation. This reaction doesn’t involve antibodies, which are used in other types of hypersensitivity, but instead is controlled by T cells. That’s why it’s called “cell-mediated.”
  • (a) Cytotoxic hypersensitivity: This involves the immune system making antibodies against the body’s own cells, leading to cell damage. An example is a blood transfusion reaction.
  • (b) Anaphylaxis hypersensitivity: This is an immediate and severe allergic reaction that involves the release of histamine from mast cells and basophils. An example is a bee sting allergy.
  • (d) Immune complex hypersensitivity: This involves immune complexes (antigen-antibody complexes) that are not effectively removed, leading to inflammation and tissue damage. An example is lupus.
  • The five classes of immunoglobulin (Ig) molecules – IgG, IgA, IgM, IgE, and IgD – are differentiated based on the structural differences in the carboxyl terminal portion of their heavy chains.
  • This region, also known as the constant region, determines the class of the antibody and its function in the immune response. For example, it influences whether the antibody can bind to other cells or proteins and what kind of immune response will be triggered. The amino terminal portion, on the other hand, is variable and is responsible for binding to specific antigens

The Ziegler-Natta catalysts are (a) stereospecific

  • They are used in the polymerization of alkenes, particularly ethylene and propylene, and can control the stereospecificity of the polymerization process.
  • Ziegler-Natta catalysts are stereospecific because they can control the orientation of the monomers as they add to the growing polymer chain. This allows for the production of polymers with a specific arrangement of atoms, which can greatly influence the properties of the resulting polymer. For example, the catalyst can control whether the polymer is a cis or trans isomer, which can affect the polymer’s density, melting point, and other physical properties. This stereospecificity is crucial in the production of many commercial polymers.
  • The efficiency of a Carnot engine is given by the formula: Efficiency = 1 – (Tcold/Thot) where Tcold is the temperature of the sink and Thot is the temperature of the source.
  • The efficiency of the engine can also be expressed as: Efficiency = (Qhot – Qcold)/Qhot where Qhot is the heat taken from the source and Qcold is the heat transferred to the sink.
  • Setting these two expressions for efficiency equal to each other, we get: 1 – (Tcold/Thot) = (Qhot – Qcold)/Qhot Solving for Qcold,
  • we get: Qcold = Qhot – Qhot*(1 – Tcold/Thot) Substituting the given values,
  • we get: Qcold = 90 J – 90 J*(1 – 200 K/300 K) Qcold = 90 J – 90 J*(1 – 2/3) Qcold = 90 J – 90 J*(1/3) Qcold = 90 J – 30 J Qcold = 60 J
  • So, the correct answer is (a) It transfers 60 J of heat to the sink at 200 K.
  • The cell voltage (Ecell) of a galvanic cell is the difference between the reduction potentials of the cathode and the anode. In a Daniell cell, copper (Cu) acts as the cathode and zinc (Zn) as the anode.
  • The cell reaction is: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
  • The cell voltage is given by: Ecell = Ecathode – Eanode Given that Ecell = 1.07 V and Ecathode (reduction potential of Cu2+/Cu) = 0.34 V,
  • we can solve for Eanode (reduction potential of Zn2+/Zn): Eanode = Ecathode – Ecell = 0.34 V – 1.07 V = -0.73 V
  • So, the reduction potential of Zn2+/Zn is -0.73 V. The correct answer is (d) -0.73 V.
  • Hapticity refers to the number of contiguous atoms of a ligand that bind to a central atom in a coordination complex.
  • In the case of Mo(C7H8)(CO)3, the cycloheptatriene (C7H8) ligand binds to the molybdenum (Mo) atom with all seven of its carbon atoms. Therefore, the hapticity of cycloheptatriene in this complex is 7.

(a) XeF4 and ClF3. Both of these compounds have two lone pairs of electrons.

  • The molecule \mathrm{C}_{60}, also known as Buckminsterfullerene or a buckyball, is a spherical molecule composed of 60 carbon atoms.
  • Its structure is similar to a soccer ball, with 12 pentagons and 20 hexagons. So, the correct answer is (c)12 pentagons and 20 hexagons.

The complex with minimum Crystal Field Stabilization Energy (CFSE) is (b) [Co(H2O)6]3+. This is because water (H2O) is a weak field ligand and results in high spin complexes with less CFSE.

  • The zero-point energy of a harmonic oscillator is given by the formula E0 = ½ħω. This is derived from the quantum mechanical model of a harmonic oscillator.
  • In quantum mechanics, a harmonic oscillator cannot have zero energy because of the Heisenberg uncertainty principle, which states that the position and momentum of a particle cannot both be precisely known at the same time.
  • This means that a particle in a harmonic oscillator always has some motion, and therefore some kinetic energy, even at absolute zero temperature. The energy levels of a quantum harmonic oscillator are quantized, meaning they can only take on certain discrete values. The lowest possible energy level, known as the ground state or zero-point energy, is given by ½ħω, where ħ is the reduced Planck’s constant and ω is the angular frequency of the oscillator.

The Diels-Alder reaction normally yields the endo-adduct as a major product due to (d) secondary orbital interaction between a diene and a dienophile. This interaction stabilizes the transition state leading to the endo product, making it more likely to form.

  • The atomic term symbol for a helium atom with the electronic configuration 1s2 is (c) 1S0. This is because helium has two electrons in the 1s orbital, both with spin -1/2, giving a total spin S of 0.
  • The total orbital angular momentum L is also 0 because the 1s orbital has no angular momentum. The superscript 1 in the term symbol indicates the multiplicity 2S+1, which is 1 for S=0.

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