All Free Biology MCQs with Answers

2561. Some bacteria are able to grow in streptomycin containing medium due to

• A. Natural selection
• B. Induced mutation
• C. Reproductive isolation
• D. Genetic drift.

Explanation: a)Natural Selection (Correct Answer): Bacteria can develop resistance to antibiotics through natural selection. In a population of bacteria, there can be variations in the presence of certain genetic mutations. When exposed to an antibiotic like streptomycin, the bacteria without the mutations are susceptible and die, while those with mutations that confer resistance survive. Over time, the resistant bacteria reproduce, passing on their resistance genes to the next generation. This process represents natural selection favoring antibiotic-resistant bacteria. b)Induced Mutation:Induced mutations occur when external factors, such as exposure to mutagenic agents (chemicals, radiation, etc.), lead to changes in the DNA of an organism. While mutations can create genetic diversity, allowing for the potential development of antibiotic resistance, the specific resistance to streptomycin in bacteria primarily results from natural selection. c)Reproductive Isolation:Reproductive isolation refers to mechanisms that prevent different species from interbreeding. It is not directly related to the development of antibiotic resistance in bacteria. Antibiotic resistance is primarily a genetic adaptation within a single bacterial species. d)Genetic Drift:Genetic drift is a mechanism that can cause changes in the frequency of particular genetic variants in a small population due to chance events. While it can lead to genetic changes over time, the development of antibiotic resistance is more directly related to natural selection in response to antibiotic exposure. In summary, natural selection is the most relevant and widely observed mechanism by which bacteria can develop antibiotic resistance when exposed to antibiotics like streptomycin. Other mechanisms, such as induced mutation, reproductive isolation, and genetic drift, may play roles in evolutionary processes but are less directly related to antibiotic resistance.

2562. In bacteria, plasmid is

• A. Extra chromosomal material
• B. Main DNA
• C. Non functional DNA
• D. Repetitive gene.

Explanation: a)Extra-Chromosomal Material: Plasmids are small, circular pieces of DNA that exist independently of the bacterial chromosome. They are considered extra-chromosomal because they are separate from the main bacterial chromosome. b)Main DNA:The main DNA of bacteria is typically found in the bacterial chromosome, which is distinct from plasmids. Plasmids are not considered part of the primary or "main" DNA of the bacterium. c)Non-Functional DNA:Plasmids can contain functional genetic information, such as genes that provide antibiotic resistance or the ability to metabolize specific compounds. While some plasmids may carry non-functional or non-essential genes, many plasmids serve essential roles in bacterial survival and adaptation. d)Repetitive Gene: Plasmids can carry specific genes, and some plasmids may carry multiple copies of the same gene. However, not all plasmids contain repetitive genes. The presence of repetitive genes on a plasmid depends on the specific plasmid's genetic content. In summary, plasmids are indeed extra-chromosomal material in bacteria, separate from the primary bacterial chromosome. They can carry various functional genes and are not limited to non-functional DNA. While some plasmids may contain repetitive genes, this is not a defining characteristic of all plasmids.

2563. Choose the correct sequence of stages of growth curve for bacteria.

• A. Lag, log, stationary, decline phase
• B. Lag, log, decline, stationary phase
• C. Stationary, lag, log, decline phase
• D. Decline, lag, log phase, stationary

Explanation: Lag, log, stationary, decline phase Here's an explanation of each stage: Lag Phase: In this phase, bacteria are adjusting to their new environment, and there is little or no increase in the number of bacteria. They are metabolically active but not yet dividing rapidly. Log (Exponential) Phase: This is the phase of rapid bacterial growth. The population of bacteria increases exponentially as they divide and reproduce under favorable conditions. Stationary Phase: In this phase, the growth rate slows, and the number of new bacteria being produced is roughly balanced by the number dying. Resources become limited, and growth plateaus. Decline Phase: Also known as the death phase, this is when the number of dying bacteria exceeds the number of new bacteria being produced. Conditions become unfavorable, leading to a decline in the bacterial population. The sequence "Lag, log, stationary, decline phase" represents the typical pattern of bacterial growth in a closed system with limited resources.

2564. Which fungal disease spreads by seed and flowers?

• A. Loose smut of wheat
• B. Corn smut
• C. Covered smut of barley
• D. Soft rot of potato

Explanation: a)The fungal disease that spreads by seed and flowers is "Loose smut of wheat." This disease is caused by the fungus Ustilago tritici and is commonly known to infect wheat plants. It affects the wheat's seed-producing structures, leading to the contamination of seeds with fungal spores. When infected seeds are sown, the fungus can spread to new wheat plants during their growth and reproduction. Loose smut of wheat (Ustilago tritici): This fungal disease affects the reproductive structures of wheat, such as the flowers, and can lead to significant crop yield losses if not controlled. b)Corn smut: Ustilago maydis is a fungal pathogen that causes corn smut. This fungus primarily affects corn (maize) plants. It infects various parts of the plant, including the kernels, stems, and leaves. Unlike loose smut of wheat, corn smut doesn't primarily spread through seeds and flowers. c)Covered smut of barley: Covered smut of barley is caused by the fungus Ustilago hordei. Similar to loose smut of wheat, this disease affects the barley plant. However, its mode of transmission is not primarily through seeds and flowers but through the infection of developing barley kernels (seeds). d)Soft rot of potato: Soft rot is caused by bacteria rather than fungi. It is commonly caused by pectinolytic bacteria like Pectobacterium carotovorum and Dickeya spp.. Soft rot leads to the decay of potatoes and other tubers, causing them to become soft, slimy, and foul-smelling. This bacterial disease is not directly related to the spread by seeds and flowers, but it can lead to significant post-harvest losses. In summary, "Loose smut of wheat" is the fungal disease primarily known to spread through seeds and flowers. The other options, including "Corn smut," "Covered smut of barley," and "Soft rot of potato," have different modes of transmission and primarily affect the respective plants in distinct ways.

2565. Which of the following secretes toxins during storage conditions of crop plants?

• A. Aspergillus and Penicillium
• B. Colletotrichum
• C. Fusarium
• D. None of these

Explanation: a)Aspergillus and Penicillium: Both Aspergillus and Penicillium genera of fungi are known to produce mycotoxins, which can be harmful to humans and animals. These mycotoxins are often produced during the storage of crop plants, particularly in conditions of high humidity or poor storage practices. Mycotoxins can contaminate stored crops such as grains, nuts, and seeds and pose health risks when consumed. So, the fungi Aspergillus and Penicillium can secrete toxins, particularly mycotoxins, during storage conditions of crop plants, making option (a) the correct answer. b) Colletotrichum: Colletotrichum is a genus of fungi, commonly known as anthracnose, which primarily affects fruits and vegetables. While it can cause damage to crops, it is not typically associated with toxin secretion during storage conditions. c)Fusarium: Fusarium is another fungal genus known for causing various plant diseases, including Fusarium wilt. Similar to Colletotrichum, it can impact the quality and yield of crops, but it's not a common source of toxins during storage. d) None of these: This option is not correct because, as explained earlier, some fungi, such as Aspergillus and Penicillium, can indeed secrete toxins during the storage of crop plants, which can have adverse effects on human and animal health. In summary, while Colletotrichum and Fusarium can cause diseases in plants, they are not as well-known for toxin production during storage as Aspergillus and Penicillium are. Therefore, the correct answer is indeed option (a).

2566. Cauliflower mosaic virus contains

• A. ss RNA
• B. ds RNA
• C. ds DNA
• D. ss DNA.

Explanation: a)ss RNA (single-stranded RNA): This option is incorrect for Cauliflower mosaic virus. While there are RNA viruses, this virus has a ds DNA genome. b)ds RNA (double-stranded RNA): This option is not applicable to Cauliflower mosaic virus as it is a DNA virus, not an RNA virus. c)Cauliflower mosaic virus contains ds DNA (double-stranded DNA). It is a pararetrovirus, meaning it has a double-stranded DNA genome that can be reverse transcribed into RNA. This property makes it distinct from typical retroviruses like HIV, which have an RNA genome. d)ss DNA (single-stranded DNA): Cauliflower mosaic virus does not contain single-stranded DNA. It has a double-stranded DNA genome. So, the correct answer is indeed ds DNA, as Cauliflower mosaic virus is known for its double-stranded DNA genome.

2567. What is true for cyanobacteria?

• A. Oxygenic with nitrogenase
• B. Oxygenic without nitrogenase
• C. Non oxygenic with nitrogenase
• D. Non oxygenic without nitrogenase

Explanation: Cyanobacteria are gram negative prokaryotes which are popularly known as bluegreen algae. Although cyanobacteria are true prokaryotes, but their photosynthetic system closely resembles with that of eukaryotes because they have chlorophyll a and photosystem II and they carry out oxygenic photosynthesis. Like the red algae, cyanobacteria use phycobiliproteins as accessory pigments. Photosynthetic pigments and electron transport chain components are located in thylakoid membranes lined with particles called phycobilisomes, which contain phycobilin pigments, particularly phycocyanin and transfer energy to photosystem II. They contain nitrogenase enzyme for nitrogen fixation. This enzyme becomes inactive in the presence of oxygen but the thick walled heterocysts provide suitable anaerobic enviornment for nitrogenase activity even in aerobic conditions

2568. What is true for archaebacteria?

• A. All halophiles
• B. All photosynthetics
• C. All fossils
• D. Oldest living beings

Explanation: Archaebacteria, also known as Archaea, are a group of single-celled microorganisms that are distinct from both bacteria and eukaryotes. Let's examine each of the options you provided and determine which one is true for archaebacteria: All Halophiles: Halophiles are microorganisms that thrive in environments with high salt concentrations. While some archaebacteria are indeed halophiles, not all archaebacteria fall into this category. So, this option is not entirely true for archaebacteria. All Photosynthetics: Archaebacteria include a group known as "extremophiles," which can perform photosynthesis in extreme environments, but not all archaebacteria are photosynthetic. Therefore, this statement is not accurate for all archaebacteria. All Fossils: Archaebacteria are believed to be among the oldest organisms on Earth, and they have been found in fossil records. However, it is not true that all archaebacteria are fossils. Some archaebacteria are still alive today. Oldest Living Beings: This option is the most accurate among the provided choices. Archaebacteria are often considered some of the oldest living beings on Earth. They have been around for billions of years and are often associated with extreme environments such as hot springs and deep-sea hydrothermal vents. Many scientists believe that archaebacteria resemble some of the earliest life forms on Earth, making them one of the oldest living organisms. Summary: The statement "Oldest Living Beings" is true for archaebacteria. They are considered some of the oldest living organisms on Earth and have been around for billions of years. While some archaebacteria are halophiles, some can perform photosynthesis, and they have been found in the fossil record, the most accurate description is that they are among the oldest living beings.

2569. Difference in gram positive and gram negative bacteria is due to

• A. Cell wall
• B. Cell membrane
• C. Ribosome
• D. Cytoplasm

Explanation: The primary difference between Gram-positive and Gram-negative bacteria is due to differences in their cell wall. Let me explain each of the options and provide the correct answer along with details: Cell Wall: Gram-Positive Bacteria: Gram-positive bacteria have a thick peptidoglycan layer in their cell walls. This thick layer retains the crystal violet stain during the Gram staining process, making them appear purple or violet after staining. Gram-Negative Bacteria: Gram-negative bacteria have a thinner peptidoglycan layer in their cell walls, located in the periplasmic space between the inner and outer membranes. Their cell walls also contain an outer membrane with lipopolysaccharides (LPS). This thinner peptidoglycan layer does not retain the crystal violet stain, causing them to appear red or pink after staining. Cell Membrane (Plasma Membrane): The cell membrane is a common feature in all bacteria, both Gram-positive and Gram-negative. It serves as a selectively permeable barrier that controls the passage of substances in and out of the cell. Ribosome: Ribosomes are cellular structures responsible for protein synthesis and are present in both Gram-positive and Gram-negative bacteria. While there may be slight differences in ribosomal composition, these differences are not the primary factor that distinguishes the two groups. Cytoplasm: The cytoplasm is the fluid-filled interior of bacterial cells where various cellular processes occur. While there may be variations in the content of the cytoplasm in different bacterial species, the cytoplasm itself is not the primary distinguishing factor between Gram-positive and Gram-negative bacteria. Correct Answer: The difference between Gram-positive and Gram-negative bacteria is primarily due to the composition and structure of the cell wall. Summary: The primary distinction between Gram-positive and Gram-negative bacteria is the composition of their cell walls. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, while Gram-negative bacteria have a thinner peptidoglycan layer located between the inner and outer membranes. Other cellular components, such as the cell membrane, ribosomes, and cytoplasm, may vary between different bacterial species, but they are not the primary factors that differentiate the two groups.

2570. Adhesive pad of fungi penetrate the host with the help of

• A. mechanical pressure and enzymes
• B. hooks and suckers
• C. softening by enzymes
• D. Only by mechanical pressure

Explanation: Fungal pathogens use various mechanisms to penetrate their host organisms. Let's examine each of the options and provide the correct answer along with details: Mechanical Pressure and Enzymes: Some fungal pathogens use both mechanical pressure and enzymatic action to penetrate the host. Mechanical pressure can help the fungus physically breach the host's surface, and enzymes, such as proteases and cellulases, are secreted to break down host cell walls and tissues, facilitating the invasion. Hooks and Suckers: This option is not typically associated with fungal pathogens. Hooks and suckers are more commonly seen in some parasitic worms and arthropods (e.g., ticks). Fungi generally do not have these specialized structures for host penetration. Softening by Enzymes: Fungal pathogens often employ enzymes to soften the host's tissues by breaking down cell walls, thereby facilitating penetration. Enzymatic softening is a crucial step in the infection process. Only by Mechanical Pressure: While mechanical pressure can play a role in initial contact with the host, fungal pathogens rely heavily on enzymatic activity to degrade and soften the host's tissues. Mechanical pressure alone is generally insufficient for successful host penetration. Correct Answer: Fungal pathogens penetrate the host with the help of both mechanical pressure and enzymes. Summary: Fungal pathogens use a combination of mechanical pressure and enzymatic action to penetrate their host organisms. While mechanical pressure can assist in initial contact, enzymes are crucial for breaking down host cell walls and tissues, making it easier for the fungus to invade the host. Hooks and suckers are not typically associated with fungal pathogens, and relying solely on mechanical pressure is not a common or effective strategy for host penetration by fungi.