All Free Biology MCQs with Answers

2571. Black rust of wheat is caused by

• A. Puccinia
• B. Ustilago
• C. Albugo
• D. Phytophthora

Explanation: "Black rust of wheat" is a plant disease, and it is caused by a specific group of fungi. Let's examine each of the options and provide the correct answer along with details: Puccinia: The correct answer is Puccinia. Puccinia species are responsible for various plant diseases, including rust diseases. Black rust of wheat, also known as stem rust, is caused by Puccinia graminis. It's a serious disease that affects wheat and other cereal crops. Ustilago: Ustilago species are commonly associated with smut diseases, which affect various plants, including cereals. Smut diseases are different from rust diseases like black rust, and Ustilago is not responsible for black rust of wheat. Albugo: Albugo species cause white rust diseases in various plants. These diseases are characterized by the appearance of white pustules on plant leaves, stems, and other parts. Albugo is not associated with black rust in wheat. Phytophthora: Phytophthora is a genus of oomycetes (water moulds) that cause diseases in plants, but it is not responsible for black rust in wheat. Phytophthora is known for causing diseases like late blight in potatoes and downy mildew in various crops. Correct Answer: Black rust of wheat is caused by Puccinia. Summary: Black rust of wheat, also known as stem rust, is caused by the fungus Puccinia graminis. It is a significant disease affecting wheat and other cereal crops. The other options (Ustilago, Albugo, and Phytophthora) are associated with different plant diseases and are not responsible for causing black rust in wheat.

2572. A system of classification, in which a large number of traits are considered, is

• A. Natural system
• B. Phylogenetic system
• C. Artificial system
• D. Synthetic system

Explanation: Natural System: A natural system of classification is based on overall similarity and considers a large number of traits. It aims to reflect the true evolutionary relationships and history of organisms. This system groups organisms together based on their shared characteristics and their common ancestry. It seeks to identify monophyletic groups, where all members share a common ancestor. In a natural system, the traits used for classification are those that are inherited from a common ancestor and reflect the true evolutionary history of the organisms. Phylogenetic System: A phylogenetic system, also known as cladistics, focuses on constructing evolutionary trees or phylogenetic trees based on shared ancestry and common evolutionary history. It uses specific traits that are inherited from a common ancestor and aims to identify monophyletic groups (clades) within the tree. The phylogenetic system is rooted in the concept of common ancestry and seeks to represent the evolutionary relationships among organisms. Artificial System: An artificial system of classification categorizes organisms based on specific, often superficial or arbitrary criteria. This system does not necessarily reflect evolutionary relationships or natural affinities. Instead, it is often used for practical purposes, like organizing plants or animals based on easily observable characteristics. Artificial systems are primarily concerned with ease of identification and organization for human purposes. Synthetic System: The term "synthetic system" is not a commonly recognized term in biological classification. It may refer to a system that combines various approaches, including both natural and artificial criteria, to classify organisms. Correct Answer: The correct answer may vary depending on the context and terminology used. If "Natural System" is the correct answer in your context, it emphasizes that this system considers numerous traits and aims to reflect the actual evolutionary relationships among organisms. Summary: A "Natural System" emphasizes evolutionary relationships and history, considering multiple traits to group organisms based on shared characteristics and common ancestry. A "Phylogenetic System" focuses on constructing phylogenetic trees based on shared ancestry and inherited traits to represent evolutionary relationships. An "Artificial System" categorizes organisms based on practical, often arbitrary criteria for ease of identification and organization. The term "Synthetic System" is not a standard term in biological classification and may refer to a system that combines various approaches.

2573. Photosynthetic bacteria have pigments in:

• A. Chromoplasts
• B. Chromatophores
• C. Leucoplasts
• D. Chloroplasts.

Explanation: Photosynthetic bacteria, often referred to as photosynthetic prokaryotes, are unique microorganisms capable of photosynthesis. These bacteria contain specialized structures for photosynthesis, which are not chloroplasts as found in plant cells. Let's examine each of the options and provide the correct answer along with details: Chromoplasts: Chromoplasts are organelles found in plant cells that contain pigments responsible for giving fruits and flowers their various colors. They are not found in photosynthetic bacteria. Chromatophores: Chromatophores are the correct answer. Photosynthetic bacteria have pigments in chromatophores, which are specialized structures that house pigments required for photosynthesis. These pigments capture light energy and carry out the photosynthetic process in these bacteria. Leucoplasts: Leucoplasts are organelles in plant cells that are involved in the synthesis and storage of starches and oils. They do not play a role in photosynthesis in photosynthetic bacteria. Chloroplasts: Chloroplasts are the organelles responsible for photosynthesis in plant cells, but they are not present in photosynthetic bacteria. Chloroplasts are exclusive to eukaryotic organisms, such as plants and algae. Correct Answer: Photosynthetic bacteria have pigments in "Chromatophores." Summary: Photosynthetic bacteria utilize specialized structures called chromatophores to house pigments required for photosynthesis. These chromatophores are different from chloroplasts, which are found in plant cells, and are not present in photosynthetic bacteria. Other organelles like chromoplasts and leucoplasts are also not involved in photosynthesis in photosynthetic bacteria.

2574. In the five-kingdom classification system, which single kingdom out of the following can include blue-green algae, nitrogen-fixing bacteria, and methanogenic archaebacteria?

• A. Plantae
• B. Protista
• C. Monera
• D. Fungi

Explanation: The five-kingdom classification system was an earlier system used to classify living organisms into five main groups based on their characteristics. Let's examine each of the options and provide details:Plantae (Kingdom Plantae):The Kingdom Plantae includes all green plants and multicellular organisms that photosynthesize. This kingdom does not include blue-green algae, nitrogen-fixing bacteria, or methanogenic archaebacteria.Protista (Kingdom Protista):The Kingdom Protista is a diverse group that includes unicellular eukaryotic organisms like protozoa, algae, and some other simple multicellular organisms. While it may include some algae, it generally does not include blue-green algae (cyanobacteria), nitrogen-fixing bacteria, or methanogenic archaebacteria.Monera (Kingdom Monera):The Kingdom Monera consists of prokaryotic organisms, which include bacteria. Blue-green algae (cyanobacteria), nitrogen-fixing bacteria, and methanogenic archaebacteria would all fall within the Kingdom Monera. So, this is the correct answer.Fungi (Kingdom Fungi):The Kingdom Fungi includes various fungi, such as mushrooms and yeast. It does not include cyanobacteria or other prokaryotic organisms like bacteria or archaebacteria.Correct Answer: In the five-kingdom classification system, the single kingdom that can include blue-green algae (cyanobacteria), nitrogen-fixing bacteria, and methanogenic archaebacteria is "c. Monera."Summary:In the five-kingdom classification system, the Kingdom Monera includes a wide range of prokaryotic organisms, including blue-green algae (cyanobacteria), nitrogen-fixing bacteria, and methanogenic archaebacteria. Other kingdoms like Plantae, Protista, and Fungi do not encompass these specific types of microorganisms.

2575. Transfer of genetic information from one bacterium to another in the transduction process is through:

• A. Bacteriophages released from the donor bacterial strain.
• B. Another bacterium having special organ for conjugation.
• C. Physical contact between donor and recipient strains.
• D. Conjugation between opposite strain bacterium.

Explanation: The transfer of genetic information from one bacterium to another in transduction occurs through bacteriophages. Let's explain each option and provide the correct answer with details: Bacteriophages released from the donor bacterial strain: This is the correct answer. Transduction is a method of horizontal gene transfer in bacteria where genetic material is transferred from one bacterium to another via a bacteriophage. Bacteriophages are viruses that infect bacteria and can carry bacterial DNA from one host to another during the infection cycle. Another bacterium has a special organ for conjugation: This option refers to conjugation, which is another method of horizontal gene transfer in bacteria. Conjugation involves the direct physical contact between two bacterial cells and the transfer of genetic material through a conjugation pilus. It is different from transduction. Physical contact between donor and recipient strains: While physical contact is essential for conjugation, it is not how transduction works. Transduction involves the use of bacteriophages to transfer genetic material, and it does not rely on direct physical contact between the donor and recipient strains. Conjugation between opposite strain bacterium: This option also describes conjugation, which is a different process from transduction. Correct Answer: The transfer of genetic information from one bacterium to another in the transduction process occurs through "a. Bacteriophages released from the donor bacterial strain." Summary: Transduction is a process of genetic transfer in bacteria in which bacteriophages (viruses that infect bacteria) carry bacterial DNA from a donor bacterium to a recipient bacterium. It is a mechanism of horizontal gene transfer and plays a role in bacterial evolution and genetic diversity. Conjugation, on the other hand, involves direct physical contact between bacterial cells and the transfer of genetic material through a conjugation pilus.

2576. The DNA of E.coli is:

• A. double stranded and linear
• B. Double stranded and circular
• C. Single stranded and linear
• D. Single stranded and circular

Explanation: The DNA of Escherichia coli (E. coli) is double-stranded and circular. Let's explain each option and provide the correct answer with details: Double-stranded and linear (a): This is not the correct answer. E. coli has double-stranded DNA, but its DNA is circular, not linear. Double-stranded and circular (b): This is the correct answer. E. coli, like many prokaryotic organisms, has a double-stranded, circular DNA molecule. This circular DNA is found in the nucleoid region within the bacterial cell. Single-stranded and linear (c): E. coli does not have single-stranded DNA, and its DNA is not linear. Single-stranded DNA is often associated with certain types of viruses and is not a characteristic of the genomic DNA of E. coli. Single-stranded and circular (d): E. coli does not have single-stranded DNA as its genomic DNA. Its DNA is double-stranded and circular. Correct Answer: The DNA of E. coli is "b. Double-stranded and circular." Summary: The DNA of E. coli is double-stranded and arranged in a circular form. This circular, double-stranded DNA molecule is located in the nucleoid region of the bacterial cell and is a key feature of prokaryotic genomes.

2577. The main role of bacteria in the carbon cycle involves:

• A. Chemosynthesis.
• B. Digestion or breakdown of organic compounds.
• C. Photosynthesis.
• D. Assimilation of nitrogenous compounds.

Explanation: Bacteria play a crucial role in the carbon cycle, primarily through the digestion or breakdown of organic compounds. Let's explain each option and provide the correct answer with details: Chemosynthesis: Chemosynthesis is a process by which certain bacteria can produce organic compounds using inorganic substances as an energy source. While chemosynthetic bacteria are essential in some ecosystems, their primary role in the carbon cycle is not related to carbon fixation or cycling. Digestion or breakdown of organic compounds: This is the correct answer. Bacteria play a significant role in decomposing organic matter, including dead plants, animals, and other organic material. During this decomposition, bacteria break down complex organic compounds into simpler forms, releasing carbon dioxide (CO2) as a byproduct. This CO2 is then returned to the atmosphere, completing the carbon cycle. Photosynthesis: Photosynthesis is a process carried out by plants, algae, and some bacteria, not just bacteria. It involves the conversion of carbon dioxide and sunlight into organic compounds, but it is not the primary role of bacteria in the carbon cycle. Assimilation of nitrogenous compounds: The assimilation of nitrogenous compounds is related to the nitrogen cycle, not the carbon cycle. Bacteria are involved in various processes in the nitrogen cycle, such as nitrogen fixation and nitrification, but this does not directly relate to their role in the carbon cycle. Correct Answer: The main role of bacteria in the carbon cycle involves the "Digestion or breakdown of organic compounds." Summary: Bacteria play a crucial role in the carbon cycle by breaking down complex organic compounds into simpler forms, which releases carbon dioxide (CO2) into the atmosphere. This process is a fundamental part of carbon cycling, as it helps to recycle carbon from organic matter back into the environment, maintaining the balance of carbon in ecosystems.

2578. A few organisms are known to grow and multiply at temperatures of 100 to 105 oC. They belong to:

• A. Thermophilic sulphur bacteria.
• B. Hot spring bluegreen algae.
• C. Methanogenic archaebacteria.
• D. Marine archaebacteria.

Explanation: Organisms that are known to grow and multiply at temperatures of 100 to 105°C are typically thermophilic and are often associated with extreme environments. Let's examine each option and provide details: Thermophilic Sulphur Bacteria (a): Some thermophilic sulfur bacteria can thrive at high temperatures, including those found in hot springs and hydrothermal vents. However, temperatures as high as 100 to 105°C are quite extreme, and only a limited number of organisms can survive at such temperatures. Hot Spring Blue-Green Algae (b): Hot spring blue-green algae, also known as cyanobacteria, are photosynthetic microorganisms that can be thermophilic. Some cyanobacteria are adapted to high-temperature environments, such as hot springs. While they can thrive in elevated temperatures, not all can grow at temperatures as high as 100 to 105°C. Methanogenic Archaebacteria (c): Methanogenic archaebacteria are often associated with extreme environments, including high-temperature environments. They are known to grow at elevated temperatures, and some may be thermophilic. However, not all of them can withstand temperatures as high as 100 to 105°C. Marine Archaebacteria (d): Marine archaebacteria can inhabit various environments, but their temperature tolerance may not extend to the range of 100 to 105°C. They are generally found in marine environments, which may not reach such high temperatures. Correct Answer: Organisms that are known to grow and multiply at temperatures of 100 to 105°C are primarily associated with "Thermophilic Sulphur Bacteria." Summary: While various extremophiles can survive and thrive in high-temperature environments, including thermophilic sulfur bacteria, hot spring blue-green algae (cyanobacteria), and methanogenic archaebacteria, it is often the thermophilic sulfur bacteria that are capable of growing at temperatures as high as 100 to 105°C. These organisms have unique adaptations that enable them to withstand and even utilize the extreme heat found in specific environments like deep-sea hydrothermal vents and some hot springs.

2579. Which of the following is free-living aerobic non-photosynthetic nitrogen-fixing bacterium?

• A. Nostoc.
• B. Azospirillum.
• C. Rhizobium.
• D. Azotobacter.

Explanation: The correct answer is d. Azotobacter. Let's explain each option and provide more details: a. Nostoc: Nostoc is a cyanobacterium, not a bacterium. It is a photosynthetic microorganism capable of fixing atmospheric nitrogen, but it is not a free-living aerobic non-photosynthetic nitrogen-fixing bacterium. b. Azospirillum: Azospirillum is a free-living, nitrogen-fixing bacterium, but it is facultatively anaerobic, meaning it can grow in the absence of oxygen as well. It does not strictly meet the criteria of being aerobic, as specified in the question. c. Rhizobium: Rhizobium is a nitrogen-fixing bacterium, but it forms a symbiotic relationship with leguminous plants and is not free-living. It establishes nodules on the roots of legumes and fixes nitrogen in this symbiotic association. d. Azotobacter: Azotobacter is a free-living, aerobic, and non-photosynthetic nitrogen-fixing bacterium. It can be found in various soil and aquatic environments and plays a crucial role in nitrogen cycling by converting atmospheric nitrogen into a form that plants can use for growth. Summary: Among the options provided, Azotobacter is the free-living aerobic non-photosynthetic nitrogen-fixing bacterium, and it is known for its ability to fix atmospheric nitrogen independently in various environments.

2580. The site of respiration in bacteria is.

• A. Ribosome.
• B. Microsome.
• C. Episome.
• D. Mesosome.

Explanation: The site of respiration in bacteria is not associated with any of the options provided. Respiration in bacteria primarily occurs in the bacterial cell membrane or the plasma membrane. Let's explain each option and provide the correct information: Ribosome (a): Ribosomes are cellular structures involved in protein synthesis. They have no direct role in respiration. Microsome (b): Microsomes are small vesicles derived from the endoplasmic reticulum in eukaryotic cells. They are not related to bacterial respiration. Episome (c): Episomes are plasmids that can integrate into the bacterial chromosome. They are genetic elements and not sites for respiration. Mesosome (d): This is the correct term for bacterial cell membranes, which can invaginate, forming mesosomes. Mesosomes are believed to play a role in bacterial respiration by increasing the surface area of the cell membrane where respiratory enzymes and electron transport components are located. While the role of mesosomes has been debated, they are thought to be involved in respiration and other cellular processes. Correct Answer: The site of respiration in bacteria is primarily the "Mesosome" (d). Summary: The site of respiration in bacteria is associated with the mesosome, which refers to invaginations of the cell membrane that increase the surface area for respiratory processes. These structures are believed to play a role in bacterial respiration by providing a location for respiratory enzymes and electron transport components. Other options listed do not represent sites of respiration in bacteria.