All Free Biology MCQs with Answers

2631. The phenomenon of transcription in HIV life cycle occurs in:

• A. Nucleus
• B. Cytoplasm
• C. Inside viral capsid
• D. None of these

Explanation: Option A is correct.In HIV infected helper T-cells, the process of transcription occurs in nucleus while the process of reverse transcription occurs in cytoplasm.The HIV virus undergoes a number of processes in the nucleus, cytoplasm, and viral capsid during its replication cycle.In the nucleus:Reverse transcription: The viral RNA is reverse transcribed into DNA by the reverse transcriptase enzyme.Integration: The DNA is integrated into the DNA of the host cell by the integrase enzyme.Transcription: The integrated DNA is transcribed into RNA by the host cell's DNA polymerase.In the cytoplasm:Translation: The RNA is translated into proteins by the host cell's ribosomes.Protease cleavage: The proteins are cleaved by the protease enzyme into their active forms.Assembly: The viral proteins and RNA assemble into new virions.In the viral capsid:Maturation: The virions are matured by the removal of the proteins that are not needed for infection.Budding: The virions bud from the host cell membrane and are released into the extracellular environment.

2632. The major cell infected by the HIV is:

• A. Leukocyte
• B. Monocyte
• C. Helper T-lymphocyte
• D. B-lymphocyte

Explanation: Option C is correct.Here are the definitions of leukocytes, monocytes, helper T-lymphocytes, and B-lymphocytes:Leukocyte: A leukocyte is a type of white blood cell that is involved in the immune system. There are five main types of leukocytes: neutrophils, eosinophils, basophils, monocytes, and lymphocytes.Monocyte: A monocyte is a type of leukocyte that is derived from bone marrow and matures in the blood. Monocytes are phagocytic cells, which means that they can engulf and destroy foreign invaders. Monocytes also produce cytokines, which are proteins that help to regulate the immune response.Helper T-lymphocyte: A helper T-lymphocyte is a type of lymphocyte that helps to coordinate the immune response. Helper T-lymphocytes produce cytokines that activate other immune cells, such as B-lymphocytes and cytotoxic T-lymphocytes. This one is the correct answer as HIV virus attack on helper T- lymphocytes and causes AIDS.B-lymphocyte: A B-lymphocyte is a type of lymphocyte that produces antibodies. Antibodies are proteins that bind to specific antigens, which are foreign substances that the body recognizes as being harmful. B-lymphocytes are activated by helper T-lymphocytes, and they then produce antibodies that help to fight infection.

2633. Reverse transcriptase is used to make DNA copies of:

• A. Host RNA
• B. Viral RNA
• C. Host DNA
• D. Viral DNA

Explanation: Option B is correct.HIV is equipped with various enzymes and among them: reverse transcriptase is of great importance. It takes HIV genomic dsRNA as template and reverse transcribe it to form viral dsDNA in the cytoplasm of the infected cells. The following steps take place in the reverse transcription process: 1) Initiation: Reverse transcriptase binds to the HIV RNA genome and recruits a primer, which is a short piece of RNA that is used to start DNA synthesis. 2)Elongation: Reverse transcriptase uses the primer to synthesize a DNA copy of the HIV RNA genome. 3) Termination: Reverse transcriptase terminates DNA synthesis when it reaches a specific sequence in the HIV RNA genome. Host RNA and host DNA are not used as templates by reverse transcriptase. The reverse transcriptase enzyme works in a few steps:The enzyme binds to the viral RNA.Then it unzips the RNA molecule, exposing the single-stranded RNA.The enzyme then uses the single-stranded RNA as a template to synthesize a DNA copy.The enzyme uses the single-stranded RNA as a template to synthesize a DNA copy.The reverse transcriptase enzyme works in a few steps:The enzyme binds to the viral RNA.Then it unzips the RNA molecule, exposing the single-stranded RNA.The enzyme then uses the single-stranded RNA as a template to synthesize a DNA copy.The enzyme uses the single-stranded RNA as a template to synthesize a DNA copy.

2634. Which of the following cells are mainly infected by HIV?

• A. T-killer lymphocytes
• B. T-helper lymphocytes
• C. B-plasma cells
• D. B-memory cells

Explanation: Option B is correct.A)T-killer lymphocytes, also known as cytotoxic T-cells, are a type of T-cell that is responsible for killing infected cells. They are a part of the adaptive immune system and play a role in cell-mediated immunity. T-killer lymphocytes are derived from T-cells that mature in the thymus gland. In the thymus, T-cells are exposed to a variety of antigens, which helps them to develop the ability to recognize and respond to specific invaders. HIV attacks on helper T-lymphocytes so this option is incorrect.B)Helper T-lymphocytes, also known as CD4+ T-cells, are a type of T-cell that helps other immune cells to fight infection. They are a part of the adaptive immune system and play a role in cell-mediated immunity. Helper T-lymphocytes are derived from T-cells that mature in the thymus gland. In the thymus, T-cells are exposed to a variety of antigens, which helps them to develop the ability to recognize and respond to specific invaders. This is the correct answer.C)B-plasma cells are mature B-cells that produce antibodies. They are found in the bone marrow and lymph nodes. B-plasma cells are responsible for the humoral immune response, which is the part of the immune system that fights infection using antibodies. HIV attacks on helper T-lymphocytes so this option is incorrect.D)B-memory cells are a type of B-cell that is produced during the primary immune response. They are long-lived cells that can survive for many years. B-memory cells are important for the secondary immune response, which is the faster and more robust immune response that occurs when a person is exposed to an antigen that they have been exposed to before.HIV attacks on helper T-lymphocytes so this option is incorrect.

2635. HIV is classified as:

• A. Bacteriophage
• B. Oncovirus
• C. Retrovirus
• D. Icosahedral virus

Explanation: Option C is correct.A)A bacteriophage, also known as a phage, is a virus that infects and replicates within bacteria. Phages are typically much simpler than other viruses, consisting of only a protein coat and a nucleic acid genome. The genome of a bacteriophage can be either DNA or RNA, and it can be single-stranded or double-stranded. It is not HIV virus so it is not correct.B) An oncovirus is a virus that can cause cancer. Oncoviruses are a type of retrovirus, which means that they carry their genetic material in the form of RNA. The RNA is reverse transcribed into DNA by the virus's reverse transcriptase enzyme. The DNA is then integrated into the DNA of the host cell, where it can be replicated and transcribed into new viral RNA. It is not HIV virus so it is not correct.C) Retrovirus are a type of viruses that uses RNA as its genetic material. After infecting their host cells, the retrovirus uses an enzyme called reverse transcriptase to convert its genomic RNA into DNA. The retrovirus then integrates its viral DNA into the DNA of the host cell, which allows the retrovirus to replicate. One typical example of retrovirus is HIV.So, this option is the correct one. D) An icosahedral virus is a virus that has a protein shell that is made up of 20 triangular faces. The icosahedral shape is the most common shape for viruses, and it is found in a wide variety of viruses, including poliovirus, rhinovirus, and adenovirus. It is not HIV virus so it is not correct.

2636. Cilia and flagella are absent in:

• A. Viruses
• B. Bacteria
• C. Protists
• D. Lower animals

Explanation: Cilia and flagella are absent in in viruses other organisms may contain these structures.1. **Viruses**: Viruses are acellular entities, consisting of genetic material (either DNA or RNA) enclosed in a protein coat (capsid). They lack cellular structures, including cilia and flagella. Viruses do not possess the machinery for movement.2. **Bacteria**: While some bacteria have flagella for locomotion, not all bacteria possess cilia or flagella. Many bacteria move using other mechanisms, like twitching or gliding. The presence of cilia or flagella varies among bacterial species.3. **Protists**: Many protists, which are eukaryotic microorganisms, have cilia or flagella. These structures are commonly used for locomotion and feeding in protists. However, there are exceptions, and not all protists have cilia or flagella. Some may use other structures for movement, such as pseudopods.4. **Lower Animals**: Cilia and flagella are often present in lower animals, including protozoans and various simple multicellular organisms. These structures are used for movement and other functions. Lower animals exhibit a wide diversity in their use of cilia and flagella.In summary, the presence of cilia and flagella varies among different organisms. While they are generally absent in viruses, their presence or absence in bacteria, protists, and lower animals can vary depending on the specific organism and its evolutionary adaptations.

2637. In viruses, a combined structure formed by core (nucleic acid) and capsid is:

• A. Nucleocapsid
• B. Envelope
• C. Capsomeres
• D. Prions

Explanation: Option A is correct.A)A nucleocapsid is the protein coat that surrounds the nucleic acid of a virus particle. It is made up of one or more types of proteins that are specifically encoded by the virus genome. The nucleocapsid protects the nucleic acid from damage and helps to direct the replication of the virus.It is made of nucliec acid(genome) and protein coat (capsid).It is the correct option.B) A virus envelope is the outermost layer of some viruses. It is made up of a lipid bilayer that is derived from the host cell membrane. The envelope contains viral proteins that help the virus to attach to and enter host cells. This option is incorrect.C) A capsomere is a protein subunit that makes up the capsid, an outer covering of protein that protects the genetic material of a virus. Capsomeres self-assemble to form the capsid.The capsomeres protect against physical, chemical, and enzymatic damage and are multiply redundant; having a few protein subunits that are repeated. This option is incorrect.C) A capsomere is a protein subunit that makes up the capsid, an outer covering of protein that protects the genetic material of a virus. Capsomeres self-assemble to form the capsid.The capsomeres protect against physical, chemical, and enzymatic damage and are multiply redundant; having a few protein subunits that are repeated. C) A capsomere is a protein subunit that makes up the capsid, an outer covering of protein that protects the genetic material of a virus. Capsomeres self-assemble to form the capsid.The capsomeres protect against physical, chemical, and enzymatic damage and are multiply redundant; having a few protein subunits that are repeated. D)Prions are infectious agents that are composed of misfolded proteins. Prions are not like other infectious agents, such as viruses or bacteria, because they do not contain any nucleic acids. Instead, they are simply misfolded proteins that can induce other proteins to fold abnormally. This can lead to the accumulation of misfolded proteins in the brain, which can cause damage and eventually death.This option is incorrect.

2638. Capsid, the protective coat of a virus is made up of _ subunits known as capsomers.

• A. Lipid
• B. Protein
• C. RNA
• D. DNA

Explanation: Option B is correct.Chemically, a virion is made up core of genetic material: either DNA or RNA surrounded by a protective coat called a capsid which is made up of protein.Capsid of viruses is made up of protein subunits known as capsomeres. Number of capsomeres is characteristic of a particular virus. The capsid is not made up of lipids, RNA and DNA both.

2639. _ enzyme is not found in human immunodeficiency virus.

• A. DNA polymerase
• B. Reverse transcriptase
• C. RNA polymerase
• D. Ligase

Explanation: Option B is correct.HIV contains three enzymes that are essential for its replication:1)Reverse transcriptase: Reverse transcriptase is an enzyme that is responsible for converting the RNA genome of HIV into DNA. This DNA can then be integrated into the host cell's genome, where it can be replicated and expressed. 2)Protease: Protease is an enzyme that is responsible for cleaving the HIV polyprotein into its functional proteins. These proteins are then used to assemble new HIV virions. 3)Integrase or ligase: Integrase is an enzyme that is responsible for integrating the HIV DNA into the host cell's genome. This integration allows the HIV DNA to be replicated and expressed by the host cell. The ligase in HIV is called integrase. Integrase is a very important enzyme for HIV replication.So, HIV particles are equipped with reverse transcriptase or DNA polymerase, integrase or ligase and proteases while RNA polymerase is absent in HIV. RNA polymerase is an enzyme that is used by cells to make RNA from DNA. It is not found in HIV, because HIV does not need to make RNA from DNA. Instead, it uses the RNA that it already has to make new copies of the virus.

2640. The filterable agents were first purified in 1935, when Stanley was successful in crystallizing the _

• A. Polio virus
• B. Tobacco mosaic virus
• C. Hepatitis virus
• D. Influenza virus

Explanation: Option B is correct.Karl Landsteiner and Erwin Popper discovered the poliovirus in 1908. Landsteiner was an Austrian-American pathologist and immunologist, while Popper was an Austrian physician. They isolated the virus from the spinal cord of a child who had died of poliomyelitis. Stanley was the first scientist who crystallized tobacco mosaic virus in 1935.TMV is transmitted from infected plant to healthy plant through contact, such as by touching an infected leaf or by using the same tools on both infected and healthy plants. TMV can also be transmitted through the air, but this is less common.Baruch Blumberg, who discovered the hepatitis B virus (HBV) in 1967. Blumberg was an Australian-American physician and immunologist who was awarded the Nobel Prize in Physiology or Medicine in 1976 for his discovery. Wilson Smith, Christopher H. Andrewes, and Patrick P. Laidlaw are credited with the discovery of the influenza virus in 1933. They were working at the National Institute for Medical Research in London, England, when they isolated the virus from the throat washings of a patient with influenza.