Biochemistry
5 questionsWhich of the following is not a precursor in the synthesis of pyrimidines?
What is attached to the 3' end of mRNA after transcription?
What does Chargaff's rule state regarding the base pairing in DNA?
Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?
What is the rate-limiting enzyme in heme synthesis?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 401: Which of the following is not a precursor in the synthesis of pyrimidines?
- A. Glutamine
- B. Carbon dioxide (CO2)
- C. Aspartic acid
- D. Thymidine (Correct Answer)
Explanation: ***Thymidine*** - **Thymidine** is a *nucleoside* consisting of deoxyribose and thymine. It is a *product* and a component of DNA, not a precursor in the *de novo synthesis* of pyrimidine bases. - While it can be incorporated into DNA via the *salvage pathway*, it does not serve as an initial building block for the pyrimidine ring itself. *Glutamine* - **Glutamine** provides the **nitrogen atoms** crucial for the formation of the pyrimidine ring, specifically N3 in the pyrimidine base. - It is a key donor of *amino groups* in various anabolic pathways, including nucleotide synthesis. *Carbon dioxide (CO2)* - **Carbon dioxide (CO2)** contributes one of the carbon atoms (C2) to the pyrimidine ring. - It combines with **ammonia** (derived from glutamine) to form **carbamoyl phosphate**, an essential intermediate. *Aspartic acid* - **Aspartic acid** provides four atoms (N1, C4, C5, C6) of the pyrimidine ring. - Its carbon skeleton and amino group are directly incorporated into the pyrimidine structure during the *de novo synthesis* pathway.
Question 402: What is attached to the 3' end of mRNA after transcription?
- A. CCA
- B. Intron
- C. 7-methylguanosine
- D. Poly-A tail (Correct Answer)
Explanation: ***Poly-A tail*** - A **poly-A tail**, consisting of multiple adenosine monophosphates, is added to the **3' end of mRNA** after transcription to protect it from degradation. - This modification aids in the **transport of mRNA from the nucleus to the cytoplasm** and in its translation. *CCA* - The **CCA sequence** is found at the **3' end of tRNA**, not mRNA, and is critical for amino acid attachment. - It is added post-transcriptionally to tRNA molecules by the enzyme **tRNA nucleotidyltransferase**. *Intron* - **Introns** are non-coding regions within a gene that are transcribed into mRNA but are subsequently removed during **RNA splicing**, not added to the 3' end. - Their removal ensures that only the **coding regions (exons)** are translated into protein. *7-methylguanosine* - **7-methylguanosine** forms the **5' cap** of mRNA, which is added to the 5' end, not the 3' end. - This cap is important for **mRNA stability**, ribosome binding, and protection against degradation.
Question 403: What does Chargaff's rule state regarding the base pairing in DNA?
- A. A=T, G=C (Correct Answer)
- B. A=G, T=C
- C. A=C, G=T
- D. Any combination possible
Explanation: ***A=T, G=C*** - **Chargaff's rules** state that in any double-stranded DNA, the amount of **adenine (A)** is approximately equal to the amount of **thymine (T)**, and the amount of **guanine (G)** is approximately equal to the amount of **cytosine (C)**. - This equivalency reflects the specific **base pairing** in the DNA double helix, where A always pairs with T, and G always pairs with C. *A=G, T=C* - This statement is incorrect as it proposes an atypical and biologically inaccurate pairing between a **purine (A)** and another **purine (G)**, and a **pyrimidine (T)** with a **pyrimidine (C)**. - This combination would disrupt the uniform diameter of the DNA double helix required for its structural stability. *A=C, G=T* - This option is incorrect because it suggests pairing a purine (A) with a pyrimidine (C) and a purine (G) with a pyrimidine (T) in a way that is not observed in natural DNA. - Such pairings would also lead to an irregular width of the DNA molecule, destabilizing its structure. *Any combination possible* - This statement is false; base pairing in DNA is **highly specific** and not random due to chemical and structural constraints. - The specific pairing rules (**A with T, G with C**) are crucial for maintaining the consistent structure of the DNA double helix and for accurate DNA replication and transcription.
Question 404: Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?
- A. Transcribed from nuclear DNA.
- B. Has a lower molecular weight than hn-RNA. (Correct Answer)
- C. Contains uracil instead of thymine.
- D. Sugar is ribose.
Explanation: ***Has a lower molecular weight than hn-RNA.*** - **Mature mRNA** undergoes **splicing**, which removes **introns** (non-coding regions) from the heterogeneous nuclear RNA (hnRNA) precursor. - The removal of these introns results in a **shorter, more compact molecule** with a lower molecular weight compared to the original hnRNA. *Transcribed from nuclear DNA.* - While mRNA is indeed **transcribed from DNA**, this statement is true for **all types of RNA (rRNA, tRNA, and mRNA)**, not just mature cytoplasmic mRNA specifically, and does not differentiate it. - The initial transcript is **hnRNA**, which is then processed into mature mRNA. *Contains uracil instead of thymine.* - This is a characteristic of **all RNA molecules**, not just mature cytoplasmic mRNA, and is a fundamental difference between RNA and DNA. - DNA contains **thymine**, while RNA contains **uracil**. *Sugar is ribose.* - This is a distinguishing feature of **all RNA molecules**, indicating that the sugar component of its nucleotides is **ribose**, whereas DNA contains **deoxyribose**. - This statement is not unique to mature cytoplasmic mRNA.
Question 405: What is the rate-limiting enzyme in heme synthesis?
- A. ALA synthase (Correct Answer)
- B. HMG CoA reductase
- C. ALA dehydratase
- D. Uroporphyrinogen 1 synthase
Explanation: ***ALA synthase*** - **Aminolevulinate synthase** (ALA synthase) is the first and **rate-limiting enzyme** in the heme synthesis pathway. - Its activity is tightly regulated, and its overexpression or deficiency can lead to disorders like **acute intermittent porphyria**. *Hmg coa reductase* - **HMG-CoA reductase** is the **rate-limiting enzyme** in the **cholesterol biosynthesis pathway**, not heme synthesis. - It is the target enzyme for statin medications, which lower cholesterol levels. *ALA dehydratase* - **ALA dehydratase** (also known as porphobilinogen synthase) is the second enzyme in the heme synthesis pathway, responsible for converting two molecules of **ALA to porphobilinogen**. - While critical, it is not the rate-limiting step; inhibition of this enzyme can lead to **lead poisoning**. *Uroporphyrinogen 1 synthase* - **Uroporphyrinogen I synthase** (also called hydroxymethylbilane synthase or porphobilinogen deaminase) catalyzes the formation of **hydroxymethylbilane** from four molecules of **porphobilinogen**. - A deficiency in this enzyme is associated with **acute intermittent porphyria**, but it is not the rate-limiting enzyme of the overall pathway.
Microbiology
1 questionsWhich of the following is the most potent stimulator of Naive T-cells?
NEET-PG 2012 - Microbiology NEET-PG Practice Questions and MCQs
Question 401: Which of the following is the most potent stimulator of Naive T-cells?
- A. Macrophages
- B. B-cell
- C. Mature dendritic cells (Correct Answer)
- D. Follicular dendritic cells
Explanation: ***Mature dendritic cells*** - **Mature dendritic cells** are the most potent professional antigen-presenting cells (APCs) for activating **naive T cells** due to their efficient antigen processing, presentation abilities, and high expression of costimulatory molecules (e.g., CD80, CD86) and MHC-peptide complexes. - Activated by pathogens or inflammatory signals, they migrate to secondary lymphoid organs where they initiate primary immune responses by presenting antigens to and activating naive T cells. *Follicular dendritic cells* - **Follicular dendritic cells** primarily present intact antigens to **B cells** in germinal centers of secondary lymphoid organs, playing a crucial role in B cell maturation, selection, and antibody production. - They lack MHC class II molecules and thus cannot directly present antigens to naive T cells. *Macrophages* - While **macrophages** are professional APCs, they are generally less efficient than mature dendritic cells at activating **naive T cells**, especially in the initiation of primary immune responses. - They are more involved in presenting antigens to already activated T cells and clearing pathogens, often acting as secondary APCs. *B-cell* - **B cells** can act as APCs, but they are generally less efficient than **dendritic cells** in activating **naive T cells**, especially for the primary immune response. - Their primary role in antigen presentation is to present processed antigens to **helper T cells** to receive costimulation for their own activation and differentiation into plasma cells, often after being activated themselves.
Pathology
3 questionsLines of Zahn occur in -
What is the term for a localized malformation composed of an excessive but disorganized arrangement of cells and tissues indigenous to the site?
In which organ do atheromatous changes of blood vessels typically occur early in the disease process?
NEET-PG 2012 - Pathology NEET-PG Practice Questions and MCQs
Question 401: Lines of Zahn occur in -
- A. Thrombus (Correct Answer)
- B. Embolus
- C. Infarct
- D. Postmortem clot
Explanation: ***Thrombus*** - **Lines of Zahn** are alternating layers of **platelets** (lighter bands) and **red blood cells** (darker bands) that are characteristic of a **thrombus** formed in flowing blood. - Their presence indicates that the clot was formed in a vessel where there was **blood flow** *Infarct* - An **infarct** is an area of **ischemic necrosis** caused by occlusion of either the arterial supply or venous drainage in a particular tissue. - While a thrombus can cause an infarct, an infarct itself does not contain Lines of Zahn; rather, it is the consequence of the thrombus. *Embolus* - An **embolus** is a detached intravascular solid, liquid, or gaseous mass that is carried by the blood to a site distant from its origin. - An embolus can be a fragment of a thrombus and therefore could contain Lines of Zahn, but the primary structure where these lines are formed is the stationary thrombus within a vessel. *Postmortem clot* - A **postmortem clot** forms after death and is typically gelatinous, poorly attached to the vessel wall, and has a dark red dependent portion (due to red cell settling) and a yellowish upper portion (like "chicken fat"). - It does not exhibit the layered architecture of platelets and red blood cells seen in **Lines of Zahn**, as there is no active blood flow or coagulation process at play.
Question 402: What is the term for a localized malformation composed of an excessive but disorganized arrangement of cells and tissues indigenous to the site?
- A. Hamartoma (Correct Answer)
- B. Malignant tumor
- C. Choristoma
- D. None of the options
Explanation: ***Hamartoma*** - A **hamartoma** is an overgrowth of cells and tissues that are normally found in the affected area, but in a disordered fashion, creating a tumor-like growth [1]. - It's a **benign (non-cancerous)** lesion, often congenital, that grows at the same rate as the surrounding tissues. *Malignant tumor* - A **malignant tumor** is characterized by uncontrolled cell growth that invades surrounding tissues and can metastasize to distant sites. - Unlike a hamartoma, a malignant tumor consists of **abnormal, dysplastic cells** that do not resemble the normal tissues of the organ. *Choristoma* - A choristoma is a **benign tumor-like growth** consisting of normal cells or tissues that are **heterotopic**, meaning they are located in an abnormal site. - An example is the presence of pancreatic tissue in the wall of the stomach, which is normal tissue in an abnormal location, unlike a hamartoma which has normal tissue in the correct location but in a disorganized manner. *None of the options* - This option is incorrect because **hamartoma** accurately describes the overgrowth of a skin structure at a localized region made of normal, but disorganized, tissue [1]. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Disorders Involving Inflammatory And Haemopoietic Cells, pp. 651-652.
Question 403: In which organ do atheromatous changes of blood vessels typically occur early in the disease process?
- A. Kidney
- B. Heart (Correct Answer)
- C. Liver
- D. Spleen
Explanation: ***Heart*** - The **coronary arteries**, which supply the heart, are particularly susceptible to **atherosclerosis** due to high blood flow turbulence and shear stress [1]. - Early atheromatous changes often begin in these arteries, leading to conditions like **coronary artery disease (CAD)** [1]. *Kidney* - While the kidneys can be affected by **atherosclerosis** (renal artery stenosis), it typically occurs later in the disease process or in the presence of more widespread disease [1]. - The primary early site for systemic atherosclerosis is generally not the renal arteries. *Liver* - The liver is not a primary site for the development of **atherosclerosis** within its own blood vessels. - Liver disease can influence lipid metabolism, but directly developing atheroma within hepatic arteries is uncommon. *Spleen* - The spleen is rarely the primary or early site for **atheromatous changes**. - Its blood vessels are generally less prone to the turbulent flow and plaque formation seen in major arteries. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 499-508.
Physiology
1 questionsWhen differentiated cells transform to form cells characteristic of other tissues, the process is called as -
NEET-PG 2012 - Physiology NEET-PG Practice Questions and MCQs
Question 401: When differentiated cells transform to form cells characteristic of other tissues, the process is called as -
- A. De-differentiation
- B. Re-differentiation
- C. Trans-differentiation (Correct Answer)
- D. Sub-differentiation
Explanation: ***Trans-differentiation*** - **Trans-differentiation** refers to the direct conversion of one differentiated cell type into another differentiated cell type without entering a pluripotent stem cell state. - This process is achieved by altering the **gene expression profile** of existing cells to adopt the characteristics of a different lineage. *De-differentiation* - **De-differentiation** is the process where a specialized cell loses its specific characteristics and reverts to a more primitive or stem cell-like state. - This is often observed in certain disease processes, like cancer, or in response to injury where cells regain limited proliferative capacity. *Re-differentiation* - **Re-differentiation** typically describes a cell that has undergone de-differentiation and then differentiates again into a new or its original cell type. - This process is often seen in tissue repair, where progenitor cells proliferate and then re-differentiate to replace damaged tissue. *Sub-differentiation* - **Sub-differentiation** is not a standard or recognized term in cell biology or developmental biology to describe the transformation of stem cells into other tissue types. - The term does not have a defined meaning within the context of cellular lineage alterations.