Biochemistry
9 questionsApo B48 is synthesized in -
What is the end product of purine metabolism in most mammals?
Which of the following statements is true regarding the sigma factor?
What is a key similarity between the processes of replication and transcription?
What are Okazaki fragments?
What is the first purine nucleotide synthesized in de novo purine biosynthesis?
C4, C5, and N7 in the purine ring are derived from which of the following?
What is the most important tool used in genetic engineering?
If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 321: Apo B48 is synthesized in -
- A. Liver
- B. Kidney
- C. Intestine (Correct Answer)
- D. RBCs
Explanation: ***Intestine*** - **Apo B48** is a truncated form of apolipoprotein B-100, uniquely synthesized in the **intestine** through RNA editing. - It is a crucial structural component of **chylomicrons**, which are lipoprotein particles responsible for transporting exogenous dietary lipids from the intestine to other tissues. *Liver* - The liver primarily synthesizes **Apo B100**, which is a full-length apolipoprotein B and a major component of VLDL, IDL, and LDL. - It does not produce Apo B48. *Kidney* - The kidneys are involved in filtering waste products and regulating fluid balance, but they do not play a role in the synthesis of apolipoproteins like Apo B48. - Kidney cells are not equipped with the specific machinery for Apo B mRNA editing. *RBCs* - Red blood cells (RBCs) are primarily responsible for oxygen transport and lack a nucleus and most organelles, including those required for protein synthesis. - Therefore, RBCs cannot synthesize proteins such as Apo B48.
Question 322: What is the end product of purine metabolism in most mammals?
- A. Glycogen
- B. Pyrimidine
- C. Histidine
- D. Allantoin (Correct Answer)
Explanation: ***Allantoin*** - **Allantoin** is the primary end product of **purine metabolism** in **most mammals** (except humans and higher primates), formed by the oxidation of uric acid by the enzyme **uricase**. - This conversion makes purine waste products more **water-soluble** and easier to excrete via the kidneys. - **Important clinical note:** Humans lack functional uricase, so **uric acid** is the end product in humans; this distinction is why hyperuricemia and gout occur in humans but not in most other mammals. *Glycogen* - **Glycogen** is a complex carbohydrate and serves as a primary **energy storage molecule** in animals, derived from glucose metabolism, not purine catabolism. - Its metabolism is regulated by hormones like **insulin** and **glucagon**, involved in maintaining blood glucose levels. *Pyrimidine* - **Pyrimidine** is a type of nitrogenous base, structurally distinct from purines, and is a component of DNA and RNA, not an end product of purine catabolism. - **Pyrimidine metabolism** involves the synthesis and breakdown of bases like cytosine, thymine, and uracil, which follows a separate biochemical pathway. *Histidine* - **Histidine** is an **essential amino acid**, a building block of proteins, and is involved in various metabolic processes, including histamine synthesis. - It plays no role as an end product of purine degradation; rather, its own metabolism leads to products like **urocanic acid**.
Question 323: Which of the following statements is true regarding the sigma factor?
- A. It is a subunit of DNA polymerase.
- B. It is a subunit of RNA polymerase. (Correct Answer)
- C. It initiates DNA replication.
- D. It is a subunit of the 50s ribosome.
Explanation: ***It is a subunit of RNA polymerase.*** - The **sigma factor** is a crucial component of **bacterial RNA polymerase**, guiding it to specific promoter regions on the DNA. - It plays a vital role in **initiation of transcription** by recognizing and binding to the **-10 and -35 boxes** of the promoter. *It is a subunit of DNA polymerase.* - **DNA polymerase** is primarily involved in **DNA replication and repair**, not transcription. - Its subunits, such as the **beta clamp** or **alpha subunit**, are distinct from the sigma factor. *It initiates DNA replication.* - **DNA replication** is initiated by **DNA helicases** unwinding the double helix and **primase** synthesizing RNA primers. - The sigma factor's role is in **transcription**, the synthesis of RNA from a DNA template. *It is a subunit of the 50s ribosome.* - The **50S ribosomal subunit** is a component of the **ribosome**, responsible for **peptide bond formation** during translation. - Its subunits are ribosomal proteins and ribosomal RNA molecules, not the sigma factor.
Question 324: What is a key similarity between the processes of replication and transcription?
- A. Use RNA primers for initiation.
- B. Use ribonucleotides as precursors.
- C. Are semi-conservative events.
- D. Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction. (Correct Answer)
Explanation: ***Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction.*** - Both DNA replication and RNA transcription synthesize nucleic acid polymers by forming **phosphodiester bonds** between incoming nucleotides. - The new strand in both processes is always elongated in the **5' to 3' direction**, as new nucleotides are added to the 3' hydroxyl group of the growing strand. *Use RNA primers for initiation.* - **DNA replication** requires **RNA primers** to initiate synthesis of new DNA strands, as DNA polymerase cannot start a new strand *de novo*. - **Transcription (RNA synthesis)** does not require a primer; **RNA polymerase** can initiate transcription *de novo* at a promoter sequence. *Use ribonucleotides as precursors.* - **Transcription** uses **ribonucleotides** (ATP, UTP, CTP, GTP) as precursors to synthesize RNA. - **Replication** primarily uses **deoxyribonucleotides** (dATP, dTTP, dCTP, dGTP) to synthesize DNA, although it temporarily uses ribonucleotides for RNA primers. *Are semi-conservative events.* - **DNA replication** is a **semi-conservative process**, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. - **Transcription** is **not semi-conservative**; it involves synthesizing an RNA molecule from a DNA template, leaving the original DNA template unchanged.
Question 325: What are Okazaki fragments?
- A. Long pieces of DNA on the lagging strand.
- B. Short pieces of DNA on the lagging strand. (Correct Answer)
- C. Short pieces of DNA on the leading strand.
- D. Long pieces of DNA on the leading strand.
Explanation: ***Short pieces of DNA on the lagging strand.*** - **Okazaki fragments** are the short, newly synthesized DNA fragments that are formed on the **lagging strand** during DNA replication. - The lagging strand is synthesized discontinuously because DNA polymerase can only add nucleotides in the **5' to 3' direction**, requiring it to move away from the replication fork as the DNA unwinds. *Long pieces of DNA on the lagging strand.* - The lagging strand is synthesized discontinuously in **short fragments**, not long continuous pieces. - The enzyme **DNA ligase** eventually joins these short fragments together to form a continuous strand. *Short pieces of DNA on the leading strand.* - The **leading strand** is synthesized continuously in one long stretch, moving towards the replication fork. - It does not require the synthesis of short fragments like the lagging strand. *Long pieces of DNA on the leading strand.* - While the leading strand is synthesized in a continuous, long piece, this statement does not accurately describe Okazaki fragments, which are specific to the lagging strand. - The leading strand's continuous synthesis is due to its **3' to 5' template orientation**, allowing DNA polymerase to proceed uninterrupted.
Question 326: What is the first purine nucleotide synthesized in de novo purine biosynthesis?
- A. AMP
- B. GMP
- C. IMP (Correct Answer)
- D. UMP
Explanation: ***IMP (Inosine Monophosphate)*** - **IMP** is the first complete purine nucleotide synthesized during the **de novo purine biosynthesis pathway**. - It serves as a branch point, from which **AMP** and **GMP** are subsequently synthesized through separate pathways. *AMP (Adenosine Monophosphate)* - **AMP** is a derivative of **IMP**, synthesized by the addition of an amino group from **aspartate** to IMP. - This step occurs after the formation of the complete purine ring structure in IMP. *GMP (Guanosine Monophosphate)* - **GMP** is also derived from **IMP**, through a pathway involving the oxidation of IMP to **XMP** (xanthosine monophosphate) and subsequent amination. - Its synthesis occurs downstream from IMP. *UMP (Uridine Monophosphate)* - **UMP** is a **pyrimidine nucleotide**, not a purine, and is synthesized via a completely different de novo pathway. - Pyrimidine biosynthesis involves forming the ring structure first, then attaching it to ribose-phosphate, unlike purine synthesis which builds the ring on a pre-existing ribose-phosphate.
Question 327: C4, C5, and N7 in the purine ring are derived from which of the following?
- A. CO₂
- B. Aspartate
- C. Glutamine
- D. Glycine (Correct Answer)
Explanation: ***Glycine*** - The entire **glycine molecule** contributes C4, C5, and N7 to the purine ring structure. - This amino acid provides a significant portion of the backbone to the imidazole ring within the purine. *Aspartate* - **Aspartate** contributes N1 to the purine ring. - It does not involve C4, C5, or N7, which are distinct atoms within the purine molecule. *CO₂* - **CO₂** contributes C6 to the purine ring through a carboxylation step. - It is not involved in providing the atoms at positions C4, C5, or N7. *Glutamine* - The nitrogen atoms N3 and N9 in the purine ring are derived from the **amide nitrogen of glutamine**. - Glutamine's contributions are different from the carbons and nitrogen provided by glycine.
Question 328: What is the most important tool used in genetic engineering?
- A. Topoisomerase
- B. DNA Ligase
- C. Restriction endonuclease (Correct Answer)
- D. Helicase
Explanation: ***Restriction endonuclease*** - **Restriction endonucleases** are crucial for genetic engineering as they specifically cut DNA at particular recognition sites, allowing the insertion or deletion of genes. - This precise cutting ability is fundamental for creating **recombinant DNA** molecules. *Helicase* - **Helicase** is primarily involved in unwinding the DNA double helix during processes like DNA replication and transcription. - While essential for cellular functions, it does not directly manipulate DNA for gene insertion or modification in the way restriction enzymes do. *Topoisomerase* - **Topoisomerase** enzymes are responsible for managing DNA supercoiling, preventing tangling during DNA replication and transcription by cutting and rejoining DNA strands. - It plays a role in DNA structure but is not directly used for targeted gene editing or insertion. *DNA Ligase* - **DNA ligase** is essential for joining DNA fragments, which is a critical step in genetic engineering after restriction endonucleases have cut the DNA. - However, while it acts as a "molecular glue" to seal nicks and re-form phosphodiester bonds, it cannot initiate the precise cutting required to isolate genes.
Question 329: If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?
- A. 15%
- B. 85%
- C. 70%
- D. 35% (Correct Answer)
Explanation: ***35%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If A = 15%, then T must also be 15%. This means A + T = 30%. Since the total percentage of all bases is 100%, G + C must be 100% - 30% = 70%. As G = C, then G = 70% / 2 = 35%. *15%* - This would only be correct if guanine paired with adenine, which it does not; guanine pairs with **cytosine**. - This answer incorrectly assumes that all four bases are present in equal proportions, or that G equals A, which violates **Chargaff's rules**. *85%* - This percentage would imply an incorrect base pairing or an imbalanced ratio of purines and pyrimidines, violating the fundamental structure of DNA. - An 85% guanine content would mean that G + C far exceeds 100% or that T is extremely low, which is biologically impossible. *70%* - This represents the combined percentage of **guanine and cytosine**, not guanine alone. - While it correctly acknowledges the remaining proportion of bases, it fails to divide this sum between the two equal components, **G and C**.
Physiology
1 questions'Flare' in Triple response is mediated by :
NEET-PG 2015 - Physiology NEET-PG Practice Questions and MCQs
Question 321: 'Flare' in Triple response is mediated by :
- A. Axon reflex (Correct Answer)
- B. Arteriolar dilation
- C. Histamine release
- D. Local hormones
Explanation: ***Axon reflex*** - The "flare" component of the triple response is caused by an **axon reflex**, where sensory nerve endings release **vasoactive neuropeptides** such as substance P and calcitonin gene-related peptide (CGRP). - These neuropeptides cause **vasodilation** in the surrounding area, leading to the characteristic red, irregularly shaped halo around the wheal. *Arteriolar dilation* - While arteriolar dilation is a component of the triple response and contributes to the **redness (flush)** and **flare**, it is not the direct mediator of the flare itself. - The initial arteriolar dilation is primarily due to **histamine** acting directly on the vessels, whereas the flare is a broader, neurally mediated spread of vasodilation. *Histamine release* - **Histamine** release from mast cells is the primary mediator of the initial **redness (flush)** and the formation of the **wheal** (swelling due to capillary permeability). - While histamine plays a role in the overall response, it does not directly mediate the "flare" component, which involves neuronal signaling via the axon reflex. *Local hormones* - While various **local mediators** (which could be broadly considered "local hormones" in a sense) are involved in inflammatory responses, the specific term "local hormones" is too general and does not precisely describe the mechanism of the flare. - The axon reflex, involving specific **neuropeptides**, is the precise mechanism for the flare, not a general category of local hormones.