NEET-PG 2015 Practice Questions

Q: Which type of glial cell is derived from mesodermal origin?

Microglial cells. ***Microglial cells*** - **Microglial cells** are unique among glial cells as they originate from **mesoderm**, specifically from **monocyte/macrophage precursors** in the bone marrow [1]. - They function as the **immune cells of the central nervous system (CNS)**, scavenging for plaques, damaged neurons, and infectious agents [1]. *Macroglial cells* - This is a broad category that includes **astrocytes, oligodendrocytes, and ependymal cells**, all of which are derived from **neuroectoderm**, not mesoderm [1]. - They perform various supportive roles but are distinct in origin from microglial cells [1]. *Oligodendrocytes* - **Oligodendrocytes** are derived from **neuroectoderm** and are responsible for forming the **myelin sheath** around axons in the CNS [2]. - Myelination is crucial for rapid and efficient nerve impulse conduction. *Ependymal cells* - **Ependymal cells** are derived from **neuroectoderm** and line the **ventricles of the brain** and the **central canal of the spinal cord**. - They play a role in the production and circulation of **cerebrospinal fluid (CSF)**.

Q: What are Okazaki fragments?

Short pieces of DNA on the lagging strand.. ***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.

Q: What is the first purine nucleotide synthesized in de novo purine biosynthesis?

IMP. ***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.

Q: How many molecules of Acetyl CoA are produced from β-oxidation of palmitic acid?

8 acetyl CoA. ***8 acetyl CoA*** - Palmitic acid is a **16-carbon saturated fatty acid (C16:0)**. During β-oxidation, each cycle cleaves two carbons as **acetyl CoA**. - The formula for acetyl CoA produced is **n/2**, where n = number of carbons. For palmitic acid: 16/2 = **8 acetyl CoA molecules**. - Alternatively: Palmitic acid undergoes **7 cycles of β-oxidation** [(n/2) - 1 = 7], each producing 1 acetyl CoA (7 total), plus the final 2-carbon fragment forming the 8th acetyl CoA. *3 acetyl CoA* - This number is too low for a 16-carbon fatty acid. **Short-chain fatty acids** would produce fewer acetyl CoA molecules. - This value corresponds to β-oxidation of a **6-carbon fatty acid** (hexanoic acid), not palmitic acid. *6 acetyl CoA* - This number is also too low for a 16-carbon fatty acid. - This quantity would be produced from a **12-carbon fatty acid** (lauric acid), not palmitic acid. *16 Acetyl CoA* - This number is too high and would incorrectly imply that each carbon forms an acetyl CoA independently. - Sixteen acetyl CoA molecules would be produced from a **32-carbon fatty acid**, which is extremely rare in biological systems.

Q: What is the most important tool used in genetic engineering?

Restriction endonuclease. ***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.

Q: If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?

35%. ***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**.

Q: The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:

DNA Ligase. ***DNA Ligase*** - **DNA ligase** forms a **phosphodiester bond** between the **3'-OH group** of one Okazaki fragment and the **5'-phosphate group** of the adjacent fragment, effectively sealing the nicks. - After **DNA polymerase I** removes the **RNA primers** and fills in the gaps, DNA ligase completes the synthesis of the **lagging strand** during DNA replication. - This enzyme is essential for maintaining the **integrity of the DNA backbone**. *DNA Helicase* - **DNA helicase** functions to **unwind the DNA double helix**, separating the two strands to create a replication fork. - It does not participate in joining DNA fragments. *DNA Phosphorylase* - **DNA phosphorylase** is not a standard enzyme involved in the direct sealing of DNA fragments during replication. - This is not the enzyme responsible for ligating Okazaki fragments. *DNA Topoisomerase* - **DNA topoisomerase** relieves the **supercoiling tension** that builds up in the DNA double helix ahead of the replication fork due to unwinding. - It does not have a role in forming phosphodiester bonds between newly synthesized DNA fragments.

Q: What primarily forms the core of chylomicrons?

Triglycerides. ***Triglycerides*** - Chylomicrons are primarily responsible for transporting **dietary triglycerides** from the intestines to other tissues. - Their large core, composed mainly of **triglycerides**, allows efficient transport of these hydrophobic molecules. *Triglycerides and Cholesterol together* - While **cholesterol** is present in chylomicrons, it is less abundant than **triglycerides** and primarily exists as **cholesterol esters** in the core. - The core is not an equal mixture; **triglycerides** overwhelmingly dominate the volume. *Free fatty acids* - **Free fatty acids** are transported in the blood primarily bound to **albumin**, not within the core of chylomicrons. - Chylomicrons typically carry **esterified fatty acids** as part of triglycerides. *Triglyceride, Cholesterol and Phospholipids* - **Phospholipids** form the outer monolayer of the chylomicron, along with apoproteins, making them **amphipathic**. - They do not constitute a core component but rather the **surface interface** with the aqueous environment.

Q: Which of the following is not classified as a chaperone protein?

Calbindin. ***Calbindin*** - **Calbindin** is a **calcium-binding protein** that helps regulate intracellular calcium levels, particularly in the brain and intestines. - It does not assist in **protein folding** or assembly like chaperone proteins. *Calnexin* - **Calnexin** is a **chaperone protein** located in the endoplasmic reticulum (ER). - It assists in the proper folding and quality control of newly synthesized **glycoproteins**. *Protein disulfide isomerase* - **Protein disulfide isomerase (PDI)** is an ER enzyme that **catalyzes the formation and rearrangement of disulfide bonds** in newly synthesized proteins, which is crucial for proper folding. - Due to its role in enabling correct protein folding, it is considered a **chaperone-like protein**. *Calreticulin* - **Calreticulin** is another **calcium-binding chaperone protein** found in the endoplasmic reticulum. - It works synergistically with calnexin to ensure the **proper folding of glycoproteins**.

Q: What is the definition of lipofuscin?

Wear and tear pigment. ***Wear and tear pigment*** - Lipofuscin is known as **wear and tear pigment** that accumulates in cells over time, especially in aging cells [1]. - It is a byproduct of **cellular lipid peroxidation** and protein degradation, indicative of oxidative stress [1]. *Form of calcification* - Not to be confused with calcification, lipofuscin is a **pigment** and not related to calcium deposition [1]. - Calcification usually occurs in response to tissue injury or necrosis, which differs fundamentally from lipofuscin accumulation. *Fat deposits* - Lipofuscin is made up of **an insoluble complex** and is not classified simply as fat or fat deposits [1]. - It is the result of the **degradation of cellular components**, rather than the accumulation of unutilized fats [1]. *Blood pigment* - Lipofuscin is not derived from **hemoglobin** or any blood components, distinguishing it from true blood pigments like **bilirubin**. - It is associated with **cellular aging** rather than with any specific blood function or metabolism [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 75-77.

Question 1

Which type of glial cell is derived from mesodermal origin?

Accepted Answer

Microglial cells

Answer

Macroglial cells

Answer

Oligodendrocytes

Answer

Ependymal cells

Question 2

What are Okazaki fragments?

Accepted Answer

Short pieces of DNA on the lagging strand.

Answer

Long pieces of DNA on the lagging strand.

Answer

Short pieces of DNA on the leading strand.

Answer

Long pieces of DNA on the leading strand.

Question 3

What is the first purine nucleotide synthesized in de novo purine biosynthesis?

Accepted Answer

IMP

Answer

AMP

Answer

GMP

Answer

UMP

Question 4

How many molecules of Acetyl CoA are produced from β-oxidation of palmitic acid?

Accepted Answer

8 acetyl CoA

Answer

3 acetyl CoA

Answer

16 Acetyl CoA

Answer

6 acetyl CoA

Question 5

What is the most important tool used in genetic engineering?

Accepted Answer

Restriction endonuclease

Answer

Topoisomerase

Answer

DNA Ligase

Answer

Helicase

Question 6

If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?

Accepted Answer

35%

Answer

15%

Answer

85%

Answer

70%

Question 7

The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:

Accepted Answer

DNA Ligase

Answer

DNA Helicase

Answer

DNA Phosphorylase

Answer

DNA Topoisomerase

Question 8

What primarily forms the core of chylomicrons?

Accepted Answer

Triglycerides

Answer

Triglycerides and Cholesterol together

Answer

Free fatty acids

Answer

Triglyceride, Cholesterol and Phospholipids

Question 9

Which of the following is not classified as a chaperone protein?

Accepted Answer

Calbindin

Answer

Calnexin

Answer

Protein disulfide isomerase

Answer

Calreticulin

Question 10

What is the definition of lipofuscin?

Accepted Answer

Wear and tear pigment

Answer

Fat deposits

Answer

Blood pigment

Answer

Form of calcification

NEET-PG 2015

Anatomy

Biochemistry

Pathology