NEET-PG 2012 - Biochemistry Practice Questions

Q: What does Chargaff's rule state regarding the base pairing in DNA?

A=T, G=C. ***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.

Q: Which of the following is a termination codon?

UAA. ***UAA*** - **UAA** is one of the three **stop codons** (UAA, UAG, UGA) that signals the termination of protein synthesis during translation. - When the ribosome encounters a UAA codon, no corresponding tRNA with an anticodon binds, and release factors bind instead, leading to the dissociation of the polypeptide chain. *AUG* - **AUG** is the universal **start codon** in most organisms, encoding for methionine in eukaryotes and N-formylmethionine in prokaryotes. - Its presence signals the initiation of protein synthesis, not its termination. *AUA* - **AUA** is a codon that codes for the amino acid **Isoleucine**. - It is a **sense codon** and does not act as a signal for termination. *AGG* - **AGG** is a codon that codes for the amino acid **Arginine**. - Similar to AUA, it is a **sense codon** and participates in elongating the polypeptide chain, rather than terminating it.

Q: DNA microarrays allow detection of gene mutations through which process?

Hybridization. ***Hybridization*** - DNA microarrays detect mutations by **hybridizing labeled patient DNA** to **thousands of oligonucleotide probes** containing known DNA sequences immobilized on a solid surface. - A mismatch between the patient's DNA and the probe results in **reduced or absent hybridization**, indicating a **mutation or genetic variation**. - This principle allows **high-throughput screening** of multiple genes simultaneously. *Polymerase Chain Reaction* - PCR is used to **amplify specific DNA sequences exponentially**, creating millions of copies from minimal starting material. - While PCR may be used to **prepare DNA samples** before microarray analysis, it is **not the detection mechanism** on the chip itself. *Cloning* - Cloning involves creating **identical copies of DNA fragments, cells, or organisms** using vectors and host cells. - It's a method for **producing large quantities** of specific DNA sequences but **not a detection technique** for mutations on microarrays. *Southern Blotting* - Southern blotting detects specific DNA sequences through **gel electrophoresis, membrane transfer, and probe hybridization**. - While it also uses hybridization, it is a **low-throughput technique** analyzing one sample at a time, unlike the **high-throughput parallel analysis** of DNA microarrays.

Q: In sickle cell anemia, the mutation at codon 6 results in the substitution of which amino acid?

Valine for glutamic acid. ***Valine for glutamic acid*** - In **sickle cell anemia**, the normal **glutamic acid** at codon 6 of the $\beta$-globin chain is replaced by **valine**. - This single amino acid substitution is responsible for the abnormal **hemoglobin S (HbS)** and the characteristic sickling of red blood cells. *Isoleucine for valine* - This substitution is **not characteristic** of sickle cell anemia. - While other hemoglobinopathies exist, this specific change does not lead to the sickle cell phenotype. *Valine for isoleucine* - This substitution is **not the primary genetic defect** found in sickle cell anemia. - The mutation in sickle cell anemia involves the replacement of a negatively charged amino acid with a neutral one. *Glutamic acid for valine* - This represents the **reverse substitution** of what occurs in sickle cell anemia. - In sickle cell, valine replaces glutamic acid, not the other way around.

Q: Which macronutrient has the highest thermogenic effect?

Proteins. ***Proteins*** - Proteins have the **highest thermogenic effect** (also known as the **thermic effect of food** or TEF) among macronutrients, typically ranging from **20-30%** of their caloric content. - This high TEF is due to the energy required for their **digestion**, **absorption**, and **metabolism** (e.g., deamination, protein synthesis), making them more metabolically "expensive" to process than fats or carbohydrates. *Fat* - Fat has the **lowest thermogenic effect**, accounting for approximately **0-3%** of its caloric content. - This low TEF is because fats are **easily absorbed** and stored, requiring minimal energy for their processing. *Carbohydrate* - Carbohydrates have an intermediate thermogenic effect, typically ranging from **5-10%** of their caloric content. - The energy expenditure for carbohydrate metabolism involves processes like **digestion**, **absorption**, and **glucose oxidation** or conversion to glycogen. *Alcohol* - While alcohol has a relatively high thermogenic effect (**20-25%**), it is **not classified as a macronutrient** in traditional nutritional science. - The three primary macronutrients are proteins, carbohydrates, and fats. Among these, **proteins** have the highest TEF.

Q: Which of the following is not affected in Abetalipoproteinemia ?

HDL. ***HDL*** - **Abetalipoproteinemia** is caused by a defect in the **microsomal triglyceride transfer protein (MTP)**, which is essential for the assembly and secretion of **chylomicrons**, **VLDL**, and subsequently **LDL** and **IDL**. - **HDL synthesis** and secretion occur independently of MTP, as nascent HDL particles are formed in the plasma from lipids and apolipoproteins (primarily apoA-I) released from other lipoproteins and cells. *LDL* - **LDL** is critically affected in abetalipoproteinemia because it is a metabolic product of **VLDL**. - Since **VLDL** production is severely impaired due to the MTP defect, there is a profound deficiency of **LDL** in the plasma. *VLDL* - **VLDL** production is severely impaired in abetalipoproteinemia because **microsomal triglyceride transfer protein (MTP)** is required for its assembly and secretion from the liver. - The inability to load triglycerides onto apoB leads to very low or absent plasma **VLDL** levels. *IDL* - **IDL** is an intermediate lipoprotein in the metabolism of **VLDL** to **LDL**. - Given that both **VLDL** and **LDL** are severely deficient in abetalipoproteinemia, **IDL** levels are also consequently very low or absent.

Q: The primary site of lipogenesis is:

Liver. ***Liver*** - The **liver** is the principal organ for **de novo lipogenesis**, converting excess carbohydrates into fatty acids and triglycerides. - This process is highly active in response to a high-carbohydrate diet, with the synthesized lipids packaged into **VLDL** for transport. *Skeletal muscles* - **Skeletal muscles** primarily utilize fatty acids for **energy production** rather than synthesizing large amounts of new lipids. - While they can store some triglycerides, their capacity for de novo lipogenesis is significantly lower compared to the liver. *Myocardium* - The **myocardium** (heart muscle) primarily relies on fatty acids for its continuous **energy demands** and has limited capacity for de novo lipogenesis. - Its metabolic focus is on efficient **ATP generation** to maintain cardiac function. *Lungs* - The **lungs** are not a primary site for general lipogenesis, though they are involved in the synthesis of specific lipids like **surfactant**. - Surfactant synthesis is a specialized process crucial for lung function, distinct from general energy storage lipogenesis.

Q: Which of the following is monoenoic acid ?

Oleic acid. ***Oleic acid*** - **Oleic acid** is a **monounsaturated fatty acid** (MUFA), meaning it has **one double bond** in its hydrocarbon chain. - Its presence in many natural fats and oils makes it a significant component of the human diet. *Arachidonic acid* - **Arachidonic acid** is a **polyunsaturated fatty acid** (PUFA) containing **four double bonds**. - It is a precursor for **eicosanoids**, including prostaglandins and leukotrienes, involved in inflammation and other physiological processes. *Linoleic acid* - **Linoleic acid** is an **essential omega-6 polyunsaturated fatty acid** with **two double bonds**. - It is crucial for human health and serves as a precursor for other fatty acids like arachidonic acid. *Linolenic acid* - **Linolenic acid** refers to two essential fatty acids: **alpha-linolenic acid (ALA)**, an omega-3 fatty acid with **three double bonds**, and **gamma-linolenic acid (GLA)**, an omega-6 fatty acid also with three double bonds. - Both are **polyunsaturated fatty acids** with multiple double bonds.

Q: Major source of energy for brain in fasting/starvation?

Ketone bodies. ***Ketone bodies*** - During **prolonged fasting or starvation**, the body depletes its **glycogen stores** and begins to break down fatty acids. The liver converts these fatty acids into **ketone bodies**, such as **acetoacetate and beta-hydroxybutyrate**. - These **ketone bodies** can cross the **blood-brain barrier** and be used by the brain as an alternative energy source when glucose becomes scarce, preventing protein breakdown for gluconeogenesis. *Glucose* - While **glucose** is the primary and preferred energy source for the brain under normal physiological conditions, its availability significantly decreases during **prolonged fasting or starvation**. - The brain requires a continuous supply of glucose, but in states of severe caloric restriction, the body must conserve glucose for other critical functions and adapt by using alternative fuels. *Glycogen* - **Glycogen** is a stored form of glucose found predominantly in the **liver and muscles**. - The brain itself has minimal **glycogen stores**, which are rapidly depleted during fasting, and thus cannot be a major long-term energy source. *Fatty acids* - **Fatty acids** are a major energy source for many tissues in the body, especially during fasting, but they **cannot directly cross the blood-brain barrier** in significant amounts to fuel the brain. - Instead, **fatty acids** are metabolized into **ketone bodies** in the liver, which then serve as the brain's alternative fuel.

Question 1

What does Chargaff's rule state regarding the base pairing in DNA?

Accepted Answer

A=T, G=C

Answer

A=G, T=C

Answer

A=C, G=T

Answer

Any combination possible

Question 2

Which of the following is a termination codon?

Accepted Answer

UAA

Answer

AUG

Answer

AUA

Answer

AGG

Question 3

DNA microarrays allow detection of gene mutations through which process?

Accepted Answer

Hybridization

Answer

Polymerase Chain Reaction

Answer

Cloning

Answer

Southern Blotting

Question 4

In sickle cell anemia, the mutation at codon 6 results in the substitution of which amino acid?

Accepted Answer

Valine for glutamic acid

Answer

Isoleucine for valine

Answer

Valine for isoleucine

Answer

Glutamic acid for valine

Question 5

Which of the following best describes the difference between glucokinase and hexokinase?

Accepted Answer

Glucokinase has higher Km for glucose compared to hexokinase.

Answer

Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase.

Answer

Glucokinase has a low affinity for glucose.

Answer

Glucokinase activity increases with glucose concentration while hexokinase remains saturated.

Question 6

Which macronutrient has the highest thermogenic effect?

Accepted Answer

Proteins

Answer

Fat

Answer

Carbohydrate

Answer

Alcohol

Question 7

Which of the following is not affected in Abetalipoproteinemia ?

Accepted Answer

HDL

Answer

LDL

Answer

IDL

Answer

VLDL

Question 8

The primary site of lipogenesis is:

Accepted Answer

Liver

Answer

Skeletal muscles

Answer

Myocardium

Answer

Lungs

Question 9

Which of the following is monoenoic acid ?

Accepted Answer

Oleic acid

Answer

Linoleic acid

Answer

Linolenic acid

Answer

Arachidonic acid

Question 10

Major source of energy for brain in fasting/starvation?

Accepted Answer

Ketone bodies

Answer

Glucose

Answer

Glycogen

Answer

Fatty acids

NEET-PG 2012 — Biochemistry