Nucleic Acid Biochemistry Practice Questions

Q: The helical structure model of two polynucleotide chains of DNA was proposed by whom?

Watson and Crick. ***Watson and Crick*** - **James Watson** and **Francis Crick** are credited with proposing the **double helix structure** of DNA in 1953, based on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins. - Their model elucidated the complementary base pairing and antiparallel nature of the two polynucleotide strands. *Linus Pauling* - **Linus Pauling** proposed a **triple-helical structure for DNA** in 1953, which was later found to be incorrect. - He was a prominent American chemist who made significant contributions to the fields of quantum chemistry and molecular biology, but his DNA model was not accurate. *Craig Venter* - **J. Craig Venter** is known for his work on the **Human Genome Project** and for being a pioneer in synthetic genomics. - He later founded Celera Genomics to compete with the publicly funded Human Genome Project in sequencing the human genome. *Michael Bishop and Harold Varmus* - **Michael Bishop** and **Harold Varmus** were awarded the Nobel Prize in Medicine in 1989 for their discovery that **oncogenes can arise from normal cellular genes (proto-oncogenes)**. - Their work focused on the genetic basis of cancer, specifically the role of retroviruses in transforming host cells.

Q: During DNA replication, which bond breaks?

Hydrogen bonds. ***Hydrogen bond*** - During DNA replication, the **double helix unwinds** as the two strands separate. - This separation occurs through the breaking of **hydrogen bonds** that link complementary base pairs (adenine with thymine, guanine with cytosine) between the two strands. *Phosphodiester bonds* - **Phosphodiester bonds** form the sugar-phosphate backbone of a single DNA strand. - These bonds are generally stable during replication and are not broken to separate the strands; rather, they hold the individual nucleotides together within each strand. *Phosphate bond* - This term is often used generally, but specifically in DNA, it refers to the **phosphodiester bonds** within the backbone or the high-energy bonds in ATP. - The direct breaking of "phosphate bonds" in this context is not the primary mechanism for separating the DNA strands. *Glycosidic bonds* - **Glycosidic bonds** link the nitrogenous base to the deoxyribose sugar within a nucleotide. - These bonds remain intact during replication, as they are crucial for maintaining the structure of individual nucleotides.

Q: The poly(A) tail of eukaryotic mRNA primarily functions to:

Enhancing mRNA stability. ***Enhancing mRNA stability*** - The **primary function** of the poly(A) tail is to **protect mRNA from degradation** by 3' to 5' exonucleases - **Poly(A)-binding proteins (PABPs)** bind to the tail and shield it from enzymatic degradation - The **length of the poly(A) tail** directly correlates with mRNA half-life - shorter tails lead to faster degradation - This is the most fundamental and universally recognized function of the poly(A) tail in all eukaryotic cells - According to standard biochemistry references (Harper's, Lehninger), mRNA stability is the primary role *Facilitating ribosome binding* - The poly(A) tail does contribute to translation efficiency through the **"closed-loop" model** - PABPs interact with translation initiation factors (eIF4G) to circularize mRNA - However, this is considered a **secondary function** that depends on the tail's presence for stability - This role is indirect and contingent on the mRNA being stable enough to be translated *Promoting nuclear export* - The poly(A) tail is required for **mRNA nuclear export** as PABPs interact with export machinery - This is an important but **tertiary function** - a prerequisite for cytoplasmic localization - Once in the cytoplasm, the tail's primary ongoing role is maintaining stability *All of the above* - While the poly(A) tail does contribute to all these functions, the question asks for its **PRIMARY function** - In molecular biology, the primary function is defined as the most direct, fundamental, and universally critical role - **Enhancing mRNA stability** is the primary function, while others are secondary or supportive roles

Q: The Watson-Crick double helix model of DNA is

Right-handed anti-parallel. ***Right-handed anti-parallel*** - The **Watson-Crick model** describes DNA as a double helix that twists in a **right-handed direction**. - The two strands are **anti-parallel**, meaning they run in opposite 5' to 3' directions relative to each other. *Left-handed anti-parallel* - While DNA strands are **anti-parallel**, the primary form of DNA, **B-DNA**, is **right-handed**, not left-handed. - A less common form, **Z-DNA**, is left-handed, but it's not the canonical Watson-Crick model. *Left-handed parallel structure* - The **Watson-Crick DNA model** is **right-handed** and its strands are **anti-parallel**, not parallel. - A **parallel DNA structure** would have both strands oriented in the same 5' to 3' direction, which is not found in nature for the double helix. *Right-handed parallel structure* - Although **DNA is right-handed**, the two strands are **anti-parallel**, not parallel. - A **parallel DNA structure** is a theoretical arrangement and does not represent the biologically functional form of the Watson-Crick double helix.

Q: Most common physiological form of DNA is

B-form. ***B-form*** - **B-DNA** is the most common and stable form of DNA under **physiological conditions** (high humidity, aqueous environment). - Its structure is a **right-handed double helix**, characterized by 10 base pairs per turn, a wider diameter, and a deep major groove along with a shallow minor groove. *A-form* - **A-DNA** is a **right-handed double helix** that forms under conditions of **low humidity** or desiccation. - It is shorter and wider than B-DNA, with 11 base pairs per turn, and a much shallower major groove and a deeper minor groove. *Z-form* - **Z-DNA** is a **left-handed double helix** that can form in regions with alternating purine-pyrimidine sequences (e.g., GCGCGC). - It has a more irregular, zigzag backbone and is thought to play a role in gene regulation, but it is not the most common physiological form. *C-form* - **C-DNA** is another **right-handed double helical** form of DNA, which is observed under even lower humidity and in the presence of certain ions, such as lithium ions. - It has 9.3 base pairs per turn, making it slightly less compact than B-DNA, but it is not the predominant physiological form.

Q: Which type of RNA is characterized by the presence of a 7-methylguanosine cap at its 5' end?

mRNA. ***mRNA*** - The **7-methylguanosine cap** at the 5' end of mRNA is crucial for **ribosome binding** during translation initiation in eukaryotes. - This cap also protects the mRNA from degradation by **exonucleases**, thus increasing its stability and half-life in the cytoplasm. *tRNA (transfer RNA)* - tRNA molecules are characterized by a conserved **cloverleaf secondary structure** and an **acceptor stem** for amino acid attachment, not a 7-methylguanosine cap. - Their primary function is to **carry specific amino acids** to the ribosome during protein synthesis. *rRNA (ribosomal RNA)* - rRNA molecules are the main structural and catalytic components of **ribosomes**, where protein synthesis occurs. - They do not possess a 7-methylguanosine cap; instead, they undergo extensive **post-transcriptional modifications** and folding to form functional ribosomal subunits. *snRNA (small nuclear RNA)* - **snRNA** molecules are key components of the **spliceosome**, which catalyzes the removal of introns from pre-mRNA. - Unlike mRNA, snRNA molecules have a **trimethylguanosine cap** (not 7-methylguanosine) and are primarily localized in the nucleus for RNA splicing functions.

Q: Simple viruses that can be crystallized, such as tobacco mosaic virus, have been found to be composed of what?

Nucleoproteins. ***Nucleoproteins*** - Simple viruses like tobacco mosaic virus consist primarily of **nucleic acid** (RNA) encased in a protein coat. - This combination of nucleic acid and protein forms **nucleoproteins**, which are the fundamental structural components of these viruses. *Nucleotides* - **Nucleotides** are the building blocks of nucleic acids (DNA and RNA), but they are not the complete structural component of a virus. - While viruses contain nucleic acids made of nucleotides, the question asks about the overall composition of the virus particle. *Phospholipids* - **Phospholipids** are key components of cell membranes and some enveloped viruses, forming a lipid bilayer. - Simple viruses such as tobacco mosaic virus are **non-enveloped** and therefore lack a phospholipid membrane. *Scleroproteins* - **Scleroproteins** (also known as fibrous proteins) are structural proteins found in animal tissues, like **collagen** and **keratin**. - They are not a primary component of viruses; viral structural proteins are typically globular in nature, forming capsids.

Q: Two strands of DNA are joined by:

Hydrogen bond. ***Hydrogen bond*** - The two strands of DNA are held together by **hydrogen bonds** formed between complementary nitrogenous bases. - **Adenine (A)** pairs with **thymine (T)** via two hydrogen bonds, and **guanine (G)** pairs with **cytosine (C)** via three hydrogen bonds. *Glycosidic bond* - A **glycosidic bond** links a nitrogenous base to the deoxyribose sugar in a single nucleotide. - It is an intramolecular bond within one strand, not between the two strands. *Covalent bond* - **Covalent bonds**, specifically **phosphodiester bonds**, form the sugar-phosphate backbone of each individual DNA strand. - These bonds are strong and define the primary structure of a single polynucleotide chain. *Ionic bond* - **Ionic bonds** involve the electrostatic attraction between oppositely charged ions and are not the primary forces holding the two DNA strands together. - While ions (like Mg2+) can play roles in DNA structure and stability, they do not directly join the strands.

Q: During DNA replication, if the template strand has the sequence 5'-GATTACA-3', what is the sequence of the complementary strand?

5'-TGTAATC-3'. **5'-TGTAATC-3'** - DNA replication involves **base pairing rules**: **adenine (A)** pairs with **thymine (T)**, and **guanine (G)** pairs with **cytosine (C)**. - The complementary strand is synthesized in an **antiparallel direction**: if the template is 5'-GATTACA-3', the new strand will be 3'-CTAATGT-5'. When written in the conventional 5' to 3' direction, this becomes 5'-TGTAATC-3'. *5'-GATTACA-3'* - This sequence is identical to the template strand, which would only occur if the DNA were to replicate in a **non-complementary manner**, violating base pairing rules. - Direct duplication of the template sequence does not produce a complementary strand. *3'-GATTACA-5'* - This sequence is the **template sequence written in the antiparallel direction** but is not the complementary strand. - It fails to apply the correct base pairing rules (A with T, G with C). *5'-ACATTAG-3'* - This sequence incorrectly pairs the bases and does not maintain the **antiparallel orientation** correctly. - For example, the first base G in the template would pair with C, not A.

Q: What is meant by the melting of double-stranded DNA?

Splitting of double strands into single strands. ***Splitting of double strands into single strands*** * **DNA melting**, also known as **DNA denaturation**, refers to the process where the two complementary strands of a **double-stranded DNA** molecule separate to form two individual single strands. * This process involves the breaking of the **hydrogen bonds** between the paired bases (**A-T and G-C**) due to increased temperature or changes in pH. * The temperature at which 50% of the DNA is denatured is called the **melting temperature (Tm)**, which depends on GC content (higher GC = higher Tm due to three hydrogen bonds vs. two in AT pairs). *Splitting of DNA into fragments* * The splitting of DNA into fragments is referred to as **DNA fragmentation**, which typically occurs due to processes like **restriction enzyme digestion**, mechanical shearing, or programmed cell death (apoptosis). * This process involves the breaking of the **phosphodiester bonds** within the DNA backbone, not just the hydrogen bonds between strands. *Formation of triple helix* * The formation of a **triple helix** (triplex DNA) is a less common DNA structure where a third oligonucleotide strand binds into the major groove of a **B-form DNA duplex**. * This process is distinct from DNA melting, which involves the *separation* of existing double strands rather than the *addition* of a third strand. *Separation of double-stranded bases* * The term "double-stranded bases" is imprecise terminology; bases are paired (e.g., A with T, G with C) within the double helix structure. * While the separation of base pairs does occur during melting, the more accurate description is the **separation of the entire double helix into two single strands**, not just the individual bases.

Question 1

The helical structure model of two polynucleotide chains of DNA was proposed by whom?

Accepted Answer

Watson and Crick

Answer

Craig Venter

Answer

Linus Pauling

Answer

Michael Bishop and Harold Varmus

Question 2

During DNA replication, which bond breaks?

Accepted Answer

Hydrogen bonds

Answer

Phosphodiester bonds

Answer

Phosphate bond

Answer

Glycosidic bonds

Question 3

The poly(A) tail of eukaryotic mRNA primarily functions to:

Accepted Answer

Enhancing mRNA stability

Answer

Facilitating ribosome binding

Answer

All of the above

Answer

Promoting nuclear export

Question 4

The Watson-Crick double helix model of DNA is

Accepted Answer

Right-handed anti-parallel

Answer

Left-handed anti-parallel

Answer

Left-handed parallel structure

Answer

Right-handed parallel structure

Question 5

Most common physiological form of DNA is

Accepted Answer

B-form

Answer

A-form

Answer

Z-form

Answer

C-form

Question 6

Which type of RNA is characterized by the presence of a 7-methylguanosine cap at its 5' end?

Accepted Answer

mRNA

Answer

snRNA (small nuclear RNA)

Answer

tRNA (transfer RNA)

Answer

rRNA (ribosomal RNA)

Question 7

Simple viruses that can be crystallized, such as tobacco mosaic virus, have been found to be composed of what?

Accepted Answer

Nucleoproteins

Answer

Nucleotides

Answer

Phospholipids

Answer

Scleroproteins

Question 8

Two strands of DNA are joined by:

Accepted Answer

Hydrogen bond

Answer

Glycosidic bond

Answer

Covalent bond

Answer

Ionic bond

Question 9

During DNA replication, if the template strand has the sequence 5'-GATTACA-3', what is the sequence of the complementary strand?

Accepted Answer

5'-TGTAATC-3'

Answer

5'-GATTACA-3'

Answer

3'-GATTACA-5'

Answer

5'-ACATTAG-3'

Question 10

What is meant by the melting of double-stranded DNA?

Accepted Answer

Splitting of double strands into single strands

Answer

Splitting of DNA into fragments

Answer

Formation of triple helix

Answer

Separation of double-stranded bases

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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