The 15 base-paired nucleic acid molecule shown below is dissolved in an aqueous buffer of pH 7.3. 5' -A G U U C G G A U A U C G U G-3' 3' -U C A U G C C U A U A G C A C -5' Which one of the following statements is INCORRECT?
It can be double-stranded RNA molecule in a B-form helix
Nucleic Acid Identification
The provided nucleic acid sequence is:
5' -A G U U C G G A U A U C G U G-3'
3' -U C A U G C C U A U A G C A C -5'
Observing the sequences, we see the presence of Uracil (U). Uracil is typically found in RNA molecules, replacing Thymine (T), which is found in DNA. Although unusual base pairs or modified bases can exist, the presence of U throughout the sequence strongly suggests that this molecule is a double-stranded RNA (dsRNA) molecule.
Nucleic Acid Helix Forms
Nucleic acids can adopt different helical structures. The most common forms are the A-form helix and the B-form helix.
- B-form helix: This is the most common conformation for double-stranded DNA (dsDNA) under physiological conditions (aqueous solution, physiological salt concentration). It has a wider minor groove and a narrower major groove.
- A-form helix: This conformation is typically adopted by double-stranded RNA (dsRNA) and DNA-RNA hybrids. It is a shorter, wider helix than the B-form, with a narrower minor groove and a wider major groove. It is also adopted by dsDNA under dehydrating conditions.
The structural difference between DNA and RNA (the presence of a 2'-hydroxyl group on the ribose sugar in RNA) causes dsRNA to strongly favor the A-form helix. The 2'-OH group would clash with the sugar-phosphate backbone if RNA were to adopt the B-form conformation, making the B-form less stable and generally unfavorable for dsRNA under normal physiological conditions.
Analyzing the Statements
Based on the presence of Uracil, we identify the molecule as likely double-stranded RNA. Let's evaluate each statement:
- Statement 1: It can be a double-stranded DNA molecule in a B-form helix. This is unlikely because the molecule contains Uracil (U) instead of Thymine (T), which is characteristic of standard DNA. Therefore, it is not standard dsDNA.
- Statement 2: It can be a double-stranded DNA molecule in an A-form helix. Similar to Statement 1, the presence of Uracil makes it unlikely to be standard dsDNA, whether in A-form or B-form.
- Statement 3: It can be a double-stranded RNA molecule in an A-form helix. This statement is correct. The molecule contains Uracil, consistent with RNA. Double-stranded RNA is known to typically exist in the A-form helix under physiological conditions.
- Statement 4: It can be double-stranded RNA molecule in a B-form helix. This statement is generally incorrect under physiological conditions. While B-form dsRNA has been observed under specific, non-physiological conditions (e.g., very low salt, in organic solvents), it is not the typical, stable, or preferred conformation for dsRNA due to the steric clash involving the 2'-OH group of the ribose sugar. Double-stranded RNA overwhelmingly favors the A-form helix. Therefore, claiming it "can be" in a B-form is the least likely or most incorrect statement among the options in a biological context.
The question asks for the INCORRECT statement. Based on the typical structures and stability of nucleic acid helices, the statement that a double-stranded RNA molecule can be in a B-form helix is generally considered incorrect under physiological conditions, making it the INCORRECT statement among the given options.
Important Questions from Conformation of Proteins and Nucleic acids
Which one of the following statements is true regarding amino acids?
A form and Z form of double stranded DNA differ in the handedness of their helices, nucleotide sequences, and configuration of base to sugar. Based on these properties, which one of the following statements defines a correct combination for A and Z forms of DNA?
The following statements are made
A. B form of DNA has ~10 base pairs/turn and A form of DNA has ~2.3Å helix rise per base pair
B. Both the A and B form of DNA have wider major groove and narrow minor groove
C. The crystalline nature of cellulose is brought about by α (1 → 4) linkage between the glucose subunits.
D. The double bonds in natural lipids are always cis, which provides fluidity to the plasma membrane.
Which of the following combinations represent the correct statements?
Analysis of a homotetrameric protein and a double stranded DNA (that had been incubated in standard buffer) on native gels revealed that they migrated true to their physical states (tetrameric nature of the protein and double stranded nature of the DNA). Following hypotheses were made for the effect of adding high salt to the incubation mix and subsequent analysis on native gels.
A. The protein would migrate as a homotetramer and DNA in double stranded form.
B. The protein would migrate as a monomer but DNA in double stranded form.
C. The protein would migrate as a homotetramer but the DNA in single stranded form.
D. The protein would migrate as a monomer and the DNA in single stranded form.
Which of the following combination of hypotheses is most likely?
Heating of some nucleic acids shows an increase in the absorbance at 260 nm (A260) typified by the plot shown above. The sharp transition midpoint is defined as melting temperature (Tm). Which one of the following nucleic acid samples is NOT expected to generate such a typical profile upon heating of its solution?
