Which one of the following statements is TRUE?
A, and B DNA helices are right-handed, Z DNA helix is left-handed.
Understanding DNA Helices Handedness
DNA, or Deoxyribonucleic Acid, is a molecule that carries genetic instructions. While the most common form is the famous double helix, DNA can exist in different structural forms depending on hydration levels, salt concentration, and the specific DNA sequence. These different forms, like A-DNA, B-DNA, and Z-DNA, have distinct physical properties, including the direction in which the helix twists, known as handedness.
Handedness of DNA Forms
The handedness refers to whether the helix twists to the right (right-handed) or to the left (left-handed) as it moves away from the viewer.
- B-DNA: This is the most common form of DNA found in living cells under physiological conditions. B-DNA is a right-handed helix. Its structure was famously described by Watson and Crick.
- A-DNA: This form appears when DNA is less hydrated than B-DNA. It is wider and shorter than B-DNA. A-DNA is also a right-handed helix.
- Z-DNA: This form is a much rarer, left-handed double helix. It has a zigzag appearance, which is where the 'Z' in its name comes from. Z-DNA can form in specific DNA sequences, often involving alternating purine-pyrimidine stretches like GCGCGC.
Comparing DNA Helix Handedness
Let's summarize the handedness of the three main types of DNA helices:
| DNA Form | Handedness |
|---|---|
| A-DNA | Right-handed |
| B-DNA | Right-handed |
| Z-DNA | Left-handed |
Based on this summary, A-DNA and B-DNA are right-handed helices, while Z-DNA is a left-handed helix.
Evaluating the Statements
Let's look at the given statements:
- Statement 1: "A, B, and Z DNA helices are left-handed." - This is incorrect because A and B DNA are right-handed.
- Statement 2: "A, and B DNA helices are right-handed, Z DNA helix is left-handed." - This matches our understanding: A and B are right-handed, and Z is left-handed.
- Statement 3: "A, and Z DNA helices are left-handed, B DNA helix is right-handed." - This is incorrect because A DNA is right-handed, not left-handed.
- Statement 4: "A, and B DNA helices are left-handed, Z DNA helix is right-handed." - This is incorrect because A and B DNA are right-handed, and Z DNA is left-handed.
Therefore, the statement that is TRUE is that A and B DNA helices are right-handed, and the Z DNA helix is left-handed.
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?
