Problem 70 - Objective Biology for NEET

Question

Which of the following is correct about DNA? (a) Double helical structure in which two strands of polynucleotide runs antiparallel. (b) Backbone is formed by Sugar-Phosphate-Sugar chain. (c) \(\mathrm{N}_{2}\) -bases projected more or less perpendicular to back bone and faces inside. (d) All of these

Step-by-Step Solution

Answer

The correct options are (a) and (c).

Step 1: Analyze each statement

Firstly, examine the assertions made in each option individually to determine their accuracy.

Step 2: Investigate First Statement

Statement (a) claims that DNA has a double helical structure and two polynucleotide strands run antiparallel. This statement is accurate since DNA is indeed a double helix, and the two polynucleotide chains run in opposite directions.

Step 3: Investigate Second Statement

Statement (b) claims that the DNA backbone is formed by a Sugar-Phosphate-Sugar chain. In reality, the backbone of DNA consists of alternating sugar and phosphate molecules. Therefore, this statement is false.

Step 4: Investigate Third Statement

Statement (c) suggests nitrogenous bases project more or less perpendicular to the backbone and face inside. This is accurate since the nitrogenous bases in DNA are indeed positioned perpendicular to the sugar-phosphate backbone and oriented inward.

Step 5: Assessment of the Options

Considering the above analysis, it can be concluded that statement (a) and (c) are correct, while statement (b) is incorrect. Thus, the answer is not (d) - all of these.

Key Concepts

Double Helix

The double helix is an iconic structure deeply associated with DNA. This shape was described by James Watson and Francis Crick in 1953. They illustrated how DNA forms a spiral or "twisted ladder" shape. The double helix consists of two strands that twist around each other, resembling a twisted spiral staircase.
The beauty of the double helix is not just its aesthetics; it's a functional necessity. This configuration ensures efficient replication and stability of the genetic material. Furthermore, this winding confers the necessary compactness allowing meters of DNA to fit inside tiny cells. The two complementary strands allow DNA replication to be tightly controlled, a crucial aspect for genetic inheritance.

Polynucleotide Strands

Polynucleotide strands are the fundamental units forming the DNA double helix. Each polynucleotide strand is a chain of repeating units known as nucleotides.
Nucleotides are composed of three parts: a sugar molecule, a phosphate group, and one of four types of nitrogenous bases (adenine, thymine, cytosine, or guanine). The two strands of the DNA double helix run antiparallel to each other. This means they run in opposite directions but follow the same pattern allowing them to complement each other.
This antiparallel arrangement is essential for the efficient replication and function of DNA.

Sugar-Phosphate Backbone

The sugar-phosphate backbone is a critical element of the DNA structure. It forms the "sides" of the DNA "ladder". Each strand of DNA has an alternating sugar and phosphate backbone. This alternating series forms a strong, covalent bond, providing structural support to the DNA molecule.
The sugar in DNA is deoxyribose, and it connects to the phosphate group forming the "rails" of the ladder. This repetitive structure provides a stable, resilient framework upon which the nucleotide bases can attach. Although the backbone itself does not store genetic information, it plays a supporting role that ensures the integrity and stability of the molecule.

Nitrogenous Bases Orientation

The orientation of nitrogenous bases in the DNA structure contributes to the unique properties of DNA. These bases include adenine, thymine, guanine, and cytosine. In DNA, they pair specifically: adenine with thymine and guanine with cytosine.
The bases project inward, like the "rungs" of a ladder, and are held together by hydrogen bonds. This inward orientation is perpendicular to the sugar-phosphate backbone.
This configuration not only facilitates the locking mechanism of base pairing but also ensures that the genetic information stored is protected within the tightly wound helix. This arrangement of bases is pivotal for accurate DNA replication and for encoding the instructions necessary for life.