Problem 74 - Objective Biology for NEET

Question

Which one is correct about DNA? (a) DNA exist as double helix. (b) Two strands of polynucleotide in DNA are antiparallel. (c) The nitrogen bases are projected more or less perpendicular to this backbone but face inside. (d) All the above

Step-by-Step Solution

Answer

The correct answer is (d) All the above. Each statement provided about DNA is accurate.

Step 1: Understanding DNA structure

Breaking down the task is important in order to solve this problem. DNA is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions.

Step 2: Check option (a)

Option (a) states that DNA exists as a double helix. This is correct. The structure of DNA is in the form of a double helix, which consists of two spiral chains of DNA.

Step 3: Check option (b)

Option (b) says that the two strands of polynucleotide in DNA are antiparallel. This is also true. Antiparallel here refers to the fact that the two sugar-phosphate backbones run in opposite directions.

Step 4: Check option (c)

Option (c) suggests that the nitrogen bases in DNA are projected perpendicular to the backbone and face inside. This is a fact. The nitrogenous bases in DNA do project more or less perpendicular to the backbone, and face inside.

Step 5: Final evaluation

Considering the information examined from each option, it concluded that each statement provided about DNA is accurate. Therefore, the correct answer is (d) - All of the above.

Key Concepts

DNA's Double Helix Configuration

The structure of DNA is fascinating and crucial to understanding how genetic information is stored and transmitted. It's not just a string of chemicals, but a beautifully intricate structure termed a 'double helix.' Picture a twisted ladder; the sides represent sugar-phosphate backbones, and the steps are pairs of nitrogenous bases. These base pairs—a match-up of adenine with thymine and guanine with cytosine—form a rung-like pattern that holds the two strands together.

In 1953, scientists James Watson and Francis Crick described this form, which elegantly solved the puzzle of how DNA replicates. The double helix is like a zip code containing instructions for building and maintaining an organism, all coiled up in the nucleus of each cell. Imagine if you could twist all the genetic instructions needed to create an organism into a molecule small enough to fit inside a single cell—well, nature found a way, and it’s called the double helix.

Antiparallel Polynucleotide Strands

Diving into the double helix reveals more intriguing details. The 'antiparallel' nature of polynucleotide strands is a fundamental aspect. This term might seem technical, but it's actually a simple concept. The two strands run in opposite directions, which is critical for several cellular processes, including DNA replication and transcription.

The antiparallel orientation means one strand runs from the 5' end (five prime end) to the 3' end (three prime end), while the other runs from 3' to 5'. This arrangement is essential for the enzymes that copy DNA. Think about how you use a zipper—one side needs to be inverted to connect properly. Similarly, the antiparallel strands ensure that the cellular machinery reads the DNA correctly and that the genetic code is accurately copied.

Nitrogenous Bases in DNA

At the heart of DNA’s structure are the nitrogenous bases, critical players in genetic coding. These molecules - adenine (A), thymine (T), guanine (G), and cytosine (C) - pair up in a very specific way. A always pairs with T, and G pairs with C. This complementary base pairing is key to genetic fidelity during DNA replication.

These bases are not just floating around; they're attached to a sugar molecule and a phosphate group, forming a nucleotide. The order of these bases spells out the genetic instructions, much like letters form words in a sentence. The enzyme DNA polymerase reads these instructions and assembles a new strand of DNA during replication, ensuring that each new cell has an exact copy of the DNA. The nitrogenous bases' precise alignment and pairing are akin to the precision of a well-designed locking mechanism, allowing genetic information to be securely stored and accessed.