Question

Newsdate: March \(1,2030 .\) A unique creature has been discovered during exploration of outer space. Recently, its genetic material has been isolated and analyzed. This material is similar in some ways to DNA in its chemical makeup. It contains in abundance the 4 -carbon sugar erythrose and a molar equivalent of phosphate groups. In addition, it contains six nitrogenous bases: adenine (A), guanine (G), thymine (T), cytosine (C), hypoxanthine (H), and xanthine (X). These bases exist in the following relative proportions: \(A=T=H\) and \(C=G=X\) X-ray diffraction studies have established a regularity in the molecule and a constant diameter of about \(30 \AA\). Together, these data have suggested a model for the structure of this molecule. (a) Propose a general model of this molecule. Describe it briefly. (b) What base-pairing properties must exist for \(\mathrm{H}\) and for \(\mathrm{X}\) in the model? (c) Given the constant diameter of \(30 \AA,\) do you think that either (i) both \(\mathrm{H}\) and \(\mathrm{X}\) are purines or both pyrimidines, or (ii) one is a purine and one is a pyrimidine?

Short answer

Answer: One of them must be a purine and the other a pyrimidine to maintain the constant diameter of 30 Angstroms.

Step-by-step solution

Propose a general model

The genetic material contains erythrose, phosphate groups, and six nitrogenous bases (A, G, T, C, H, X). The relative proportions are \(A=T=H\) and \(C=G=X\). This structure is similar to DNA, which consists of a sugar-phosphate backbone and nitrogenous bases. Given the similarities and the given proportions, we can propose a double-stranded helix model, with erythrose as the sugar component and nitrogenous bases forming base pairs across the helix.

Determine base-pairing properties for H and X

In DNA, the base-pairing properties are determined by hydrogen bonds between complementary bases. Adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G). Since we have base proportions of \(A=T=H\) and \(C=G=X\), we can assume that H will pair with A and T, while X will pair with C and G. This means H must form hydrogen bonds with A and T, while X must form hydrogen bonds with C and G.

Analyze H and X structures based on given diameter

Given the constant diameter of \(30 \AA\), we must determine if H and X are either both purines, both pyrimidines, or one of each. In DNA, purines (A and G) pair with pyrimidines (T and C) maintaining a constant diameter. If both H and X are purines or both pyrimidines, the molecule would not have a constant diameter. Therefore, one of them must be a purine and the other a pyrimidine to maintain the constant diameter of \(30 \AA\).

Key concepts

Base-Pairing Properties

In the realm of genetics, base-pairing properties refer to the specific ways in which nitrogenous bases on strands of DNA or similar genetic materials form hydrogen bonds to create the structure of the molecule.

Think of these nitrogenous bases as the rungs of a ladder in the famous double helix structure. They connect via hydrogen bonds in very particular ways — adenine (A) always pairs with thymine (T), and cytosine (C) pairs with guanine (G) in DNA. Every base is like a puzzle piece that fits perfectly with its counterpart, ensuring the stability of the genetic structure.

For the unique extraterrestrial genetic material in the problem, two additional bases, hypoxanthine (H) and xanthine (X), are introduced. These must also have specific partners to maintain structural integrity. In order for the constant diameter of the helix to be maintained, just like DNA, one would speculate that H must pair with T or A, and X with C or G. The consistency in diameter suggests a perfect pairing system, which is essential for the replication and function of the genetic material.

Nitrogenous Bases

The nitrogenous bases are fundamental components of genetic material. These organic molecules contain nitrogen atoms and are categorized into two types: purines and pyrimidines.

Purines, like adenine (A) and guanine (G), have a double-ring structure, while pyrimidines, such as thymine (T) and cytosine (C), have a single-ring structure. In the extraterrestrial genetic material presented in the problem, besides the standard nitrogenous bases found in Earth's DNA, we also encounter hypoxanthine (H) and xanthine (X).

The introduction of these additional bases suggests an expanded genetic code, potentially allowing for more complex traits or abilities in the organism. As part of the exercise, understanding which bases are purines and which are pyrimidines is critical, as this distinction is key to predicting base-pairing patterns and maintaining the helical structure's diameter.

DNA Model

The DNA model most widely recognized today is the double helix structure first described by James Watson and Francis Crick. It consists of two long strands twisted around each other, with the nitrogenous bases forming the steps between them.

The backbone of the double helix is made up of a sugar and a phosphate molecule. In typical earthly DNA, the sugar is deoxyribose, but in this unique extraterrestrial case, the sugar is erythrose, a 4-carbon sugar.

When proposing a general model for an alien genetic material, recognizing similarities to DNA's structure gives us a starting point. Using erythrose to form the sugar-phosphate backbone and the nitrogenous bases to form base pairs across the two strands preserves the helical structure. This design would support the genetic functions necessary for life, including the storage and transmission of genetic information.

X-ray Diffraction Studies

X-ray diffraction studies are a powerful technique used to understand the detailed structure of crystalline materials — in this case, the crystalline structure of genetic material. By shining X-rays on these structures, scientists can observe patterns that reveal how atoms are arranged.

Rosalind Franklin's X-ray diffraction studies were essential in deciphering the double helix structure of DNA, showing the characteristic 'X' shape that indicated a helical structure. In this extraterrestrial genetic material, X-ray studies suggested a regularity indicating a similarly ordered structure with a constant diameter.

This constant diameter of approximately 30 angstroms is a crucial clue to the arrangement of the bases within the helix, alluding to a structured and uniform pairing system much like that found in Earth-based DNA. The understanding gleaned from these studies helps us deduce that, for a consistent diameter, the extraterrestrial genetic material likely pairs a purine with a pyrimidine, mirroring what we see in DNA.