Question

Chemical analysis of some viral DNA has given the following results: 20 percent of the bases are adenines, 35 percent are thymines, 15 percent are cytosines, and 30 percent are guanines. What must be true of this DNA? A. It is in the process of being transcribed into mRNA. B. It is mitochondrial DNA. C. It is single stranded. D. It is in the S stage of the cell life cycle.

Short answer

The DNA is single stranded (C).

Step-by-step solution

- Understand DNA base pairing

In double-stranded DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This means, ideally, the percentage of adenine should equal thymine, and the percentage of cytosine should equal guanine.

- Analyze given percentages

The given percentages are: adenine (A) = 20%, thymine (T) = 35%, cytosine (C) = 15%, guanine (G) = 30%. Notice that the percentages of adenine and thymine are not equal, and the percentages of cytosine and guanine are also not equal.

- Compare with double-stranded DNA properties

In double-stranded DNA, the percentages of adenine should equal thymine, and cytosine should equal guanine. This is not the case here, which suggests the DNA is not double-stranded.

- Determine the nature of the DNA

Since the given DNA does not follow the classic base-pairing rules of double-stranded DNA, it must be single-stranded. This aligns with option C.

Key concepts

Base Pairing Rules

To understand DNA and its function, it's essential to know the base pairing rules. DNA is made up of four types of bases: adenine (A), thymine (T), cytosine (C), and guanine (G). In double-stranded DNA, these bases pair in a specific way: adenine with thymine, and cytosine with guanine.

• Adenine (A) always pairs with Thymine (T).
• Cytosine (C) always pairs with Guanine (G).

This is known as the base pairing rule. Each base pair is held together by hydrogen bonds, making the DNA structure stable. This rule ensures that the amount of adenine always equals the amount of thymine, and the amount of cytosine always equals the amount of guanine in double-stranded DNA.

In the given problem, the adenine was 20%, but thymine was 35%. Similarly, cytosine was 15%, and guanine was 30%. These unequal proportions suggest the DNA is not following the base pairing rules, leading to the conclusion that the DNA must be single-stranded.

DNA Structure

DNA (deoxyribonucleic acid) is the hereditary material in humans and almost all other organisms. Its structure is a double helix, consisting of two long strands that run in opposite directions, connected by base pairs. Think of it like a twisted ladder.

Each strand is made up of a chain of nucleotides, where each nucleotide contains:

• A phosphate group.
• A sugar molecule (deoxyribose).
• One of the four nitrogenous bases (A, T, C, G).

The double-helix structure allows DNA to be incredibly stable and durable, enabling it to carry genetic information across generations.

Double-stranded DNA conforms to strict base pairing rules, as mentioned earlier. However, in some organisms, such as certain viruses, the DNA can be single-stranded. This means there isn't a complementary strand to pair the bases in a specific ratio, leading to uneven percentages of the bases as seen in the exercise example.

Viral DNA

Viruses can have DNA or RNA as their genetic material, which can be single-stranded or double-stranded. Understanding viral DNA is critical for grasping viral replication and infection.

The DNA in viruses doesn't always follow the same rules as human DNA. Many viruses have single-stranded DNA, not the double-helix structure seen in human cells. This single-stranded nature means the base pairing rules don't apply, resulting in the unequal base percentages observed in the exercise.

The uniqueness of viral DNA makes it a key point of study in virology and helps scientists develop vaccines and antiviral drugs. Single-stranded viral DNA can be more variable in its base composition, which can affect how the virus behaves and how our immune system can recognize and fight it.