Question

Which of the following statements is (are) true? (a) The two strands of DNA run parallel from their 5 ' to their 3 " ends. (b) An adenine-thymine base pair contains three hydrogen bonds. (c) Positively charged counterions are associated with DNA. (d) DNA base pairs are always perpendicular to the helix axis.

Short answer

(c) and (d)

Step-by-step solution

Evaluate Statement (a)

The two DNA strands run in opposite directions, meaning they are antiparallel. One runs from 5' to 3' and the other runs from 3' to 5'. Therefore, statement (a) is false.

Evaluate Statement (b)

An adenine (A) - thymine (T) base pair forms two hydrogen bonds, not three. Therefore, statement (b) is false.

Evaluate Statement (c)

DNA is negatively charged due to its phosphate backbone. Positively charged counterions, such as sodium (Na+), are associated with DNA to neutralize this charge. Therefore, statement (c) is true.

Evaluate Statement (d)

In the double helix structure of DNA, the base pairs are indeed perpendicular to the helix axis. Therefore, statement (d) is true.

Key concepts

Antiparallel Strands

The two strands of DNA are known to exhibit a unique arrangement called antiparallel orientation. This means that one strand runs in the 5' to 3' direction, while the complementary strand runs in the opposite direction, from 3' to 5'.

This arrangement is crucial for the processes of DNA replication and transcription. It ensures that the enzymes involved in these processes, such as DNA polymerase and RNA polymerase, can work efficiently. The antiparallel nature allows for the formation of the double helix structure, which is stabilized by hydrogen bonds between complementary bases.

Without this antiparallel orientation, the precise and accurate copying of DNA wouldn't be possible, leading to potential issues in genetic information transfer.

Hydrogen Bonding in Base Pairs

Hydrogen bonds play an essential role in stabilizing the DNA double helix structure by holding the complementary base pairs together.

In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds, while cytosine (C) pairs with guanine (G) via three hydrogen bonds. This specificity in pairing is crucial for the fidelity of DNA replication and transcription.

The hydrogen bonds not only contribute to the stability of the DNA molecule but also ensure that the genetic information is stored accurately. If these bonds did not form correctly, it could lead to mutations and other genetic abnormalities.

Positive Counterions

DNA is inherently negatively charged due to the presence of phosphate groups along its backbone. To neutralize this negative charge, positively charged ions, known as counterions, are associated with DNA.

Common positive counterions include sodium (Na+), potassium (K+), and magnesium (Mg2+). These ions help stabilize the DNA structure by mitigating the repulsive forces between the negatively charged phosphate groups.
The presence of these counterions is essential for maintaining the structural integrity of DNA, especially in cellular environments where the ionic composition can vary.

Helix Axis

The DNA double helix is characterized by its helical structure, where the base pairs are aligned perpendicular to the helix axis. This perpendicular alignment of base pairs is crucial for the overall stability and functionality of the DNA molecule.

The helix axis serves as the central point around which the two strands of DNA twist. This arrangement allows for efficient stacking interactions between base pairs, which contribute to the stability of the double helix.

The perpendicular orientation of base pairs also ensures that the genetic information is accessible during processes such as DNA replication and transcription. It allows the necessary enzymes to interact with the base pairs in a precise manner, ensuring accurate copying and expression of genetic information.