Problem 49 - General Chemistry: Principles and Modern Applications

Question

If one strand of a DNA molecule has the base sequence of \(\mathrm{AGC}\), what must be the sequence on the opposite strand? Draw a structure of this portion of the double helix, showing all hydrogen bonds.

Step-by-Step Solution

Answer

The sequence on the opposite DNA strand must be TCG. The structure will show two strands with the base pairs A-T and G-C connected by hydrogen bonds.

Step 1: Understand the DNA structure

DNA molecules are double-stranded structures with each strand composed of nucleotides that are linked together by phosphodiester bonds. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. The bases from the two strands are held together through hydrogen bonds, forming base pairs.

Step 2: Determine the complementary sequence

According to the base pairing rules in DNA (Chargaff's rules), adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). So, if one DNA strand has a sequence of \(AGC\), the sequence of its complementary strand must be \(TCG\). Thus, our complementary pair is \(TCG\) for the given sequence \(AGC\).

Step 3: Draw the DNA structure

Start by drawing two parallel lines to represent the DNA strands, and then label them as 5'-TCG-3' and 3'-AGC-5' respectively (the 5' and 3' notation refers to the directionality of the DNA strands). Then, draw and label the bases for the sequence (T,A,C and G) within the two strands, ensuring that T pairs with A, and C pairs with G. Finally, add dashes to represent the hydrogen bonds between the nitrogenous base pairs: two hydrogen bonds between A and T, and three between C and G. Do this for each of the base pairs in the sequence.

Key Concepts

Complementary Base Pairing

Understanding the concept of complementary base pairing is crucial for grasping DNA structure and function. In the world of genetics, complementary base pairing refers to the specific alignment of nucleotide bases in the DNA molecule. Each base on one strand pairs with a specific partner: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This is not just a chance pairing; it's due to the size, shape, and chemical properties of the bases.

These pairings are held together by hydrogen bonds, a type of weak chemical bond that is nevertheless strong enough to stabilize the DNA structure. Adenine and thymine pair through two hydrogen bonds, while cytosine and guanine pair through three. This difference in bonding contributes to the very specific pairing mechanisms and ensures that the DNA helix has a consistent width throughout.

The importance of complementary base pairing cannot be overstated. It ensures accurate DNA replication, crucial for the preservation of genetic information across generations, and plays a key role in DNA repair and gene expression.

Chargaff's Rules

Chargaff's rules are named after Erwin Chargaff, a pioneer in biochemistry who made seminal contributions to understanding DNA. These rules state that in any given DNA molecule, the amount of adenine (A) is approximately equal to thymine (T), and the amount of guanine (G) is virtually identical to cytosine (C). This is summarized succinctly as A=T and G=C.

These groundbreaking rules helped uncover the double helical structure of DNA, as they suggested that A paired with T and G with C, hinting at the base pairing mechanism within the helix. Chargaff's findings were a central piece in the puzzle that led to the correct modeling of DNA by Watson and Crick. For students unlocking the mysteries of DNA, understanding Chargaff's rules is akin to learning the alphabet before forming words—it's fundamental.

Hydrogen Bonds in DNA

Hydrogen bonds are the superglue of the DNA world. These bonds may be weaker compared to covalent bonds, but they are the heroes when it comes to the stability of the double helix. Each hydrogen bond forms between a hydrogen atom in one nucleotide base and an electronegative atom (usually oxygen or nitrogen) in its complementary base.

In the classic A-T pairing, only two hydrogen bonds are needed to maintain the connection, while the G-C pairing requires three, giving the latter a touch more stability. The presence of these hydrogen bonds allows the DNA strands to separate during replication and transcription, yet remain closely aligned in a way that makes re-zipping them efficient and accurate.

Understanding hydrogen bonds in DNA also begs attention to environmental conditions. Factors like temperature and pH can affect bond stability, which is why extreme conditions can lead to 'denaturation', where the DNA helix separates. Hence, the entire genetical structure relies heavily on these tiny, vital hydrogen bonds.