Question

If one strand of a DNA molecule has the base sequence TCT, what must be the sequence on the opposite strand? Draw a structure of this portion of the double helix, showing all hydrogen bonds.

Short answer

The opposite sequence to TCT on a DNA strand would be AGA.

Step-by-step solution

Understand DNA Base Pairing Rules

Within a DNA molecule, every adenine (A) forms a base pair with thymine (T) and every guanine (G) forms a base pair with cytosine (C). This pairing is consistent throughout all DNA molecules due to the structures of the chemical bases.

Identify the Opposite Sequence

Knowing the base pairing rules, one can determine the sequence on the opposing DNA strand. Given the one strand has the sequence TCT, the opposing sequence would be AGA, because A pairs with T and G pairs with C.

Draw DNA Structure

After identifying the opposite sequence, a structure of this portion of the DNA molecule can be created. The two strands should be drawn as parallel lines with the bases represented as perpendicular lines between them illustrating the base pairings (A with T and G with C). The hydrogen bonds can be drawn as dotted lines between the corresponding base pairs.

Key concepts

Nucleotide Sequences

The fundamental building blocks of DNA are known as nucleotides, which are organic molecules that serve as the individual units of the nucleic acid polymers. Each nucleotide consists of three components: a sugar (deoxyribose in DNA), a phosphate group, and one of four nitrogenous bases—adenine (A), thymine (T), guanine (G), or cytosine (C). In DNA, these nucleotides link together to form long chains, with the sugar and phosphate groups creating the backbone and the bases extending from it.

When discussing nucleotide sequences, we refer to the precise order of these nitrogenous bases along a strand of DNA. This sequence is critically important as it encodes genetic information—the instructions for building and maintaining an organism. For example, if one strand of DNA has a sequence TCT, it is complementary to another sequence on the opposing strand. With the base pairing rules, we can deduce its complementary sequence as AGA, with A always pairing with T, and C with G. This precision in matching sequences is vital for DNA replication and function.

DNA Structure

The structure of DNA is referred to as a double helix, which was famously described by James Watson and Francis Crick in 1953. This structure appears like a twisted ladder or spiral staircase. Each side of the staircase is made up of alternating sugar and phosphate groups, which together form the sugar-phosphate backbone of the molecule. The steps of the staircase consist of pairs of nitrogenous bases, connected via hydrogen bonds.

The double helix structure is not arbitrary; it allows DNA to carry genetic information in a stable form that can be accurately copied during cell division. The underlying reason for this stability is the way the bases pair with each other. They do so in such a fashion that the helix has a uniform diameter. Adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds, maintaining the geometry of the DNA strand. Visualizing DNA as a structure with a specific geometry and understanding its three-dimensional shape can help students grasp how the molecule functions within the cell.

Hydrogen Bonds

Hydrogen bonds are a type of weak chemical bond that are nevertheless of crucial importance in biological systems. They occur when a hydrogen atom, bound to a more electronegative atom like nitrogen or oxygen, is attracted to another electronegative atom from a different molecule or group.

In the context of DNA, these hydrogen bonds are responsible for the specific pairing of bases across the double strands. A always pairs with T through two hydrogen bonds, while G pairs with C through three hydrogen bonds. These bonds are strong enough to hold the two strands together but weak enough to allow them to separate during DNA replication or protein synthesis. This balance provides both stability and flexibility—a hallmark of DNA's unique chemical and biological properties.