Question

If the sequence \(\mathrm{T}-\mathrm{A}-\mathrm{C}-\mathrm{C}-\mathrm{C}-\mathrm{T}\) appears on the informational strand of DNA, what sequence appears opposite it on the template strand? Label your answer with \(3^{\prime}\) and \(5^{\prime}\) ends.

Short answer

3' A-T-G-G-G-A 5'

Step-by-step solution

Understand DNA pairing rules

DNA is made up of nucleotides, which pair as A (adenine) with T (thymine) and C (cytosine) with G (guanine). This means if you have a letter on one strand, its complement on the opposite strand will be the corresponding pair.

Identify the ends of the template strand

An informational DNA strand runs from the 5' (five prime) end to the 3' (three prime) end. Consequently, the template strand that is complementary runs antiparallel, from the 3' end to the 5' end.

Write the complementary sequence

Using the base pairing rules, the complementary sequence to \( \text{T-A-C-C-C-T} \) is \( \text{A-T-G-G-G-A} \).

Label the template strand with the correct orientation

Since the template strand runs in the opposite direction, label the complementary sequence starting from 3' to 5'. Thus, the sequence is \( 3' \text{A-T-G-G-G-A} 5' \).

Key concepts

Nucleotide Pairing

DNA is composed of building blocks called nucleotides. Each nucleotide includes a sugar, a phosphate group, and a nitrogenous base. One of the most crucial aspects of DNA is how these bases pair. In nucleotide pairing, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This specific pairing is not random but follows fixed rules that ensure the structural integrity and function of the DNA molecule.
Understanding this pairing concept is fundamental for comprehending how genetic information is stored and replicated. The complementarity provides a mechanism for DNA to replicate itself during cell division, ensuring genetic material is passed correctly to new cells.
Here’s a quick recap of the pairing rules:

• A always pairs with T
• C always pairs with G

These rules allow the sequences of nucleotides in DNA to be complementary, a concept we will explore further.

Complementary DNA Strands

The principle of complementary DNA strands refers to how two strands of the DNA molecule fit together like a lock and key. This complementarity is due to the specific nucleotide pairing. Each base on one strand has a corresponding partner on the other strand.
When examining a DNA strand, knowing the sequence of one strand allows you to infer the sequence of its complementary strand by applying the nucleotide pairing rules. For example, if one strand has a sequence T-A-C-C-C-T, its complementary strand will be A-T-G-G-G-A.
Complementarity is key to DNA's ability to replicate itself. During replication, the strands separate, and new complementary bases are added to each single strand, forming two identical DNA molecules. This process showcases how genetic information is efficiently preserved and passed down.

Antiparallel DNA Structure

DNA strands run in opposite directions, a concept termed antiparallel structure. This means that if one strand runs in the 5' to 3' direction, its complementary strand runs 3' to 5'. The numbering refers to the carbon atoms in the sugar component of the DNA's backbone.
The antiparallel nature is crucial for DNA replication and repair. Enzymes that replicate DNA read each template strand in the 3' to 5' direction, synthesizing a new strand in the 5' to 3' direction. This orientation ensures that the cellular machinery can effectively access and duplicate the genetic code.
When labeling DNA sequences, it’s important to consider this structure. For example, an informational strand might run 5'-T-A-C-C-C-T-3', while its template strand would run 3'-A-T-G-G-G-A-5', perfectly illustrating the antiparallel nature of DNA.