Imagine a single DNA strand containing a section with the following base sequence: \(5^{\prime}-\) GCATTGG \(C-3^{\prime}\) . What is the base sequence of the complementary strand?

Understanding the base pairing rules is crucial to comprehend how DNA strands are formed and replicate. DNA molecules are made up of four different types of nucleotides, each containing a unique nitrogenous base: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases form the 'rungs' of the DNA double helix ladder, connecting the phosphate and sugar backbone of each strand.

In DNA, specific pairing occurs between these bases according to the base pairing rules: adenine always pairs with thymine (A-T), and cytosine always pairs with guanine (C-G). This is due to the shape and structure of the bases allowing hydrogen bonds to form between A and T with two hydrogen bonds and between C and G with three hydrogen bonds.

• Adenine (A) pairs with Thymine (T) using two hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) using three hydrogen bonds.

These hydrogen bonds give the DNA molecule its stability and ensure that each base is always paired with its correct complement, safeguarding the genetic information.

A nucleotide sequence is like a sentence where each letter represents a nucleotide base. In DNA, this sequence dictates the genetic instructions and is written in a specific orientation from the 5' end to the 3' end of the strand.

Orientation of DNA Strands

The two ends of a DNA strand are labeled 5' (five prime) and 3' (three prime), which refer to the numbering of carbons in the DNA's sugar backbone. The 5' end has a phosphate group attached to the fifth carbon, while the 3' end has a hydroxyl group attached to the third carbon. When we look at the sequence of a single DNA strand, such as the one given in the exercise, it's essential to recognize this orientation to properly determine the complementary strand. For instance, a given strand reading 5'-GCATTGGC-3' orients the nucleotide G with its phosphate group towards the 5' end and the nucleotide C towards the 3' end.In a double-stranded DNA, the nucleotide sequence of one strand determines the sequence of its complement due to the specific base pairing rules. Therefore, a corresponding complementary sequence will be formed extending from the 3' to the 5' end to match with the original strand.

DNA strand complementation ensures the correct transmission of genetic information during DNA replication and cell division. Each strand of the DNA serves as a template for creating its complementary strand. The process of complementation involves matching each base of the original strand with its partner following the base pairing rules.

Complementary and Antiparallel Nature

A key aspect of DNA strand complementation is its antiparallel nature. That is, the two strands run in opposite directions. As seen in the given exercise, the original strand running in the 5' to 3' direction will have its complement running antiparallel from 3' to 5'. Therefore, to create a complementary sequence for the given strand 5'-GCATTGGC-3', we read the original sequence from the 3' end and apply the base pairing rules to form the sequence 3'-CGTAACCG-5'. This antiparallel arrangement is vital for the enzymes that replicate and repair DNA, ensuring that the genetic code is accurately transmitted.