In a DNA double helix an adenine of one strand always pairs with a(n) of the complementary strand, and a guanine of one strand always pairs with a(n) of the complementary strand.

adenine pairs with Thymine  and guanine always pairs with cytosine respectively

Explanation:

In DNA nucleotide subunits, there are four nitrogenous bases:

Each of these bases can be divided into two categories: purine bases and pyrimidine bases.

Adenine and guanine are examples of purine bases. This means their structure is a nitrogen-containing six atom ring joined with a nitrogen-containing five atom ring that share two atoms to combine the two rings.

Thymine and cytosine are examples of pyrimidine bases.

Note that  RNA replaces thymine with a different pyrimidine base called uracil (U).

The complementary base pairing rule, Chargaff's rule states that DNA base pairs are always adenine with thymine (A-T) and cytosine with guanine (C-G). A purine always pairs with a pyrimidine and vice versa. However, A doesn't pair with C, despite that being a purine and a pyrimidine.

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these macromolecules could either be amino acids, proteins or nucleic acids. they are responsible for the chemical changes and reactions that affects largely the cell and its composition. take for instance the cytoplasm of the cell where these organelles are settled. these simple organelles are composed of macromolecules which ignites and catalyses different functions that enables cells, in macro-perspective in motion and metabolism. in intestines for example, metabolism happens and breaking down parts of a food to simpler compounds that are used and these nutrients delivered throughout the body and again broken down by into smaller components.