Here's why:

* DNA Transcription: The process of creating mRNA from DNA is called transcription. During transcription, the DNA double helix unwinds, and one of the strands, called the template strand, is used as a template for mRNA synthesis.

* Base Pairing: The mRNA molecule is built using the base pairing rules:

* Adenine (A) in DNA pairs with Uracil (U) in RNA

* Thymine (T) in DNA pairs with Adenine (A) in RNA

* Guanine (G) in DNA pairs with Cytosine (C) in RNA

* Complementary Sequence: Because of these base pairing rules, the sequence of bases in the mRNA molecule is complementary to the sequence of bases in the template strand of DNA.

Example:

If the template strand of DNA has the sequence:

```

3'-TTCAGTCGT-5'

```

Then the complementary mRNA sequence would be:

```

5'-AAGUCAGCA-3'

```