Mission to Mars (2000) Part I - "That DNA Looks Human"

Spoiler Level – Moderate-to-high

The Movie – After the first journey to Mars meets with catastrophe, a rescue team arrives and finds a message from the planet's previous inhabitants.

This movie has lots of bad science. Most of it involves bad astronomy, but there's also some stunningly bad biology. The worst of it is a repeated misrepresentation of DNA. At one point, a scientist makes a double helix out of M&M's and claims it's the exact genetic make up of his perfect women. I choose to believe he means he wants a woman made of delicious chocolate. I'm going to focus on a later scene that can't be interpreted as charitably. 

The Scene – The astronauts decode a message being sent from a mysterious structure on the Martian surface. It's revealed to be a three-dimensional representation of a double helix. They identify it as a model of DNA and suggest that it is a signature left behind from ancient Martians. One of the scientists says, "That DNA looks human." I'll get to the rest of the scene next time.

It's good enough that I know I'm supposed to be looking at DNA. But there are just so many problems with the details. From Mission to Mars.

The Science – This scene reveals a fundamental misunderstanding of what DNA is and how it works. DNA does indeed take the form of a double helix. It does look like a twisted ladder with two parallel backbones connected by rungs. That's about as much as they got right. Here's what a good model of what DNA looks like:

The different colored orbs represent the individual atoms that make up the DNA. The purple ribbons indicate which atoms are included in the backbone. From Wikimedia Commons. 

Some of the differences are subtle. For instance, the movie DNA twists the wrong way and doesn't have unequal grooves like the DNA pictured here. There is actually a form of DNA that has those movie properties but it's far less common than what I've shown and not a good choice to use if you want to say that alien and human DNA are identical. 

However, there are bigger problems that relate to the core purpose of DNA. Our DNA contains all the information that makes us human. Just like a computer stores information with a specific sequence of 0's and 1's, DNA uses a specific sequence of four chemicals that can be thought of as A's, C's, G's, and T's. Theses are called bases, and each rung of the ladder consists of a base pair.

The colors added to a model of DNA will always be artificial, but in the movie they're also misleading. By making the spheres in the backbone different colors but showing the rungs as identical, you would think that the information content is in the backbone. Each color sphere is also associated with a distinct set of smaller orbs, further leading you to believe that this is where DNA varies. In reality, the reverse is true. The backbone along the double helix is made of identical repeating units, but the rungs differ and represent the A's, C's, G's and T's.

Now you may notice that there are only about 15 rungs on the DNA shown in the movie and you may think that 15 base pairs couldn't encode very much information. You'd be correct. Humans have billions of base pairs in each cell. Even the tiniest bacterium has hundreds of thousands. And one of the beautiful parts of biology is that the basic structure of bacterial and human DNA is identical! Every living thing on Earth shares the same basic building blocks. So for the scientists to see 15 base pairs and say it's human would be as ridiculous a programmer looking at the sequence 010011101100100 and saying it looks like the Windows operating system.

Fixing the Scene – Well, there are a couple ways to go with this. You could show something called a karyotype, which could unmistakably be identified as human. But that's just not as iconic as the double helix. I'd suggest showing a proper double helix and rewriting the dialogue. Just say that it looks like the same DNA that all organisms on Earth possess. For Martian DNA to have the same structure as Earth DNA would be a huge enough revelation by itself. 

Next Week – The scene gets worse from there. I'll use the next line of dialogue to clear up more misunderstandings about DNA.