Mission to Mars (2000) Part II - "It's Missing the Last Pair of Chromosomes"

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. The science was bad before extraterrestrials started leaving us notes. It gets worse from there.

The Scene – The astronauts decode a Martian message and find it's an image of a double helix, and one of them incorrectly claims it looks human. Another scientist says he's wrong, and I very briefly get my hopes up that she will introduce some good science. Instead, she points out that two spheres from the backbone aren't present and says, "It's missing the last pair of chromosomes." "Chromosome" is a real scientific term, and it is connected to DNA, but it has no relationship to what they were looking at. Let's use this as an opportunity to discuss what scientists mean when they talk about DNA, genes, chromosomes, and genomes.

In fairness to the aliens, maybe they just invested so heavily in space travel that they were never able to properly research genetics. From Mission to Mars.

The Science – Previously, I described that what's on the screen is a poor depiction of about 15 base pairs of DNA. The rungs of that ladder are the bases, while the backbone is composed of alternating chemicals: a sugar called deoxyribose and a phosphate group (see below). So the missing orbs would just represent that sugar and phosphate.

Left: Deoxyribose sugar. Right: A phosphate group. The back bone of DNA is made up of an alternating sequence of these groups. From Wikimedia Commons.

I mentioned before that the double helix shown in the picture would represent only a tiny tiny fraction of all the DNA in a human cell. To understand chromosomes, we have to go bigger. I'll go over some DNA-related terms to help prevent confusion in the future.

My favorite analogy for dealing with DNA is a library. I already described the different bases of DNA as letters: A's, C's, G's, and T's. Now these bases can be arranged into three-letter words called codons. Each codon represents an amino acid. Amino acids are the building blocks for proteins, which perform a wide variety of jobs for our cells.

As a book is a series of words that tell a full story, a gene is a series of codons that provides the sequence for a complete protein. Most genes consist of hundreds or thousands of base pairs. So the double helix on the screen was far too small to even be a gene, let a alone what comes next.

A chromosome could best be compared to a bookshelf. It is a single continuous ladder of DNA that includes anywhere from dozens to thousands of genes. Some creatures keep all of their DNA on a single chromosome, while others use many. People have 23 pairs of chromosomes, each with millions of base pairs.

Finally, we have the genome. That's equivalent to the entire library. It's the complete set of instructions for a given creature. For a human, that would include all 23 pairs of chromosomes, a total size of over 4 billion base pairs! And nearly every cell in our bodies contains one full copy of the genome.

Now, there's a lot more going on in our DNA and some of my explanation were overly simplistic, but that seems like enough of an introduction for now.

Fixing the Scene – I mentioned before that a karyotype could be used. A karyotype shows all of a creature's chromosomes, so it would indeed be possible to leave out a pair, but I still don't like the idea. The astronauts eventually determine that the message is a puzzle that they need to solve by filling in the missing pieces. I think it would be best to stick with a double helix (though a better depiction of one), but leave out a couple tiny pieces. Then point out that it looks like earth DNA but is missing part of the backbone, and have that be the solution to the puzzle.

Mission to Mars has more problems after the astronauts solve the puzzle, but those scenes will have to wait for another time.

Next Week – On a 2008 episode of Heroes, writers appears to come up with their science dialogue by throwing darts at a biology textbook.