Free Ancestor by Scott Sigler Page B

proud.”
    “Not really. I reprogrammed it so I could win. Maybe you should try playing some video chess. Let your mind do something else for a little bit.”
    She shrugged. She wasn’t about to lecture a grown man on the value of hard work.
    “Come on, Jian. Go to bed.”
    “I will,” she said. “Let me finish sequencing the four new samples first, then I will sleep.”
    “Promise?”
    She nodded.
    “All right,” Tim said. “Then you’re on your own. I’m pooped. Cheating at Tetris will really take it out of you. Night.”
    He turned and walked out of the room. She rubbed her eyes. She
was
tired. But it wouldn’t take that long to finish this process.
    They’d long ago collected samples of every living mammal known to man. After that, Danté had started acquiring samples from extinct species. Each time they digitized one of those additional genomes, the God Machine’s viability rate went up. Would the four new samples Bobby had delivered take them over 80 percent?
    The myriad forms of animals on Earth take many shapes, but every last one is made from a simple set of four nucleotides:
adenine, guanine, cytosine
and
thymine
. Those four basic nucleotides create the double helix structure that is deoxyribonucleic acid, or DNA. Some people didn’t understand
double helix
, but everyone got Jian’s favorite description—the twisting ladder.
    Variety between the strands, across the
rungs
of the DNA ladder, is limited even further, to just
two
combinations: adenine can only bind with thymine, and guanine can only bond with cytosine. But the combinations along the
sides
of the ladder, the four letters
A, G, T
and
C
, combine in infinite ways.
    Those infinite combinations were what Jian wanted to analyze, to digitize, so the God Machine could see the full genome of each animal and compare it with the master ancestor sequence.
    First, she extracted the cellular DNA of the four extinct mammals and placed each in a vial. To each vial, she added her sequencing master mix. The mix consisted of a DNA polymerase, random primers and the four basic nucleotides. The mix also included dideoxynucleotides, which were nucleotides with a slightly different chemical structure that contained a fluorescent section critical to the final stage of the process.
    She slid the vials into a polymerase chain reactor, a machine designed to produce billions of copies of the target DNA. First the PCR machine “unzipped” the DNA by heating it to ninety-five degrees Celsius, which broke the hydrogen bonds in the rungs. That split the double helix, leaving two single strands of DNA. The machine then cooled the mixture to fifty-five degrees Celsius. This brought the prefabricated random primers into play. A primer is to a strand of DNA what a foundation is to a brick wall: DNA strands can’t form at random, they have to begin with a primer. Lowering the temperature allowed the primers to lock in to complementary sections on the single DNA strand, so that a primer with the combination ACTGA would make rungs that created a combination of TGACT on the other sideof the ladder.
A
binds with
C, T
binds with
G
, and click, a starting point locks down.
    Then, more heat.
    As the temperature rose to seventy-two Celsius, the DNA polymerase started at the random primers and moved down the open strand, locking free nucleotides onto the open-ended single DNA strands—just like a train engine building the track underneath it as it goes. The end result was two perfect copies of the original DNA strand. From there, the process quickly repeated over and over—two copies became four, then eight, then sixteen, an exponential increase that added up fast.
    In years past, there had been more steps she had to follow, but now the entire process was automated. Her machine created
millions
of identical copies, peppered with the little fluorescent dideoxynucleotide chunks that marked segments. The computer used a laser to make those chunks fluoresce, then counted off the