Database Reference
In-Depth Information
Thinking of DNA as Source Code
At the cellular level, your body is a completely distributed system. Nothing is centralized.
If you think of your DNA as source code, here are some things to consider:
▪ The source is comprised of only four characters: A, C, T, and G.
▪ The source has two contributors, your mother and father, who contributed 3.2 billi-
on letters each. In fact, the reference genome provided by the Genome Reference
Consortium (GRC) is nothing more than an ASCII file with 3.2 billion characters
▪ The source is broken up into 25 separate files called
chromosomes
that each hold
varying fractions of the source. The files are numbered, and tend to get smaller in
size, with chromosome 1 holding ~250 million characters and chromosome 22
holding only ~50 million. There are also the X, Y, and mitochondrial chromo-
somes. The term
chromosome
basically means “colored thing,” from a time when
biologists could stain them but didn't know what they were.
▪ The source is executed on your biological machinery three letters (i.e., a codon) at
a time, using the genetic code explained previously — not unlike a Turing machine
that reads chemical letters instead of paper ribbon.
▪ The source has about 20,000 functions, called
genes
, which each create a protein
when executed. The location of each gene in the source is called the
locus
. You can
think of a gene as a specific range of contiguous base positions on a chromosome.
For example, the BRCA1 gene implicated in breast cancer can be found on chro-
mosome 17 from positions 41,196,312 to 41,277,500. A gene is like a “pointer” or
“address,” whereas alleles (described momentarily) are the actual content. Every-
one has the BRCA1 gene, but not everyone has alleles that put them at risk.
▪ A
haplotype
is similar to an object in object-oriented programming languages that
holds specific functions (genes) that are typically inherited together.
▪ The source has two definitions for each gene, called
alleles
— one from your
mother and one from your father — which are found at the same position of paired
chromosomes (while the cells in your body are
diploid
— that is, they have two al-
leles per gene — there are organisms that are
triploid
,
tetraploid
, etc.). Both alleles
are executed and the resultant proteins interact to create a specific
phenotype
. For
example, proteins that make or degrade eye color pigment lead to a particular
phenotype, or an observable characteristic (e.g., blue eyes). If the alleles you inher-
it from your parents are identical, you're
homozygous
for that allele; otherwise,
you're
heterozygous
.
