I wanted to write this post to help answer some bare-bones questions about DNA and the human genome. Since diving head-first into human genetics at the graduate school level, I often find myself saying, “Hold the phone. I’m confused about what is actually known and not known about our genome?”
Sometimes it feels like we know everything – the human genome is sequenced, we can perform all these funky DNA tests, the Discovery channel just showed me this great 3D image of DNA unwinding and processing genes. And sometimes…it’s like we’re living in the genetic stone age – we haven’t really sequenced the entire human genome, we have no idea why an extra chromosome really causes the Down syndrome phenotype, we can’t really sequence everyone’s individual genome because that would be ridiculously expensive and time-consuming…
OK, so what are some facts about the human genome?
There are about 3 billion base pairs in the human genome (or 6 billion total). Base pairs are those A-T, C-G nucleotides that line up and wind up on our 23 pairs of chromosomes. Scientists estimate that two strangers differ once every 1,200 to 1,500 DNA bases.
There are about 25,000 genes, which means only about 1.5% our entire genome actually codes for proteins (or RNA). The rest is non-coding, regulatory, or what some people call “junk” DNA – up for debate.
And so, if we sequenced the entire human genome, but people differ in their sequences, what does it mean to sequence THE HUMAN GENOME?
The key is that we’re all 99.9% similar in our DNA sequences. In that way, the Human Genome Project is actually “representative” of all humans, even though only a few were sequenced. It’s somewhat like a blueprint for our heritable instructions for development.
But I’m a unique snowflake…
Exactly. And the Human Genome Project certainly won’t tell us exactly how one person differs from another, but we have identified nucleotides or sequences that tend to differ between people (“hot spots” of variation). We also know that everyone has mutations, both new mutations and ancestral mutations. But mutations is an ugly word, so it’s often nice to say, “We all have variations“.
What’s been going on since the Human Genome Project then?
I’m going to save that for Part Deux. But, as you might imagine, there have since developed a lot of cool projects that have further investigated our genetic variations and attempted to map out our ancestors. And, of course, there has been a lot of research to uncover and explore those disease causing regions of the genome.