system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How did scientists "map the human genome" and what does that mean for the rest of us?
gpt
The human genome was fully sequenced in the early 2000 by an international team of researchers. A human has 3,000,000,000 bases (letters) in the genome. Each letter is a chemical, one of four that we assign a name or letter to. These are cytosine (C), guanine (G), adenine (A), thymine (T). Each letter is attached to another in a long string. Inside the body, this long chemical body is read as instructions. A typical stretch of the sequence might read 

ATGCATGGTTAG

Imagine that but 3 billion letters long and split across multiple separate chains called chromosomes. 

To map the human genome, we had to sequence all of those letters, in order. That was a massive undertaking. For one reason, you have a lot of repetitive regions. And at the time, you might only be able to sequence about 800 letters at a time. Once you do all of that, it is pretty hard to connect all of these sequences are pieces back together in an ultimate jigsaw. 

There were some tricks to figure out how to connect all of these sequences back together even if we couldn't just from the 800 letter long pieces. But once we did this, we could use it to understand a lot about how humams work. We could count how many genes (parts of the genome that have a particular function, like making the protein actin, which helps power muscles). Not only can we count them, but we can figure out what there functions are. This is much faster than looking at then one at a time, as was done before the whole genome was published. 

One really useful thing about the genome is to look at genes that are mutated in people are are ill. If for example, you have cancer, you can sequence the tumor cells and the healthy cells around it to find chances in the letter only found in the cancer. Now we have a whole reference genomes published, and sequencing technology became cheaper, we can now sequence even smaller chunks of DNA. Instead of doing complex and expensive things to figure out how they all connect, we can just match them to the reference genome we already have and find where the mutations are (ie where the letters do not match).