When Human Genome Project researchers announced they had successfully completed sequencing the human genome, it was only about 92% complete. There were still hundreds of gaps or missing DNA sequences. Why was it so difficult to complete the sequence? Let’s break it down! #T2T 
A quick refresher: DNA strands are made up of chemical units called nucleotide bases. These are adenine (A),cytosine (C), guanine (G) and thymine (T). Your body reads the order of these letters to determine the info in the strand, just as you read a word to determine its meaning!
An organism’s complete set of DNA is called its genome. Nearly every single cell in the body contains a copy of exactly 3 billion DNA base pairs that make up the human genome. You read that right — 3 BILLION. In other words, the human genome contains a massive amount of DNA!
Researchers cannot read all 3 billion base pairs from end to end. First, they determine the sequence of random pieces of DNA. Then, they use those smaller sequences to put the whole genome sequence back together. It is a massive puzzle! With all great puzzles, they take time.
Parts of our DNA are also painfully repetitive. Some sections of the human genome sequence are so long and repetitive that it can be difficult for researchers to put it in the right place.
Thankfully, researchers have been developing a new technology called long-read sequencing that helps to read longer, more difficult stretches of DNA. During the Human Genome Project, researchers could only read 500 bases at a time. Now, they can read up to 100,000!
Researchers needed those new sequencing technologies in order to finish the last, extremely difficult 8% of the human genome. It took twice as long to sequence the last 8% of the human genome as it did the first 92%! Researchers have been developing this technology for decades.
Fancy sequencing technology cannot work without the genomic researchers who are putting in the hard work, skill and dedication. These amazing researchers are true perfectionists and made completing the human genome sequence finally happen!
The complete human genome sequence could provide a lot of new insight into missing heritability and human disease.
Want to learn more about this epic puzzle and the work to finish it? Check out our infographic on finishing the human genome sequence! #T2T
This effort to finish the human genome sequence is from the researchers in the Telomere-to-Telomere (#T2T) Consortium, led by @khmiga and @aphillippy. Learn more about the program here! bit.ly/3tdSaFP
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