{"id":4048,"date":"2022-04-18T22:00:00","date_gmt":"2022-04-18T22:00:00","guid":{"rendered":"https:\/\/modernsciences.org\/staging\/4414\/?p=4048"},"modified":"2022-04-04T05:31:18","modified_gmt":"2022-04-04T05:31:18","slug":"the-human-genome-project-pieced-together-only-92-of-the-dna-now-scientists-have-finally-filled-in-the-remaining-8","status":"publish","type":"post","link":"https:\/\/modernsciences.org\/staging\/4414\/the-human-genome-project-pieced-together-only-92-of-the-dna-now-scientists-have-finally-filled-in-the-remaining-8\/","title":{"rendered":"The Human Genome Project pieced together only 92% of the DNA \u2013 now scientists have finally filled in the remaining 8%"},"content":{"rendered":"\n
\n \n
\n Over half of the human genome contains repetitive DNA sequences whose functions are still not fully understood.\n Malte Mueller\/fStop via Getty Images<\/a><\/span>\n <\/figcaption>\n <\/figure>\n\nGabrielle Hartley<\/a>, University of Connecticut<\/a><\/em><\/span>\n\n

When the Human Genome Project<\/a> announced that they had completed the first human genome in 2003, it was a momentous accomplishment – for the first time, the DNA blueprint of human life was unlocked. But it came with a catch – they weren\u2019t actually able to put together all the genetic information in the genome. There were gaps: unfilled, often repetitive regions that were too confusing to piece together.<\/p>\n\n

With advancements in technology that could handle these repetitive sequences, scientists finally filled those gaps in May 2021<\/a>, and the first end-to-end human genome was officially published on Mar. 31, 2022<\/a>.<\/p>\n\n

I am a genome biologist<\/a> who studies repetitive DNA sequences and how they shape genomes throughout evolutionary history. I was part of the team that helped characterize the repeat sequences<\/a> missing from the genome. And now, with a truly complete human genome, these uncovered repetitive regions are finally being explored in full for the first time.<\/p>\n\n

<\/p>\n\n

The missing puzzle pieces<\/h2>\n\n

German botanist Hans Winkler coined the word \u201cgenome<\/a>\u201d in 1920, combining the word \u201cgene\u201d with the suffix \u201c-ome,\u201d meaning \u201ccomplete set,\u201d to describe the full DNA sequence contained within each cell. Researchers still use this word a century later to refer to the genetic material that makes up an organism. <\/p>\n\n

One way to describe what a genome looks like is to compare it to a reference book. In this analogy, a genome is an anthology containing the DNA instructions for life. It\u2019s composed of a vast array of nucleotides (letters) that are packaged into chromosomes (chapters). Each chromosome contains genes (paragraphs) that are regions of DNA which code for the specific proteins that allow an organism to function.<\/p>\n\n

\n \"Diagram<\/a>\n
\n Genetic material is made of DNA tightly packaged into chromosomes. Only select regions of the DNA in a genome contain genes coding for proteins.<\/span>\n VectorMine\/iStock via Getty Images Plus<\/a><\/span>\n <\/figcaption>\n <\/figure>\n\n

While every living organism has a genome, the size of that genome varies from species to species. An elephant uses the same form of genetic information as the grass it eats and the bacteria in its gut. But no two genomes look exactly alike. Some are short, like the genome of the insect-dwelling bacteria Nasuia deltocephalinicola<\/em><\/a> with just 137 genes across 112,000 nucleotides. Some, like the 149 billion nucleotides of the flowering plant Paris japonica<\/em><\/a>, are so long that it\u2019s difficult to get a sense of how many genes are contained within.<\/p>\n\n

But genes as they\u2019ve traditionally been understood \u2013 as stretches of DNA that code for proteins \u2013 are just a small part of an organism\u2019s genome. In fact, they make up less than 2% of human DNA<\/a>. <\/p>\n\n

The human genome<\/a> contains roughly 3 billion nucleotides and just under 20,000 protein-coding genes – an estimated 1% of the genome\u2019s total length. The remaining 99% is non-coding DNA sequences that don\u2019t produce proteins. Some are regulatory components that work as a switchboard to control how other genes work. Others are pseudogenes<\/a>, or genomic relics that have lost their ability to function. <\/p>\n\n

And over half<\/a> of the human genome is repetitive, with multiple copies of near-identical sequences. <\/p>\n\n

What is repetitive DNA?<\/h2>\n\n

The simplest form of repetitive DNA are blocks of DNA repeated over and over in tandem called satellites<\/a>. While how much satellite DNA<\/a> a given genome has varies from person to person, they often cluster toward the ends of chromosomes in regions called telomeres<\/a>. These regions protect chromosomes from degrading during DNA replication. They\u2019re also found in the centromeres<\/a> of chromosomes, a region that helps keep genetic information intact when cells divide.<\/p>\n\n

Researchers still lack a clear understanding of all the functions of satellite DNA. But because satellite DNA forms unique patterns in each person, forensic biologists and genealogists use this genomic \u201cfingerprint\u201d<\/a> to match crime scene samples and track ancestry. Over 50 genetic disorders are linked to variations in satellite DNA, including Huntington\u2019s disease<\/a>.<\/p>\n\n

\n \"46<\/a>\n
\n Satellite DNA tends to cluster toward the ends of chromosomes in their telomeres. Here, 46 human chromosomes are colored blue, with white telomeres.<\/span>\n NIH Image Gallery\/flickr<\/a>, CC BY-NC<\/a><\/span>\n <\/figcaption>\n <\/figure>\n\n

Another abundant type of repetitive DNA are transposable elements<\/a>, or sequences that can move around the genome.<\/p>\n\n

Some scientists have described them as selfish DNA because they can insert themselves anywhere in the genome, regardless of the consequences. As the human genome evolved, many transposable sequences collected mutations repressing<\/a> their ability to move to avoid harmful interruptions. But some can likely still move about. For example, transposable element insertions are linked to a number of cases of hemophilia A<\/a>, a genetic bleeding disorder.<\/p>\n\n

\n