Altmann, G. (n.d.) DNA molecules [Digital Art]. Pixabay. https://pixabay.com/images/id-3539309/
On April 14, 2003, the Human Genome Project was completed. Researchers had sequenced 92% of the total human genome, the largest amount possible with the technologies of the time. The Human Genome Project, started in 1990, was an international effort that included hundreds of scientists from across the world. The completed genome was publicly released, allowing the data to be used freely by scientists and biotechnology companies. The human genome sequence can be used to improve human health and enhance scientific understanding of genomics and the function of genes.
Sequencing the human genome involved identifying the order of the nitrogenous bases in the DNA. Nitrogenous bases are the part of the DNA that codes for the physical and behavioral traits of organisms. Each nucleotide building block of DNA consists of one base. The order that these bases are arranged determines the proteins that are made and thus the traits in that organism. There are approximately 3 billion of these bases in the human genome.
The human genome sequenced in this project was not the genome of one specific person. Rather, it was a combination of the genomes of several anonymous volunteers, mostly 11 people from Buffalo, New York.
To sequence this DNA, the project utilized a method of sequencing known as automated Sanger DNA sequencing. Sanger DNA sequencing involves the enzyme DNA polymerase, which makes copies of a strand of DNA, and modified nucleotides called deoxyribonucleotides (ddNTPs). ddNTPs stop DNA polymerase from adding more nucleotides onto the copy it is making. There are four different types of ddNTPs, each of which cuts short the copy at a different one of the four bases. To sequence the human genome, scientists had to combine the Sanger method with what is known as the "shotgun" method. With the shotgun method, DNA is separated into short segments and sequenced using Sanger sequencing, then the segments are overlapped to form a long, continuous sequence.
The 8% of the human genome not sequenced by the Human Genome Project contained repetitive sequences that the DNA sequencing technology of the time was unable to handle. Only around 1% of human DNA actually codes for proteins, and the 92% sequenced by the Human Genome Project contained around 99% of those gene-coding regions. However, the other 99% of the genome still has functions involved in regulating other genes. In fact, alterations in repetitive DNA are involved in many genetic disorders.
There are two main types of repetitive DNA. Satellite DNA is found in the telomere region at the ends of chromosomes, where it protects the chromosomes from degrading, and in the centromere, which connects the two halves of a chromosome during cell division. Transposable elements are a type of repetitive DNA that can move to different locations in the genome which can either disrupt the genome or help to regulate other genes.
In March of 2022, the Telomere-to-Telomere Consoritorium announced that it had filled nearly all of the remaining gaps in the human genome. The project was named for the telomere region at the end of a chromosome. They were able to fill in these gaps because of improvements in DNA sequencing technology that allowed for the sequencing of 100,000 bases at once, producing longer continuous strands of DNA that were easier to assemble into the correct order.
The advancements made by the Human Genome Project and Telomere-to-Telomere Consoritorium can help scientists understand the relationship between genetics and human health and are also useful for comparative genomics. Specifically, the fully sequenced human genome can be used to identify mutations in genes that are involved in disease. Comparative genomics allows scientists to learn more about genes and their function by comparing the human genome and those of other species. An important next step in human genome sequencing is the full sequencing of the genomes of a diverse variety of individuals to learn more about human genetic variation and the genes involved in diseases.
About the Author
Marcella is a Junior at Northwood High School who is interested in a career as a Conservation Biologist. She enjoys reading, playing the piano, and being outside.
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