FILLING THE GAPS by Laura M. Zahn published in the Science presents a research by the Telomere-to-Telomere (T2T) Consortium, which has deciphered the human genome in its entirety.

Since its initial release in 2000, the human reference genome has covered only the euchromatic fraction of the genome, leaving important heterochromatic regions unfinished i.e only 92% of human genome could be sequenced till 2003.

The Telomere-to-Telomere (T2T) Consortium, a team of nearly 100 researchers, presented a complete 3.055 billion-base pair sequence of a human genome, T2T-CHM13, that includes gapless assemblies for all chromosomes except Y, corrects errors in the prior references, and introduces nearly 200 million base pairs of sequence containing 1956 genepredictions, 99 of which are predicted to be protein coding.

What is Human Genome Project?

The Human Genome Project (HGP)is the world’s largest collaborative biological international project designed to decode the human genetics. The primary work of the project is to develop three research tools that will allow scientists to identify genes involved in both rare and common diseases.

HGP researchers deciphered the human genome in three major ways: determining the order, or “sequence,” of all the bases in our genome’s DNA; making maps that show the locations of genes for major sections of all our chromosomes; and producing what are called linkage maps, through which inherited traits (such as those for genetic disease) can be tracked over generations.

What is a genome?

A genome is an organism’s complete set of deoxyribonucleic acid (DNA), a chemical compound that contains the genetic instructions needed to develop and direct the activities of every organism. DNA molecules are made of two twisting, paired strands. Each strand is made of four chemical units, called nucleotide bases. The bases are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases on opposite strands pair specifically; an A always pairs with a T, and a C always with a G.

The human genome contains approximately 3 billion of these base pairs, which reside in the 23 pairs of chromosomes within the nucleus of all our cells. Each chromosome contains hundreds to thousands of genes, which carry the instructions for making proteins. Each of the estimated 30,000 genes in the human genome makes an average of three proteins.

Sequencing means determining the exact order of the base pairs in a segment of DNA. Human chromosomes range in size from about 50,000,000 to 300,000,000 base pairs. Because the bases exist as pairs, and the identity of one of the bases in the pair determines the other member of the pair, scientists do not have to report both bases of the pair.

A telomere is the end of a chromosome. Telomeres are made of repetitive sequences of non-coding DNA that protect the chromosome from damage. Each time a cell divides, the telomeres become shorter. Eventually, the telomeres become so short that the cell can no longer divide.

High-resolution assembly string graph of the CHM13 genome

Stages of Human Genome Project

Human Genome Project was employed with two phases, the first phase, called the shotgun phase, divided human chromosomes into DNA segments of an appropriate size, which were then further subdivided into smaller, overlapping DNA fragments that were sequenced. The Human Genome Project relied upon the physical map of the human genome established earlier, which served as a platform for generating and analyzing the massive amounts of DNA sequence data that emerged from the shotgun phase. Next, the second phase of the project, called the finishing phase, involved filling in gaps and resolving DNA sequences in ambiguous areas not obtained during the shotgun phase.

In 2003, an accurate and complete human genome sequence was finished and made available to scientists and researchers two years ahead of the original Human Genome Project schedule. 92% of the human genome was then deciphered. The remaining 8% of the human genome was too complex to resolve and it took nearly two decades to complete the sequencing.

View entire HGP timeline

Read the reseach paper

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