Since the release of the draft human genome sequence in 2001, sections were left unsequenced, and some sequence information was incorrect. Now, two decades later, we have a much more complete version.

What is the human genome sequence?

• The human genome sequence is contained in our DNA and is made up of long chains of “base pairs” that form our 23 chromosomes.
• Along our chromosomes are the base pair sequences that form our 30,000 genes.
• All humans share a great degree of similarity in their genome sequences – the same genes are ordered in the same manner across the same chromosomes.
• Each of us is unique (except for identical twins) in terms of the exact base pair sequence that makes up our genes and thus our DNA/chromosomes.
• It is this similarity that, in a genetic sense, defines us as “human” and the specific variation that defines us as individuals.

The Human Genome Project

• As early as the 1980s, momentum was gathering behind activities that supported, and would eventually define, the Human Genome Project.
• Conversations had turned into workshops that likened characterization of the human genome to characterization of the human anatomy that had centuries earlier revolutionized the practice of medicine.
• In 1990, with continued support from the US and widespread international collaboration and cooperation, the $3 billion dollar Human Genome Project was launched.
• The project aimed to determine the sequence of the human genome within 15 years.
• By 2000 (well ahead of schedule) a working draft of the human genome was announced.
• This was followed by regular updates and refinements and today we all have access to a human “reference genome sequence”.

Why did it take 20 years?

• Much of the newly sequenced material is the “heterochromatic” part of the genome.
• This is more “tightly packed” than the euchromatic genome and contains many highly repetitive sequences that are very challenging to read accurately.
• These regions were once thought not to contain any important genetic information but they are now known to contain genes that are involved in fundamentally important processes such as the formation of organs during embryonic development.
• Among the 200 million newly sequenced base pairs are an estimated 115 genes predicted to be involved in producing proteins.

Two key factors made the completion of the human genome possible:

1. Choosing a very special cell type

• The new sequence was created using human cells derived from a very rare type of tissue called a complete hydatidiform mole, which occurs when a fertilized egg loses all the genetic material contributed to it by the mother.
• Most cells contain two copies of each chromosome, one from each parent and each parent’s chromosome contributing a different DNA sequence.
• A cell from a complete hydatidiform mole has two copies of the father’s chromosomes only, and the genetic sequence of each pair of chromosomes is identical.
• This makes the full genome sequence much easier to piece together.

2. Advances in sequencing technology

• A new method called “shotgun sequencing”, involved breaking the genome into very small fragments of about 200 base pairs, cloning them inside bacteria, deciphering their sequences, and then piecing them back together like a giant jigsaw.
• This was the main reason the original draft covered only the euchromatic regions of the genome — only these regions could be reliably sequenced using this method.
• The latest sequence was deduced using two complementary new DNA-sequencing technologies.

Is the genome now completely sequenced?

• Well, no. An obvious omission is the Y chromosome, because the complete hydatidiform mole cells used to compile this sequence contained two identical copies of the X chromosome.
• However, this work is underway and the researchers anticipate their method can also accurately sequence the Y chromosome, despite it having highly repetitive sequences.
• Even though sequencing the (almost) complete genome of a human cell is an extremely impressive landmark, it is just one of several crucial steps towards fully understanding humans’ genetic diversity.

What’s next?

• The next job will be to study the genomes of diverse populations (the complete hydatidiform mole cells were European).
• Once the new technology has matured it will be better positioned to make a more significant impact on our understanding of human history, biology and health.
• Both care and technological development are needed to ensure this research is conducted with a full understanding of the diversity of the human genome to prevent health disparities.

Answer this PYQ in the comment box:

Q.With reference to the recent developments in science, which one of the following statements is not correct?

(a) Functional chromosomes can be created by joining segments of DNA taken from cells of different species.

(b) Pieces of artificial functional DNA can be created in laboratories.

(c) A piece of DNA taken out from an animal cell can be made to replicate outside a living cell in a laboratory.

(d) Cells taken out from plants and animals can be made to undergo cell division in laboratory petri dishes.

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