GENOME MAPPING

1. Context

2. Human Genome Project (HGP)

• One of the most comprehensive genome mapping projects in the world is the Human Genome Project (HGP), which began in 1990 and reached completion in 2003.
• The international project, which was coordinated by the National Institutes of Health and the US Department of Energy, was undertaken with the aim of sequencing the human genome and identifying the genes that contain it.
• The project was able to identify the locations of many human genes and provide information about their structure and organization.

3. Genome Mapping

• Gene mapping refers to the technique used to identify a gene's location and distance between genes.
• The distances between various sites inside a gene can also be described through gene mapping.
• Placing several molecular markers at specific locations on the genome is the fundamental element of all genome mapping.
• There are many types of molecular markers. When creating genome maps, genes can be observed as a particular class of genetic markers mapped similarly to other markers.

4. Types of Gene Mapping

• Genetic linkage maps and physical maps are the two main categories of "Maps" used in gene mapping.
• Both maps consist of genetic markers and gene loci. While physical maps involve actual physical distances, often measured in a number of base pairs, distances of genetic maps are based on genetic linkage information.
• There are many gene mapping methods, including comparative, physical, and genetic-linkage mapping. However, physical, and genetic-linkage mapping are more common.

5. What does genome mapping tell us?

• According to the Human Genome Project, there are estimated to be over 20,500 human genes.
• Genome refers to an organism's complete set of DNA, which includes all its genes, and mapping these genes simply means finding out the location of these genes in a chromosome.
• In humans, each cell consists of 23 pairs of chromosomes for a total of 46 chromosomes, which means that for 23 pairs of chromosomes in each cell, there are roughly 20,500 genes located on them.
• Some of the genes are lined up in a row on each chromosome, while others are lined up quite close to one another and this arrangement might affect the way they are inherited.
• For Example, if the genes are placed sufficiently close together, there is a probability that they get inherited as a pair.
• Genome mapping, therefore, essentially means figuring out the location of a specific gene on a particular region of the chromosome and also determining the location of relative distances between other genes on that chromosome.
• Significantly, genome mapping enables scientists to gather evidence if a disease transmitted from the parent to the child is linked to one or more genes.
• Furthermore, mapping also helps in determining the particular chromosome which contains that gene and the location of that gene in the chromosome.
• According to the National Human Genome Research Institute (NHGRI), genome maps have been used to find out genes that are responsible for relatively rare, single-gene inherited disorders such as cystic fibrosis and Duchenne muscular dystrophy.
• Genetic maps may also point out scientists to the genes that play a role in more common disorders and diseases such as asthma, cancer, and heart disease among others.

6. Why it is more important?

• A complete human genome makes it easier to study genetic variation between individuals or between populations.
• A genome refers to all of the genetic material in an organism, and the human genome is mostly the same in all people, but a very small part of the DNA does vary between one individual and another.
• By constructing a complete human genome, scientists can use it for reference while studying the genome of various individuals, which would help them understand which variations, if any, might be responsible for the disease.

7. What was missing?

• The genetic sequence made available in 2003 from the Human Genome Project, an international collaboration between 1990 and 2003, contained information from a region of the human genome known as the euchromatin.
• Here, the chromosome is rich in genes, and the DNA encodes for protein. The 8% that was left out was in the area called heterochromatin. This is a smaller portion of the genome and does not produce protein. 
• There were at least two key reasons why heterochromatin was given lower priority. This part of the genome was thought to be “junk DNA” because it had no clear function.
• Besides, the euchromatin contained more genes that were simpler to sequence with the tools available at the time.
• Now, the fully sequenced genome is the result of the efforts of a global collaboration called the Telomere-2- Telomere (T2T) project.
• The invention of new methods of DNA sequencing and computational analysis helped complete the reading of the remaining 8% of the genome. 

8. What is in the 8%?

• The new reference genome, called T2TCHM13, includes highly repetitive DNA sequences found in and around the telomeres (structures at the ends of chromosomes) and the centromeres (at the middle section of each chromosome).
• The new sequence also reveals long stretches of DNA that are duplicated in the genome and are known to play important roles in evolution and disease.
• The fact that the sequences are repetitive is enlightening. The findings have revealed a large number of genetic variations, and these variations appear in large part within these repeated sequences.
• A significant amount of human genetic material turns out to be long, repetitive sections that occur over and over.
• Although every human has some repeats, not everyone has the same number of them. And the difference in the number of repeats is where most of the human genetic variation is found,” the University of Connecticut said in a press release.
• Many of the newly revealed regions have important functions in the genome even if they do not include active genes. 

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