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What is the DNA, the genome and what are genes?

Currently, much has been said and heard about genetics in the media, mainly in the field of precision medicine or preventive medicine and genetic editing.

At the end of 2018, the whole world heard about the creation of the first genetically modified babies by Chinese scientists. Long before that, in 2013, the Hollywood actress Angelina Jolie announced that she underwent double mastectomy (breast removal) after diagnosing through genetic tests an increased risk of developing breast cancer.

News about advances in genetics is increasingly in evidence, however, before understanding the new techniques and applications of the genetic universe it is important to understand the basis of everything: the DNA. In the following topics we will explain what DNA is, what it is composed of and how DNA is found in the cells of the human body.

What is the DNA and what does it do?

Deoxyribonucleic acid or DNA is a type of nucleic acid, where we find all the information necessary for protein synthesis, and therefore, it is responsible for storing all the characteristics of an individual.

What is DNA composed of?

The DNA is made up of a series of nucleotides linked in specific and unique combinations – the polynucleotide chains.

Each nucleotide consists of a pentose sugar, that is, a sugar that has 5 carbon atoms (called deoxyribose), a phosphate group and a nitrogenous base, which can be:

  • A – adenine
  • C – cytosine
  • G – guanine
  • T – thymine

The bases C and T are called pyrimidines and have only one aromatic ring. While bases A and G are called purines and have two aromatic rings.

The order, or sequence, of these bases determines which biological instructions are contained in a DNA strand. To illustrate for example, the ATCGTT sequence can determine the development of blue eyes, while ATCGCT can determine brown eyes.

In 1953, Watson and Crick proposed the double helix model for the DNA structure, in which two nucleotide strands, or also called complementary strands, are matched by means of hydrogen bonds (relatively weak bonds): always a T base with a A for 2 hydrogen bonds, and a C base with a G for 3 hydrogen bonds.

The complementarity between the nitrogenous bases admits that the knowledge of the sequence of nucleotides in one filament automatically allows to determine the sequence of bases of the other filament, an important characteristic for the accomplishment of the DNA sequencing.

Where is the DNA found?

In eukaryotic organisms, such as humans, the DNA is found within a special area of ​​the cell called the nucleus.

All tissues of the human body, such as skin, liver, heart, lung, etc are made up of countless cells. Each cell has many DNA molecules. In order for all this DNA to be disposed of within the cells it needs to be compacted and this packaged form of DNA is called a chromosome.

During the cell cycle, the compacted DNA is unwound both to allow it to be copied (a phenomenon called replication) and also so that its instructions can be used to produce proteins (phenomena known as transcription and translation) and other biological processes.

What is the genome?

The Human Genome consists of the totality of an individual’s DNA (Figure 2), organized into the 22 pairs of chromosomes, plus the X chromosome (one in men, two in women) and/or a Y chromosome (only in men) contained in the nuclei of all cells (called the nuclear genome) and in a small circular DNA molecule found within the individual mitochondria (called the mitochondrial genome).

What is a gene?

The content of the human genome is commonly divided between DNA coding sequences and non-DNA coding sequences. The coding DNA is composed of the sequences that can be transcribed and translated into proteins during the human life cycle; also called Gene.

There are about 19,000 to 20,000 genes encoding human proteins, and these coding sequences represent a very small fraction of the genome – approximately 1.5%. Genes are the basic unit of heredity, that is, they are transmitted from parents to their children.

It is through the transmission of genes that physical and behavioral traits are inherited in families. Therefore, any change that occurs in the sequence of genes will affect the development and/or normal functioning of the individual and may cause physical or cognitive defects, or diseases, at birth or in adulthood.

The non-coding DNA consists of all sequences (about 98% of the genome) that are not used to code proteins, that act in the regulation of the DNA or are sequences in which the function has not yet been discovered.

The alterations in the sequences of non-coding DNA, although they do not directly alter the genes, can cause alterations in the way in which the genes are expressed in the individual. Currently, the search for information about the function and evolutionary origin of non-coding DNA is an important objective of contemporary genomic research.

The complete sequences of the human genome were published by the Human Genome Project and Celera Corporation in 2004 and their total length exceeds 3 billion base pairs.

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