Friday, April 19, 2024

What is DNA? – science and life

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Maria Gill
Maria Gill
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Double helix-shaped filaments: This is revealed by the famous image 51 obtained by X-ray diffraction in 1952. If history has (almost) forgotten its name, it was the British chemist Rosalind Franklin who first laid eyes on deoxyribonucleic acid ( DNA), a molecule found at the heart of every one of our cells. It was she who determined his own structure. This crucial discovery laid the foundations of molecular biology and paved the way, ten years later, for the decoding of the genetic code. A guide to our organism, which will be the 23 pairs of chromosomes, supporting the genetic material, are the seasons, and the four nucleotides (A, C, G, T), the chemical units of DNA that make up them, will represent the letters. It contains all the information necessary for the growth and functioning of the body. In fact, most of these functions, such as the heartbeat, are common to all humans, so 99.9% of our DNA is the same.

Only 0.1% of DNA is considered completely unique and unique to all, except for identical twins. But given the enormous size of the human genome, which consists of about 3 billion pairs of nucleotides, this tiny difference is enough to identify a person and compare genetic traits.

To do this, we must concern ourselves with the non-coding sequences of our DNA, which make up 95% to 98% of our genome and which are located between genes (about 25,000 in number) coding for proteins. It does not specify any phenotypic trait, such as eye color, but it does involve polymorphism: short, repeating sequences of nucleotides, called STRs (short tandem repetitions) or microsatellites, the number of times they are repeated varies from person to person. This is like comparing the length of the A, C, G and T chains. Each of these chains has a specific location, called a locus, on the chromosome. By analyzing about twenty of them, it is possible to establish a genetic profile of an individual.

Regulation of DNA in the cell

At the core of each cell, our 46 chromosomes represent about 2 meters of tightly coiled DNA in nuclei between 10 µm and 100 µm in diameter. Each consists of a double helix of DNA, complementary chains of 4 chemical elements called nucleotides: A (adenine), C (cytosine), G (guanine) and T (thymine). The molecule first wraps around proteins called histones twice, like a thread around the coil, to form a complex called the nucleosome. These structures still roll on themselves, forming more or less dense chromatin fibers, which themselves condense into chromosomes.


genome: All information that constitutes the genetic heritage of an individual or species. Its size is measured by the number of base pairs (or nucleotides).

Non-coding DNA: These are genome sequences that do not code for the production of proteins, unlike DNA that codes. Long referred to as junk DNA, it actually plays an essential role in regulating gene activity.

Phenotype: The set of observable anatomical and physiological characteristics of an individual, which depends on the expression of genes, but also on the environment.

polymorphism: Individual differences in DNA sequences, these are called “marks” in forensic medicine.

Place: The physical location of the tag on the chromosome, designated by a specific symbol.

Micro-satellite (STR): Short DNA strands of 2 to 6 nucleotide pairs are repeated up to several tens of times. The number of repetitions varies from person to person.


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Aristotle was the first to formulate the theory of epigenetics, according to which living species transmit their hereditary characteristics.

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1869 Friedrich Mischer first identified DNA, which he then called ‘nuclein’, as a complete molecule within the cell nucleus.

1909 Wilhelm Johansen coined the term “gene” to refer to the elements in a cell through which physical characteristics are transmitted between parents and children.

1953 James Watson and Francis Crick publish in temper nature Model the structure of DNA, shaped like a double helix, thanks to the initial work of Rosalind Franklin.

1985 Alec Jeffries discovers polymorphisms in the structure of DNA, regions that vary greatly from one individual to the next.

2003 All three billion base pairs of the human genome have been sequenced, after fifteen years of work.

According to Science & Vie No. 296

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