Definition
Definition
DNA (deoxyribonucleic acid) is the molecule that carries hereditary genetic information.
Generalities
More precisely, the cistron is the set of several constituent units of the genetic code of DNA, forming a triplet corresponding to a sequence of three nucleotides each characterized by a nitrogenous base (see below). Le cistron holds under its control the manufacture of a protein as follows: each lonely is at the origin of the synthesis (determines the development) of one of the amino acids which constitute the protein in question.is therefore the smallest functional unit of the chromosome. It was formerly called the gene. Le lonely is made up of theexon and theintron.
The exon (from the Greek exô : outside) is the part of a gene that carries the genetic code information for protein synthesis (for specialists: during the copying of this DNA by transfer DNA ). The other part of the gene is called the intron ; no role can be assigned to it at present.
History
The term was used by Friedrich Miescher in 1869 to refer to DNA (most of which is located in the cell nucleus) as a large molecule that makes up chromosomes and their various segments: genes (also called cistrons). It was Avery O.T. in 1944 who identified DNA as the carrier of hereditary traits. The genetic code is the complete set of genetic information stored on DNA through codons.
At the beginning of the 20th century, thanks to three other American biologists ( Avery , MacLeod , MacCarthy ), it was discovered that the molecule responsible for the transmission of hereditary characteristics is a molecule of deoxyribonucleic acid: DNA.
This acid, called nucleic acid , which is made up of a giant molecule (macromolecule), has actually been known since 1889. The discovery of the structure of DNA , in 1953, is due to James Watson and Francis Crick who elucidated the details of its organization.
Symptoms
physiology
Very quickly, the possibility of highlighting chromosomes will make it possible to determine in advance whether a baby will have a disease linked to a chromosomal abnormality. This is possible thanks to the karyotype (a technique allowing, after having burst the cells taken from the liquid amniotic, to color and visualize the chromosomes, to photograph them, then to cut them out and arrange them). Le karyotype has shown its limitations, it does not allow the detection of chromosomal diseases, related to chromosome abnormalities visible under the microscope. Now, most diseases are dependent on small fragments, so small that it is impossible to see them under a microscope: genes. If we take the example of chromosome number 21, responsible for the trisomie, this contains many genes, which themselves carry information likely to cause genetic diseases. Thanks to biologists, we now know that DNA is made up of two strands twisted like a long rope ladder twisted on itself. The drawing of the molecule of DNA is now present in everyone's mind, it represents the very symbol of genetics.
The two uprights of this long helix of approximately 1 m are connected by bars made up of three types of molecules:
- Phosphoric acid.
- A sugar (deoxyribose).
- A base.
It is the bases , according to their position in this sequence, that determine how the instructions for the transmission of hereditary traits are given from generation to generation. The DNA molecule is made up of a very long chain of bases. Between cell divisions, the DNA is folded back on itself to form a chromosome (its structure does not change, only its shape). Outside of cell divisions, it is fully extended and can, using the bases (listed below), give instructions.
These bases all contain nitrogen . They are identified by the following initials:
- A or Ade corresponds to adenine : purine base entering into the composition of nucleic acids also formerly called vitamin B4 or antiagranulocytic.
- T or Thy corresponds to thymine : pyrimidine base.
- G corresponds to guanine : purine base.
Purine bases are made up of substances called purines (adenine, guanine, also xanthine and hypoxanthine ). Purines are either obtained from food or synthesized by the body, and their breakdown results in uric acid, an excessive amount of which in the blood (hyperuricemia) causes gout.
