Just today I found myself saying to students that the genetic code is simply one of the biggest evidence uncovered over the past fifty years (he was discovered in the 1960s) which confirms, in clear and elegant way, Darwinian evolution - especially when operated in its most fundamental level possible, ie the molecular level.
What is the genetic code?
We often use the term genetic code as an expression synonymous with "complete set of DNA from a cell", ie as a synonym for what we consider the genome of an organism. Technically speaking, however, when geneticists say genetic code they are not talking of DNA content of organisms. Indeed
the genetic code consists of a molecular code for passing the information contained in DNA to the information contained in proteins . To simplify the story, back a little on the theory of inheritance. We know that our cells contains 46 DNA molecules, which consist of large tape calls chromosomes. Each of our cells contains 46 chromosomes, 23 Heard from our father and our mother's legacy 23. It turns out that the DNA is considered today as a book kept on the shelf (or like a cake recipe), it contains a lot of information but it is considered chemically inert , ie it does not act effectively in the intracellular environment. The four chemical bases that make up: A, C, G and T teem a structure very similar and do not allow the realization of complex chemical interactions that form the cellular metabolism. (This argument has even been used many times - before Watson and Crick - to argue that DNA could not be the molecule of heredity.)
Anyway, today it is known that DNA, although chemically inert, contains necessary and sufficient information to direct the synthesis of other chemically active molecules, especially proteins. So that has a cell metabolism and is capable of transforming chemicals is necessary that the information contained in their DNA - and formed by chromosomal tapes containing millions of base pairs A, C, G and T arranged in a row - is converted into protein molecules. DNA, therefore, has two main functions, namely: (i) have the code of heredity and (ii) contain the information for making proteins. function (i) has to do with evolution and the fact that children resemble their parents, it is studied mainly by discipline of genetic . Since the function (ii) is related to how DNA controls the metabolism of a cell, and is academically studied mainly in the disciplines of molecular biology and biochemistry. Environmental cues to stimulate DNA transcription perform certain pieces of its molecule. Transcription is the process where the DNA form what is called messenger RNA (for details, click here ). This mRNA contains the information of the gene, which now leaves the nucleus (safe dwelling DNA) and to the cytoplasm will be translated into proteins. Information flow DNA to proteins is known as the central dogma of molecular biology , and was proposed by Watson and Crick some time after the discovery of the structure of DNA double helix (1953).
The central dogma of molecular biology indicates how the DNA information, which is chemically inert, information is transformed into protein. The protein molecules are mainly responsible for the control of cellular metabolism and, therefore, of life at its molecular level. [1]
Proteins are polymeric molecules made up of 20 different types of monomers, called amino acids. A polymer can be considered as a string of pearls, where each pearl is the monomer of the polymer paste. DNA is a polymer whose monomers (beads) are the nucleotides A, C, G and T. A small "necklace of DNA" could be formed by the following set of nucleotides: ACTCGGACATTTTACAGACACACGGAC. Since the protein is a polymer whose monomers are 20 amino acids, namely: (i) Alanine, (ii) cysteine, (iii) Aspartic acid, (iv) Glutamic Acid (v) Phenylalanine, (vi) Glycine, (vii ) Histina, (viii) isoleucine, (ix) Lysine, (x) Leucine, (xi) methionine (Xii) Asparagine (xii) Proline, (xiv) glutamine, (xv) Arginine, (xvi) serine, (xvii) Threonine, (xviii) Valine, (xix) Tyrosine, (xx) Tryptophan. Unlike nucleotides, which are chemically inert, each amino acid has different chemical properties differ, and some are positively charged, while others are negatively charged, while others are hydrophobic. The chemical diversity of amino acids is what allows the proteins have different roles in cellular metabolism, being responsible for some molecules bind in aqueous media, other functioning as pores for passage through the plasma membrane, other traveling the blood to other tissues and function as hormones. Finally, the combination of these 20 "pearl" necklaces aminoacidic to form protein allows proteins to have an incredible diversity and chemistry that can link different molecules that interact with our bodies. And to top it off the chemical diversity, these collars proteins are also able to curl up in space and form three-dimensional structures are highly complex, which greatly influence the shape of the operation of our cell metabolism.
Chemical formulas of the 20 amino acids that make proteins. Divided into chemical classes, amino acids can be charged positively or negatively, may still be polar or nonpolar without charge. This diversity makes the protein chemistry (polymers containing between 30 to 1000 amino acids, typically) highly reactive molecules. Besides the chemical diversity, the proteins also have structural diversity, given by three-dimensional conformation of amino acids in space. Figure obtained this site.
Although other molecules such as nucleic acids, lipids (fats) and sugars (carbohydrates) are highly important for cell metabolism, biologists molecular believe the key molecules that control and command the operation of a cell are the same proteins. E is the DNA that has the code for the formation of these proteins. Therefore, DNA and acts indirectly controls cell metabolism.
Finally the genetic code was perhaps the major discovery made after Watson & Crick had discovered the double helix structure of DNA, the understanding of this code allows us to understand how a code of four letters (nucleotides) of DNA is able to produce a code of 20 letters (amino acids) of proteins. The comparison of DNA and protein with letters and words made here is not merely a way of simplifying or analogy; teem DNA and proteins, as well as languages, syntax and semantics that can be learned from his observation and analysis. Interestingly, the story of the discovery of the genetic code is a very interesting topic that promise to be back here to tell. What matters here is that, when we speak of genetic code, we're on the mechanisms of transformation of a molecule of nucleic acid in a protein molecule . The origin of the highly complex mechanism of translation is still a mystery (that's right, the process is called translation and works similarly to the translation between languages), but experimentally the researchers were able to discover that for every three adjacent letters found in the coding region of a gene, the cell is able to produce a specific amino acid. See the table design of the genetic code below:
The table of the genetic code. Although the genetic code word is often confused with the gene content or genome of an organism or species for geneticists is the genetic code in this table, ie the code by which a triplet of nucleotides in DNA is converted into an amino acid that will be part of a protein.
The table above has been demonstrated experimentally in a variety of organisms, and it represents the translation between the encoded language of DNA chemistry and chemical language of proteins. The manufacture of proteins often begins with the crack (codon) ATG (or AUG), which encodes the amino acid methionine (Met). And so, the letters of the DNA (processed in the intermediate messenger RNA) below enabling us to translate the information in a language-of-information in DNA to a language-of-proteins. A sequence such as ATG-CCT-CCA-GGT-CAG-GGA-GTC-TGA in the DNA in the cell will be transformed - by the mechanism known as translation - in a protein with the amino acids Methionine-Proline-Proline-Glutamine-Glycine-Glycine -valine (which can be also described as Met-Pro-Pro-Gly-Gln-Gly-Val, or, more briefly, as MPPGQPV). The last codon TGA (or UGA) is the signal for the end of protein synthesis where DNA signals to the cell that it should not continue translating it and that code for proteins that ends there.
How and why the genetic code is consistent with Darwinism?
Given the enormous diversity organisms on Earth, would can think of each different organism could use a different genetic code to produce its protein. It is theoretically possible to think of alternative genetic codes, where the assignment was different crack-amino acid shown in this figure. (Francis Crick himself has even proposed a different genetic code that would make such an allocation between DNA nucleotides and amino acids in proteins. And although the code non-punctuated Crick was beautiful and elegant, it was not observed in any organism. And the development science always requires experimental confirmation to prove that if you wish.) Meanwhile, molecular biologists have investigated hundreds of thousands of living organisms, from the most ancient bacteria, through animals in the sea, in fresh water under the earth, the mountains, we investigated the fish, amphibians , reptiles, plants, algae, fungi, primates, marsupials, cyanobacteria, the archaebacteria and, finally, all known organisms. In none of these organisms the genetic code was very different and we know that this is represented in the table above. Only a few bodies teem either codon (triplet of nucleotides), which makes a different assignment, and these consist of just the exception that proves the rule. [2]
The fact that all living organisms have the same genetic code - or at least a very similar genetic code - is considered by scientists is evidence of two facts: (i) all living organisms few are descended from organisms that lived in the past and (ii) the emergence of the genetic code and translation engine brought a huge evolutionary advantage for organisms that were able to use it . The fact that (i) is the main sub-theories of Darwin, which is called common ancestry. Charles Darwin was the first person to say that all organisms on Earth have a common ancestor that lived in the past and was being distinguished in living species. Darwin did not live long enough to see the molecular corroboration of his ideas, but it certainly would have found it brilliant to see how all life on our planet that we know has the same basic code for making proteins. If organisms are not descended from a common ancestor, they could have completely different codes and their proteins could make a totally different and creative. They could even have some kind of metabolism that was not based primarily on the interaction of proteins and metabolites from each other. But such a body ever seen and we believe that there is at least on Earth. So the fact that all living organisms have the same genetic code is one of the strongest evidence of common ancestry among them and was a key corroboration of Darwinism made in the second half of the twentieth century, little more than a century after the publication original work "The Origin of Species" in 1859.
we focus now on the point (ii) presented above, which says that the first organism in which the genetic code appeared (and therefore the translation engine) has had an enormous adaptive advantage relative to other organisms that lived in his time . No I want here to extend the discussion of how the bodies were in a world where the genetic code did not exist. If I did, would be returning to the subject of the origin of life, highly controversial topic that can generate an entire work just heated discussions about very interesting details. I wish here only to show that by the time the genetic code emerged, there should be several types of proto-cells that were able to produce proteins from DNA in a highly inefficient. While the emergence of the complex mechanism of translation (and the genetic code) has been a rather large "evolutionary leap" still unexplained, since this mechanism has emerged (from quite similar to that which exists today), the bodies were able to use this code evolutionary won the war against other organisms that can coexist in the same season. And the proof is the same as that used in (i) there is no other body that is able to do differently this protein that uses the translation engine and where broken DNA are translated into amino acids according to the genetic code the figure. If there was another efficient way of producing proteins by living organisms would be able to see any animal or plant or fungus that would use any alternative mechanism, which does not happen. So the first guy who was able to make proteins according to a proto-code like this that we have today, he subdued all its competitors in the evolutionary race and what we see today in all the diversity of life on our planet, are the descendants of ancient organism that was able to use sequences of code- three chemical letters of nucleic acids to produce a letter of protein chemistry.
The human genome is the set of all genes that humans possess, and also non-genic regions. Although only 1% of our genome is responsible for doing what we call the genes encoding-de-proteins, these regions are now considered as the most important of our genome. Recent estimates speak that human beings have something around 30 000 protein-coding genes and are mainly those interactions between these proteins within our cells that make our body work coordinated in a highly complex and wonderful. The same goes for most other living organisms, also possessing tens of thousands of genes in their genomes, are able to turn those genes into proteins through translation engine that performs the reading of the genetic code and allows the existence of cellular metabolism . This amazing and wonderful code that when present, almost unchanged in all living organisms, consists one of the greatest tests that Darwinism really is not just theory or supposition: it is fact.
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[1] Another important and necessary predisposition "theory of the central dogma" is that once the DNA information is written in the form of proteins, it no longer returns - ever - to be DNA. This fact was also interpreted as the definitive refutation of the Lamarckian theory of inheritance of acquired characteristics. If that ever happens to the proteins back to DNA, then there is nothing like that happens in the life of an organism to cause impressions in their genetic material, material that is passed to this your child. It is worth noting that today there is evidence that some more subtle features can be re-printed in the DNA in the form of changes in the bases C, which may be methylated, or structure of the proteins that surround the DNA (chromatin) - see posting on epigenetic .
[2] Notes to the author, when you want to actually publish this text in the book: to give examples of alternative genetic codes, such as the mitochondrion, explaining the differences. Also explain how the roles have evolved codon-amino acid according to the theory of Savio / Romeo .
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