The Age of DNA
Science and Life No. 2-2008
Doctor of Physical and Mathematical Sciences M. FRANK-KAMENETSKY.Of all that surrounds us, life seems to be the most inexplicable.
We got used to it being always around us and in ourselves, and lost the ability to be surprised. But go to the forest, look as if you saw them for the first time, at the trees, grass, flowers, birds and ants... Is there really something in common in the whole world around us, something that unites all living beings, be it a person or a microbe that is not visible to the eye?
These questions are as old as the world, but it was only in the second half of the XX century that it was possible to get answers to them for the first time. In fact, the answers were not too complicated and, most importantly, dazzlingly beautiful. The book "The Age of DNA" tells about what secrets scientists have managed to uncover and what, in fact, they consist of. It was written by a well-known scientist, Doctor of Physical and Mathematical Sciences Maxim Davidovich Frank-Kamenetsky.
These are unusual but true stories about the life of cells, about how living cells "learn" from dead cells, about why some cells keep other people's genes with them, just like people keep dogs or cats, and about how genes can be tied in a knot. The book helps to understand what is the DNA molecule — the basis of the foundations of the origin of life? We suggest reading one of the chapters of the book in an abbreviated version.
MEET ME: THE MOST IMPORTANT MOLECULEThe plan of how each of us will turn out is ready at the moment when the germ cells of our parents, mom and dad, merge into one whole, called a zygote or a fertilized egg.
The plan is contained in the nucleus of this single cell, in its DNA molecule, and there is a lot in it: what the color of our eyes and hair will be, how tall our height will be, what shape our nose is, how thin our musical ear is, and much more. Of course, our future depends not only on DNA, but also on the vicissitudes of fate. But to a very large extent it is determined by the qualities inherent in our genes from birth.
Each cell carries information about the structure of the whole organism. It is as if each brick of the building contained a miniature plan of the entire structure. I wish architects had done this since ancient times! Then the restorers would not have to puzzle over, say, what the Pergamon altar once looked like, even if only a single stone had been preserved from it...
The fact that one cell of an entire organism actually knows how the whole organism works was first discovered and demonstrated by the British biologist J. Gurdon. He took the cell nucleus from the intestines of an adult frog and, using the finest microsurgical technique, transplanted it into a frog egg, from which its own nucleus was removed.
A normal tadpole or even a frog grew out of a hybrid egg — absolutely identical to the one whose cell nucleus was taken. Nature itself sometimes creates such doubles. This happens when, after the first division of the zygote, the daughter cells do not stay together, but diverge and each turns out to be its own organism. This is how identical, or identical, twins are born. Twins have exactly the same DNA molecules, which is why twins are so similar...
IT LOOKS LIKE... A CORKSCREWHow does the DNA molecule, this queen of a living cell, work?
It looks like a rope ladder, and this ladder is curled into a right spiral. It resembles a corkscrew, but the corkscrew is double. Each of the DNA strands forms a right helical line, exactly like on a corkscrew (Fig. 1 a, b).
DNA consists of alternating molecular links. Its length depends on which organism it belongs to. The DNA of the simplest viruses contains only a few thousand links, bacteria — several million, and higher organisms — billions. If you line up all the DNA molecules contained in only one human cell, you will get a thread about 2 m long and the length of this thread will be a billion times its thickness.
IT LOOKS LIKE A WINDOW PANEPhysicists started studying DNA because they understood the importance of checking all the details of its structure.
They discovered that DNA is like a solid body, and the links are stacked in it, as in a crystal.
The one-dimensional DNA crystal (it looks like a thread) has intrigued scientists terribly. Isn't it a semiconductor? Or maybe a superconductor, and even at room temperature? The DNA was subjected to another examination. No, it's not a semiconductor, and certainly not a superconductor. It turned out to be an ordinary insulator, like a window pane. Yes, it is transparent, like glass. An aqueous solution of DNA (and it dissolves very well in water) is just a transparent liquid.
The similarity with glass does not end there. Ordinary glass, including window glass, is transparent to visible light and absorbs ultraviolet rays very strongly. DNA also absorbs ultraviolet light. But, unlike glass, to which such rays are not harmful, DNA is very sensitive to them. Ultraviolet rays are so destructive to the DNA molecule that the cell has learned to resist their effects on its own during evolution and to protect the information contained in it from damage.
IT MELTS, BUT NOT LIKE ICEWhat can a one-dimensional DNA crystal turn into when melting?
To understand this, let's remember why ice melts.
Ice is a crystal made of water molecules. There is a strict order in which water molecules are connected to each other by the maximum possible number of intermolecular bonds. Some of these bonds break, others deform during the transition to a liquid state.
What causes water to be liquid at temperatures above 0 ° C? Having lost some of the bonds, having weakened others, water molecules acquire the ability to move much more freely (move and rotate). With even greater heating, the water molecules sacrifice the last bonds with each other for the sake of complete freedom — a transition from a liquid to a gaseous state occurs.
All this fully applies to DNA — with increasing temperature, the existence of a double helix becomes unprofitable. Intermolecular bonds holding two chains near each other break, and two single-stranded chains are formed from one double-stranded molecule, while each chain feels much freer, can acquire many more different configurations in space.
Despite the analogy, DNA melting is fundamentally different from ice melting. The difference is that the melting of DNA occurs in a wide temperature range (it is equal to several degrees), and the melting of ice is strictly at one point of the temperature scale. This is the so-called phase transition. With such a transition, the phase state of a substance changes abruptly — from solid it becomes liquid, from liquid it becomes gaseous.
We face a phase transition every day when we boil a kettle. During the boiling process, the "water — steam" system is at the point of phase transition — the temperature of the water in the kettle will not exceed 100 ° C by one iota until all of it boils off. The same thing will happen when ice or snow is heated. The temperature rises to 0 ° C, then the growth stops until all the ice completely melts, and then the temperature will go up again.
When DNA is heated, the temperature increases continuously, with its increase, all new sections of molecules pass from the helical state to the molten state when its two strands are separated.
How are the threads bred? What plays the role of an iron that can melt a piece of DNA? Their special enzymes are unwound and straightened. The enzyme binds firmly to the DNA and begins to move along it, unwinding all new sections on its way. At these sites, the synthesis of the RNA molecule begins. The sections of the gene from which the enzyme "moved out" are again "slammed" into a double helix, and the resulting RNA molecule enters the solution. An intracellular structure, the ribosome, floats up to it and, according to the genetic information recorded in RNA, protein synthesis is started on the ribosome. Schematically, this is shown in Fig. 3.
IT IS SIMILAR TO THE PATH OF A PERSON WHO GOT LOST IN THE FORESTWhy can a person who tries to walk only forward in the forest get lost in cloudy weather when it is impossible to navigate by the sun?
Why will he return again and again to the place where he has already been? There are different beliefs about this. Some say that a person walks in a circle because he has one leg slightly shorter than the other. Others see the reason that we have different steps — one is longer, the other is shorter. In fact, the reason is different. A person tries to walk straight, but, having no distant landmarks in front of him, he constantly strays from a straight line. This loss of memory about the original direction occurs the faster the thicker and more monotonous the forest. The path of a person in this case is random and does not look like a movement in a circle at all.
It's not just people who wander. Molecules also wander — they try to move straight, but due to collisions with each other, their path is curved. This is how the famous Brownian motion arises...
But what does all this have to do with DNA? Believe me, the most direct. Like the path of a person in the forest and the movement of a particle in the environment, the DNA molecule tends to stretch into one straight line... But thermal motion spoils the whole thing. The DNA molecule is bombarded by the surrounding water molecules, and it begins to wriggle, like a worm, curls up into a polymer ball, constantly changing shape.
The fact that the double helix is able to bend has considerable biological significance. The fact is that if the DNA molecule was very rigid, like a knitting needle, then it could not fit inside the cell, let alone the cell nucleus. We already know that the length of DNA in a human cell is about 2 m. If you allow it to curl into a ball, its diameter will be about 0.5 mm — this is a thousand times larger than the diameter of the core. How does it fit in there anyway?
The fact is that in the cells of higher organisms there is a special mechanism for forcibly bending the double helix. The molecule is wound, like a thread on a coil, on a special complex of nuclear proteins (histones). On each "coil" the molecule makes about two revolutions, then moves to the next "coil" and so on. A "coil" with DNA wound on it is called a nucleosome, so the DNA in the nucleus of higher organisms is a necklace of nucleosomes.
Of course, this necklace is not elongated in one line, but is compactly stacked in a very complex way in special corpuscles called chromosomes. It is in this cunning way that the cell manages to do a trick that only a skilled magician can do — to fit a polymer tangle into a core whose diameter is less than a micrometer.
The discovery of the structure of DNA played the same role in the development of biology as the discovery of the atomic nucleus in physics. Finding out the structure of the atom led to the birth of a new, quantum physics, and the discovery of the structure of DNA led to the birth of a new, molecular biology. But the parallel does not end there.
Theoretical, fundamental research of the atom allowed a person to master an almost inexhaustible source of energy and radically changed our daily life thanks to the computer, the Internet, and the mobile phone. The development of molecular biology has opened up the possibility to interfere in an unheard-of way with the properties of a living cell, to change heredity in a targeted way. And this is already beginning to affect people's lives no less radically than mastering the energy of the atomic nucleus. The age of DNA has arrived.
DictionaryThe gene is the basic concept of classical genetics.
This term refers to a section of DNA on which information about the amino acid sequence of one protein is recorded in the form of a sequence of nucleotides. But in some cases, we mean a continuous section, only part of which corresponds to the protein chain, and in others — a set of sections corresponding to the whole protein molecule. Or it may be that the same piece of DNA belongs to two or even three genes at once.
Genetics is the science of heredity.
Genome — all the genetic information of an organism.
Histones are proteins that are part of chromosomes. They form the protein core of nucleosomes.
DNA is deoxyribonucleic acid. A molecule that contains genetic information. It consists of two polynucleotide chains forming a double helix.
A zygote is a fertilized egg from which an entire organism grows.
The nucleosome is the main structural element of a chromosome. It is a protein (histone) core on which DNA is wound.
A nucleotide is a monomeric link of DNA and RNA.
Germ cells are cells that serve for procreation (spermatozoa and eggs).
The ribosome is a complex complex of RNA and proteins that carries out the process of protein synthesis in the cell.
RNA is ribonucleic acid. A biological polymer very close to DNA in its chemical structure. It is capable of forming a double helix, but in nature, as a rule, it exists in the form of a single chain.
A spiral is often called a helical line, the shape of which has a DNA molecule.
Ultraviolet rays are electromagnetic radiation not visible to the eye with a wavelength of less than 400 nm.
An enzyme is a protein molecule that catalyzes one of the chemical reactions in a cell. Being biological catalysts, the enzymes themselves do not change during the reaction, but their presence greatly accelerates the reaction.
A chromosome is a complex complex of DNA and proteins located in the cell nucleus, in which genetic information is stored.
Portal "Eternal youth" www.vechnayamolodost.ru04.05.2008