DNA replication: new data
Biologists for the first time filmed the process of DNA doubling
For the first time, scientists from the USA were able to obtain photographs and videos of the process of doubling the DNA of E. coli, which turned out to be much more random than biologists previously thought, published in the journal Cell (Graham et al., Independent and Stochastic Action of DNA Polymerases in the Replisome).
"The speed of this process can change dramatically during the assembly of the molecule by tens of times. It turned out that the work of proteins in the "assembly line" of DNA does not synchronize in any way – everything happens by chance and they act completely independently from each other," said Stephen Kowalczykowski from the University of California at Davis (in a press release Close–Up View of DNA Replication Yields Surprises - VM).
One of the distinguishing features of living organisms that separates them from viruses and inanimate nature is their ability to independently create copies of their own genetic code encoding all components and processes occurring inside cells. This process, the so-called DNA replication, is one of the most complex chemical reactions in the universe.
In this process, as the experiments of recent years show, several dozen proteins are involved, each of which performs different functions. First, the chromosomes are "unwound" using the FACT protein, then the DNA helix "unravels" the enzyme helicase, and then the protein "anchor" of primase joins them, and special proteins, which scientists call DNA polymerases, begin the copying process by reading the helix and assembling its analogue from individual molecular "letters"-nucleotides.
The problem, as Kowalczukowski says, is that DNA consists of two helices, which polymerases, as scientists initially assumed, copy simultaneously. The first observations of this process showed that in fact one of them is copied faster than the second. The second polymerase periodically "slows down", so that the protein molecules and their "servants" do not interfere with each other.
For this reason, many scientists believed that the work of polymerases was somehow synchronized with each other, but the mechanism of this synchronization remained a mystery to them.
Kowalczukowski and his colleagues tried to find an answer to this question by tracing the copying of short DNA strands that scientists extracted from E. coli and "glued" using a modified version of primase to the surface of a glass plate.
Biologists placed these plates in a solution containing DNA polymerases, a cellular "energy currency" of ATP and a special set of nucleotides labeled with glowing protein molecules. These proteins glowed only when the nucleotide attached to them was "attached" to a double strand of DNA, which allowed Kovalchukovsky's team to track how copies of E. coli chromosomes grew.
As it turned out, the secret of the polymerases was that there was no synchronization between them – the replication process of both threads was completely random. When there were "collisions" between the DNA collectors, the process of lengthening the thread simply began anew.
The discovery of this random mechanism of work, scientists believe, will help us understand how errors occur when copying DNA and find out how features in the work of polymerases affect the rate of evolution and accumulation of mutations.
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16.06.2017