What Is Gene Duplication?

Gene replication refers to the replication process of DNA double strands before cell division. The result of replication is that one double strand becomes two identical double strands (if the replication process is normal), and each double strand is the same as the original double strand. . This process is successfully completed through a mechanism called semi-retained replication.

Gene replication

Right!

Gene replication refers to the replication process of DNA double strands before cell division. The result of replication is that one double strand becomes two identical double strands (if the replication process is normal), and each double strand is the same as the original double strand. . This process is successfully completed through a mechanism called semi-retained replication.

Chinese name

Gene replication

Foreign name

Gene replication

Aka

DNA

process

Replication can be divided into the following phases:

Initiation of DNA replication

The priming phase of replication includes the unfolding of the double-stranded DNA origin, synthesis of RNA molecules through the transcription activation step, synthesis of RNA primers, and DNA polymerase adding the first deoxynucleotide to the 3'-OH end of the primer RNA. The key step induced by replication is the synthesis of the leading strand DNA. Once the polymerization of the leading strand DNA begins, the DNA synthesis on the lagging strand also starts. There is a necessary step before all the leading strands begin to polymerize. RNA polymerase (not Primer enzyme) transcribes a short RNA molecule along the lagging template.

In some DNA replications (such as the plasmid ColE), the RNA molecule is added as a primer for DNA replication. However, in most DNA replication, this RNA molecule has no primer effect. Its role only seems to separate the two DNA strands, exposing certain specific sequences so that the initiator can bind to it, and start to synthesize RNA primers on the template DNA of the leader strand. This process is called transcriptional activation, and the replication of the leader strand Other proteins are required during the priming process, such as the dnaA protein of E. coli. These two proteins can bind to four 9bp long sequences that are highly conserved on the DNA at the origin of replication, and their specific functions are unclear.

It is possible that the combination of these proteins with the origin of DNA replication can promote the assembly of seven proteins of the DNA polymerase III complex into functional holoenzymes at the origin of replication. At the beginning of DNA replication, the DNA helicase first dissociates the double-stranded DNA at the origin of replication. The RNA molecule synthesized by transcription activation also functions to separate the two DNA strands, and then the single-stranded DNA-binding protein binds to the dissociated strand on. Preprimosome consisting of replication factor X (n protein), replication factor Y (n 'protein), n "protein, i protein, dnaB protein and dnaC protein. It interacts with single-stranded DNA to form an intermediate, which is a pre-priming process.

The priming precursor is further assembled with a primase to form a primosome. The initiator can move on single-stranded DNA and recognize the origin of DNA replication under the action of the dnaB subunit. First, a primer is catalyzed by a primer enzyme on the leading strand to synthesize a piece of RNA primer. Then, the initiator moves continuously on the lag strand in the direction of 5 ' 3'. See later). RNA primers are repeatedly synthesized over a certain distance for DNA polymerase III to synthesize Okazaki fragments. The functions of many protein factors in the priming body are unclear.

However, these components must work together to make the initiator move on the lagging strand, identify the appropriate primer synthesis position, and aggregate the nucleotides into the RNA primer at the priming position. Because the initiator moves on the lag strand template in the opposite direction to its synthetic primers, the RNA primers synthesized on the lag strand are very short, typically only 3-5 nucleotides long. Moreover, these primers have similar sequences in cells of the same organism, indicating that the primers must synthesize RNA primers at a specific position (sequence) on the DNA hysteresis template.
Why do we need RNA primers to trigger DNA replication? This may be related to minimize mutations at the beginning of DNA replication. A few nucleotides at the beginning of DNA replication are most prone to errors. Therefore, even if an error occurs with RNA primers, it will eventually be excised by DNA polymerase I, which improves the accuracy of DNA replication. After the RNA primer is formed, the first deoxynucleotide is catalyzed by DNA polymerase III to be added to the 3'-OH end of the RNA primer according to the principle of base complementarity to enter the extension stage of the DNA strand.

DNA strand extension

The synthesis of the nascent strand of DNA is catalyzed by DNA polymerase III. However, the double strand must be released by the helicase while moving at the replication fork. This creates a topological problem: due to the melting of DNA, a positive supercoil is bound to be generated in the double-stranded region of DNA, which is more obvious in circular DNA. When it reaches a certain level, it will cause replication forks to be difficult Go ahead and stop DNA replication. However, DNA replication in cells does not stop due to topological problems.

There are two mechanisms to prevent this phenomenon: [1] DNA itself is a supercoil in biological cells. When the DNA is melted to produce a positive supercoil, it can be neutralized by the original negative supercoil; [2] DNA topoisomerase I must open a strand to transform the positive supercoil state into a relaxed state, and DNA topoisomerase II (rotase) can continue to introduce negative supercoil into the double strand in front of DNA melting DNA. These two mechanisms ensure the smooth melting of both circular and open-loop DNA replication, and a new DNA strand is synthesized by DNA polymerase III.

It has been mentioned that the extension of the DNA growth chain is mainly catalyzed by a DNA polymerase, which is a polymer composed of 7 kinds of proteins (polypeptides) and is called a total enzyme. All subunits in the whole enzyme are necessary to complete DNA replication. The subunit has polymerization function and 5 ' 3' exonuclease activity, and the subunit has 3 ' 5' exonuclease activity. In addition, there is an ATP molecule in the whole enzyme. It is necessary for DNA polymerase III to catalyze the first deoxyribonucleotide to be linked to the RNA primer. The functions of other subunits are unclear.
At the DNA replication fork, two sets of DNA polymerase III must be able to replicate the DNA leading and lagging strands at the same time. If the lagging template surrounds the DNA polymerase III holoenzyme and passes through DNA polymerase III, and then folds in the same direction as the unmelted double-stranded DNA, the synthesis of the lagging strand can be in the same direction as the synthesis of the leading strand Carried on.
In this way, when the DNA polymerase III moves along the lagging strand template, the RNA primer synthesized by the specific primer enzyme can be extended by the DNA polymerase III. When the synthesized DNA strand reaches the position of the previously synthesized Okazaki fragment, the lagging strand template and the newly synthesized Okazaki fragment are released from DNA polymerase III. At this time, as the replication fork continued to move forward, another single-stranded lagging strand template was generated, which re-encircled the DNA polymerase III holoenzyme and began to synthesize a new lagging chain Okazaki fragment through DNA polymerase III. Through such a mechanism, the synthesis of the leader strand will not exceed the lagging strand too much (the length of only one Okazaki fragment in the end). Furthermore, the initiator moves on the DNA strand at the same speed as DNA polymerase III.
According to the above-mentioned DNA replication mechanism, near the replication fork, a ribosome-sized complex composed of two sets of DNA polymerase III holoenzyme molecules, an initiator, and a helix is formed, which is called a DNA replisome. When the replica moves on the DNA leading and lagging template, a continuous DNA leading strand and a lagging strand composed of many Okazaki fragments are synthesized. In the process of DNA synthesis extension, it is mainly the role of DNA polymerase III. After the Okazaki fragment was formed, the DNA polymerase I excised the RNA primer on the Okazaki fragment through its 5 ' 3' exonuclease activity, and at the same time, used the latter Okazaki fragment as a primer to synthesize DNA from 5 ' 3'. The last two Okazaki fragments are joined by DNA ligase to form a complete DNA lagging strand.

Termination of DNA replication

It used to be thought that once DNA replication started, all of the DNA molecules would be copied before terminating its DNA replication. However, recent experiments have shown that there is also a replication termination site in DNA, and DNA replication will terminate at the replication termination site, and it does not necessarily wait for all DNA synthesis to be completed. However, little is known about the structure and function of replication termination sites. One problem that is confusing during the termination phase of DNA replication is how do two ends of a linear DNA molecule complete its replication? RNA primers are known to participate in DNA replication. When the RNA primer is removed, the gap left in the middle is filled by DNA Polymerization I. However, the synthesis of a lagging strand with 5 ' 3' as a template at both ends of a linear molecule cannot be filled with DNA polymerase after the RNA primers at the ends are cut off.
This problem was partially resolved when studying T7DNA replication. The DNA sequence region at both ends of T7DNA has a 160bp long identical sequence. Moreover, during T7DNA replication, the offspring DNA molecules produced are not one unit of T7DNA length, but many units of T7DNA are connected end to end. Both progeny DNA molecules of T7DNA will have a single stranded tail at the 3 'end, and the 3' end tails of the two progeny DNAs will complement each other to form a linear connection of two units of T7DNA. It is then filled with DNA polymerase I and ligated with DNA ligase, and continues to replicate to form four unit-length T7 DNA molecules. This replication can form multiple T7DNA molecules of unit length. Such a T7 DNA molecule can be cleaved by a specific endonuclease, and a double-stranded T7 DNA molecule identical to the parent DNA is filled with a DNA polymerase.
In studying poxvirus replication, we discovered a second way for linear DNA molecules to complete terminal replication. Poxvirus DNA forms a hairpin loop at both ends. When DNA is copied, it starts at a replication origin in the middle of the linear molecule and proceeds in both directions, turning the hairpin loop structure into double-stranded circular DNA. Then, different DNA strands are cut in the center of the hairpin to denature the DNA molecules and separate the double strands. In this way, the formation of a single-stranded tail at the ends of each molecule is self-complementary, forming a complete hairpin structure, just like the parent DNA molecule. When eukaryotic chromosomal linear DNA molecules are replicated, it is unclear how the terminal replication process proceeds. It may also replicate and form hairpin structures like poxviruses.

But recent experiments show that eukaryotic DNA replication at the end of a chromosome is a special enzyme that adds a new terminal DNA sequence to the end of the DNA that has just been copied. This mechanism was first discovered in Tetrahymena. The biological cell has 30-70 copies of the 5'TTGGGG3 'sequence at the end of the linear DNA molecule. An enzyme exists in the cell to add the TTGGGG sequence to the TTGGGG sequence at the end of the single bond DNA that already exists. In this way, a single-stranded DNA with a long terminal end can be re-primed by a primer enzyme or other enzyme protein to synthesize an RNA primer, which is then converted into a double-stranded DNA by a DNA polymerase. This prevents the DNA from becoming shorter as the replication continues.
The above-mentioned problems do not exist in the termination phase of the replication of circular DNA. The circular DNA is copied to the end, and the double-stranded DNA is cut by DNA topoisomerase II to separate the two DNA molecules into two complete progeny DNAs that are the same as the parent DNA molecules.
DNA replication in the context of high school biology. DNA replication is a process of copying while spinning. At the beginning of replication, the DNA molecule first uses the energy provided by the cell to unwind the double strands of the two spirals under the action of helicase. This process is called unwinding. Then, using each unbroken parent chain as a template and four kinds of deoxynucleotides in the surrounding environment as raw materials, in accordance with the principle of base pairing and complementary pairing, under the action of DNA polymerase, each is synthesized to be complementary to the parent chain. A sub-chain. As the unwinding process proceeds, the newly synthesized daughter strands also continue to extend. At the same time, each daughter strand and its parent strand are coiled into a double helix structure, thereby forming a new DNA molecule. In this way, after replication, a DNA molecule is divided into two daughter cells through cell division!