Open survey. In: Facts In the Cell. This is carried out by an enzyme called helicase which breaks the hydrogen bonds holding the complementary bases of DNA together A with T, C with G. The two separated strands will act as templates for making the new strands of DNA.
As a result of their different orientations, the two strands are replicated differently: An illustration to show replication of the leading and lagging strands of DNA. Related Content:. What is a genome? What is DNA? Genetics: A Conceptual Approach, 2nd ed. Replication errors can also involve insertions or deletions of nucleotide bases that occur during a process called strand slippage. Sometimes, a newly synthesized strand loops out a bit, resulting in the addition of an extra nucleotide base Figure 3.
Other times, the template strand loops out a bit, resulting in the omission, or deletion, of a nucleotide base in the newly synthesized, or primer , strand. Regions of DNA containing many copies of small repeated sequences are particularly prone to this type of error.
DNA polymerase enzymes are amazingly particular with respect to their choice of nucleotides during DNA synthesis, ensuring that the bases added to a growing strand are correctly paired with their complements on the template strand i.
Nonetheless, these enzymes do make mistakes at a rate of about 1 per every , nucleotides. That might not seem like much, until you consider how much DNA a cell has. In humans, with our 6 billion base pairs in each diploid cell, that would amount to about , mistakes every time a cell divides! Fortunately, cells have evolved highly sophisticated means of fixing most, but not all, of those mistakes. Some of the mistakes are corrected immediately during replication through a process known as proofreading , and some are corrected after replication in a process called mismatch repair.
During proofreading, DNA polymerase enzymes recognize this and replace the incorrectly inserted nucleotide so that replication can continue. After replication, mismatch repair reduces the final error rate even further. Incorrectly paired nucleotides cause deformities in the secondary structure of the final DNA molecule. During mismatch repair, enzymes recognize and fix these deformities by removing the incorrectly paired nucleotide and replacing it with the correct nucleotide.
Incorrectly paired nucleotides that still remain following mismatch repair become permanent mutations after the next cell division.
This is because once such mistakes are established, the cell no longer recognizes them as errors. Consider the case of wobble-induced replication errors. When these mistakes are not corrected, the incorrectly sequenced DNA strand serves as a template for future replication events, causing all the base-pairings thereafter to be wrong.
For instance, in the lower half of Figure 2, the original strand had a C-G pair; then, during replication, cytosine C is incorrectly matched to adenine A because of wobble.
In this example, wobble occurs because A has an extra hydrogen atom. In the next round of cell division, the double strand with the C-A pairing would separate during replication, each strand serving as a template for synthesis of a new DNA molecule. At that particular spot, C would pair with G, forming a double helix with the same sequence as its original i.
This type of mutation is known as a base, or base-pair, substitution. Base substitutions involving replacement of one purine for another or one pyrimidine for another e. Likewise, when strand-slippage replication errors are not corrected, they become insertion and deletion mutations. Much of the early research on strand-slippage mutations was conducted by George Streisinger in the s. Streisinger, a professor at the University of Oregon and a fish hobbyist, is known by some as the "founding father of zebrafish research.
Streisinger used this virus to show that most nucleotide insertion and deletion mutations occur in areas of DNA that contain many repeated sequences also called tandem repeats , and he formulated the strand-slippage hypothesis to explain why this was the case Streisinger et al.
In Figure 3, notice the series of repeat T's on the template strand where the slippage has occurred. When slippage takes place, the presence of nearby duplicate bases stabilizes the slippage so that replication can proceed.
During the next round of replication, when the two strands separate, the insertion or deletion on either the template or primer strand, respectively, will be perpetuated as a permanent mutation.
Scientists have collected enough evidence to confirm Streisinger's strand-slippage hypothesis, and this type of mutagenesis remains an active field of scientific research. Figure 3: Strand slippage during DNA replication. When strand slippage occurs during DNA replication, a DNA strand may loop out, resulting in the addition or deletion of a nucleotide on the newly-synthesized strand.
Although most mutations are believed to be caused by replication errors, they can also be caused by various environmentally induced and spontaneous changes to DNA that occur prior to replication but are perpetuated in the same way as unfixed replication errors.
As with replication errors, most environmentally induced DNA damage is repaired, resulting in fewer than 1 out of every 1, chemically induced lesions actually becoming permanent mutations. The same is true of so-called spontaneous mutations. Rather, they are usually caused by normal chemical reactions that go on in cells, such as hydrolysis. These types of errors include depurination , which occurs when the bond connecting a purine to its deoxyribose sugar is broken by a molecule of water, resulting in a purine-free nucleotide that can't act as a template during DNA replication, and deamination , which results in the loss of an amino group from a nucleotide, again by reaction with water.
Again, most of these spontaneous errors are corrected by DNA repair processes. But if this does not occur, a nucleotide that is added to the newly synthesized strand can become a permanent mutation.
Mutation rates vary substantially among taxa, and even among different parts of the genome in a single organism. Scientists have reported mutation rates as low as 1 mistake per million 10 -8 to 1 billion 10 -9 nucleotides, mostly in bacteria , and as high as 1 mistake per 10 -2 to 1, 10 -3 nucleotides, the latter in a group of error-prone polymerase genes in humans Johnson et al.
Even mutation rates as low as 10 can accumulate quickly over time, particularly in rapidly reproducing organisms like bacteria.
This is one reason why antibiotic resistance is such an important public health problem; after all, mutations that accumulate in a population of bacteria provide ample genetic variation with which to adapt or respond to the natural selection pressures imposed by antibacterial drugs Smolinski et al.
Take E. The genome of this common intestinal bacterium has about 4. Assuming a mutation rate of 10 -9 i. That may not seem like much. At that point, approximately 10, of these bacteria will have accumulated at least one mutation. As the number of bacteria carrying different mutations increases, so too does the likelihood that at least one of them will develop a drug-resistant phenotype.
Likewise, in eukaryotes, cells accumulate mutations as they divide. In humans, if enough somatic mutations i. Or, less frequently, some cancer mutations are inherited from one or both parents; these are often referred to as germ-line mutations.
One of the first cancer-associated somatic mutations was discovered in , when researchers found that a mutated HRAS gene was associated with bladder cancer Reddy et al. HRAS encodes for a protein that helps regulate cell division.
Since then, scientists have identified several hundred additional "cancer genes. Of course, not all mutations are "bad. Figure 3: Replication of the leading DNA strand is continuous, while replication along the lagging strand is discontinuous. After a short length of the DNA has been unwound, synthesis must proceed in the 5' to 3' direction; that is, in the direction opposite that of the unwinding.
Figure Detail. The fragments of newly synthesized DNA along the lagging strand are called Okazaki fragments, named in honor of their discoverer, Japanese molecular biologist Reiji Okazaki. Okazaki and his colleagues made their discovery by conducting what is known as a pulse-chase experiment, which involved exposing replicating DNA to a short "pulse" of isotope-labeled nucleotides and then varying the length of time that the cells would be exposed to nonlabeled nucleotides.
This later period is called the "chase" Okazaki et al. The labeled nucleotides were incorporated into growing DNA molecules only during the initial few seconds of the pulse; thereafter, only nonlabeled nucleotides were incorporated during the chase.
The scientists then centrifuged the newly synthesized DNA and observed that the shorter chases resulted in most of the radioactivity appearing in "slow" DNA. The sedimentation rate was determined by size: smaller fragments precipitated more slowly than larger fragments because of their lighter weight. As the investigators increased the length of the chases, radioactivity in the "fast" DNA increased with little or no increase of radioactivity in the slow DNA. The researchers correctly interpreted these observations to mean that, with short chases, only very small fragments of DNA were being synthesized along the lagging strand.
As the chases increased in length, giving DNA more time to replicate, the lagging strand fragments started integrating into longer, heavier, more rapidly sedimenting DNA strands. Today, scientists know that the Okazaki fragments of bacterial DNA are typically between 1, and 2, nucleotides long, whereas in eukaryotic cells, they are only about to nucleotides long. Bacterial and eukaryotic cells share many of the same basic features of replication; for instance, initiation requires a primer, elongation is always in the 5'-to-3' direction, and replication is always continuous along the leading strand and discontinuous along the lagging strand.
But there are also important differences between bacterial and eukaryotic replication, some of which biologists are still actively researching in an effort to better understand the molecular details. One difference is that eukaryotic replication is characterized by many replication origins often thousands , not just one, and the sequences of the replication origins vary widely among species. On the other hand, while the replication origins for bacteria, oriC, vary in length from about to 1, base pairs and sequence, except among closely related organisms, all bacteria nonetheless have just a single replication origin Mackiewicz et al.
Eukaryotic replication also utilizes a different set of DNA polymerase enzymes e. Scientists are still studying the roles of the 13 eukaryotic polymerases discovered to date. In addition, in eukaryotes, the DNA template is compacted by the way it winds around proteins called histones. This DNA-histone complex, called a nucleosome , poses a unique challenge both for the cell and for scientists investigating the molecular details of eukaryotic replication.
What happens to nucleosomes during DNA replication? Scientists know from electron micrograph studies that nucleosome reassembly happens very quickly after replication the reassembled nucleosomes are visible in the electron micrograph images , but they still do not know how this happens Annunziato, Also, whereas bacterial chromosomes are circular, eukaryotic chromosomes are linear. During circular DNA replication, the excised primer is readily replaced by nucleotides, leaving no gap in the newly synthesized DNA.
In contrast, in linear DNA replication, there is always a small gap left at the very end of the chromosome because of the lack of a 3'-OH group for replacement nucleotides to bind. As mentioned, DNA synthesis can proceed only in the 5'-to-3' direction. If there were no way to fill this gap, the DNA molecule would get shorter and shorter with every generation. However, the ends of linear chromosomes—the telomeres —have several properties that prevent this. DNA replication occurs during the S phase of cell division.
In eukaryotes, the pace is much slower: about 40 nucleotides per second. The coordination of the protein complexes required for the steps of replication and the speed at which replication must occur in order for cells to divide are impressive, especially considering that enzymes are also proofreading , which leaves very few errors behind.
The study of DNA replication started almost as soon as the structure of DNA was elucidated, and it continues to this day. Currently, the stages of initiation, unwinding, primer synthesis, and elongation are understood in the most basic sense, but many questions remain unanswered, particularly when it comes to replication of the eukaryotic genome. Scientists have devoted decades to the study of replication, and researchers such as Kornberg and Okazaki have made a number of important breakthroughs.
Nonetheless, much remains to be learned about replication, including how errors in this process contribute to human disease. Annunziato, A. Split decision: What happens to nucleosomes during DNA replication? Journal of Biological Chemistry , — Bessman, M. Enzymatic synthesis of deoxyribonucleic acid.
General properties of the reaction. Kornberg, A. The biological synthesis of deoxyribonucleic acid. Nobel Lecture, December 11, Biological synthesis of deoxyribonucleic acid. Science , — Lehman, I. Preparation of substrates and partial purification of an enzyme from Escherichia coli.
Losick, R. DNA replication: Bringing the mountain to Mohammed. Mackiewicz, P. Where does bacterial replication start? Rules for predicting the oriC region. Nucleic Acids Research 32 , — Ogawa, T. Molecular and General Genetics , — Okazaki, R.
Mechanism of DNA chain growth. Possible discontinuity and unusual secondary structure of newly synthesized chains. Proceedings of the National Academy of Sciences 59 , —
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