However, the replication fork is bi-directional; one strand is oriented in the 3' to 5' direction leading strand while the other is oriented 5' to 3' lagging strand. The two sides are therefore replicated with two different processes to accommodate the directional difference. The leading strand is the simplest to replicate. The primer always binds as the starting point for replication.
Primers are generated by the enzyme DNA primase. Enzymes known as DNA polymerases are responsible creating the new strand by a process called elongation.
There are five different known types of DNA polymerases in bacteria and human cells. In bacteria such as E. DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication. In eukaryotic cells, polymerases alpha, delta, and epsilon are the primary polymerases involved in DNA replication. Because replication proceeds in the 5' to 3' direction on the leading strand, the newly formed strand is continuous.
The lagging strand begins replication by binding with multiple primers. Each primer is only several bases apart. This process of replication is discontinuous as the newly created fragments are disjointed. Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands.
These primers are then replaced with appropriate bases. Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand. The ends of the parent strands consist of repeated DNA sequences called telomeres.
Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing. A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. In the end, replication produces two DNA molecules , each with one strand from the parent molecule and one new strand. DNA replication would not occur without enzymes that catalyze various steps in the process.
Enzymes that participate in the eukaryotic DNA replication process include:. Each molecule consists of a strand from the original molecule and a newly formed strand. Prior to replication, the DNA uncoils and strands separate. A replication fork is formed which serves as a template for replication. This addition is continuous in the leading strand and fragmented in the lagging strand. After the formation of both the continuous and discontinuous strands, an enzyme called exonuclease removes all RNA primers from the original strands.
The gaps where the primer s had been are then filled by yet more complementary nucleotides. A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA.
Telomeres are regions of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Think of shoelace caps. Telomeres are also a biomarker of aging, with telomeres shortening with each cellular division or, in other words, as you advance in age. Basically, shorter telomeres make you more susceptible to a number of diseases, such as cancer or cardiovascular disease.
Finally, the parent strand and its complementary DNA strand coils into the familiar double helix shape. The result is two DNA molecules consisting of one new and one old chain of nucleotides. Remarkably, it takes very little time for our biological machinery to copy something this exceedingly long. Every cell completes the entire process in just one hour! Tibi is a science journalist and co-founder of ZME Science. He writes mainly about emerging tech, physics, climate, and space. In his spare time, Tibi likes to make weird music on his computer and groom felines.
Nitrogenous bases on each strand are represented by blue, orange, red, or green horizontal rectangles attached to each segment of the sugar-phosphate backbone. The bases form rungs of red-green or blue-orange between the grey cylinders. Helicase is bound to the ends of several nitrogenous bases on the lower strand. Beside it, four nitrogenous bases, each attached to a sugar molecule, have been annealed to complementary nitrogenous bases on the bottom strand.
About three dozen individual nucleotides float in the background. Meanwhile, as the helicase separates the strands, another enzyme called primase briefly attaches to each strand and assembles a foundation at which replication can begin. This foundation is a short stretch of nucleotides called a primer Figure 2. As DNA polymerase makes its way down the unwound DNA strand, it relies upon the pool of free-floating nucleotides surrounding the existing strand to build the new strand. The nucleotides that make up the new strand are paired with partner nucleotides in the template strand; because of their molecular structures, A and T nucleotides always pair with one another, and C and G nucleotides always pair with one another.
This phenomenon is known as complementary base pairing Figure 4 , and it results in the production of two complementary strands of DNA. Base pairing ensures that the sequence of nucleotides in the existing template strand is exactly matched to a complementary sequence in the new strand, also known as the anti-sequence of the template strand.
Later, when the new strand is itself copied, its complementary strand will contain the same sequence as the original template strand. Thus, as a result of complementary base pairing, the replication process proceeds as a series of sequence and anti-sequence copying that preserves the coding of the original DNA. In the prokaryotic bacterium E. In comparison, eukaryotic human DNA replicates at a rate of 50 nucleotides per second.
In both cases, replication occurs so quickly because multiple polymerases can synthesize two new strands at the same time by using each unwound strand from the original DNA double helix as a template. One of these original strands is called the leading strand, whereas the other is called the lagging strand.
The leading strand is synthesized continuously, as shown in Figure 5. In contrast, the lagging strand is synthesized in small, separate fragments that are eventually joined together to form a complete, newly copied strand. This page appears in the following eBook. Aa Aa Aa. How is DNA replicated? What triggers replication?
Figure 1: Helicase yellow unwinds the double helix. The initiation of DNA replication occurs in two steps. First, a so-called initiator protein unwinds a short stretch of the DNA double helix.
Then, a protein known as helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, thereby pulling apart the two strands. As the helicase moves along the DNA molecule, it continues breaking these hydrogen bonds and separating the two polynucleotide chains Figure 1. How are DNA strands replicated?
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