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What is the difference between a DNA replication and a chromosomal duplication? What would a double stranded semi-conservative DNA replication look like, if the DNA that was being replicated was a triple-helical strand of DNA? What is the difference between a replication fork and a.
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During replication, these strands are separated. Each strand of the original DNA molecule then serves as a template for the production of its counterpart, a process referred to as semiconservative replication. As a result of semi-conservative replication, the new helix will be composed of an original DNA strand as well as a newly synthesized strand. In a cell , DNA replication begins at specific locations, or origins of replication , in the genome.

A number of proteins are associated with the replication fork to help in the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new strands by adding nucleotides that complement each template strand. DNA replication occurs during the S-stage of interphase. DNA exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotides. Nucleotides in DNA contain a deoxyribose sugar, a phosphate , and a nucleobase.

The four types of nucleotide correspond to the four nucleobases adenine , cytosine , guanine , and thymine , commonly abbreviated as A, C, G and T. Adenine and guanine are purine bases, while cytosine and thymine are pyrimidines. These nucleotides form phosphodiester bonds , creating the phosphate-deoxyribose backbone of the DNA double helix with the nucleobases pointing inward i.

Nucleobases are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine two hydrogen bonds , and guanine pairs with cytosine three hydrogen bonds. These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. The pairing of complementary bases in DNA through hydrogen bonding means that the information contained within each strand is redundant.

Phosphodiester intra-strand bonds are stronger than hydrogen inter-strand bonds. This allows the strands to be separated from one another. The nucleotides on a single strand can therefore be used to reconstruct nucleotides on a newly synthesized partner strand. To begin synthesis, a short fragment of RNA, called a primer , must be created and paired with the template DNA strand. The energy for this process of DNA polymerization comes from hydrolysis of the high-energy phosphate phosphoanhydride bonds between the three phosphates attached to each unincorporated base.

Free bases with their attached phosphate groups are called nucleotides ; in particular, bases with three attached phosphate groups are called nucleoside triphosphates. When a nucleotide is being added to a growing DNA strand, the formation of a phosphodiester bond between the proximal phosphate of the nucleotide to the growing chain is accompanied by hydrolysis of a high-energy phosphate bond with release of the two distal phosphates as a pyrophosphate. Enzymatic hydrolysis of the resulting pyrophosphate into inorganic phosphate consumes a second high-energy phosphate bond and renders the reaction effectively irreversible.

In general, DNA polymerases are highly accurate, with an intrinsic error rate of less than one mistake for every 10 7 nucleotides added. Finally, post-replication mismatch repair mechanisms monitor the DNA for errors, being capable of distinguishing mismatches in the newly synthesized DNA strand from the original strand sequence.

Together, these three discrimination steps enable replication fidelity of less than one mistake for every 10 9 nucleotides added. The mutation rate per base pair per replication during phage T4 DNA synthesis is 1. DNA replication, like all biological polymerization processes, proceeds in three enzymatically catalyzed and coordinated steps: initiation, elongation and termination.

For a cell to divide , it must first replicate its DNA. Four distinct mechanisms for DNA synthesis are recognized:. The first is the best known of these mechanisms and is used by the cellular organisms. In this mechanism, once the two strands are separated, primase adds RNA primers to the template strands. The leading strand receives one RNA primer while the lagging strand receives several. The leading strand is continuously extended from the primer by a DNA polymerase with high processivity , while the lagging strand is extended discontinuously from each primer forming Okazaki fragments.


RNase removes the primer RNA fragments, and a low processivity DNA polymerase distinct from the replicative polymerase enters to fill the gaps. When this is complete, a single nick on the leading strand and several nicks on the lagging strand can be found. Ligase works to fill these nicks in, thus completing the newly replicated DNA molecule.

The primase used by archaea and eukaryotes, in contrast, contains a highly derived version of the RNA recognition motif RRM. It assembles into a replication complex at the replication fork that exhibits extremely high processivity, remaining intact for the entire replication cycle. As DNA synthesis continues, the original DNA strands continue to unwind on each side of the bubble, forming a replication fork with two prongs. In contrast, eukaryotes have longer linear chromosomes and initiate replication at multiple origins within these.

It is created by helicases, which break the hydrogen bonds holding the two DNA strands together in the helix. The resulting structure has two branching "prongs", each one made up of a single strand of DNA. These two strands serve as the template for the leading and lagging strands, which will be created as DNA polymerase matches complementary nucleotides to the templates; the templates may be properly referred to as the leading strand template and the lagging strand template.

Since the leading and lagging strand templates are oriented in opposite directions at the replication fork, a major issue is how to achieve synthesis of nascent new lagging strand DNA, whose direction of synthesis is opposite to the direction of the growing replication fork. The leading strand is the strand of nascent DNA which is synthesized in the same direction as the growing replication fork.

This sort of DNA replication is continuous. The lagging strand is the strand of nascent DNA whose direction of synthesis is opposite to the direction of the growing replication fork. Because of its orientation, replication of the lagging strand is more complicated as compared to that of the leading strand.

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As a consequence, the DNA polymerase on this strand is seen to "lag behind" the other strand. The lagging strand is synthesized in short, separated segments. On the lagging strand template , a primase "reads" the template DNA and initiates synthesis of a short complementary RNA primer. A DNA polymerase extends the primed segments, forming Okazaki fragments. In all cases the helicase is composed of six polypeptides that wrap around only one strand of the DNA being replicated. The two polymerases are bound to the helicase heximer. In eukaryotes the helicase wraps around the leading strand, and in prokaryotes it wraps around the lagging strand.

This process results in a build-up of twists in the DNA ahead. Topoisomerases are enzymes that temporarily break the strands of DNA, relieving the tension caused by unwinding the two strands of the DNA helix; topoisomerases including DNA gyrase achieve this by adding negative supercoils to the DNA helix. Bare single-stranded DNA tends to fold back on itself forming secondary structures ; these structures can interfere with the movement of DNA polymerase.

To prevent this, single-strand binding proteins bind to the DNA until a second strand is synthesized, preventing secondary structure formation. Clamp proteins form a sliding clamp around DNA, helping the DNA polymerase maintain contact with its template, thereby assisting with processivity. The inner face of the clamp enables DNA to be threaded through it. Once the polymerase reaches the end of the template or detects double-stranded DNA, the sliding clamp undergoes a conformational change that releases the DNA polymerase.

Clamp-loading proteins are used to initially load the clamp, recognizing the junction between template and RNA primers. At the replication fork, many replication enzymes assemble on the DNA into a complex molecular machine called the replisome. The following is a list of major DNA replication enzymes that participate in the replisome: [24]. In the replication machineries these components coordinate. In most of the bacteria, all of the factors involved in DNA replication are located on replication forks and the complexes stay on the forks during DNA replication.

These replication machineries are called replisomes or DNA replicase systems. These terms are generic terms for proteins located on replication forks.

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In eukaryotic and some bacterial cells the replisomes are not formed. Since replication machineries do not move relatively to template DNAs such as factories, they are called a replication factory. In the replication factory model, after both DNA helicases for leading strands and lagging strands are loaded on the template DNAs, the helicases run along the DNAs into each other.

The helicases remain associated for the remainder of replication process. Peter Meister et al. They detected DNA replication of pairs of the tagged loci spaced apart symmetrically from a replication origin and found that the distance between the pairs decreased markedly by time. That is, couples of replication factories are loaded on replication origins and the factories associated with each other.

Subsequent research has shown that DNA helicases form dimers in many eukaryotic cells and bacterial replication machineries stay in single intranuclear location during DNA synthesis. The replication factories perform disentanglement of sister chromatids. The disentanglement is essential for distributing the chromatids into daughter cells after DNA replication. Because sister chromatids after DNA replication hold each other by Cohesin rings, there is the only chance for the disentanglement in DNA replication. Fixing of replication machineries as replication factories can improve the success rate of DNA replication.

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If replication forks move freely in chromosomes, catenation of nuclei is aggravated and impedes mitotic segregation. Eukaryotes initiate DNA replication at multiple points in the chromosome, so replication forks meet and terminate at many points in the chromosome. Because eukaryotes have linear chromosomes, DNA replication is unable to reach the very end of the chromosomes. Due to this problem, DNA is lost in each replication cycle from the end of the chromosome.

Telomeres are regions of repetitive DNA close to the ends and help prevent loss of genes due to this shortening. Shortening of the telomeres is a normal process in somatic cells. This shortens the telomeres of the daughter DNA chromosome. As a result, cells can only divide a certain number of times before the DNA loss prevents further division. This is known as the Hayflick limit. Within the germ cell line, which passes DNA to the next generation, telomerase extends the repetitive sequences of the telomere region to prevent degradation.

Telomerase can become mistakenly active in somatic cells, sometimes leading to cancer formation. Increased telomerase activity is one of the hallmarks of cancer. Termination requires that the progress of the DNA replication fork must stop or be blocked. Whilst employing local artists and training others on this project he also involved another people from the local area or passing by or even tourists visiting the city to lend a hand on various workshops along the lower level over the years.

Every year Mark receives more commissions for people to be added to the mural where they can either be depicted at the lower level as a historical character or in one of the many windows or even 3 inches high on the map itself. We have already people on their favourite places on the beach, in their car, van, home, business premises, boats even names on headstones in the graveyard. So many stories are told on the mural one was a couple depicted where they met 50 years ago in the same clothes they wore, this was a surprise anniversary present to his wife.

At the original planning stages, Mark not only wanted to use some of the best paints Keim mineral paints in the world but also protect the wall from advertising billboards for future generations. He achieved this by securing a year lease of the wall from the housing association who owns the building. The first Strand Map Mural won the best landscaping award from the Portsmouth Society in and the completed mural was unveiled by the Lord Mayor.

We celebrated the grand finale with a free family fun day for the community with, tug of war, raffles, music, food and drinks, face painting, hair braiding and celebrity football players to celebrate and bring people together.