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DNA Replication

Page history last edited by Charles Forstbauer 14 years, 2 months ago

Closed 1/14/10

Totaled 1/10/10 Mr F

Totaled 1/5 /10 Mr F 

Totaled 12/22 Mr F

 

Before a cell divides, its DNA is replicated (duplicated.) Because the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically supplies the information needed to produce its partner.  If the two strands of a DNA molecule are separated, each can be used as a pattern or template to produce a complementary strand.  Each template and its new complement together then form a new DNA double helix, identical to the original.

Before replication can occur, the length of the DNA double helix about to be copied must be unwound.  In addition, the two strands must be separated, much like the two sides of a zipper, by breaking the weak hydrogen bonds that link the paired bases.   Once the DNA strands have been unwound, they must be held apart to expose the bases so that new nucleotide partners can hydrogen-bond to them. 

The enzyme DNA polymerase then moves along the exposed DNA strand, joining newly arrived nucleotides into a new DNA strand that is  complementary to the template. 

Though DNA polymerase can elongate a polynucleotide strand by adding new nucleotides, it cannot start a strand from scratch because it can only bond new nucleotides to a free sugar (3') end of a nucleotide chain. DNA polymerase requires the assistance of a primer, a previously existing short strand of DNA (or RNA) that is complementary to the first part of the DNA segment being copied.  This small strand of nucleotides binds by complementary base pairing to the beginning of the area being copied. With the primer in place, DNA polymerase is then able to continue adding the rest of the pairs of the segment until a new double starnd of DNA is completed. Primers are formed from free nucleotides in the cell by enzymes called DNA primases.

 

Replication occurs differently on antiparallel strands of

DNA.

 

That nucleotides can be added only to the sugar or 3' end of the growing complementary chain presents no problem for the side of the DNA chain opening at its phosphate or 5' end.  The primer that binds to the first few exposed bases will end with a sugar (3') where the phosphate of a new nucleotide can be attached.  From there on, DNA polymerase can continuously synthesize the growing complementary strand.  This strand of DNA is called the leading strand.  A nice little animation of DNA synthesis on the leading strand can be seen at the Nobel Prize e-museum site athttp://www.nobel.se/medicine/educational/dna/a/replication/replication_ani.html.

A different challenge faces DNA polymerase when the complementary side of the DNA molecule begins unzipping from its sugar (3') toward its phosphate (5') end.  A primer of complementary molecules attaching to the opening end of this chain would have a phosphate not a sugar at its exposed end so that new nucleotides could not be joined. To get around this problem, this strand is synthesized in small pieces backward from the overall direction of replication.  This strand is called the lagging strand The short segments of newly assembled DNA from which the lagging strand is built are calledOkazaki fragments. As replication proceeds and nucleotides are added to the 3' end of the Okazaki fragments, they come to meet each other.  The primer fragments are then booted out by enzymes and replaced by appropriate DNA nucleotides.  The whole thing is then stitched together by another enzyme called DNA ligase.  The Nobel e-museum also has an animation of this process athttp://www.nobel.se/medicine/educational/dna/a/replication/lagging_ani.html .

 

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This is a video with the dna replication song based off of a popular blink 182 song. hopefully it will help you learn about how dna replication happnes

 

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This is another video with a song. This is backstreet boys but its made by college students so you know its right.

 

Reiji Okazaki, a Japanese molecular biologist, discovered the function of Okazaki fragments with his wife Tsuenko Okazaki in 1968. His expiriment was conducted using E. Oli, and figured out how the lagging strand of DNA was replicated. Their study proved that half of DNA is fully put together, while the other half is in pieces. He died of luekemia seven years after his important discovery from radiation caused by the Hiroshima atomic bomb.  

 

 This is a picture of Tsuenko Okazaki, Reiji Okazaki's widow. She jointly discovered the function of Okazaki fragment's with her late husband.

 

 

 

Figure 2

 

 

This video is set to backstreet boys i want it that way and could possibly help you remember what dna replication is about even though the singing is not that great. 

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First step: Unwind DNA

  • helicose enzyme
  • stablized by single stranded binding protiens or SBB's

 Here is a picture of the Helicose enzyme doing it's job of unwinding the DNA. You can see how it splits the DNA strand in half and how the two sides would fit perfectly together.

 

This graphic shows the unwind of DNa but it is labeled very clearly.

Second Step: Build daughter strand

  • add new bases
  • DNA  polymerase III 

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This is a picture clearly showing the building of the daugter strands of DNA. The strand on top is going in the correct 5' to 3' direction while the strand on bottom is going from 3' to 5'. This means that the strand on bottom needs to be built using the Okazaki fragments which are clearly illustrated in the red boxes with blue arrows.

 

Nucleotides arrive as nucleosides

  • DNA bases with PPP
  • P-P-P = energy for bonding
  • bonded by DNA polyerase III

Adding new bases:

  • you can only add nucleotides to the 3' end of a growing strand
  • needs to have a starter nucleotide to bond too
  • strand only grows 5' -> 3'
  • there's no energy to form a bond when going from 3' end to 5' end.

 

Only the 5' end can initiate the bond.  This is because it has 3 phosphates (or a triphosphate).  No energy can is there to form a bond when you try going from the 3' end to the 5' end.  To do the opposite way (3'-5') you have to start from the center, and go backwards towards the 5' end.  Once you have finished there, you start at the bottom again and go until you can't go anymore.  Once you have reached there, you still need to connect the bond.  Thus, you bring in the enzyme ligase to connect those missing bonds. 

 

 

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^I found this video helpful since it explained all of the parts for the replication process very well.

 

 

After the replication process is done, two DNA molecules, identical to each other and identical to the original, have been produced.  Each strand has stayed in place and is a template for the synthesis of a complementary strand.  This mode of replication is called semi-conservative:  one half of each new DNA molecule is new and the other have is old.  Watson and Crick had suggested that this was the way DNA would turn out to be replication.  Meselson and Stahl proved this with their experiments.

 

Speed of Replication:

Bacteria: 

 

The single molecule of DNA that is the E. Coli genome.  It contains 4.7 x 106 nucleotide pairs. DNA replication begins at a single, fixed location in this molecule, the "replication origin", proceeds at about 1000 nucleotides per second, and thus is done in no more than 40 minutes. And thanks to the precision of the process (which includes a "proof-reading" function), the job is done with only about one incorrect nucleotide for every 109 nucleotides inserted. In other words, more often than not, the E. coli genome (4.7 x 106) is copied without error.

 

 

 

 

In general, DNA is replicated by uncoiling of the helix, strand separation by breaking of the hydrogen bonds between the complementary strands, and synthesis of two new strands by complementary base pairing. Replication begins at a specific site in the DNA called the origin of replication. DNA replication is bidirectional from the origin of replication. To begin DNA replication, unwinding enzymes called DNA helicases cause the two parent DNA strands to unwind and separate from one another at the origin of replication to form two "Y-shaped" replication forks. These replication forks are the actual site of DNA copying. Helix destabilizing proteins bind to the single-stranded regions so the two strands do not rejoin. Enzymes called topoisimerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication. As the strands continue to unwind and separate in both directions around the entire DNA molecule, the hydrogen bonding of free DNA nucleotides with those on each parent strand produce new complementary strands. As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds. Actually, the nucleotides lining up by complementary base pairing are deoxynucleoside triphosphates, composed of a nitrogenous base, deoxyribose, and three phosphates. As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding. Finally each parent strand serves as a template to synthesize a complementary copy of itself, resulting in the formation of two identical DNA molecules.  

 

This is great quiz that really helps to test your knowledge on DNA replication

http://library.thinkquest.org/27819/cgi-bin/quiz.cgi?quiz=6_5

 

 
 
 
 
As  a DNA strand peels apart, it immediately begins to replace the other have of its strand. in the diagram above, the red helix represents the original DNA strand that is replicating itself. the Blue represents the the newly constructed section of the DNA which is the opposite of what the original strand is made up of.
 
Image: A Simple Diagram of Replication
This illistration breaks down sequentially how a new DNA strand is formed. First the strand splits apart and then the new bases link togetherto form a new dna strain. after this, the new strands will undergo a similar process until the cell can no longer regenerate itself
 
 
The diagram above displays how in each 5 ring carbon chain, each carbon is given a number. the 3 and 5 carbons are the ones that link together to form a chain. when the DNA replicates, it only goes in the 5 to 3 direction which then gets into the leading strands and the lagging strands.
 
     Helicase splits up the DNA to make the DNA fork.  Helicase splits the DNA in two in order to allow it to replicate into daughter cells.  Two helicase are used in each bubble in order to split it both ways.  SSB follows helicase in order to keep it in order.
 
     Before the DNA could be replicated, a primer needs to be layed down.  RNA primer is layed down, off of which polymerase lll builds DNA off of.  The direction in which it grows in okzaki fragments there are multiple RNA primers layed down in spaces, with polymerase lll building DNA off of it.
 
     Once the DNA has built off of the RNA primase, the RNA needs to be replaced so that it can be DNA.  Polymerase 1 does this by taking out the RNA and getting new nucleotydes to replace it.
 
I found this video very helpful in understanding how Okazaki fragments work and form.  The instructor breaks down all of the details simply so it is easy to understand.  I highly recommend watching it! I was really confused until I watched it. 
 
 
 
When DNA replicates, the replication process is usually started at the T'A'T'A' box. Helicase begins unwinding at the T-A A-T formation because a TATA box/2 base pair combo has the weakest bonds between them so the box is the easiest to unravel out of all the other double pairs. The TATA base pairs only have a 2 hydrogen bond between them. Therefore, the T'A'T'A' box is usually the origin of replication during DNA replication.
 
There are several important enzymes involved in the replication process
DNA Ligase - Glues together pieces of DNA that have been created by Polymerase I and III. It adds phosphate in the remaining gaps of the sugar-phosphate backbone.

In molecular biology, DNA ligase is a special type of ligase that can link together two DNA strands that have double-strand break (a break in both complementary strands of DNA). The alternative, a single-strand break, is fixed by a different type of DNA ligase using the complementary strand as a template but still requires DNA ligase to create the final phosphodiester bond to fully repair the DNA.

DNA ligase has applications in both DNA repair and DNA replication. In addition, DNA ligase has extensive use in molecular biology laboratories for Genetic recombination experiments
 
DNA Polymerase - Sticks primer on an open DNA strand and builds the second strand in a 5' to 3' direction.
A DNA polymerase is an enzyme that catalyzes the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best-known for their role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand. This process copies a piece of DNA. The newly-polymerized molecule is complementary to the template strand and identical to the template's original partner strand. DNA polymerases use a magnesium ion for catalytic activity.
 
DNA Primase - Lays down the initial RNA primers so that Polymerase III can get to work, and begin replicating

DNA primase is an RNAP enzyme involved in the replication of DNA.

Primase catalyzes the synthesis of a short RNA segment (called a primer) complementary to a ssDNA template. Primase is of key importance in DNA replication because no known DNA polymerases can initiate the synthesis of a DNA strand without an initial RNA or DNA primer (for temporary DNA elongation).

 
DNA Helicase - Unwinds the helix and prys apart the double strands so primase, polymerase and ligase have access to single strands at one time
Helicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands (i.e. DNA, RNA, or RNA-DNA hybrid) using energy derived from ATP hydrolysis.
 
This website includes more information and an animation about the function of helicase and how it works to unwind DNA.
 
Single Strand DNA Binding Proteins (SSB or SSBP) - Work together to bind individual strands in a DNA double stranded helix and aid the helicases in opening it up into single strands. Also stabilizes the unwound single stranded conformation. It also prevents premature reannealing. The strands of DNA have a tendency to revert back to duplex form, but the SSBPs bind to the single strands to keep them separate. This allows the replication "machinery" to perform its assigned function.
 
I found an animation that clearly explains how each individual enzyme of DNA replication works in sequence. The animation allows you to rewind and pause at certain areas in case you or unsure about how a specific enzyme functions. At the top of the animation you can highlight an enzyme to get a breif explanation about what its purpose in DNA replication is.
 
 

DNA synthesis always starts with a RNA primer

 

 

 

An RNA polymerase that synthesises a short RNA primer sequence to initiate DNA replication

 

 

 

 

 

DNA polymerase

DNA polymerase adds nucleotides to the 3' end of a strand of DNA. If a mismatch is accidentally incorporated, the polymerase is inhibited from further extension. Proofreading removes the mismatched nucleotide and extension continues.

 

 

DNA polymerases are a family of enzymes that carry out all forms of DNA replication. A DNA polymerase can only extend an existing DNA strand paired with a template strand; it cannot begin the synthesis of a new strand. To begin synthesis of a new strand, a short fragment of DNA or RNA, called a primer, must be created and paired with the template strand before DNA polymerase can synthesize new DNA.

Once a primer pairs with DNA to be replicated, DNA polymerase synthesizes a new strand of DNA by extending the 3' end of an existing nucleotide chain, adding new nucleotides matched to the template strand one at a time via the creation of phosphodiester bonds. The energy for this process of DNA polymerization comes from two of the three total phosphates attached to each unincorporated base. (Free bases with their attached phosphate groups are called nucleoside triphosphates.) When a nucleotide is being added to a growing DNA strand, two of the phosphates are removed and the energy produced creates a phosphodiester (chemical) bond that attaches the remaining phosphate to the growing chain. The energetics of this process also help explain the directionality of synthesis - if DNA were synthesized in the 3' to 5' direction, the energy for the process would come from the 5' end of the growing strand rather than from free nucleotides.

DNA polymerases are generally extremely accurate, making less than one error for every 10 nucleotides added. Even so, some DNA polymerases also have proofreading ability; they can remove nucleotides from the end of a strand in order to correct mismatched bases. If the 5' nucleotide needs to be removed during proofreading, the triphosphate end is lost. Hence, the energy source that usually provides energy to add a new nucleotide is also lost.

 

Origins of replication:

For a cell to divide, it must first replicate its DNA. This process is initiated at particular points within the DNA, known as "origins", which are targeted by proteins that separate the two strands and initiate DNA synthesis. Origins contain DNA sequences recognized by replication initiator proteins (eg. dnaA in E coli' and the Origin Recognition Complex in yeast). These initiator proteins recruit other proteins to separate the two strands and initiate replication forks.

Initiator proteins recruit other proteins to separate the DNA strands at the origin, forming a bubble. Origins tend to be "AT-rich" (rich in adenine and thymine bases) to assist this process, because A-T base pairs have two hydrogen bonds (rather than the three formed in a C-G pair)—strands rich in these nucleotides are generally easier to separate due the positive relationship between the number of hydrogen bonds and the difficulty of breaking these bonds.Once strands are separated, RNA primers are created on the template strands. More specifically, the leading strand receives one RNA primer per active origin of replication while the lagging strand receives several; these several fragments of RNA primers found on the lagging strand of DNA are called Okazaki fragments, named after their discoverer. DNA polymerase extends the leading strand in one continuous motion and the lagging strand in a discontinuous motion (due to the Okazaki fragments). RNAse removes the RNA fragments used to initiate replication by DNA Polymerase, and another DNA 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.

As DNA synthesis continues, the original DNA strands continue to unwind on each side of the bubble, forming 2 replication forks. In bacteria, which have a single origin of replication on their circular chromosome, this process eventually creates a "theta structure" (resembling the Greek letter theta: θ). In contrast, eukaryotes have longer linear chromosomes and initiate replication at multiple origins within these.

 

 

The replication fork:

Many enzymes are involved in the DNA replication fork.

When replicating, the original DNA splits in two, forming two "prongs" which resemble a fork (hence the name "replication fork"). DNA has a ladder-like structure; imagine a ladder broken in half vertically, along the steps. Each half of the ladder now requires a new half to match it. Because DNA polymerase can only synthesize a new DNA strand in a 5' to 3' manner, the process of replication goes differently for the two strands comprising the DNA double helix.

Leading strand

The leading strand is that strand of the DNA double helix that is oriented in a 5' to 3' manner.

On the leading strand, a polymerase "reads" the DNA and adds nucleotides to it continuously. This polymerase is DNA polymerase III (DNA Pol III) in prokaryotes and presumably Pol ε[10][11] in eukaryotes.

Lagging strand

The lagging strand is that strand of the DNA double helix that is orientated in a 3' to 5' manner. Because of its orientation, opposite to the working orientation of DNA polymerase III which is in a 3' to 5' manner, replication of the lagging strand is more complicated than that of the leading strand.

On the lagging strand, primase "reads" the DNA and adds RNA to it in short, separated segments. In eukaryotes, primase is intrinsic to Pol α.[12] DNA polymerase III or Pol δ lengthens the primed segments, forming Okazaki fragments. Primer removal in eukaryotes is also performed by Pol δ.[13] In prokaryotes, DNA polymerase I "reads" the fragments, removes the RNA using its flap endonuclease domain, and replaces the RNA nucleotides with DNA nucleotides (this is necessary because RNA and DNA use slightly different kinds of nucleotides). DNA ligase joins the fragments together.

Dynamics at the replication fork
The assembled human DNA clamp, a trimer of the protein PCNA.

As helicase unwinds DNA at the replication fork, the DNA ahead is forced to rotate. This process results in a build-up of twists in the DNA ahead.[14] This build-up would form a resistance that would eventually halt the progress of the replication fork. DNA topoisomerases are enzymes that solve these physical problems in the coiling of DNA. Topoisomerase I cuts a single backbone on the DNA, enabling the strands to swivel around each other to remove the build-up of twists. Topoisomerase II cuts both backbones, enabling one double-stranded DNA to pass through another, thereby removing knots and entanglements that can form within and between DNA molecules.

Bare single-stranded DNA has a tendency to fold back upon itself and form 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.[15]

Clamp proteins form a sliding clamp around DNA, helping the DNA polymerase maintain contact with its template and 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 which releases the DNA polymerase. Clamp-loading proteins are used to initially load the clamp, recognizing the junction between template and RNA primers.

 

Regulation of replication:

The cell cycle of eukaryotic cells.
Eukaryotes:

Within eukaryotes, DNA replication is controlled within the context of the cell cycle. As the cell grows and divides, it progresses through stages in the cell cycle; DNA replication occurs during the S phase (Synthesis phase). The progress of the eukaryotic cell through the cycle is controlled by cell cycle checkpoints. Progression through checkpoints is controlled through complex interactions between various proteins, including cyclins and cyclin-dependent kinases.[16]

The G1/S checkpoint (or restriction checkpoint) regulates whether eukaryotic cells enter the process of DNA replication and subsequent division. Cells which do not proceed through this checkpoint are quiescent in the "G0" stage and do not replicate their DNA.

Replication of chloroplast and mitochondrial genomes occurs independent of the cell cycle, through the process of D-loop replication.

Bacteria:

Most bacteria do not go through a well-defined cell cycle and instead continuously copy their DNA; during rapid growth this can result in multiple rounds of replication occurring concurrently.[17] Within E coli, the most well-characterized bacteria, regulation of DNA replication can be achieved through several mechanisms, including: the hemimethylation and sequestering of the origin sequence, the ratio of ATP to ADP, and the levels of protein DnaA. These all control the process of initiator proteins binding to the origin sequences.

Because E coli methylates GATC DNA sequences, DNA synthesis results in hemimethylated sequences. This hemimethylated DNA is recognized by a protein (SeqA) which binds and sequesters the origin sequence; in addition, dnaA (required for initiation of replication) binds less well to hemimethylated DNA. As a result, newly replicated origins are prevented from immediately initiating another round of DNA replication.[18]

ATP builds up when the cell is in a rich medium, triggering DNA replication once the cell has reached a specific size. ATP competes with ADP to bind to DnaA, and the DnaA-ATP complex is able to initiate replication. A certain number of DnaA proteins are also required for DNA replication — each time the origin is copied the number of binding sites for DnaA doubles, requiring the synthesis of more DnaA to enable another initiation of replication.

 

Rolling circle replication:

Rolling circle replication

Another method of copying DNA, sometimes used in vivo by bacteria and viruses, is the process of rolling circle replication.[20] In this form of replication, a single replication fork progresses around a circular molecule to form multiple linear copies of the DNA sequence. In cells, this process can be used to rapidly synthesize multiple copies of plasmids or viral genomes.

 A plasmid is a circular, double stranded extra chromosal DNA, which is capable of replicating independently from the chromosal DNA.

A viral genome is can either be single stranded or double stranded. The single stranded viral genomes consist of unparied nucleic acid, while the double stranded genomes consist of two complementary paired nucleic acids. The size of genomes can also vary greatly, RNA viruses tend to have a smaller sizes than DNA viruses because of the high error rate when replicating.

In the cell, rolling circle replication is initiated by an initiator protein encoded by the plasmid or virus DNA. This protein is able to nick one strand of the double-stranded, circular DNA molecule at a site called the double-strand origin (DSO) and remains bound to the 5' phosphate end of the nicked strand. The free 3' hydroxyl end is released and can serve as a primer for DNA synthesis. Using the unnicked strand as a template, replication proceeds around the circular DNA molecule, displacing the nicked strand as single-stranded DNA. Continued DNA synthesis produces multiple single-stranded linear copies of the original DNA in a continuous head-to-tail series. In vivo these linear copies are subsequently converted to double-stranded circular molecules.

Rolling circle replication can also be performed in vitro and has found wide uses in academic research and biotechnology, often used for amplification of DNA from very small amounts of starting material. Replication can be initiated by nicking a double-stranded circular DNA molecule or by hybridizing a primer to a single-stranded circle of DNA. The use of a reverse primer (or random primers) produces hyperbranched rolling circle amplification, resulting in exponential rather than linear growth of the DNA molecule.

 

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This video shows how a virus such as HIV would replicate itself and spread throughout the host. This is a very good video and really helped me understand how viruses spread.

 

 

 

Telomeres
 

A telomere is a region of repetitive DNA at the end of a chromosome, which protects the end of the chromosome from destruction.  Telomeres compensate for incomplete semi-conservative DNA replication at chromosomal ends. In most prokaryotes, chromosomes are circular and thus do not have ends to suffer premature replication termination. A small fraction of bacterial chromosomes (such as those in Streptomyces and Borellia) are linear and possess telomeres, which are very different from those of the eukaryotic chromosomes in structure and functions. The known structures of bacterial telomeres take the form of proteins bound to the ends of linear chromosomes, or hairpin loops of single-stranded DNA at the ends of the linear chromosomes.

 

In most multicellular eukaryotic organisms telomerase is active only ingerm cells,stem cells and certain white blood cells. There are theories that the steady shortening of telomeres with each replication in somatic (body) cells may have a role in senescense and in the prevention of cancer. This is because the telomeres act as a sort of time-delay "fuse", eventually running out after a certain number of cell divisions and resulting in the eventual loss of vital genetic information from the cell's chromosome with future divisions.

 

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Although this is a simple video that doesnt have much to say for its visual effects, it accuratly and clearly describes how the telomeres act as a template to complete the DNA sequence. This video really helped me understand exactly what role the telomeres played in DNA replication.

 

Lengthening Telomeres

 

It has been hypothesized that there is a trade-off between cancerous tumor suppression and tissue repair capacity, in that lengthening telomeres might slow aging and in exchange increase vulnerability to cancer. There are studies done indicating that there is a correlation between lengthening telomeres and a longer lifespan. Two groups of worms were studied which differed in the amount of protein their cells produced, resulting in telomere lengthening in the mutant worms. The worms with the longer telomeres lived 24 days on average, about 20 percent longer than the normal worms. Techniques to extend telomeres could be useful for tissue engineering, because they might permit healthy, noncancerous mammalian cells to be cultured in amounts large enough to be engineering materials for biomedical repairs. Changing telomere lengths are usually associated with changing speed of senescence, however this telomere shortening might be a consequence of and not a reason for aging. That the role of telomeres is far from being understood is demonstrated by two recent studies on long-lived seabirds.

 

 

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