when and why does dna replicate?
154157
DNA replicates for cell division.  The new cell gets the same DNA as the parent cell.  DNA replicates during mitosis.  DNA also replicates into RNA.  This is done inside the nucleus.  RNA is the exact opposite copy of DNA.  RNA aids in the synthesis of proteins.  The RNA goes to the ribosomes and synthesizes proteins.
325191
Actually it makes perfect sense.  Things naturally pair together, whether it be by gravity, magnetism, parasitism or whatever reason.  eventually enough atoms will link together for convienience to form a protein, enough protiens will link together to form a cell, then we have life.  cells will then pair together to form tissues, which will pair with other tissues to increase their survival rate giving us more complex organisms.  See the steps yet?  Following that we get to higher functioning organisms.  An organism's cells reproduce millions of times in that organisms lifetime and every time the DNA is replicated there is a chance for a mistake.  How many typos do you make typing a paper?  This is where variation comes from.  Evolution and extinction come from natural selection.  As the world changes things have to adapt those that don't die. If they didn't die we wouldn't call it evolution.
124712
Microsatellites are expansive repeats of two, three, four, or five nucleotides (ex: GCCGCCGCCGCC...). They occur when the DNA template "slips" and the DNA replication machinery reorients itself on the template incorrectly. For example, imagine that there are 25 "GCC" repeats, and suppose that the DNA replicating machinery is on repeat 4. The template "slips" and the DNA replicating machinery goes back to repeat 2. Why does it not know that it has done this? Because there are so many GCC that it doesn't recognize the mistake.  So it was replicating #4 GCC, but now it's replicating #2 GCC. That means that the final DNA strand being replicated will now have two additional GCC...and thus, an increase in the microsatellite region.
541814
DNA is a molecule, not a living object. It can stay there as long as it is not disturbed and swept away.
493756
A virus needs a cell to replicate itself. You see viruses have absolutely no ability to reproduce on there own. They basically "hijack" a cell and then the cell makes a copy of the virus.
1032144
RNA, most assuredly.\n\nDNA is a great replicator, but a lousy enzyme.\n\nProteins make great enzymes, but lousy replicators.\n\nRNA makes a pretty good replicator, and a pretty good enzyme, so is a much better candidate than DNA for an early replicator that becomes its own catalyst.  However, RNA replication is far more error-prone than DNA, so it is not sufficient for coding for complex organisms.\n\nSo what we have now is DNA which is stable (error-correcting) enough to support *huge* DNA molecules that store information for tens of thousands of genes through an indefinite number of generations ... but RNA is used as an intermediary carrier of information during protein synthesis.
500215
DNA is the building blocks of life (I know you've heard that one). Basically, DNA encodes everything. It's what creates cells.\n\nIn a nutshell, DNA is comprised of a basic 5-carbon sugar with an attached nucleic acid (one of four, adanine, guinine, cytosine, and thiamine). These form long chaings that if stretched out fully would encompass the earth around 5 times. They encode (in a lengthy process) for protiens and certain protein chains that in the end, make your cells. And more cells, and more, until you get an organ. Enough organs and you get an organism. \n\n-Eric
413070
Alternatively, if you meant why do cells stop dividing after a certain time it is because their telomeres get too short. Telomeres are the parts of DNA at the end of chromosomes. DNA polymerase requires a primer (a piece of DNA to sit on). It starts making new DNA from this point, but doesn't copy the part it sits on at first, so the copy is a tiny bit shorter than the original. Each time cells divide and copy their DNA, their telomeres get a little bit shorter. When they are too short the cell recognizes this and stops dividing.
776889
DNA has the form of a double helix. It is also anti parallel.
737692
Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the biological development of a cellular form of life or a virus. All known cellular life and some viruses have DNAs. DNA is a long polymer of nucleotides (a polynucleotide) that encodes the sequence of amino acid residues in proteins, using the genetic code.\nDNA is responsible for the genetic propagation of most inherited traits. In humans, these traits range from hair color to disease susceptibility. The genetic information encoded by an organism's DNA is called its genome. During cell division, DNA is replicated, and during reproduction is transmitted to offspring.\n\nIn eukaryotic cells, such as those of plants, animals, fungi and protists, most of the DNA is located in the cell nucleus, and each DNA molecule is usually packed into a chromosome that are passed to daughter cells during cell division. By contrast, in simpler cells called prokaryotes, including the eubacteria and archaea, DNA is found directly in the cytoplasm (not separated by a nuclear envelope) and is circular. The cellular organelles known as chloroplasts and mitochondria also carry DNA. DNA is thought to have originated approximately 3.5 to 4.6 billion years ago.[1]\n\nIn humans, the mother's mitochondrial DNA together with 23 chromosomes from each parent combine to form the genome of a zygote, the fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother. Lineage studies can be done because mitochondrial DNA only comes from the mother, and the Y chromosome only comes from the father
910568
During prophase. But it actually doesn't happen in all organisms. Yeast for example has a nuclear duplication and division without ever breaking down the nuclear membrane.
1186162
Deoxyribonucleic acid (DNA) is a nucleic acid —usually in the form of a double helix— that contains the genetic instructions specifying the biological development of all cellular forms of life, and most viruses. DNA is a long polymer of nucleotides (a polynucleotide) and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides.\n\nIn complex eukaryotic cells such as those from plants, animals, fungi and protists, most of the DNA is located in the cell nucleus. By contrast, in simpler cells called prokaryotes, including the eubacteria and archaea, DNA is not separated from the cytoplasm by a nuclear envelope. The cellular organelles known as chloroplasts and mitochondria also carry DNA.\n\nDNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most inherited traits. In humans, these traits can range from hair colour to disease susceptibility. During cell division, DNA is replicated and can be transmitted to offspring during reproduction. Lineage studies can be done based on the facts that the mitochondrial DNA only comes from the mother, and the male Y chromosome only comes from the father.\n\nEvery person's DNA, their genome, is inherited from both parents. The mother's mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome of a zygote, the fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother.\n\nThe discovery that DNA was the carrier of genetic information was a process that required many earlier discoveries. The existence of DNA was discovered in the mid 19th century. However, it was only in the early 20th century that researchers began suggesting that it might store genetic information. This gained almost universal acceptance after the structure of DNA was elucidated by James D. Watson and Francis Crick in their 1953 Nature publication. Watson and Crick proposed the central dogma of molecular biology in 1957, describing the process whereby proteins are produced from nucleic DNA. In 1962 Watson, Crick, and Maurice Wilkins jointly received the Nobel Prize for their determination of the structure of DNA.
19715
DNA replication is not organism dependent. It involves 3 basic steps, initiation, replication, and termination. More detailed information is below.
909375
Cells that are undergoing constant mitosis such as hematopoetic stem cells (cells in your bone marrow that give rise to blood cells) and keratinocytes (skin cells) generally replicate in an ongoing manner about once a day, or faster.  I should say that it is only the skin cells of the bottom two layers of epidermis that undergo replication (stratum basale, and stratum spinosum).  Most of the replication occurs in the bottom layer of the epidermis (basale).\n\nMost of the time will be spent in the pre-mitotic cell cycle stages, G1, S, and G2, giving the cell ample time to replicate its DNA, and prepare for the division.  These stages are collectively referred to as 'interphase'.  The G1 phase varies (it will be at least 4 hours), the G2 phase lasts aroud 4-5 hours, and the S phase is usually around 6-8 hours (this is when the DNA is replicating).  So the entire interphase will be anywhere from 14-24 hours.\n\nThe mitotic process (the M phase) only takes about an hour.  Prophase is usually the longest phase of mitosis, and is in itself often broken down into a few different substages.  Once the metaphase stage begins, things move along pretty swiftly.  \n\nHope that helps!
619772
Because the basic mechanisms of self-replicating molecules such as RNA and DNA were SO successful that they continued to replicate and mutate, to thrive and survive.  Success breeds success.\n\nIf other mechanisms evolved to compete RNA and DNA, they might have lost to competition for resources and so their lineages died out.
584877
It's not expected to be the same. \n\nIn double stranded DNA, A=T and C=G, therefore (A+C)/(G+T) = 1 always. The number of A is not necessarily equal as C. Therefore, (A+T)/(C+G) can be any number, different than 1.
289884
The insertion of foreign DNA fragments into a cell/organism which will replicate the DNA and integrate it into its own genome