In answering your question, the first thing we should consider is the stepwise processes of chemical evolution. I think this helpful diagram speaks volumes:

Creationist view of abiogenesis:

Simple molecules------------------>Bacteria

Real view of abiogenesis:

Simple molecules--->Adsorption surface, piezoelectric properties exhibited by crystalline surfaces forms the basis of catalysis---->Simple organic pro-polymers like PNA---->Pre-RNA world---->The formation of pre-ribozymes allows for first replicative systems----->Outphasing of pre-RNA by RNA---->Autocatalysis via ribozymal RNA templates----->Natural selection of ribozymes for self-replicative catalytic properties---->RNA world---->Formation of vesicles from phospholipids thermodynamically faborable in the water---->Segregation of ribozymes allows for mutual cooperation and selection----->Formation of first biological replicative system from the duplication of RNA----->Outphasing of RNA by DNA----->Formation of dsDNA templated polymerization-------->Cooperation of DNA as templated polymer of replicative heridary information and RNA ribozymes as the catalytic molecules of pre-cellular life in membranous vesicles---->Formation of the first polypeptides by RNA ribozymes with peptidyl transferase properties (this gives the ribozyme a selective advantage)----->Formation of active sites at pre-polypeptides leads to the outphasing of RNA as the central catalytic molecule of pre-cellular life----->Ribozymes are retained in certain necessary functions like pre-ribosomal translation---->Ability of ribozymes to hold polypeptides allows for development of tRNA---->Mutual cooperation of tRNA and ribozymes allows for the creation of an mRNA based translation system----->This gives rise to the first vesicular membranes holding cooperative systems which resemble the modern system of transcription and translation, forming the precursors of bacterial organisms----->Natural selection of the replicative hereditary molecules in these pre-bacterial organisms allows for more complex protein-based functions such as pumps, regulated ion transport etc.----->Formation of the first bacteria

So what we should consider is

a) What does this mean

and

b) What does DNA need in order to self-replicate?

DNA did not arise as a biological molecule until a sophisticated system of intracellular vesicular comparments holding RNA-based catalysis systems formed the basis of the first remotely biological entities on planet Earth.

The process of formation of organic autocatalysis is time consuming. It begins with Piezoelectric systems on crystalline surfaces, which form the precursors of ribozymes. The first biological molecules on Earth may have been formed by metal based catalysis on the crystalline surface of minerals.

In principle, an elaborate system of molecular synthesis and breakdown called metabolism could have existed as such long before the first cells. Life requires molecules which catalyze reactions which lead directly or indirectly to replication of more molecules like themselves. Catalysts with this self promoting property can use raw materials to reproduce themselves and therefore divert the same materials from the production of their substances. In modern cells the most versatile catalysts are polypeptides. However, they cannot propagate self-replication, they do not replicate. There needs to be a molecule which can act as a catalyst and guide its own replication. Such a molecule does exist: RNA.

To understand this more fully, we must understand the relation between protein, DNA, and RNA. Now, the key principle to grasp is the central dogma of molecular biology. Note that the word dogma is not used because the principle has religious adherence, but rather because it seems to be obeyed by all biological life.

Here is where we should consider precisely how DNA replicates in order to understand that in principle any double stranded DNA can replicate, including a 5-pair polymer.

Although many people vaguely allude to DNA as I have described it above, as being a “blueprint” for life encoding “information”, such terms are vague and allusion to metaphors such as these are of no real help when we wish to understand biology at the molecular level. Therefore, later i this section, it shall be introduced (a) In what way precisely DNA holds the “information” to be duplicated and (b) the method of said duplication.

 If all biological organisms carry molecules which contain the information encoding them to be passed to progeny, such molecules must be capable of guiding their own synthesis. 

This is called templated polymerization. It occurs as follows:

 The DNA is replicated by using one strand of DNA as a template strand for a complementary sequence. The result is that both strands of double stranded DNA is used as templates for their complementary sequences. Since one strand of ssDNA is used as a template for the new strand, and that new DNA is passed onto progeny, DNA replicates semiconservitavely, with only one strand being passed onto progeny in a DNA double helix being "new".

-DNA is made up of four nucleotides, Adenine, Thymine, Guanine and Cytosine.  These are linked together by sugar-phosphate backbone bonds. The sugar-phosphate constitutes the backbone of the ladder, whereas base pairs constitute the rungs.

-Thymine forms hydrogen bonds with adenine and cytosine with guanine, hence four permutations for a base pair (AT, TA, CG, GC). This is the principle which underlies DNA replication. The base pairs one one strand are used to construct the complementary strand from free nucleotides

This means that any piece of DNA, no matter how small, can serve as a template for another piece, even a single nucleotide, which means our question becomes how DNA arose as a biological molecule in the first place:

In modern organisms, the process of getting DNA to replicate is complicated, regulated and involves a battery of proteins in order to unwind the helices, decondense the chromatin, catalyze the polymerization, undo the torsion on the helices, remove errors and mark the DNA for signals. Originally, however this would have been far simpler, and in order to understand this, we need to understand the RNA world in detail:

Proteins, being the primary structure and material of all cellular life, are encoded by the information in DNA, which is transcribed to an RNA intermediate before being translated into protein. This process is central to all biological life and is universal in its usage. However, it was not always the case. Prior to the existence of DNA, RNA was used as a ribozyme, prior to the usage of protein as the catalytic compound which would be necessary for the self-replicative properties of biological life. Hence:

Pre-RNA>Pre-RNA

would eventually be superseded by:

RNA>RNA

The manner in which this could occur and form has been detailed above, since RNA have a similarity to polypeptides in their ability to form an active site for homogenous catalysis. It is important to note that ribozymal catalysis still exists in the most primite and ancient mechanisms common to all cellular life. Particular processes worthy of note are protein biosynthesis, snoRNA and snRNA manufacture, SSI (Self-splicing intron/exon junctions) and snRNP splicing (small nuclear ribonucleoprotein complexing), miRNA regulation of translation and RNAi mediated destruction of viral mRNA:

The process of natural selection would naturally favor those RNA which could hold amino acids or small stretches of such at their active sites, by the process detailed above. Hence, eventually the order above would have been superseded by the following:

RNA>RNA and RNA>Protein

DNA has several advantages over RNA. The superseding of RNA as the primary encoder of the codons for polypeptides would have been a slower process. In modern organisms, translation is done by virtue of a set of RNA molecules which hold amino acids and match them to the mRNA called tRNA adaptors. As mentioned previously, RNA molecules that could hold information to guide polypeptide synthesis would have had a massive advantage over its catalytic counterparts because polypeptides are much more efficient at catalysis. Now, ribozymes can perform the function of tRNA-like molecules called psuedo-tRNAs. Finally, this crude form of the peptidyl-transferase would have been naturally selected for its ability to hold the amino acid at the catalytic side of the string of RNA that can hold its code. This development was probably what led to the modern system of codons. Hence we have the following:

mRNA (via psuedo-ribosomes)> translates Protein

Finally, the sequence we see today:

DNA>RNA>Protein

Would have arisen later. The chemical similarities between the two molecular chains, that is, DNA and RNA, allows for the superseding to occur without interrupting the central processes of this primitive form of biological life. But DNA would be naturally selected for as the primary sequence for the transcription of chains to hold the codons because deoxyribose is more stable than ribose and thymine more so than uracil.

Polynucleotides Can Both Store Information and Catalyze Chemical Reactions. RNA can propagate itself by means of complementary base pairing. However, this process without catalysis is slow, error prone and inefficient. Today, such processes are catalyzed by a massive battery of complex interactions of RNA and proteins.

In the RNA world, the RNA molecules themselves would have acted as catalysts. A pre-RNA world probably Predates the RNA One. It is unlikely RNA was the first self-replicating propogater. It is difficult to imagine that they could form through nonenzymatic means. The ribonucleotides are hard to form enzymatically, also RNA polymers entail a 5 to 3 chain which must compete with other linkages that are possible including 2 to 5 and 5 to 5. It has been suggested that RNA was preceded by molecules with similar properties, but that were similar. Candidates for pre-RNA include p-RNA and PNA (peptide nucleic acid)

The transition from pre-RNA to RNA would have occurred through the synthesis of RNA via these simpler components as template and catalyst. Laboratory experiments demonstrate this as plausible. PNA can act as a template for RNA molecules. Once the first RNA molecules had been produced, they could have outphased their antecedents leading to the RNA world

Single-Stranded RNA molecules can fold into highly elaborate structures Comparisons between many RNA structures reveal conserved motifs, short structural elements used over and over again as part of larger structures. Common motifs include:

Single strands, double strands, single nucleotide bulges, triple nucleotide bulges, hairpin loops, symmetric internal loops, asymmetric internall loops, two stem junction, three stem junctions and four stem junctions. RNA molecules can also form common conserved interactions such as psuedoknots and “kissing hairpins” and hairpin-loop bulge contacts, like in this picture:

-Protein catalysts require a surface of unique countours. RNA molecules with appropriate folds can also served as enzyme. Many of the ribozymes work by positioning metal ions at the catalytic sites. Relatively few catalytic RNA exist in modern day cells, being the polypeptides work much better.

An example of In vitro selection of synthetic ribozymes:

-A large pool of dsDNA each with a randomly generated sequence. Transcription and folding into randomly generated RNA molecules. Addition of ATP derivative containing a sulfer in place of oxygen Only a rare RNA has the ability to phosphorylate itself. This is captured by elution of the phosphorylated material

These experiments and others like them have created RNAs that can catalyze a wide variety of reactions:

Peptide bond formation in protein synthesis, RNA cleavage and DNA ligation, DNA cleaving, RNA splicing, RNA polymerization, RNA and DNA phosphorylation, RNA aminoacylation, RAN alkylation, Amide bond formation, amide bond cleavage, glycosidic bond formation and porphyrin metalation, since, like proteins, ribozymes undergo allosteric conformation change

Self-Replication Molecules Undergo Natural Selection

-The 3D structure is what gives the ribozyme chemical properties and abilities. Certain polynucleotides therefore will be especially successful at self-replication. Errors inevitably occur in such processes, and therefore variations will occur over time. Consider an RNA molecule that helps catalyze template polymerization, taking any RNA as a template

-This molecule can replicate. It can also promote the replication of other RNA. If some of the other RNA have catalytic activity that help the RNA to survive in other ways, a set of different types of RNA may evolve into a complex system of mutual cooperation.

One of the crucial events leading to this must have been the development of compartments. A set of mutually beneficial RNA could replicate themselves only if the specialized others were to remain in proximity

Selection of a set of RNA molecules according to the quality of replication could not occur efficiently until a compartment evolved to contain them and therefore make them available only to the RNA that had generated them. A crude form of this may have simply been simple absorption on surfaces or particles.

The need for more sophisticated containment fulfilled by chemicals with the simple physiochemical properties of ampipathism. The bilayers they form created closed vesicles to make a plasma membrane. In vitro RNA selection experiments produced RNA molecules that can tightly bind to amino acids. The nucleotide sequence of such RNA contains a disproportionate number of codons corresponding to the amino acid. This is not perfect for all amino acids, but it raises the possibility that a limited genetic code could have arisen this way. Any RNA that guided the synthesis of a useful polypeptide would have a great advantage.

It is important to realize that vescicles which are the basis of cell membranes as well as intracellular organelle membranes, are the origin of sealed compartments holding and containing biological molecules in which segregation and selection can occur. These vesicles will spontaneously assemble because of basic thermodynamic properties. A phospholipid molecule is ampipathic because it contains a hydrophilic head group (usually choline, glycerol and phosphate) and a hydrophobic tail (fatty acyl). As such they will usually spontaneously array into a bilayer or a micelle. The former is the only geometric outcome for a bilayer such that all the hydrophobic parts are kept free from water and the hydrophilic parts are kept in contact with water. This will produce a sealed vesicle where none of the hydrophobic tails contact the water:

What is the conclusion we should draw here? Whenever we examine the requisites for certain systems in biological life, it is important to remember that because of the process of coevolution, their antecedants did not start out that way. The modern mitochondria and eukaryota both need each other because they have been in symbiosis for so long that mtDNA has ingrained itself in the nuclear genome hence making it an irreversible addition to the Eukaryota. Without it, the organelle dies. Without the organelle, the cell dies. But their antecedants, as they had not evolved "into" each other yet, did not require this relationship. This is true of many complex interlocking systems in biology, and the concept of mutual interlocking dependency is an enormous topic on evolutionary biology and biochemistry. The mutual dependency being discussed here is RNA with Protein and DNA and vice-versa. However, as we have seen, in the RNA world, where catalysis was run by RNA, the mutual dependency between RNA and polypeptides did not exist. Polypeptides developed under natural selection because Ribozymes that can hold amino acids, especially those that can catalyze the peptidyl transferase reaction, have a great advantage. Eventually, polypeptides superseded ribozymes as catalysts because they are more efficient at catalysis (in biochemical terms this is due to the variance of side chains on amino acids, since biological amino acids are more diverse than biological bases). This allows for particular functions such as acid-base catalysis which are not available to ribozymes. As a result, because there was less selection pressure for ribozymal autocatalysis since that was handled by polypeptides, the ribozymes eventually became dependent on polypeptides. A similar "outphasing" occured with the origin of DNA.  There was a reason for this:

-DNA is obviously a more advantegous molecule to use. It is more stable, but more importantly, it can form larger double-strands. The double-strand is enormously important in biology. It allows the information in DNA to be kept in two templates, the one holding the base pairs in question, and the one that can retrieve them via templated polymerization.

Most of it. The abilities of membranous vesicles to self assemble are well demonstrated. We can perform ribozymal natural selection based on a technique called elution chromatography, and can create RNA molecules with active sites capable of holding amino acids which would be reasonable representations of the first RNA-amino acid systems. Some of it is simply inference, of steps that would have needed to occur and how they would have occured.

Approximately 500 million years give or take 200mil.

Depending on your level of scientific literacy, I'd suggest one of two things. 

1. Go to the bookstore and look at Richard Dawkins' books.  All of them.  Copy down his bibliographies.  Go to the nearest university library.  Spend six thousand years reading his sources.

2. Read Richard Dawkins' books.  All of them.  Trust that he's one of the leading authorities on zoology and the theory of evolution in the world.

I'm happy to make the disclaimer that Dawkins has not been an active researcher since assuming the chair of his department.  He's been writing books for you and me to read.  However, there's nothing that's happened in the last ten years or so that substantially alter anything significant in any of his books.  The Selfish Gene dates from 1976, so many of his statements about what has and has not been researched are outdated, but otherwise, it's still a monumental work.

Also, for the record, Deludedgod is an authority on evolution.  He's got all the letters after his name to prove it.

The reason I mentioned Dawkins books is not to say that he should be trusted as a singular authority.  It's to get the OP to look at his bibliographies.  From just Dawkins, you can get a list of sources that will take a lifetime to read.

The simplest way to decide whether or not to trust Dawkins, or any other scientist, is to look at their standing in the scientific community.  Dawkins is no less than the man who revolutionized evolutionary theory by publishing The Selfish Gene.  Though there were scientists who had discovered the principle, the scientific community as a group had not accepted it.  Now, it is virtually incontrovertible.

There are certainly tons of authors out there who have written about evolution.  Dennet, Ridley, Wilson, Zimmer, et al.  As far as talent as a writer combined with scientific credentials, I don't know of anyone better than Dawkins.  He is a fabulous teacher and a meticulous researcher.  I'm not suggesting that you'll have it 100% right if you read Dawkins, but I don't know of a better way to get an overview of evolution sufficient to say that you are an informed person.

Thanks Guys - I have actually read the Selfish Gene several years ago and was planning to make a return to Dawkins after watching a program he made on Darwinism, where he got the Archbishop of Canterbury in a muddle trying to explain how the Christian (Protistant) church tries to absorb science and evolution while at the same time continuing in their belief in, as he put it, "The Cheap Parlour tricks of the Bible".

Just looking on Amazon - (side note - Dawkins' "The God delusion" is listed as a "great Christmas present"... anyone else see irony there?!) For those of you who have read Dawkins works, which one goes into the most detail about the formation of DNA? I already know plenty about darwinian evolution - DNA is the particular subject I'm interested in.

PS - What's the spidey sense allusion? Perhaps you think I'm a previous user of the board back in a new guise (i've seen this on other boards in the past)? Sorry to disappoint - I'm just another atheist interested in finding intelligent answers to gaps in my knowledge.

Cheers,

Ian