DNA

It is raining DNA outside. On the bank of the Oxford canal at the bottom of my garden is a large willow tree, and it is pumping downy seeds into the air. [...] Not just any DNA, but DNA whose coded characters spell out specific instructions for building willow trees that will shed a new generation of downy seeds. [...] It is raining instructions out there; it’s raining programs; it’s raining tree-growing, fluff-spreading, algorithms. That is not a metaphor, it is the plain truth. It couldn’t be any plainer if it were raining floppy discs.

Deoxyribonucleic acid (DNA) is the building block of the inheritable material on which every form of life on Earth is based on. DNA is similar to a blueprint for living organisms (not only, but also viruses); however, that analogy is limited due to the fact that a blueprint is something that is built on and later not modified; British biologist Richard Dawkins explains: "DNA is a recipe or a computer program, not a blueprint. The program is in every cell but each cell reads its own part & interprets it locally in its own way."

DNA is a nucleic acid (a class of biological macro-molecules) just like RNA (ribonucleic acid). DNA is composed of three basic components wrapped into a double helix structure: a phosphate group (PO4-), deoxyribose (a five-carbon sugar), and nitrogen bases. The phosphate and carbohydrate (carbohydrates are commonly called sugar) act as the "backbone" of the helix, while genetic information is stored in the sequence of nitrogen bases that connect the two sides of the helix (in the inside). There are four nitrogen bases found in our DNA: adenine (A), cytosine (C), guanine (G), and thymine (T), with their abbreviations in parenthesis. Adenine binds with thymine, and guanine binds with cytosine. The sequences of those nitrogen bases carry information that can be used for the construction of proteins via messenger RNAs (mRNA).

A considerable proportion of our DNA is erroneously labeled "junk" DNA, as it does not appear to have any direct specific purpose; it is quite possible that some of that "junk" DNA performs a vital role.

Human DNA
The fact that other than identical twins no two people have the same DNA allows determination of identity. So-called "DNA tests", however, do not involve a comparison of all of the DNA; a complete sequencing of the genome is rather expensive. Instead, regions of high variability in the genome are identified, and those areas are compared. The number of regions varies depending on how thorough the test is. With enough regions tested, the chances of a false positive can be made extremely small. Sometimes, this probability is given as less than one in a billion, but the exact probability cannot be calculated without assuming that the regions vary independently, which is not necessarily the case. Furthermore, it's a bit silly to talk about the chances of an error being one in a billion, when there are all sorts of ways that there could be a false positive, such accidental contamination, deliberate planting of evidence, etc. So-called "identical" twins are often assumed to have identical genomes, but research is still ongoing as to what extent that is the case. In fact, there can even be different DNA within the same person.

Similarly, DNA can be used to determine ancestry, especially in cases where paternity has to be ascertained.

Mitochondrial DNA (which is only passed down the female line as mitochondria in the sperm are lost with the flagellum or selectively destroyed after fertilisation) is used to study human evolution because it changes more slowly than cellular DNA. It has been used to trace the origin of female humans back to prehistoric Africa. Mitochondrial DNA is also used to trace the migration of humans after they left Africa. However, concern has arisen about mitochrondrial DNA dispersing through populations at a rate different from nuclear DNA.

If Jesus had existed, his DNA would have been ~98.8% similar to that of a chimpanzee. Or you. You're ~98.8% Jesus.

How does DNA actually, er… work?
Let's not get too complicated here, or too hidebound by detailed facts. Here's the telegram version.

DNA replicates by unwinding into two strands. Each strand then picks up all its complementary A,C,G, or T bases from stuff that's lying around. The result is two copies. NEVER MIND about mutations. NEVER MIND how it picks up the bases.

DNA synthesises proteins&mdash;that's what we're mostly made of&mdash;by a different kind of unwinding. In this type, the complementary strand is the same sequence, but a molecule of mRNA. NEVER MIND about Uracil.

mRNA then chuffs off and makes the protein. Every triplet of RNA bases encodes a specific amino acid. The amino acids are strung together in a long chain and there's yer protein. NEVER MIND about tRNA. NEVER MIND about start/stop codons. NEVER MIND about secondary and tertiary structure.

Don't even think about all the other things regulatory sections of DNA produce like lncRNA's, snRNA's, snoRNA's, miRNA, rRNA, and a slew of other regulatory and structural RNA's.

Genes
A gene is a section of DNA within the genome that codes for specific aspect of a living thing. Usually a gene corresponds to one or more proteins that it codes for, by transcribing the gene's DNA information to a string of RNA, which then is fed into a ribosome to produce a protein. A gene may code for more than the one protein due to the same chain of amino acids being folded in more than one way, and because of the ability for genes to be 'spliced' in such a way as to produce entirely different proteins. The gene may also be part of the gene regulation system which controls or modifies how a given gene is expressed, significantly during embryonic development.

Genes are organized into chromosomes more or less randomly, though some genes, for example the HOX developmental genes are organized by order of expression. The randomness of organization means that genes can be linked by being close enough together to avoid being split during meiosis; this results in correlations such as blue eyes and blindness in cats, which are not directly causative.

Genes code for a lot, but not all, of the features seen in life, and usually outside factors such as environment will also have a hand in shaping the final form. These outside influences are studied in epigenetics. Genes were also the inspiration behind memetics, where units of cultural information (as opposed to genetic information) are referred to as memes.

RNA
RNA stands for ribonucleic acid, a polymer produced by all living things to help translate genetic information stored in DNA into functional proteins. It is single stranded and chemically different from DNA, replacing thymine with uracil and the sugar deoxyribose with ribose in its molecular backbone.

Types
RNA comes in different types. Messenger RNA contains the genetic code, copied from DNA, that will determine which amino acids will be assembled to produce a specific protein. Ribosomal RNA forms part of the structure to assemble these proteins, and Transport RNA brings the amino acids needed to make the protein form. There are other types that help regulate the genetic processes of the cell.

Possible Role in the Formation of Life
RNA structures store information, and certain types (called ribozymes) are able to manipulate other strands of RNA or assemble proteins out of amino acids. Short sequences can also replicate themselves if certain substances are present in the environment, and these compounds have been shown to form under the natural conditions that dominated the primitive earth. These characteristics may mean that RNA had a role in the formation of life.

One prominent theory states that RNA began to assemble itself into small, fragile strands. Most of them would fall apart under the harsh conditions of the time, but some would have been able to effectively reproduce themselves. Through random structural changes, some of those might have started assembling proteins from the amino acids floating in the sea, creating a self-replicating RNA-protein complex. If these proteins helped the strand to effectively reproduce, they would be kept and enhanced by natural selection, eventually leading to the formation of lone proteins more effective at reproduction than RNA or its complexes.

Chromosome
The chromosome is the largest iteration of the DNA polymer. It is made up of genes, made up out of both introns and exons, and junk DNA. Chromosomes are nothing more than genetic filing cabinets, and each individual of a species generally has the same genes stored in the same chromosome.

In meiosis, chromosomes "cross over" and produce new variations in the distribution of genes. Genes can also split in two, through duplication of the centromere, a special point at the center of the chromosome, or join together at the tips; this is thought to be a major cause of speciation, and Homo sapiens' 46 chromosomes as opposed to chimpanzees' 48 is a prime example.

Chromosomes, which are stored in the nucleus of every cell, occur in symmetric pairs, one half of each coming from each parent (in sexually reproducing species). These pairs are then split up when forming reproductive cells (gametes), which are then combined to form the new offspring's (relatively) unique chromosome set.

In sexual reproduction, chromosomes are a key part of creating genetically different offspring. Gender is also determined by chromosomes; in humans, gender depends on the 'X' and 'Y' chromosomes, but in other species, other letters are used to represent them.

Differing numbers of chromosomes produce infertility, since meiosis will produce odd results in the hybrid of two different-numbered chromosome sets. This explains why some hybrids (i.e. horse X donkey = mule) are (usually) not fertile.

Occasionally, people can have an extra chromosome, leading to a genetic abnormality called trisomy. Examples of trisomy include Down syndrome (extra chromosome 21), Klinefelter's syndrome (Male with extra X), and everyone's favorite Triple X syndrome.