<p>The genetic code is the set of rules by which a gene is translated into a functional protein. Each gene consists of a specific sequence of nucleotides encoded in a DNA (or sometimes RNA) strand; a correspondence between nucleotides, the basic building blocks of genetic material, and amino acids, the basic building blocks of proteins, must be established for genes to be successfully translated into functional proteins. Sets of three nucleotides, known as <a class="mw-redirect" title="Codon" href="http://en.wikipedia.org/wiki/Codon">codons</a>, each correspond to a specific amino acid or to a signal; three codons are known as &quot;stop codons&quot; and, instead of specifying a new amino acid, alert the translation machinery that the end of the gene has been reached. There are 64 possible codons (four possible nucleotides at each of three positions, hence 4³ possible codons) and only 20 standard amino acids; hence the code is redundant and multiple codons can specify the same amino acid. The correspondence between codons and amino acids is nearly universal among all known living organisms.</p>

<p><a id="Transcription" name="Transcription"></a></p>

<p>The process of genetic <a title="Transcription (genetics)" href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29">transcription</a> produces a single-stranded <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a> molecule known as <a title="Messenger RNA" href="http://en.wikipedia.org/wiki/Messenger_RNA">messenger RNA</a>, whose nucleotide sequence is complementary to the DNA from which it was transcribed. The DNA strand whose sequence matches that of the RNA is known as the <a title="Coding strand" href="http://en.wikipedia.org/wiki/Coding_strand">coding strand</a> and the strand from which the RNA was synthesized is the <a class="mw-redirect" title="Template strand" href="http://en.wikipedia.org/wiki/Template_strand">template strand</a>. Transcription is performed by an <a title="Enzyme" href="http://en.wikipedia.org/wiki/Enzyme">enzyme</a> called an <a title="RNA polymerase" href="http://en.wikipedia.org/wiki/RNA_polymerase">RNA polymerase</a>, which reads the template strand in the <a class="mw-redirect" title="3' end" href="http://en.wikipedia.org/wiki/3%27_end">3'</a> to <a class="mw-redirect" title="5' end" href="http://en.wikipedia.org/wiki/5%27_end">5'</a> direction and synthesizes the RNA from <a class="mw-redirect" title="5' end" href="http://en.wikipedia.org/wiki/5%27_end">5'</a> to <a class="mw-redirect" title="3' end" href="http://en.wikipedia.org/wiki/3%27_end">3'</a>. To initiate transcription, the polymerase first recognizes and binds a <a title="Promoter" href="http://en.wikipedia.org/wiki/Promoter">promoter</a> region of the gene. Thus a major mechanism of <a class="mw-redirect" title="Gene regulation" href="http://en.wikipedia.org/wiki/Gene_regulation">gene regulation</a> is the blocking or sequestering of the promoter region, either by tight binding by <a title="Repressor" href="http://en.wikipedia.org/wiki/Repressor">repressor</a> molecules that physically block the polymerase, or by organizing the DNA so that the promoter region is not accessible.</p>

<p>In <a title="Prokaryote" href="http://en.wikipedia.org/wiki/Prokaryote">prokaryotes</a>, transcription occurs in the <a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a>; for very long transcripts, translation may begin at the 5' end of the RNA while the 3' end is still being transcribed. In <a title="Eukaryote" href="http://en.wikipedia.org/wiki/Eukaryote">eukaryotes</a>, transcription necessarily occurs in the nucleus, where the cell's DNA is sequestered; the RNA molecule produced by the polymerase is known as the <a class="mw-redirect" title="Primary transcript" href="http://en.wikipedia.org/wiki/Primary_transcript">primary transcript</a> and must undergo <a title="Post-transcriptional modification" href="http://en.wikipedia.org/wiki/Post-transcriptional_modification">post-transcriptional modifications</a> before being exported to the cytoplasm for translation. The <a title="Splicing (genetics)" href="http://en.wikipedia.org/wiki/Splicing_%28genetics%29">splicing</a> of <a title="Intron" href="http://en.wikipedia.org/wiki/Intron">introns</a> present within the transcribed region is a modification unique to eukaryotes; <a title="Alternative splicing" href="http://en.wikipedia.org/wiki/Alternative_splicing">alternative splicing</a> mechanisms can result in mature transcripts from the same gene having different sequences and thus coding for different proteins. This is a major form of regulation in eukaryotic cells.</p>

<p><a class="mw-redirect" title="Translation (genetics)" href="http://en.wikipedia.org/wiki/Translation_%28genetics%29">Translation</a> is the process by which a mature mRNA molecule is used as a template for synthesizing a new <a title="Protein" href="http://en.wikipedia.org/wiki/Protein">protein</a>. Translation is carried out by <a title="Ribosome" href="http://en.wikipedia.org/wiki/Ribosome">ribosomes</a>, large complexes of RNA and protein responsible for carrying out the chemical reactions to add new <a title="Amino acid" href="http://en.wikipedia.org/wiki/Amino_acid">amino acids</a> to a growing <a class="mw-redirect" title="Polypeptide chain" href="http://en.wikipedia.org/wiki/Polypeptide_chain">polypeptide chain</a> by the formation of <a title="Peptide bond" href="http://en.wikipedia.org/wiki/Peptide_bond">peptide bonds</a>. The genetic code is read three nucleotides at a time, in units called <a class="mw-redirect" title="Codon" href="http://en.wikipedia.org/wiki/Codon">codons</a>, via interactions with specialized RNA molecules called <a title="Transfer RNA" href="http://en.wikipedia.org/wiki/Transfer_RNA">transfer RNA</a> (tRNA). Each tRNA has three unpaired bases known as the <a class="mw-redirect" title="Anticodon" href="http://en.wikipedia.org/wiki/Anticodon">anticodon</a> that are complementary to the codon it reads; the tRNA is also <a class="mw-redirect" title="Covalent" href="http://en.wikipedia.org/wiki/Covalent">covalently</a> attached to the <a title="Amino acid" href="http://en.wikipedia.org/wiki/Amino_acid">amino acid</a> specified by the complementary codon. When the tRNA binds to its complementary codon in an mRNA strand, the ribosome ligates its amino acid cargo to the new polypeptide chain, which is synthesized from <a title="N-terminus" href="http://en.wikipedia.org/wiki/N-terminus">amino terminus</a> to <a title="C-terminus" href="http://en.wikipedia.org/wiki/C-terminus">carboxyl terminus</a>. During and after its synthesis, the new protein must <a title="Protein folding" href="http://en.wikipedia.org/wiki/Protein_folding">fold</a> to its active <a title="Tertiary structure" href="http://en.wikipedia.org/wiki/Tertiary_structure">three-dimensional structure</a> before it can carry out its cellular function.</p>

<p><a id="DNA_replication_and_inheritance" name="DNA_replication_and_inheritance"></a></p>

<p>The growth, development, and reproduction of organisms relies on <a title="Cell division" href="http://en.wikipedia.org/wiki/Cell_division">cell division</a>, or the process by which a single <a title="Cell (biology)" href="http://en.wikipedia.org/wiki/Cell_%28biology%29">cell</a> divides into two usually identical <a class="mw-redirect" title="Daughter cell" href="http://en.wikipedia.org/wiki/Daughter_cell">daughter cells</a>. This requires first making a duplicate copy of every gene in the <a title="Genome" href="http://en.wikipedia.org/wiki/Genome">genome</a> in a process called <a title="DNA replication" href="http://en.wikipedia.org/wiki/DNA_replication">DNA replication</a>. The copies are made by specialized <a title="Enzyme" href="http://en.wikipedia.org/wiki/Enzyme">enzymes</a> known as <a title="DNA polymerase" href="http://en.wikipedia.org/wiki/DNA_polymerase">DNA polymerases</a>, which &quot;read&quot; one strand of the double-helical DNA, known as the template strand, and synthesize a new complementary strand. Because the DNA double helix is held together by <a title="Base pair" href="http://en.wikipedia.org/wiki/Base_pair">base pairing</a>, the sequence of one strand completely specifies the sequence of its complement; hence only one strand needs to be read by the enzyme to produce a faithful copy. The process of DNA replication is <a title="Semiconservative replication" href="http://en.wikipedia.org/wiki/Semiconservative_replication">semiconservative</a>; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA.<sup class="reference" id="_ref-Watson_2004_1"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Watson_2004">[8]</a></sup></p>

<p>After DNA replication is complete, the cell must physically separate the two copies of the genome and divide into two distinct membrane-bound cells. In <a title="Prokaryote" href="http://en.wikipedia.org/wiki/Prokaryote">prokaryotes</a> - <a title="Bacteria" href="http://en.wikipedia.org/wiki/Bacteria">bacteria</a> and <a title="Archaea" href="http://en.wikipedia.org/wiki/Archaea">archaea</a> - this usually occurs via a relatively simple process called <a title="Binary fission" href="http://en.wikipedia.org/wiki/Binary_fission">binary fission</a>, in which each circular genome attaches to the <a title="Cell membrane" href="http://en.wikipedia.org/wiki/Cell_membrane">cell membrane</a> and is separated into the daughter cells as the membrane <a class="mw-redirect" title="Invaginate" href="http://en.wikipedia.org/wiki/Invaginate">invaginates</a> to split the <a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a> into two membrane-bound portions. Binary fission is extremely fast compared to the rates of cell division in <a title="Eukaryote" href="http://en.wikipedia.org/wiki/Eukaryote">eukaryotes</a>. Eukaryotic cell division is a more complex process known as the <a title="Cell cycle" href="http://en.wikipedia.org/wiki/Cell_cycle">cell cycle</a>; DNA replication occurs during a phase of this cycle known as <a title="S phase" href="http://en.wikipedia.org/wiki/S_phase">S phase</a>, while the process of segregating <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosomes</a> and splitting the <a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a> occurs during <a class="mw-redirect" title="M phase" href="http://en.wikipedia.org/wiki/M_phase">M phase</a>. In many single-celled eukaryotes such as <a title="Yeast" href="http://en.wikipedia.org/wiki/Yeast">yeast</a>, reproduction by <a title="Budding" href="http://en.wikipedia.org/wiki/Budding">budding</a> is common, which results in asymmetrical portions of cytoplasm in the two daughter cells.</p>

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