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Every living creature has within its cells a genetic code made of nucleotides on a sugar-phosphate backbone nucleic acid. There are two types of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). In most living things, the genetic material is DNA. For the sake of simplicity, the genetics applicable to animals is discussed further; bacteria and viruses have more complex and variable genetic assemblies that are easier to understand once the basics are mastered.

Gene expression is the central dogma of biochemistry. In animals, DNA is a double-stranded helix. This is the structure discerned by Watson and Crick in the mid-twentieth century. There are four types of nucleotides that make up the genetic code, two complementary pairs that hold the helix together via hydrogen bonds. Single stranded (separated) DNA is transcribed by the cell's machinery into single stranded RNA, known as messenger RNA (mRNA), in a process called transcription. The mRNA is then translated by additional cell machinery into protein by ribosomes and another type of RNA, transfer RNA (tRNA), reading the strand three nucleotides at a time and constructing a string of amino acids to make the peptide, the building block of proteins. This process is called translation.

The resulting protein, the gene product, is gene expression, though gene expression is sometimes measured on the mRNA level to discern between transcriptional and translational control in the laboratory.Gene regulation is essentially the control a cell exerts over gene expression.

Among the four-nucleotide sequence of DNA are blocks of sequence known as genes. Genes encode particular proteins and are read by the cellular machinery to produce a particular protein, as described above, based on its sequence. Genes are defined by start and stop sequences consisting of three nucleotides (codons). Flanking genes are also untranslated regions and regulatory sequences known as enhancers, silencers, and promoters.

The promoter region of a gene occurs before the start codon and can be short, a few hundred base pairs, or long, several kilobases, depending on the distance to the next gene. Some regulatory sequences even occur within other genes, making the regulation of gene expression very complex to discern and understand for some genes. Within the promoter region, as well as enhancers and silencers, are nucleotide sequences recognized by transcription factors, proteins that bind to a DNA sequence and affect transcription.

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