Often referred to as the molecule of life, DNA (deoxyribonucleic acid) is found in almost all living things. It acts as a type of chemical code that contains instructions, known as genes, for how the body and all its different parts grow, develop, function, and maintain themselves.
In nearly all human cells, DNA is packaged into 46 X-shaped elements called chromosomes, which are situated in the cell’s nucleus. DNA’s enormous list of instructions takes the form of long, thin molecules, one per chromosome, each taking the shape of a double-helix. Each double-helix has two long, corkscrew-like strands, which act as “backbones” for the molecule, twining around each other. These are held together by rungs, like a twisted ladder. The rungs are made of pairs of chemicals called bases: adenine (A), guanine (G), thymine (T), and cytosine (C). In each rung, A always pairs with T, and G with C. This structure gives DNA its two key features: the order of the bases contains the chromosome’s genetic code, while the way the bases cross-link enables DNA to make exact copies of itself.
One of DNA’s key functions is to provide the information to build proteins. Some proteins are the body’s major structural molecules, others form enzymes, which control chemical reactions within the body. Manufacture of proteins occurs in two main phases, transcription and translation. In transcription, information is taken from the DNA and copied to an intermediate type of molecule called mRNA (messenger ribonucleic acid). This is built from nucleotide units in a similar way to DNA. The mRNA moves out of the cell’s nucleus to protein assembly units called ribosomes. In the translation phase, the mRNA acts as a template for the formation of units of protein, known as amino acids. There are about 20 different amino acids. Their order is specified by lengths of mRNA three bases long, called triplet codons. The order of bases in each codon is the code for a particular amino acid (hence the term genetic code). The mRNA carries instructions to make a specific protein from a sequence of amino acids.
In the cell’s nucleus, the DNA strands temporarily separate, with one acting as the template for the formation of mRNA. Separate RNA nucleotides with the correct bases lock onto the exposed DNA bases in cross-linked fashion, thereby forming a mirror image of the DNA’s information.
In the cell’s cytoplasm, the mRNA attaches to a ribosome. Individual tRNA (transfer ribonucleic acid) molecules have specific amino acids attached. They can slot onto the mRNA only if the order of their bases matches, ensuring they bring the correct amino acid. As the ribosome moves along the mRNA, the tRNAs bring the correct sequence of amino acids, which fit together to construct a protein.
A gene is generally regarded as a unit of DNA needed to construct one protein. It consists of all the sections of DNA that code for all the amino acids for that protein. These sections are not necessarily on the same strand of DNA or even on the same chromosome. There may be many strands of DNA, each containing the code for one portion of the protein. Typically, lengths of DNA called introns and exons are both transcribed (see illustration) to form immature mRNA. The parts of mRNA made from the introns are then stripped out by the cell’s molecular machinery, leaving mature mRNA for translation. There are also regulatory DNA sequences that code for their own proteins, affecting the gene transcription rate.
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