Background: DNA and the central dogma

DNA and its messenger RNA
DNA is the genetic code that every organism inherits from its parent(s). If you compare a cell with a factory, the DNA is the entire set of building plans for all the different machines in the factory and any other instructions necessary for the factory to function. Genes are the parts of the DNA that correspond to the building plans. In the case of a cell, the building plans contain the instructions for the formation of proteins and other biomolecules essential to the cell’s survival. To build a machine in a factory, a copy of the correct building plan needs to be taken to a workshop. Similarly, to construct proteins in a cell, a copy of the correct gene needs to be taken to the protein-builders: the ribosomes. This copy is called messenger RNA (or mRNA).

DNA and RNA composition
When observed under a microcope, DNA looks like a twisted ladder. This structure is called a helix. It consists of two parallel strands of chemical compounds called nucleotides, each of which is loosely bound to its partner nucleotide on the parallel strand. DNA consists of four nucleotides: adenine (A), thymine (T), cytosine (C) and guanine (G). A can only loosely bind to T, and vice versa, and C can only bind to G. Thus, if one strand of the DNA helix reads ‘AGATCC’, the parallel strand, read in the same direction, will read ‘TCTAGG’. RNA also consists of nucleotides, but instead of T, it contains uracil (U). In contrast to DNA, RNA is most often single stranded.

The central dogma
When a certain protein is required in a cell, an mRNA copy has to be made from the corresponding gene. To make this copy, the DNA at the site of the gene detaches from the parallel strand. This unwinding of the helix lets RNA nucleotides bind. Once the entire gene is covered by the correct RNA nucleotides, the formed mRNA strand leaves and moves to the ribosomes, that is, the protein builders. There, the mRNA ‘building plan’ is used to make the required protein. DNA –> mRNA –> protein is known as the ‘central dogma’ of molecular biology.

Gene expression determines a cell’s function
Each cell of a multicellular organism, such as a plant, fish and yourself, contains the same DNA. However, cells within the organism can perform very diverse functions. This is possible because different genes are active in different cells. Ultimately, this results in different proteins being made and different cells being able to perform different functions: an immune cell can combat pathogens, a muscle cell can contract, a cell in a plant leaf can perform photosynthesis. Changes in the environment also influence gene activity, allowing a cell or organism to respond to the changes.

Summary
Genes are the parts of the DNA that code for proteins. To make proteins, a copy of the DNA, called mRNA, is moved towards the protein-builders, where the new protein is made. Changes in gene expression govern cell development and function and its responses to the environment. In multicellular organisms, gene expression in all the individual cells ultimately determines the appearance and fitness of the total organism.

For more information, see:
https://en.wikipedia.org/wiki/Central_dogma_of_molecular_biology
http://www.yourgenome.org/facts/what-is-the-central-dogma

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