The human genome contains a lot of DNA that does not code for protein. Much of this DNA is involved with regulating which genes are turned on or off. There are also several types of non-coding RNA, some of which aid in protein production and some that inhibit it. Although non-coding DNA and RNA do not directly code for protein to be made, they serve to regulate which genes are made into protein in many cases.
A gene is a portion of the DNA within a chromosome that contains all the necessary information for making RNA and then protein. The region of a gene that codes for protein and will be made into RNA is called the open reading frame, or ORF. The ability of the ORF to make RNA and then protein is controlled by a section of DNA called the regulatory region. This region of the DNA is very important in controlling which genes are turned on and eventually made into protein, but does not code for any protein itself.
There are several types of RNA, most of which do not code for protein. Ribosomal RNA codes only for production of the ribosome, the complex which turns RNA into protein. Transfer RNA is important for making the protein from RNA, but does not code for making protein itself. Micro RNA, or miRNA, prevents protein from being made by targeting the coding RNA to be degraded. The miRNA serves to negatively regulate which genes are turned into protein, essentially turning the genes off. This process of turning off genes with miRNA is known as RNA interference.
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When a gene is transcribed from DNA to RNA, the resultant coding RNA, or mRNA, requires further processing before it can be made into protein. The mRNA is composed of sequences known as introns and exons. The introns do not code for any protein and are removed from the mRNA before it is made into protein. The exons are the sequences that code for protein. However, some exons are removed from the mRNA as well and do not get made into protein. This process of removing introns and exons from RNA is known as gene splicing.
Some DNA has no known purpose and is therefore referred to as junk DNA. Junk DNA is commonly found in the telomeres -- the ends of the chromosomes. The telomeres of chromosomes are slightly shortened with each cell division, and over time, a significant amount of the DNA from the telomeres can be lost. It is thought that the telomeres are made of mostly junk DNA so that no important genetic information is lost when the telomeres are shortened.