that contains the instructions used in the development and functioning of all known living with the exception of some . The main role of DNA is the long-term storage of . DNA is often compared to a set of , like a recipe or a code, since it contains the instructions needed to construct other components of , such as and molecules. The DNA segments that carry this genetic information are called , but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.
DNA consists of two long of simple units called , with made of and groups joined by bonds. These two strands run in opposite directions to each other and are therefore . Attached to each sugar is one of four types of molecules called . It is the sequence of these four bases along the backbone that encodes information. This information is read using the , which specifies the sequence of the within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called .
Within cells, DNA is organized into long structures called . These chromosomes are duplicated before cells , in a process called . (, , , and ) store most of their DNA inside the and some of their DNA in , such as or . In contrast, ( and ) store their DNA only in the . Within the chromosomes, proteins such as compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.