Friday, October 15, 2010


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.