同DNA一起組成細胞遺傳物質的一類核酸。DNA在細胞核中儲存遺傳信息，而RNA將DNA的信息帶到細胞的其它部位，在那里指導蛋白質的合成。從結構來看，RNA是單鏈結構。構成RNA主體的復合糖是核糖（相反DNA是脫氧核糖），4個堿基是U、G、A、C（DNA以T代替U）。共有三種RNA，它們在把DNA信息轉達給蛋白質過程中起到媒介作用：信使RNA是在核中的DNA復制，將信息帶到解讀密碼的細胞機器所在的胞質中；轉運RNA幫助每次讀取3個堿基（一個密碼子）并將信息由RNA轉移至蛋白質；核糖體RNA是核糖體結構的一部分，核糖體是使RNA翻譯為蛋白質的機器。

A category of nucleic acids which along with DNA forms the genetic material of a cell. While DNA stores the genetic information in the nucleus of every cell, RNA is a “carrier” of this information from DNA to other parts of the cell where the message is converted to protein. Structurally, RNA is a single-stranded structure. The component sugar that forms the backbone of RNA is ribose (as opposed to deoxyribose in DNA) and the four bases in RNA are Uracil, Guanine, Adenine and Cytosine (DNA contains Thymidine instead of Uracil). There are three types of RNA, all of which play an intermediary role in converting DNA information into protein: messenger RNA (mRNA) is a copy of the DNA in the nucleus and carries it to the cytoplasm where the cellular machinery for decoding that information lies; transfer RNA (tRNA) allows the code to be read three bases (one codon) at a time and transfers the information from RNA to protein (each tRNA carries one particular amino acid); and ribosomal RNA (rRNA) has a structural role forming part of the ribosome--the machinery that converts RNA into protein.