DNA provides the instructions needed by a cell to make proteins. But the instructions are not made directly into proteins. First, a DNA message is converted into RNA in a process called transcription. Then, the RNA message is converted into proteins in a process called translation. The relationship between these molecules and processes is summed up in the central dogma, which states that information flows in one direction, from DNA to RNA to proteins.
Like DNA, RNA is a nucleic acid. It is made of nucleotides that consist of a phosphate group, a sugar, and a nitrogen-containing base. However, RNA differs in important ways from DNA: (1) RNA contains the sugar ribose, not deoxyribose; (2) RNA is made up of the nucleotides A, C, G, and uracil, U, which forms base pairs with A; (3) RNA is usually single-stranded. This single-stranded structure enables RNA to fold back on itself into specific structures that can catalyze reactions, much like an enzyme.
During transcription, a gene is transferred into RNA. Specific DNA sequences and a combination of accessory proteins help RNA polymerase recognize the start of a gene. RNA polymerase is a large enzyme that bonds nucleotides together to make RNA. RNA polymerase, in combination with the other proteins, forms a large transcription complex that unwinds a segment of the DNA molecule. Using only one strand of DNA as a template, RNA polymerase strings together a complementary RNA strand that has U in place of T. The DNA strand zips back together as the transcription complex moves forward along the gene.
Transcription makes three main types of RNA.
• Messenger RNA (mRNA) is the intermediate message between DNA and proteins. It is the only type of RNA that will be translated to form a protein.
• Ribosomal RNA (rRNA) forms a significant part of ribosomes.
• Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome during translation.
The DNA of a cell therefore has genes that code for proteins, as well as genes that code for rRNA and tRNA.