Transcription & Translation
The process of gene expression, which is modeled in this animation, occurs in two major steps. The first step is called transcription. It involves the replication of a strand of DNA, but rather than making a complementary DNA strand, an enzyme called RNA polymerase makes a complementary RNA strand. The RNA that is produced is called mRNA, or messenger RNA, and typically remains single stranded. Note that RNA uses the base uracil (U) instead of thymine (T).
The second step of gene expression is called translation. Note that the mRNA strand moves from the nucleus of the cell where it is made, to the cytoplasm. Once in the cytoplasm, the mRNA strand becomes attached to the surface of a ribosome. A group of three mRNA bases, called a codon, attracts and binds to another complementary group of three bases, called an anticodon, which is part of a transfer RNA (tRNA) molecule. Each tRNA is bound to an amino acid, which it brings to the ribosome/mRNA complex. Details of this process can be seen in the Codons, Anticodons section of this web site. As the ribosome moves along the mRNA sequence, new tRNA molecules are attracted to the mRNA codons and transfer their amino acids, which become joined by peptide bonds to form a polypeptide chain. Ultimately, the ribosome encounters a stop codon on the mRNA, and translation stops.
In this manner, the sequence of bases found in a gene directly determines the sequence of amino acids in a polypeptide.