DNA, mRNA, and Proteins

All the information necessary to make proteins is contained in DNA molecules. DNA (deoxyribonucleic acid) is a double stranded molecule which has a sugar phosphate backbone and nucleotide bases which form the “instructions” for the creation of proteins. RNA (ribonucleic acid) is an intermediary for the protein synthesis process. The sequence of nucleotides is made up of the bases adenine, thymine, guanine, and cytosine. Each base is hydrogen bonded to a base on the complimentary strand such that adenine bonds with thymine, and guanine bonds with cytosine. Protein synthesis is the process by which DNA is converted into protein and is made up of two processes, transcription and translation. Transcription results in the formation of mRNA (messenger RNA), and translation uses the mRNA to form a protein product. A diagram of a DNA molecule is shown below.

http://oak.cats.ohiou.edu/~ballardh/pbio475/Heredity/DNA-double-helix.JPG

RNA polymerase unwinds the DNA helix and transcribes the DNA into RNA. Transcription starts at the 5’ end of the DNA and moves towards the 3’ end. It starts at a sequence called the promoter and stops at the terminator sequence. As the transciptase moves along the DNA, bases of RNA are connected together. The bases in RNA are identical to those in DNA except thymine (which pairs with adenine) is replaced by uracil. After the mRNA strand is complete, processing occurs. Intron sequences (which are noncoding sequences) are removed, a cap is added to the 5’ end, and a poly A tail is added to the 3’ end.

Then the process of translation begins. The mRNA transcript is fed through a ribosome. Transciptional RNA (tRNA) contains three nucleotides (the anticodon) at one end and a specific amino acid at the other. Each group of 3 nucleotides (the codon) on the mRNA is matched by the tRNA with the correct anticodon. As each tRNA binds to the mRNA, an amino acid chain is formed. As the mRNA is fed through the ribosomal RNA, the amino acid chain grows until eventually a stop codon (specific sequence of three nucleotides) is reached, and the amino acid chain is complete.