What is mRNA? What is its job during protein synthesis?

Messenger RNA molecules (mRNA) are responsible for transcription along the length of the DNA molecule for a protein-coding gene. Transcription occurs in the nucleus.  Translation of this gene results in the synthesis of a polypeptide which uses the information encoded in the messenger RNA to assemble the appropriate amino acids into the polypeptide which ultimately will become part of a functional protein. Translation occurs in the cytoplasm. The flow of genetic information is known as the central dogma--DNA--> RNA-->protein. This idea was put forth by Francis Crick.

mRNA copies one of the strands of the DNA double helix known as the template strand. Base-- pairing rules apply except RNA lacks the base thymine and uracil is substituted during transcription. For example, if part of the DNA code reads:  TAC GGG ACA then the corresponding mRNA would read:                                    AUG CCC UGU. Note that uracil is subsituted for thymine in the mRNA.

The process of transcription occurs in the nucleus in three stages. First is initiation whereby transcription factors attach to the promoter region in DNA and the enzyme RNA polymerase unwinds the two DNA strands. Next, is the process of elongation where nucleotides are added to the growing transcript at the 3 prime end. Eventually, a termination sequence is reached called a polyadenylation signal after which the mRNA transcript is cut from the polymerase enzyme.

The initial transcript of the DNA by mRNA results in pre-mRNA. This must be further processed to produce mature mRNA. This includes splicing together of exons (containing the protein-coding information) and the removal of introns (non-protein coding sections of DNA). Also, both ends of the pre-mRNA are modified. The five prime end receives a cap made of a modified guanine molecule and the three prime end receives a poly-A tail made of many adenine nucleotides. The cap and tail will help the mRNA leave the nucleus and travel to the cytoplasm, protect the mRNA from damage by enzymes and facilitate the later attachment of ribosomes to the five prime end of the mRNA where the process of translation will begin.