Protein Synthesis

The art of protein synthesis

This incredible work of art shows a process that takes place in the cells of all living things: the production of proteins. This process is called protein synthesis and it actually consists of two processes: transcription and translation. In eukaryotic cells, transcription takes place in the nucleus. During transcription, DNA is used as a template to form a molecule of messenger RNA (mRNA). The mRNA molecule then leaves the nucleus and heads for a ribosome in the cytoplasm, where translation occurs. During translation, the genetic code on the mRNA is read and used to make a protein. These two processes are summarized in the central dogma of molecular biology: DNA → RNA → Protein.

Transcription

Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions in DNA to mRNA. During transcription, one strand of mRNA is complemented by one strand of DNA.

Transcription Steps

Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated following.

  • Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called a promoter. This tells the DNA to unwind so that the enzyme can “read” the bases on one of the DNA strands. The enzyme is ready to produce an mRNA strand with a complementary base sequence.
  • Elongation is the addition of nucleotides to the mRNA chain.
  • Termination is the end of the transcript. The mRNA chain is complete and separates from the DNA.

mRNA processing

In eukaryotes, the new mRNA is not yet ready for translation. At this stage, it is called pre-mRNA and must go through more processing before it leaves the nucleus as mature mRNA. Processing may include splicing, editing, and polyadenylation. These processes modify mRNA in several ways. Such modifications allow a single gene to be used to produce more than one protein.

  • Splicing removes the introns from the mRNA. Introns are regions that do not code for protein. The remaining mRNA consists only of regions called exons that code for protein. The ribonucleoproteins in the diagram are small proteins in the nucleus that contain RNA and are necessary for the splicing process.
  • The editing changes some of the nucleotides in the mRNA. For example, a human protein called APOB, which helps transport lipids in the blood, has two different shapes due to editing. One form is smaller than the other because the editing adds an earlier stop signal on the mRNA.
  • Polyadenylation adds a “tail” to the mRNA. The tail consists of a chain of As (adenine bases). It marks the end of the mRNA. It is also involved in the export of mRNA from the nucleus and protects the mRNA from enzymes that might break it down.

Translation

The translation is the second part of the central dogma of molecular biology: RNA → Protein. It is the process in which the genetic code in mRNA is read to make a protein. After the mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins. The ribosome reads the sequence of codons on the mRNA, and the tRNA molecules bring the amino acids to the ribosome in the correct sequence.

To understand the role of tRNA, you need to know more about its structure. Each tRNA molecule has an anticodon for the amino acid it carries. An anticodon is complementary to an amino acid codon. For example, the amino acid lysine has the codon AAG, so the anticodon is UUC. Therefore, the lysine would be carried by a tRNA molecule with the anticodon UUC. Whenever the AAG codon appears on the mRNA, a tRNA UUC anticodon is temporarily attached. While binding to the mRNA, the tRNA gives up its amino acid. With the help of rRNA, bonds are formed between amino acids as they are carried one by one to the ribosome, creating a polypeptide chain. The amino acid chain continues to grow until a stop codon is reached.