Protein Synthesis

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Introduction

Protein synthesis is important because it is through the use of genetic information to synthesize proteins that the genes in the DNA are expressed in the phenotype. Make sure you understand what proteins are and why they are important.

Protein synthesis involves two big steps and several other smaller steps depending on the organism. The two big steps are called transcription and translation. Transcription involves the synthesis of an RNA transcript using the cell's DNA as a template, leading to messenger RNA(mRNA). In eukaryotic cells transcription happens in the nucleus. Once the messenger RNA is made it then is transported to the cytoplasm where the information contained in the mRNA is translated into a sequence of amino acids making up a polypeptide.

We first examine the simplest way of looking at protein synthesis as expressed in the so called Central Dogma of Biology, namely that the direction of information flow in the cell is from DNA to mRNA to proteins.

Transcription

RNA transcriptionTranscription relies on the principle of complementarity to make the RNA transcript. The RNA's are nucleic acids as are the DNA's. The big differences are that RNA is not double sided, RNAs have the nitrogen base uracil in place of thymine and the five carbon sugar in the nucleotides is ribose, not deoxyribose.

The figure shows a section of mRNA being transcribed from the DNA template. This process involves several enzymes. Chief among them RNA polymerase. The developing RNA strand is on the right in this diagram.  Once the RNA transcript is made, if it is going to be used as messenger RNA is may be edited before it is translated.  It's important to realize that all types of RNA are made by transcription but only messenger RNA has the sequence of information that will later be translated into the sequence of amino acids that will become the polypeptide.

Just as DNA replication involves a series of enzymes, so to does transcription. The main enzyme is called RNA polymerase. In the figure, the oval shape under the side of the DNA molecule shown represents a the RNA polymerase. Only one side of  the DNA molecule needs to be shown, because typically the other side is not transcribed, at least within the region of the same gene.
 
 
 Once the RNA transcript that will become the messenger RNA is made, it is carried to the cytoplasm of the cell where it translated into, if you will, the language of proteins, namely a sequence of amino acids. A brief overview of translation is here.
 

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