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Genetics & Biotechnology



Protein Synthesis

In the cell, ribosomes make proteins, but how do ribosomes know how to make the protein that the cell needs. The instructions for how to build every protein that you body needs lies in you DNA. DNA is a blueprint for making each protein, whether it is an enzyme, a protein used in your muscles, or a protein for transporting materials, such as hemoglobin.

The process for making proteins involves more than just ribosomes and DNA. It requires RNA as well. RNA is the messenger, the copy of the DNA blueprint for how to make the right protein for the right job. There are three types of RNA used in protein synthesis. Messenger RNA (mRNA) is a copy of the section of DNA which codes for, is the blueprint for a particular protein, like hemoglobin. mRNA is made during a process called transcription. Ribosomal RNA (rRNA) is the RNA which is bound to the ribosome. It is this RNA from which proteins are directly made. Finally, transfer RNA (tRNA) is a type of RNA that is bound to amino acids. tRNA pulls the amino acids together, building a protein one amino acid at a time. Both rRNA and tRNA are used in the last step of protein synthesis called translation. In this section, I will discuss how proteins are made.

Transcription

Transcription is the first step towards making proteins. Transcription works remarkably like DNA replication, with two exceptions. First, only one small segment of the DNA is copied. Second, RNA polymerase, and not DNA polymerase, makes the copy.

Like DNA replication, the first step involves helicase, which unwinds the DNA molecule. Except that this time, helicase starts at the promoter, a section of DNA that yells to helicase, STAR HERE!!! Also, helicase only unwinds to the terminator, another section of DNA like the promoter. Except the terminator yells STOP UNWINDING ME!!!

Just like DNA polymerase, RNA polymerase (1) reads the base, (2) finds the complementary base, then (3) binds the complementary base into place. Remember that if the base is guanine, then RNA polymerase finds, then binds cytosine in place. If the base is cytosine, the RNA polymerase finds, then binds guanine in place. If the base is thymine, the RNA polymerase finds, then binds adenine in place. But here is another exception. If the base is adenine, the RNA polymerase finds urasil, and binds it in place. Urasil replaces thymine in RNA.

As soon as RNA polymerase reaches the DNA terminator, transcription stops, and mRNA is completed. Then mRNA is sent outside of the nucleus, into the cytoplasm until it reaches a ribosome. Then it is time for the next step.

Translation

Translation is the process where proteins are built from the mRNA blueprint, a copy of the DNA blueprint for the protein. DNA is coded, and so is mRNA. The code selects a specific amino acid. So when read, code after code, one amino acid is added after another amino acid. The code is based upon a codon, which is made from three bases. For example, the bases Uracil-Cytosine-Guanine codes for the amino acid serine. Uracil-Cytosine-Guanine is the codon. So the codon Adenine-Uracil-Adenine codes for the amino acid isoleucine Biologist have developed a codon table to help translate sequences of mRNA. The table is built around mRNA, so if you use tRNA to read a codon table, you will have the wrong amino acid sequences for the protein.

The job of tRNA is to ensure that the right amino acid is put in its proper place. The tRNA is the anticodon of the mRNA, so tRNA fits into the mRNA like a piece of a jigsaw puzzle. Put a little more genetically, the tRNA has the complimentary bases for the bases on the mRNA. Since the tRNA is pulling its amino acid along when it fits into the mRNA, the amino acid is put it its proper place.

So what are the actual steps in translation. First, mRNA becomes bound to a complete ribosome, so the mRNA's start codon is placed in the P site, the first position, on the ribosome. This is similar to placing a piece of paper in a typewriter, with the first line on the paper lined up with the type strokes. Next to the P site is the A site. Both sites will hold one tRNA. Once the start codon is set into the P site, translation begins.

Next, a tRNA fits into the P site, based upon the codon on the mRNA lined up with the P site. Another tRNA fits into the A site, also based upon the codon on the mRNA lined up with the A site. When both the P site and the A site have tRNA, the amino acids attached to those tRNA become bonded together, forming the first two links on the protein chain.

Third, when the two amino acids link, the tRNA in the P site is released, and the mRNA moves codon, so that the tRNA in the A site shifts over to the P site. Again, this is like moving your cursor over to type in another place on the screen.

Forth, when the A site becomes clear, a new tRNA moves into the A site, again a tRNA with the anticodon (a matching codon) for the mRNA. Then the new amino acid becomes bonded to the protein chain, and the process repeats itself.

This process continues until a stop codon is read at the A site. When that happens, the ribosome has finished synthesizing its protein. So, tRNA is not added to the protein chain, the two ribosomal units separate, and the protein chain is released from the ribosomes.