Protein Creation

RNAs role in making Protein

Building essential proteins from raw organic materials is a complicated process that is of no small cost to the individual cell.  As with most everything involving cell life the initial instruction must originate in the nucleus, home for the governing molecule – DNA.  Much as a computer program is coded entirely using a sequence of zeros and ones, the instructions regulating life in the cell are also sequences, but ones coded in chemical bonds. 

The large double helix molecule of DNA that resides in the nucleus does not directly involve itself with the mechanics needed to sustain life.  Instead it relies upon its complement, a single strand of RNA, to deliver its instructions to the appropriate component of the cell.  In order to understand how a message is coded it is important to know that both DNA and RNA are long strings of four different types of nucleotides linked together.  DNA and RNA both include the nitrogenous bases, known as adenine, guanine and cytosine.  Thymine is the fourth nitrogen base in DNA while uracil is the equivalent base in RNA. 

Transmitting coded instructions from DNA to the messenger RNA, mRNA, requires a bonding attraction between the various nucleotides.  Thus, adenine on DNA attracts the uracil of RNA and cytosine attracts guanine.  Of course, the reverse is also true – guanine bonds with cytosine and thymine with adenine.  RNA will deliver its message in a fashion complementary to the original DNA code.  It might be thought of as a mirror image but it nonetheless remains a faithful transcription of the genetic instruction.

The completed mRNA nucleic strand finds its way outside the nucleus and proceeds to a point of protein construction.  Most often this involves one of many tiny sites associated with a long, membrane that extends out from the nucleus and into the cell’s metabolizing interior.  This membrane area is called rough endoplasmic reticulum because of the ribosomes, spherical structures that generously populate its outer surface.  It is the role of the ribosome to translate nucleic information into the sequence of amino acids necessary to make a specific form of protein.

A single protein can include hundreds of amino acids of which there are only twenty basic types.  It is the order of these amino acids, when linked together, which give the protein polymer its particular molecular characteristics – what molecule is attracted to it, what type of bond is created, what function is provided the cell.  How does the ribosome radically transform nucleotides such as uracil and guanine, found on mRNA, into a protein amino acid such as tryptophan?

We should first know that there is a triplet of nucleotides that uniquely identify each amino acid type.  For instance, a sequence of uracil – guanine – guanine provides the code for the amino acid tryptophan.   Unfortunately, tryptophan needs help if it is to bond with this triplet of nitrogenous bases.  There needs to be an intermediary provided by the ribosome, and there is.   Again we call on the services of RNA.  This time it is a relatively short strand, transfer RNA (tRNA), which links to a specific amino acid on its one end, and provides a complement of nitrogenous bases on the other. 

The triplet code, or codon, for tryptophan was uracil – guanine – guanine.  Its complement for base-pairing would be the anticodon adenine – cytosine – cytosine.  When the messenger RNA gives the codon for tryptophan, the ribosome site provides the corresponding tRNA.  Tryptophan is attached to it at one end, and the appropriate anticodon at the other end provides the complementary bond.  Once the amino acid establishes its link to the previous amino acid on this developing protein sequence, the tRNA is released in order to establish another bond with an amino acid of identical type.  It is in this stitching manner that the ribosome makes available a new protein polymer for the cell to use.

The molecules associated with living forms are far larger and more complex than any molecule exclusive to inanimate matter.  The reaction cycles necessary to construct these macromolecules are also extraordinary in both their complexity and the choreography of their timing.  The cell is the center of all life.  It extracts energy from its environment, discriminately chooses necessary resources from its surroundings, manages chemical reactions to power the synthesis of new parts, and organizes and regulates all its diverse activities using only a finely tuned molecular intelligence.

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