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Gene Expression

How The Information in DNA is Translated Into an Organism's Traits  
 

One of the most important activities of a cell is the production of proteins that fulfill major roles in the cell--structural, enzymatic, hormonal, and more.  The instructions for building all the proteins an organism needs to make are located in the DNA molecules of the chromosomes which never leave the nucleus of the cell.  However, the actual synthesis is carried out by the ribosomes, small structures which either float freely in the cytoplasm or are attached to membrane networks that snake their way through the cell--in either case, outside the nucleus.  This section will explain briefly and superficially the way the instructions reach the ribosomes and how they are translated into the language of proteins.  This information is not critical for understanding the use of DNA for genealogy but does form a foundation for understanding the way genetic mutations are expressed and a basis for understanding genetic differences.

A protein is a chainlike molecule built of subunits of smaller molecules called amino acids.  We obtain most of our amino acids by digesting proteins taken in with our food.  The digestive process breaks the protein chains down into individual amino acid molecules which are then absorbed by the blood and transported to the individual body cells.

During protein synthesis, the separate amino acids are reassembled into new chains.  Each kind of protein has its own particular sequence of amino acids, which differs from the sequence in every other kind of protein.  Just the way the order of letters in a word give it its own specific form and meaning, it is the order of the amino acids in the chain that determines the protein's structure and function.

The code for ordering the amino acids of a protein is written as a sequence of bases in the DNA in the nucleus. However, since DNA never leaves the nucleus and proteins are constructed by ribosomes in the cytoplasm of the cell, the instructions must somehow be carried out of the nucleus to the ribosomes.

This is accomplished when the double spiral of DNA unwinds and unzips a little at the point where the instructions for the given protein are located.

(This section of the DNA molecule is called a gene.)  While it is unzipped, this short section of the DNA molecule acts as a pattern or template for another kind of nucleicacid called RNA (ribonucleic acid)  Each adenine of the unzipped DNA attracts a uracil, U, (instead of a thymine as in DNA).  The other bases, G, T, and C attract the same partners as they do in DNA replication, G attracts C, C attracts G, and T attracts


Figure 1

 A.  The newly formed, single chain of nucleotides is called messenger RNA It carries an accurate reproduction of the information that was recorded in the DNA.  This formation of messenger RNA is called transcription.

The molecules of messenger RNA (mRNA) leave the nucleus, carrying with them the instructions (encoded in the sequence of their nucleotides) that they picked up from the DNA molecule.  In the cytoplasm, mRNA molecules are attracted to the ribosomes. 

Also in the cytoplasm are also a second kind of smaller RNA molecules called transfer RNA (tRNA.)  One end of a tRNA molecule has a special site to which only one kind of amino acid can be attached.  There are many different types of transfer RNA molecules--actually more than one for each of 18 of the 20 different amino acids found in proteins (methionine and tryptophan being the exceptions.) (Notice that the amino acids are represented by the little dots at the ends of the tRNA molecules in the diagram above.)

The other end of each RNA molecule carries a unique tag which identifies it.  The tag is written in the usual code of a nucleic acid--a sequence of bases.  Each amino acid carrying molecule has its own three letter code.  For example, the valine tRNA is labeled AAC, the alanine-transfer RNA is labeled GGC, the phenylalanine -tRNA is labeled AAA and so on.

With the strand of messenger RNA lined up at the ribosome, the base pairs again are attracted to their partners, this time the attraction is between the complementary bases of the messenger RNA and the transfer RNA.   A sequence of three nucleotides in mRNA codes for each amino acid.  This sequence is called a codon.  For a chart of the codons for each of the twenty amino acids, click here. The transfer RNAs carrying amino acids attach to the mRNA by means of base-pairing between the messenger RNA codons and the  transfer RNA "anticodons."  Each transfer RNA then donates its amino acid, in the proper order, to the growing chain of amino acids that will become the new protein.  This assembly of amino acids to form a protein, in a sequence specified by the order of nucleotides in a molecule of messenger RNA (determined, in turn, by the sequence specified by a segment of DNA in the nucleus called a gene) is called translation (i.e. translated from the "language of nucleic acids" to the "language of proteins".) 
 

Junk DNA  
 

Interestingly, not all of the nucleotides in a chromosome are part of a code for a protein.  Long stretches of a DNA molecule, called
introns (intervening sequences) have no known function.  Often these stretches occur right in the middle of a sequence that does code for a protein (called an exon--for "expressed".).  Though their sequence is transcribed into messenger RNA, it is never translated into a protein.  Instead they are spliced out of the RNA molecule leaving a shorter, mature RNA molecule that is translated into a functional protein.   
   
Other long stretches of DNA  appear to have no known function and are never translated into protein.  


Figure 2



It is estimated that the human genome (all the genes that encode the proteins of an organism) is made up of about 35,000 genes.
 
 

Only about five percent of human DNA actually encodes protein. 

Some scientists regard introns and other noncoding DNA as "junk DNA".  Others suggest they may have some as yet undiscovered function.
 

You might enjoy an interactive flash presentation of protein synthesis at:
http://www.pbs.org/wgbh/aso/tryit/dna/index.html#

Figure 1 above is from the National Human Genome Project glossary at
http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/gene_expression.shtml

Figure 2 above is from the National Human Genome Project glossary at
http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/intron.shtml

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This web is lovingly dedicated to the memory of
Mr. James Dorsey
who so graciously and enthusiastically
donated his DNA to solve our family mystery. 


Jim Dorsey
2/12/1930 — 4-30-2002

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