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. Chromosomes never leave the nucleus of the cell. However, protein 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—both 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. Human cells can also manufacture some amino acids. However, eight of the amino acids that are essential to building human proteins must be acquired from food. They eight essential amino acids are phenylalanine, valine, leucine, isoleucine, lysine, threonine, tryptophan and methionine. Histidine is essential for infants but not for adults.
During protein synthesis, the free floating 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. In the same 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, the instructions must somehow be carried out of the nucleus to the ribosomes.
Image courtesy of National Human Genome Research Institute, modified for this website
This is accomplished when the double spiral of DNA unwinds and unzips 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, the gene acts as a pattern or template for another kind of nucleic acid called messenger RNA (mRNA). 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 A.
The newly formed, single stranded mRNA carries an accurate reproduction of the information that was recorded in the DNA. The formation of messenger RNA is called transcription.
By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
The molecules of messenger RNA (mRNA) leave the nucleus through small pores in the nuclear membrane carrying with them the instructions (encoded in the sequence of their nucleotides) that they picked up from the DNA molecule. In the cytoplasm, the mRNA molecule attaches to a small granular appearing organelle called a ribosome.
From the ribosome, mRNA molecules attract a second kind of smaller RNA molecule called transfer RNA (tRNA).
One end of a tRNA molecule has a special site which can only bind to one specific kind of amino acid. There are many different types of transfer RNA molecules. In fact, there are more than one for each of 18 of the 20 different amino acids found in proteins (methionine and tryptophan being the exceptions).
The other end of each tRNA molecule carries a unique "tag." 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 tag or code. For example, the valine tRNA is tagged AAC, the alanine-transfer RNA is tagged GGC, the phenylalanine -tRNA is tagged AAA and so on.
The three base pairs of the tRNA tags are attracted to their complementary partners on the mRNA that is lined up on the ribosome. The three letter sequences of the mRNA are called a codons. The three letter tRNA tags are called anticodons. Guided by the mRNA, each transfer RNA donates its amino acid, in the proper order, to a growing chain of amino acids that will be joined by peptide bonds to form a new polypeptide (protein).
And thus the language of nucleic acids is translated into the language of proteins.
By Steven Kuensting (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
