The Nature of the Living State
Macromolecules of Living Systems
Energy Flow in Living Systems
The Instruction Set of Life
The Instruction Set of Life
DNA only has information needed to make proteins. Some of these proteins are enzymes, which in turn, are then used to make the other macromolecules of cells. Please remember that the genetic information encoded in DNA is stored in the form of the exact sequence of bases along the two polynucleotide strands. A piece of DNA sufficient to code for a protein is called a gene.
Transcription takes place in the nucleus. During the part of the cell cycle (S) where synthesis occurs, the chromosomes are extended and the DNA is available to the cell. At certain sites along the DNA, the hydrogen bonds holding the two complementary strands of DNA together, break, leaving this region as single stranded. Once separated, ribonucleotides (A, C, G, U) which are always present in the cell, migrate to the single stranded region and begin complementary base pairing with ONE of the single DNA strands (A-U, C-G). The DNA strand that is used is called the sense strand versus the other which does not participate, which is known as the nonsense strand. At this point, an enzyme called RNA polymerase joins all the ribonucleotides together forming an RNA molecule. Notice that the sequence of occurrence of bases along the RNA molecule is the complementary image of the sequence of occurrence of bases along the sense strand of the DNA. Thus, the genetic information of the DNA is now encoded in complementary form in the RNA molecule. This type of RNA molecule is called messenger RNA or abbreviated as mRNA. Transcription is complete at this point.
The mRNA molecule breaks away and migrates to the cytoplasm where translation will take place. Once removed from the sense strand of the DNA, another copy of the mRNA molecule can be made or hydrogen bonds will re-form between the two strands of the DNA returning it to its original form. Notice that the information in the DNA has not been changed in any way. The only use of the DNA was as a template or model to make the mRNA molecule. A change in the information in DNA would be known as a mutation.
Translation takes place in the cytoplasm. Once the mRNA has migrated from the nucleus, a ribosome attaches at one end and forces the bases of the RNA molecule to face away from the ribosome in sets of three at the point of attachment.
Elsewhere in the cytoplasm, there is another special class of RNAs called transfer RNAs or tRNAs. For each type of amino acid in the cell, there is a type of tRNA which can temporarily bind to its own type of amino acid. The process of binding is known as charging. At a site on the tRNA, other than the place where amino acids bind, there is a three base "identification label" known as the anticodon. The anticodon is different for each type of tRNA.
Going back to the ribosome/mRNA complex, the three bases of the mRNA facing away from the ribosome are referred to as the codon. To this codon, charged tRNAs carrying their amino acids bind when their anticodons form a complementary match. Thus, only a single type of amino acid/tRNA can bind to the first three bases of the mRNA molecule.
At this point, the ribosome moves along the mRNA strand revealing the next three bases. Whatever aminoacid/tRNA has a complementary match (codon/anticodon match) to these three bases will then come into place. This process will continue moving more and more amino acids into the correct position. Enzymes will ultimately form peptide bonds between the amino acids creating a protein. The tRNAs break away and go back for re-charging. The ribsome moves the full length of the mRNA until it finally drops off. At this point, a new ribsome can start the process again forming an additional copy of the protein or the cell can break down the mRNA recycling the ribonucleotides. Translation is complete.
Notice that the sequence of amino acids along the protein is specified by the sequence of bases along the mRNA which in turn had been specified by the sequence of bases along the sense strand of the DNA. In this manner, the information encoded in the DNA is used to make specific kinds of proteins!
A related process is how copies of DNA are made to be available when needed during cell division. Each daughter cell must receive a full set of the DNA instructions.
Similar to transcription, sites along the DNA lose their hydrogen bonding creating single stranded regions. However, as these sites develop during DNA replication, deoxyribonucleotides begin complementary base pairing with BOTH strands of the DNA. The single stranded regions enlarge along the length of the DNA molecule with complementary base pairing filling in the spaces. An enzyme called DNA polymerase joins the deoxyribonucleotides together forming a new polynucleotide strand. This process continues until each of the original strands completely separates and a newly synthesized strand forms to each of the original strands. The two DNA molecules created have one original strand bound to a new strand. This type of process is called semi-conservative replication.