In the last episode of this series, we took a very brief look at The Many Faces of RNA. We saw that RNA is involved in the reading and decoding of DNA and in governing the process of protein synthesis among a number of other functions. Today we are going to do a brief overview of DNA to once again demonstrate that the workings of the simple cell or anything but simple.
Today we hear so much about DNA. It is used in forensics to free the innocent and convict the guilty. It is used in paternity cases and to trace family backgrounds. DNA is also being studied and used for the identification of various genetic disorders such as numerous cancers, cystic fibrosis which is caused by a defective gene on chromosome 7 called CFTR (cystic fibrosis transmembrane conductance regulator) and Huntington’s disease which is caused by a mutation in a gene on chromosome 4.
Surprisingly, most people believe that Watson and Crick were the first ones to discover DNA in 1953, but they weren’t. Swiss physician and biologist Friedrich Meischer was the first person to detect the presence of the nucleic acid in 1889. He gave it the name nuclein. Watson and Crick were made famous when they discovered the double helix structure of DNA.
DNA stands for deoxyribonucleic acid. In simple terms DNA is the instruction manual of every cell of every living organism. Not only is it the instruction manual for the cell, but it also governs and directs every aspect of a living organism. In the case of humans, DNA directs the processes from fertilization of the ovum, embryonic development all the way to our death.
DNA is composed of a deoxyribose sugar which is a pentose sugar (5 carbon atoms) that has one less oxygen molecule than the ribose sugar found in RNA, a phosphate molecule and four nitrogenous bases: adenine (A), cytosine (C), guanine (G) and thymine (T). Each phosphate molecule will bind to two additional phosphate molecules to form the backbone of the DNA strand. Each phosphate also binds to a deoxyribose sugar which binds to one of the bases. To form the double strand of DNA, the bases bind to the complementary bases of the opposing strand by means of weak hydrogen bonds. The complementary pairs are cytosine to guanine and adenine to thymine.
The primary function of DNA is to code for protein synthesis. This is accomplished when the DNA code is read by the RNA and then transcribed into the RNA code where uracil takes the place of thymine. The coding carried by the mRNA dictates which amino acids will be produced in what order. With the help of ribosomes, the amino acids are precisely assembled in a very specific order to form the right proteins.
Quite often you will see the DNA genetic code separated in groups of three as shown above. The reason for this is due to the RNA translation for specific amino acids. The chart below shows the corresponding RNA coding for the amino acids used in the body. If you recall, RNA uses uracil in place of thymine. You will also notice that there is a sequence that codes for a ‘start’ and a ‘stop’. These are used to define the entire sequence used for each protein and function.
Periodically something will happen to cause an alteration in the base unit sequence of the DNA. Ultraviolet light, radiation, a number of chemicals and various environmental factors can damage the DNA molecule. If these alterations and damages to the DNA code are referred to as mutations. Without any way of repairing the alterations and damage, one would expect thousands and perhaps hundreds of thousands of mutations to have built up in the human genome if man had been around for 2-5 million years.
However, that isn’t what we find, and for a very good reason. Cells have their own DNA repair kits. Depending upon the type of damage, different repair mechanisms are utilized. In fact, the cell has 4 basic types of repair kits: single strand, double strand, direct reversal and translesion synthesis.
Single Strand Repair Mechanism
If the damage to the DNA code occurs on only one of the two strands of DNA or chromatin, the other strand of DNA or chromatin is used as a template to repair the damaged bases. This repair is done by means of a number of mechanisms that locate the damaged base unit, removes it, reads the complementary base unit on the opposing DNA or chromatin strand, and inserts a new base unit into the damaged location. Depending upon the type of damage, there are several different types of single strand repair mechanisms that are used.
Double Strand Severance Repair Mechanism
Sometimes both strands of the double helix DNA molecule are severed like a butcher cuts off a section of salami. If not repaired, these types of damage can be very detrimental to the cell. Depending upon the cause and location of the severed section of DNA, the cell has several different repair mechanisms available to rejoin the sections.
Direct Reversal Repair Mechanism
Sometimes specific base units become damaged in such a way that the cell can chemically repair it without using the complementary strand as a template. Like the single and double strand severance repair mechanisms, the direct reversal repair mechanism has several different means of carrying out the repair, depending upon the type of damage.
Translesion Synthesis Repair Mechanism
Lesions often occur to DNA. Translesion synthesis repair takes place when the mechanism replicates former DNA lesions and switches them out for specific lesion damage. Sometimes this repair system is not as efficient as others and yet sometimes they are extremely efficient in repairing lesion damage.
There are volumes of information on DNA that could be mentioned here, but again, our main purpose for this series on the Simple Cell is to show that cells are so complex that it is impossible for all of these parts and functions to have evolved and create life by random chance. For instance, in this section on DNA, I wanted to emphasize the repair mechanisms that are in place to help maintain the integrity of the DNA. Can anyone truly explain how not one, but around ten different repair mechanisms could have evolved?
Evolution is based upon the concept of an ever increasing number of mutations building up to cause a change from one organism into another. DNA repair mechanisms go against that evolutionary premise by serving to eliminate as many mutations as possible.
God often teaches by repetition. Jesus used repletion all the time in His effort to get us to understand what He wanted to teach us. Just as in Genesis 1, when God used several expressions to convey ordinary 24 hours of creation, He also uses repetition to reinforce that He is the Creator. The cell doesn’t have just one repair system, or just two, but around ten different mechanisms in place as if He is reinforcing the fact that we did not evolve but were created by His spoken Word.