Course: SAT 170 Introductory Biotechnology

Unit 3 Learning Module

Page 3 of 3
DNA-The Molecule of Heredity (continued)

Figure 7

The results of this experiment allowed Meselson and Stahl to conclude that DNA replicates in a semi-conservative fashion. This process has since been investigated to improve our understanding significantly. There are several steps to the process of DNA replication and several enzymes that are involved in that process. Enzymes are substances, usually proteins that catalyze or speed up chemical reactions.

See Figure 7: Notice how this image illustrates the semi-conservative mode of DNA replication, with the original parent DNA strands containing the blue backbone and the new daughter strands shown with the green backbone.
- First, an enzyme known as helicase breaks the hydrogen bonds between the nucleotide bases of the two strands in the molecule, unzipping the molecule and unwinding the helix. The site where helicase binds to unravel the DNA strands is known as a replication fork. The location from which replication proceeds is known as the replication fork.
- Once the DNA strands are separated they have a tendency to come back together spontaneously, a process known as reannealing. In order to keep the parent strands separated for the replication process, a set of proteins known as single stranded binding proteins (SSB proteins) come in and bind to the separated strands holding them apart.
- With the strands fully separated the next enzyme, primase, can come in to play its role. Primase is needed to create primers for the process of DNA replication. A primer is a short fragment of RNA nucleotides; these nucleotides are complementary to the DNA template being copied.
- Next, an enzyme known as DNA polymerase can synthesize the complementary daughter strands by adding new DNA nucleotides to the 3’ end of the RNA primer in a complementary fashion to the template strand. In eukaryotic cells, replication proceeds in only one direction and since DNA polymerase can only add to the 3’ end of the growing DNA strand, each of the daughter strands must be synthesized differently. One of the strands is known as the leading strand, this is the strand that is running 5’ to 3’ parallel with the direction of replication. This strand requires only one primer and can be synthesized continuously as the parent strands are separated along the direction of replication. The opposite strand is referred to as the lagging strand. Because this strand is running 5’ to 3’ opposite to the direction of replication it must be synthesized in fragments. These fragments are known as Okasaki fragments. Each fragment requires a new primer.
- Once the two daughter strands have been completely synthesized, DNA polymerase can come back and remove the RNA primers and replace them with DNA nucleotides.
- Finally, the fragments of the lagging strand can be sealed together using an enzyme known as DNA ligase.
- Each of the two finished DNA molecules contains one old strand and one new strand.
- Since DNA polymerase is not able to add to the 5’ end of a chain of nucleotides, it is unable to replace the 5’ most primer of the lagging strand once that primer has been removed. This leaves a portion of the 3’ end of the leading strand unpaired; this single stranded portion is prone to degradation. Thus, after each round of replication, the linear chromosome is slightly shortened. In order to prevent this from affecting the genes carried on those chromosomes, there are telomeres at the ends of these chromosomes. Telomeres are sequences of nucleotides which are non-coding and function to protect the genetic material as the chromosomes shorten with continual replication. During early development, when replication of cells and DNA is happening very rapidly, cell produce an enzyme known as telomerase which is able to continually replace these telomeres. As we continue to develop, the production of telomerase slows down along with the replacement of telomeres. This is why telomeres and telomerase are considered as key factors in understanding the aging process.

See Figure 8: The roles of each of the DNA replication enzymes are clearly shown in this image.

Figure 8

Figure 9

DNA Proofreading and Repair

Once DNA replication is complete, the process of proofreading the DNA begins to ensure that no mistakes in the DNA have been made. This process of examining the quality of DNA continues long after replication during what is known as DNA repair. While DNA proofreading corrects errors due to replication, DNA repair corrects DNA damage. DNA damage can occur at any point in response to exposure to mutagens. Mutagens are things that like radiation and chemicals that can mutations of the normal structure of DNA. There are many different types of DNA repair including nucleotide excision repair, base excision repair and mismatch repair. Although these mechanisms slightly differ from one another, they all rely on the ability to recognize the abnormal nucleotides and then remove and replace those nucleotides. In the case of nucleotide excision repair, the damage being repaired was caused by UV radiation. Mismatch repair can be used as a part of proofreading or in response to a variety of types of damage. Finally, base excision repair is a mechanism used to repair damage throughout the cell’s life.

Some forms of damage are left uncorrected and are seen as genetic mutations. Often these mutations involve a change to only a single nucleotide. Such mutations are known as point mutations. In some cases these single nucleotide changes have no impact on the protein product that a particular gene is coding for, and are thus known as silent mutations. There are also missense and nonsense mutations which can result in changing a particular amino acid of a protein product into a different amino acid or a signal to stop the genetic code, respectively.

If the damage is too extensive to be properly repaired, the cell may enter a programmed cell suicide known as apoptosis.

See Figure 9: DNA repair involves molecular complexes that scan the DNA molecule for errors or mutations in order to remove and correct those problems.