Genetic Information Flow: Structure and Replication
The Flow of Genetic Information
2.1. The Structure of Classical Genes
Gene Definition: The basic unit of heredity and functionality. It is the physical region of a chromosome that carries genetic information from one generation to the next and determines the features of the body. It is responsible for much of the genetic variability.
- Molecular Definition: “One gene-one enzyme” hypothesis: A gene is responsible for the synthesis of an enzyme.
- One Gene-One Protein Hypothesis: The general principle is that all enzymes are proteins.
- One Gene-One Polypeptide Hypothesis: Proteins are composed of several polypeptides, each encoded by a different gene, a cistron.
This hypothesis falls short today because a gene has two different types of sequences: structural and regulatory. Within the structural coding sequences, there are exons and introns.
2.2. The Flow of Genetic Information
Originally, it was thought that information would flow from DNA to RNA and from RNA to protein, with a DNA-DNA loop.
The Current Scheme of the Central Dogma of Biology:
- First, the flow back from RNA to DNA indicates that RNA can serve as a template for the synthesis of DNA in RNA viruses. All tumor-producing viruses can produce an enzyme called reverse transcriptase or reverse transcriptase.
- Secondly, RNA can act as a template for its own replication; this process has been observed in a number of phages. Small RNA phages can act as messengers when they infect the cell.
Transcription Process.
3. DNA Replication
This is the process by which two daughter DNA molecules are synthesized from a parent DNA molecule, with the same original DNA sequence. It takes place in the S phase of interphase and needs to be carried out before cell division.
3.1. Replication in Prokaryotes
Proteins Involved in DNA Replication:
- DNA Polymerases: Enzymes responsible for catalyzing the formation of phosphodiester bonds. These enzymes are DNA polymerases 1, 2, and 3.
- Helicases: Enzymes that break the hydrogen bonds that hold the complementary bases and are responsible for opening the double helix.
- Topoisomerases: Unwind the DNA double helix.
- Primase: An enzyme that catalyzes the formation of an RNA fragment.
- SSB Proteins: Link to the single strands of DNA.
- DNA Ligase: Enzyme that binds two fragments of the same chain.
3.2. Replication in Eukaryotes
- Eukaryotic chromosomes contain very long DNA molecules. In several points of the chain, there are replicons.
- There are five types of DNA polymerases; correction and elongation tasks are distributed among them.
- In eukaryotic chromosomes, DNA is associated with histones, basic proteins that do not have to be duplicated in prokaryotes.
- The DNA replication process will be completed until the end of the chromosome, the telomere.
4. Transcription
The process by which one passes from a sequence of nitrogenous bases of a gene (DNA) to a complementary sequence of nitrogenous bases belonging to mRNA. With the information obtained from the mRNA, a polypeptide chain can be synthesized during the process of translation.
A DNA strand acts as a template: The RNA chains formed are complementary to only one of the two strands that form DNA; it acts as a mold.