DNA Replication and RNA Transcription in Prokaryotes and Eukaryotes
DNA Replication in Prokaryotes
In bacteria, in addition to the main DNA molecule, there are small circular DNA molecules called plasmids. These plasmids may or may not replicate simultaneously with the main chromosome using a similar mechanism. All plasmid DNA introduced into bacteria must possess a replication origin sequence to be multiplied by the replication machinery.
Eukaryotic DNA Replication Complexity
In eukaryotes, the rate of replication of nuclear DNA is more complex than in bacteria for two main reasons:
- The linear DNA molecules that make up eukaryotic chromosomes possess a much higher number of base pairs than bacterial DNA molecules and are consequently longer.
- The organization of the chromatin fiber is more complex.
Eukaryotic DNA synthesis takes place during a specific phase of the cell cycle (S phase). During this phase, different chromosomes replicate in the nucleus, accompanied by a high synthesis of histone proteins, which are needed to regenerate the characteristic chromatin structure.
RNA Synthesis: The Transcript
Transcription is a selective and sequential process resulting in the synthesis of different types of cellular RNA molecules. Each RNA molecule is synthesized individually via transcription from a specific area of DNA, known as a transcription unit. This unit is small compared to the overall length of the DNA molecule.
Of the two DNA strands, only one transcription unit is transcribed. Key terminology:
- Coding Strand (or Strand +): This strand has the same sequence as the RNA transcript (with Thymine [T] replaced by Uracil [U]).
- Template Strand (or Strand -): This strand acts as the template for RNA synthesis.
Prokaryotic Gene Transcription
Bacteria utilize a single enzyme, called DNA-dependent RNA polymerase, responsible for transcribing all genes of the bacterial chromosome, leading to the formation of three basic types of RNA: rRNA, tRNA, and mRNA.
The synthesis process is divided into three stages:
- Initiation
- Elongation
- Termination
Transcription units in prokaryotes are usually formed by more than one gene, so the resulting mRNA is called polycistronic, carrying the information for the synthesis of two or more different polypeptide chains.
In bacteria, transcription is regulated by proteins that, either directly or by interaction with other factors, combine to stimulate (positive control) or decrease (negative control) the transcription of a particular unit or gene.
Eukaryotic Transcription Mechanisms
The transcription of nuclear DNA in eukaryotes is very similar to that in bacteria concerning the process of RNA strand synthesis, although the participating enzymes, the organization of transcription units, and their promoters are very different from those of prokaryotes.
Eukaryotes utilize three different nuclear RNA polymerases, each participating in the synthesis of a particular type of RNA. Additionally, a DNA-dependent RNA polymerase is localized in the mitochondria, responsible for synthesizing mitochondrial RNA.
To perform transcription, nuclear DNA-dependent RNA polymerases require a series of auxiliary factors. These factors are involved in the initiation process or in promoting the localization of the RNA polymerase attachment site at the beginning of RNA synthesis. Many specific factors recognize specific sequences, usually located in the 5′ direction, called response elements (RE).