This week we are going back to the raw material that controls every process in our body, our genes. DNA is turned into proteins through a process of transcription and translation. This week we will focus on the transcription stage and how this differs in human and bacterial cells.
In eukaryotic cells (such as human cells) transcription occurs in the nucleus of the cell whereas translation occurs in the cytoplasm. However, bacteria being prokaryote cells do not have a distinct nucleus that separates DNA from the ribosomes in the cytoplasm. Therefore there is no barrier between the stages of transcription and translation in bacteria, unlike eukaryotic cells. In high magnification images of bacteria, the ribosomes can actually be seen translating RNA that is still being transcribed from the DNA in the cytoplasm. There are many advantages within this process for prokaryotic cells, one involves the energy required to facilitate transcription. In eukaryotic cells; this has to be generated separately. However, in bacteria energy can be generated from the large scale expenditure of the unstable nucleotide phosphates generated during the translation process. Having transcription and translation occur simultaneously in bacteria causes any changes that affect one process to also affect the other providing a process unique to prokaryotic cells. Antibacterial drugs affect the process of either transcription or translation; different antibacterial drugs have different effects (this is something we will look at in greater detail next week).
Transcription refers to the process of the unfolding of the DNA double helix and the information in a gene is transcribed to form an RNA molecule using DNA as a template.
The structure however of RNA suggests that it is synthesised in a similar way to DNA replication. The basic mechanism of RNA synthesis, the pairing of complementary bases, just like DNA, is the key process behind its manufacture.
Transcription takes place in 3 phases:-
1) Initiation – In this section of transcription the promoter site is located. The promoter site on the DNA is a sequence of bases that initiate the start of a protein sequence; the DNA sequence TAC is recognised by the RNA polymerase as the start codon. This happens upstream of the gene being transcribed and results in the DNA unwinding.
2) Elongation – which involves the use of the antisense (-) strand of DNA by RNA polymerase to synthesize a complementary RNA molecule. This synthesized complementary RNA molecule will have the same sequences of bases as the non-template strand of DNA, called the sense (+) strand (or coding strand), with the exception of thymine being replaced by uracil. The strand of DNA that is used as the template depends on which strand is the coding strand for the gene in question, signified by the TAC starting codon.
3) Termination – finally the enzyme, RNA polymerase encounters the termination signal and ends the transcription process. The RNA then dissociates from the DNA.
Next week we will look more in-depth at the translation part of protein synthesis. These mechanisms within the bacterial cells are highly targeted in antibiotic therapy, next week we will dive into this even deeper.