The Transcription of Prokaryotic Cells and Eukaryota Cells - Essay Example

Topic: Discuss the control of transcription in prokaryotic cells Cells are basically ified into two main types that are prokaryotic cells and eukaryotic cells. This essay will focus primarily on the prokaryotic cells. Prokaryotes are a very important subject of study in both microbiology and biochemistry and much research has been done on their structure and genetic expression. This essay will particularly discuss the transcription of these cells and the control and regulation of this transcription. The approximate diameter of prokaryotes is 1 to 5 micrometer. They possess a very rigid and strong cell wall. This rigidity is accounted to for by the peptidogycan which is the major component of its cell wall. They possess flagella which accounts for the motility of some prokaryotes. These organisms do not contain intracellular membrane bound organelles such as mitochondria and lysosmes. The size of their ribosomes is 70S which is small as compared to that of eukaryotic cells. The most important and distinguishing feature of prokaryotes is regarding their genetic material. They possess both DNA and RNA but their DNA is single stranded. Their genetic material does not lie in a nucleus rather in a nucleoid which does not have a nuclear membrane and consists of DNA which is in a circular form and is loose. The DNA of the prokaryotes is double stranded. This DNA is also not folded and arranged around histones. The prokaryotes do not contain sterols except for one exception which is Mycoplasma Bacterium which contains them. Prokaryotes are organisms which can survive in extreme conditions as for example at temperatures ranging as high as 80 degrees centigrade. The formation of RNA from DNA is termed as transcription. This process is very essential because it is this RNA which then codes for amino acids and hence proteins. The process of transcription is accomplished by an enzyme known as RNA polymerase. The strand of the DNA that is transcribed is in the 3’ to 5’ direction. Hence the transcript that is formed in the opposite direction that is 5’ to 3’ direction. This is because the RNA polymerase functions in the 3’ to 5’ direction on the DNA creating a complementary strand on RNA in which the Adenine always pairs with Uracil whereas the guanine always pairs with the the cytosine. Prokaryotic cells contain only one type of RNA polymerase. This RNA polymerase only is responsible for the formation of all the different types of RNA in the prokaryotes which includes the mRNA, rRNA and tRNA. RNA polymerase has a distinguishing quality that it can initiate the process of transcription by itself. There are particular sequences on the DNA strand where this enzyme binds to carry out the process of transcription. Promoters and enhancers are these sequences which are located on the DNA strand. Promoters are near to the point where transcription starts whereas enhances are many nucleotides away and their primary role is to alter the umber of times transcription takes place. The structure of the enzyme RNA polymerase in prokaryotes can be understood by considering an organism of this kingdom which is known as E. coli. In E. coli the RNA polymerase has 2 alpha subunits, one beta subunit, one beta prime subunit and a sigma factor. It is this sigma factor which helps in the binding of the RNA polymerase to the DNA strand. The region to the left of the start point of transcription is known as 5’ flanking region. On the other hand the region where the transcription moves ahead is known as the transcribed region. The first nucleotide in the 5’ flanking region is labeled -1 and the others that follow are labeled as -2, -3, -4 and so on. On the other side, the nucleotides in the transcribed region are written with a positive sign that is +1, +2, +3, and +4 and so on. For the beginning of transcription, the sigma factor binds to specific sequences within the promoter regions known as consensus sequences. These sequences lie close to the start point and are repeated in the promoter region. These sequences are rich in the nucleotides adenine and thymine and are considered to play a role in the determination of start point of transcription. In E. coli this sequence is located at -10 and the sequence is TATAAT. This sequence in the E. coli is known as the Pribnow box as well. This is not the only consensus sequence. There might be other sequences present further away and these sequences are also consensus sequences. In the prokaryotes there is a unit of genes known as an operon. An operon is actually a set of genes which gives rise to multiple proteins. It is here at the operon that the transcription of the prokaryotes is regulated. The proteins which actually bind to the operon and the site of this binding determine whether there will be an up regulation or a down regulation of the transcription process. There are two types of proteins which are involved in this regulation. They are either repressors or activators. Repressors attach to operator which is a region present on the promoter. This binding prevents the binding of the enzyme RNA polymerase to the DNA strand. Hence there is an inhibition in the process of transcription. On the other hand proteins which facilitate the binding of RNA polymerase are activators. T5hese activator proteins bind to -35 region that is present in the flanking region or even further ahead enhancing the process of transcription. Prokaryotic transcription begins with the unwinding of the double stranded SNA. This unwinding is induced by the binding of the RNA polymerase to the promoter area. The sigma factor comes into play when the RNA transcript has ten nucleotide bases. The transcription only stops after a termination signal is encountered by the RNA polymerase.The prokaryotic transcript is a polycisteronic transcript. A polycisteronic transcript is one which on being translated gives a great number of different proteins. The process of transcription is a complex process and the regulation of it has been presented. In the prokaryotes, there are other two regulatory mechanisms which can further explain the control of transcription in these prokaryotic organisms. These two mechanisms are termed as positive control and negative control. The functioning of these control mechanisms will be considered with examples. An operon which is a genetic unit that consists of genes that produce a number of genes, works in the control of prokaryotic transcription. The operon codes for a polycisteronic transcript which codes for multiple proteins. These proteins may be enzymes which function in collaboration. They can induce a negative control on transcription. An example is Allolactose. This is a lactose derivative which removes the repressor protein from the operon, thus allowing the synthesis of enzymes required for lactose breakdown. Thus lactose actually is acting as a negative control on itself for its own catalytic breakdown. This process is known as negative control. The second mechanism of control is the positive control. Over here the example of tryptophan operon can be considered. This operon codes for the formation of tryptophan. Tryptophan is coded to by five genes which are present all on the operon. There is a side by side positive control of this operon going on. But more important is the positive control. The mechanism of tryptophan generation occurs in a very coordinated way. The RNA which is coded for the formation of tryptophan is formed from the operon but this mRNA enfolds upon itself exposing the termination sequence. Hence due to the early appearance of the termination sequence the amino acid cannot be formed. But when the cell lacks this amino acid the tryptophan itself generates a positive control on itself. The enfolding which is taking place for the early exposure of the termination sequence is avoided by preventing the enfolding. At the same time the movement of the ribosome over the mRNA sequence slows down so that the termination sequence can be moved away. This thus leads to the formation of the required product which in this case is tryptophan. Hence positive control also plays a role in regulating the process of transcription in the prokaryotes. The process of transcription is very essential for the prokaryotes. It is a process which is the first step towards the generation of proteins which may be essential enzymes or perform other vital functions for these organisms. For an organism like a prokaryote which has to even survive in adverse conditions there can be no chances on its regulatory mechanism. To help with their functions their transcription process has a very complex pattern which is well integrated, coordinated and regulated. Bibliography: LEVINSON, W. (2008). Review of medical microbiology and immunology. New York, McGraw-Hill Medical MEEHAN, R. (1994). Genes V (B. Lewin). Trends in Genetics. 10, 258. MURRAY, R. K., GRANNER, D. K., & RODWELL, V. W. (2006). Harpers illustrated biochemistry. Lange medical book. New York, Lange Medical Books/McGraw-Hill. STRYER, L. (1988). Biochemistry. New York, W.H. Freeman TALARO, K. P. (2005). Foundations in microbiology. Dubuque, IA, McGraw-Hill. Read More