Transcription in prokaryotes and eukaryotes

Introduction

The deoxyribonucleic acid (DNA) its an organic compound whose molecules contain genetic instructions, its role is to store necessary information to create ribonucleic acid (RNA) and proteins. The DNA segment that encloses this information is called a gene.

The DNA molecular structure was discovered jointly by James Watson and Francis Crick in 1953.

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From a chemistry perspective the DNA is a long polymer composed of two simple units, the monomers of nucleotides, whose main chain is formed by phosphate and sugar molecules. Connected to the sugar molecule is a molecule of four nitrogenous bases. The sequence of bases along the DNA constitute the genetic information which encodes the amino acids of proteins specific sequences.

Translation occurs through the RNA messenger (mRNA) by copying part of the DNA chain through the process of transcription, subsequently this information is translated in proteins. RNA is used to synthesize proteins.

Eukaryotes such as animals, plants and funghi contain their DNA inside the nucleus while prokaryotes such as bacteria DNA is scattered in the cytoplasm.

Transcription

The term transcription is given to the first stage of gene expression, when a DNA template synthesizes RNA. Proteins are synthesized by DNA through the nucleic acid RNA. RNA and DNA are similar, however, RNA contains ribose as its sugar and a uracil base, DNA in turn contains deoxyribose and a thymine base, therefore, whilst DNA nucleotides have a base A, G, C, or T, and RNA nucleotides have a base A, G, C, or U.

DNA and RNA along with proteins are polymers that transmit information all the way through specific sequences of monomers. In both nucleic acids these monomers comprise four types of nucleotides supplying a specific sequence of nucleotide bases to each gene. In proteins the monomers are amino acids meticulously ordered in polypeptides. Transcription and translation are the means to synthesize protein from DNA.

DNA and RNA only transcribe information from molecule to molecule. DNA synthesizes RNA through transcription by providing a template for assembling a corresponding sequence of RNA nucleotides, an identical process to DNA replication where a DNA strand template synthesizes new strands. In proteins, the resultant RNA molecule is called messenger RNA (mRNA), and is a transcript of the protein gene that communicates a genetic message from DNA to the cell synthesizing the protein.

The three stages of transcription are initiation, elongation, and termination. The RNA transcript not yet translated into protein is a primary transcript. RNA synthesis is initiated in the nucleotide where the promoter gene, the RNA binding site, defines not only the beginning of transcription but also which DNA strand is the template. The promoter is where transcription begins when the RNA polymerase has attached.

In eukaryotes the transcription factors composed by a set of proteins liaise the RNA polymerase binding and the beginning of transcription. In prokaryotes the RNA polymerase identifies and binds to the promoter, RNA polymerase II only binds to the promoter once specific transcription factors have already attached, this is called the transcription initiation complex, where enzyme begins transcribing the template DNA strand.

Termination in prokaryotes is reached when the transcription proceeds through a terminator sequence in DNA, the terminator RNA which has then been transcribed is the terminator signal, and the transcript is released when the polymerase separates from the DNA which is then used as mRNA.

Termination in eukaryotes happens through the polyadenylation signal sequence when the RNA polymerase II has transcribed the DNA sequence, which codes for the polyadenylation signal (AAUAA) in the pre-mRNA. After that, the AAUAA signal nucleotides, and RNA transcript proteins detach it from the polymerase, liberating the pre-mRNA. However, the polymerase precedes transcribing DNA for nucleotides after the site where the pre-mRNA was generated.

The enzyme RNA polymerase is capable to start a chain without a primer, and therefore, reunites the RNA nucleotides while they pair off the base along the DNA template, assembling a polynucleotide just in its 5’ – 3’ direction. DNA nucleotides sequences determine the beginning and finishing of transcription.

Transcription in Prokaryotes

Prokaryotes enclose a particular type of RNA polymerase, a complex enzyme composed of five sub-units ?2??’? (holoenzyme), that form a core enzyme which when bound to DNA synthesizes RNA. The core enzyme is attached to random DNA sites and the purified polymerase is called the sigma factor (?). Attachment of sigma factor to the core enzyme begins the transcription-promoter recognition. The enzyme then divides both DNA strands in the starting site. Once about ten nucleotides have been integrated into a transcript, the enzyme is changed to a transcriptional elongation complex releasing the sigma factor.

The nucleotide at which transcription starts is designated as +1 and the previous nucleotide as -1. The previous DNA toward the 3’ end of the template is called upstream and the following DNA toward the 5 end of the template is called downstream.

Small nucleotides sequences are recognized by an RNA polymerase as DNA connection point to start transcription, the promoters have consensus sequences recognized by enzymes. These sequences occur at about 10 and 35 nucleotide pairs before the initial site of transcription. Prokaryotic cells have an array of distinct sigma factors that recognise different promoter sequences.

Elongation is performed by the core enzyme after the release of subunit. The polymerase continuously unfolds the DNA ahead and folds the DNA behind the site of transcription (already transcribed). The RNA strand will be disengaging from the DNA as that RNA polymerase moves along the double helix.

The protein RHO is necessary for termination of transcription. Termination occurs when the RNA polymerase finds a termination signal – the transcription complex dissociates and releases the RNA molecule. Sometimes, the polymerase finishes transcription without additional factors when it arrives at a terminator sequence and releases the RNA. There are two types of terminators in E. coli:-dependent terminators of RHO and independent terminators of RHO.

Transcription in Eukaryotes

In eukaryotes there are three types of different RNA polymerases acting in transcription, RNA polymerase I, RNA polymerase II, RNA polymerase III. Each RNA polymerase is responsible for transcription of a specific class of genes This difference in RNA polymerases is one of the differences between prokaryotes and eukaryotes. Other difference in transcription between prokaryotes and eukaryotes is that eukaryotes involve an array of transcription factors. Transcription factors are accessory proteins are essential for polymerase binding to DNA template throughout the whole three stages of transcription.

The three types of RNA (mRNA’s, rRNA’s and tRNA’s) are originated from RNA molecules. The first RNA is called the primary transcript, (pre-RNA). The site of DNA from which a primary transcript is transcribed is a transcription unit. The generation of RNA involves 90 to 300 nucleotides and their accessory proteins. The genes primary transcripts that encode proteins suffer modification before being transported to the cytoplasm.

RNA polymerase requires transcription factors to initiate RNA synthesis and bind to the promoter region to form a complex initiation for the binding of RNA polymerase and beginning of transcription. Polyadenylation plays an important role in the transport of RNA from the nucleus to the protein.

Conclusion

The processes of translation and transcription are related in both prokaryotes and eukaryotes, however, due to prokaryotes not possessing nuclei, their DNA is not segregated from ribosomes and protein synthesis apparatus, which allow initiation of mRNA translation while transcription is occurring. Eukaryotes transcription occurs in the nucleus where RNA transcripts are modified to generate mRNA, the transporter of information from DNA to the cell synthesizing protein, which is then translated in the cytoplasm.

Prokaryotes have only one type of RNA for protein synthesis, such as ribosomal RNA. Eukaryotes have three types of RNA polymerase in the nucleus. RNA polymerase II is responsible for mRNA synthesis. RNA polymerases I and III transcribe RNA molecules that are not translated into protein.

References

1. Campbell, N., Reece, J., Urry, L., Cain, M., Wasserman, S., Minorsky, P., Jackson, R. (2008) Biology 8th Edition. San Francisco: Pearson international Edition. pp 351-367.
2. Latchman, D. (2002) Gene Regulation: A eukaryotic perspective. 4th Edition. Cheltenham: Nelson Thornes Ltd. pp 35-65.
3. Karp, G. (2005) Cell and Molecular Biology. 4th Edition. United Sates of America: Von Hoffmann Press. pp 436-467.

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