¶ … Transcription is a process that genetic information on the DNA copies into RNA and the DNA acts as the template for the new molecules of RNA. Transcription process begins with the DNA double helix unwinding as the hydrogen bonds holding the opposing bases breaks and the DNA strands are uncoupled. The process occurs within the cytoplasm of a prokaryote and in the nucleus of eukaryotic cells. Transcription process consists of three steps; initiation, elongation, termination, and are regulated by transcription factors that include protein products of the genes. The protein products regulate at postranscriptional levels every time.
Initiation of transcription begins with enzyme RNA polymerase that identifies and attaches to DNA at the promoter and transcription of the DNA template starts. An initiation complex forms by association of 50 proteins different from each other required by RNA polymerase II. RNA polymerase synthesizes polynucleotides of RNA from the template of DNA. Transcription occurs only on one of the DNA strands in the gene (Latchman, 2009). The polymerase enzyme bind to the promoter and the helix unwinds making the two strands separate. The eukaryotic gene expression has a sequence of elements located far from the transcription start site and the elements can be upstream, downstream or in a transcription unit. A transcription initiation site can be determined by the DNA protein interaction that several components of the initiation transcription complex. Polymerase I, II, and III enzymes required for the process of transcription to start making it one of the most conserved protein in eukaryotes like yeast and complex ones like humans. Amino acids bind to the TATA box stimulating the initiation of transcription.
The elongation process occurs when the enzyme moves along the template DNA strand adding nucleotides to the 3' end of the growing chain. RNA polymerase detaches the strands and attaches nucleotides following the pairing rules of bases. Cytosine (C) bonds with guanine (G) and adenine (A) with uracil (U). Transcription unit consist of a triplet of bases that code for specific amino acids. Enhancers increase the promoter's activity while lacking promoter activity themselves. Enhancers have various binding sites for interaction to mediate transcription process. During the transcription process, there is interaction of histones with DNA that depends on conformational changes triggered by interaction with activators and coactivators.
A newly formed transcription unit ships out of the ribosome from the nucleus if processed by a series of enzymes. The addition of a 5' cap to the 5' end takes place helping to protect the RNA strand from degradation by enzymes that bind the RNA strand to the ribosome. At the 3' end, a poly (A) tail adds itself protecting the RNA from hydrolytic enzyme degradation and helps to release the RNA into the cytoplasm of the cell. Small nuclear ribonucleoproteins (snRNPs) functions in the removal of introns that are non-coding regions of the gene. RNA transcription, splicing and polyadenylation occur as a continuous process coordinated by interaction of processing factors of the transcription process (Latchman, 2009).
Termination of transcription occurs at specific sequence of bases on the template DNA strand. RNA polymerase moving along the template DNA releases the messenger RNA polymer and gets detached from DNA as it reaches the terminator sequence. The mRNA moves from the nucleus to the cytoplasm where it exists as a single strand unlike DNA that is double stranded. Transcription lead to creation of three products namely; mRNA that carry genetic information for manufacturing of polypeptides, rRNA which perform a structural function for ribosomes and tRNA which deliver amino acids to the ribosome to be assembled into proteins. During termination, the RNA product of transcription released shows a complementary image of the sequence of bases in the DNA template strand.
Question 2. Why do you suppose E. coli has three different DNA polymerases? Why do eukaryotes have more DNA polymerases?
The first DNA polymerase to gain characterization came from the E. coli and was DNA Polymerase 1. In each cell, there exist around 400 molecules of that enzyme. The weight of the enzyme is 103 kDa and is that of one huge protein. There are other two DNA polymerases with the same particular characteristics. The reason why the E. coli has these three different DNA polymerase is for the effective working of the E. coli provided the fact that these DNA polymerse may fail to carry out its enzymatic activity thus requiring the action of the other (Singer, 2011).
Further cell membrane, the cell organelles among other things. This means that eukaryotic cells require more proteins to support all this. It is thus another reason why the cell uses more DNA as opposed to the prokaryotic cells (Singer, 2011).
Question 3. Detail the process of translation. Include as many factors as you can comprehend.
Translation entails the transformation of information in the messenger RNA (mRNA) into sequences of amino acids making it an important pathway in expression of genes. The proteins produced take part in cell metabolism and coordination of overall cellular events. The translation process occurs at the ribosome and mRNA encounters transfer RNA (tRNA) molecule with the appropriate amino acids. The codon in the mRNA exposes itself towards the tRNA, and translation takes place when the ribosome moves along the mRNA. Translation process occurs in three steps; chain initiation, chain elongation and chain termination.
Protein synthesis takes place at the initiation stage where there are tRNAs having anticodons that the start codon AUG recognize and bind to the mRNA on the ribosomes small subunit. A functional ribosome forms by the addition of a large ribosomal subunit (Alvis, 2010).
The start codon AUG located in the P-site in eukaryotes, and in bacteria Shine Dalgarno sequence in prokaryotic cells. Bacteria have formylmethionine (fMet) as its first amino acid and three initiation factors; IF1, IF2, and IF3 which take part in the placement of fMet tRNA at the AUG initiation codon.
Elongation involves elongation factors that bind charged tRNAs that and can be recognized to the A-site of the ribosome, only aminoacyl-tRNAs gets attached. The bacterial cell contains several elongation factors (EF-Tu) which protect the charged tRNAs from hydrolysis that take place in the cytoplasm. The second tRNA inserted into the ribosome recognizes the UCC codon positioned in the mRNA.
The elongation factor EF-Tu recognizes the aminoacyl tRNAs ester moiety but cannot differentiate between different amino acid species. According to the information on the genetic code, the amino acid serine attaches to the tRNA. tRNA retains its position by hydrogen bonds located between the anticodon and codon bases, and peptide bond holds enzyme attached methionine and serine in position. The reaction requires energy supplied by guanosine triphosphate (GTP) and adenosine triphosphate (ATP). The release of the first tRNA occurs dropping the methionine molecule on the amino acid chain, and the tRNA moves one codon right exposing the codon (GCC) and tRNA attaches to amino acid alanine. The ribosome gets excited by the serine attached tRNA making chain elongation process continue as the ribosome moves to expose the next codon. During the elongation process, shifting of tRNA from the A-site to the P-site occurs with the help of elongation factor (EF-G) and hydrolysis of guanosine triphosphate. The elongation cycle repeats itself until a termination codon is reached (Alvis, 2010).
Chain termination involves the addition of tRNAs and transfer of elongating polypeptides to the incoming charged tRNA until the ribosome reaches the stop codon (UGA). Termination occurs since there is no tRNA to recognize the stop codons. The termination codons lines up at the A-site and release factors RF1 and RFR2 bind to the codon catalysing the hydrolysis of the peptidyl-moiety in the P-site to terminate protein synthesis. Releasing factors bind to the regions where the tRNAs normally attach. Release of polypeptides sets in and the large and small ribosomal subunits disassembled to mark the termination of the translation process. Polypeptides twist into their primary and secondary structures during protein synthesis made sure by chaperons and cytoplasmic proteins that cause perfect folding process.…
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