Transcription produces which of the following quizlet

With the genes bound in a nucleus, the eukaryotic cell must transport protein-encoding RNA molecules to the cytoplasm to be translated. Protein-encoding primary transcripts , the RNA molecules directly synthesized by RNA polymerase, must undergo several processing steps to protect these RNA molecules from degradation during the time they are transferred from the nucleus to the cytoplasm and translated into a protein.

For example, eukaryotic mRNAs may last for several hours, whereas the typical prokaryotic mRNA lasts no more than 5 seconds.

The primary transcript also called pre-mRNA is first coated with RNA-stabilizing proteins to protect it from degradation while it is processed and exported out of the nucleus. In addition to preventing degradation, factors involved in subsequent protein synthesis recognize the cap, which helps initiate translation by ribosomes. This modification further protects the pre-mRNA from degradation and signals to cellular factors that the transcript needs to be exported to the cytoplasm.

Eukaryotic genes that encode polypeptides are composed of coding sequences called exons ex -on signifies that they are ex pressed and intervening sequences called introns int -ron denotes their int ervening role. Transcribed RNA sequences corresponding to introns do not encode regions of the functional polypeptide and are removed from the pre-mRNA during processing.

It is essential that all of the intron-encoded RNA sequences are completely and precisely removed from a pre-mRNA before protein synthesis so that the exon-encoded RNA sequences are properly joined together to code for a functional polypeptide. If the process errs by even a single nucleotide, the sequences of the rejoined exons would be shifted, and the resulting polypeptide would be nonfunctional.

The process of removing intron-encoded RNA sequences and reconnecting those encoded by exons is called RNA splicing and is facilitated by the action of a spliceosome containing small nuclear ribonucleo proteins snRNPs. Although they are not translated, introns appear to have various functions, including gene regulation and mRNA transport. On completion of these modifications, the mature transcript , the mRNA that encodes a polypeptide, is transported out of the nucleus, destined for the cytoplasm for translation.

Introns can be spliced out differently, resulting in various exons being included or excluded from the final mRNA product. This process is known as alternative splicing. The advantage of alternative splicing is that different types of mRNA transcripts can be generated, all derived from the same DNA sequence. In recent years, it has been shown that some archaea also have the ability to splice their pre-mRNA. See how introns are removed during RNA splicing here.

In the emergency department, a nurse told Travis that he had made a good decision to come to the hospital because his symptoms indicated an infection that had gotten out of control.

Within the affected area, a rash had begun, blistering and small gas pockets underneath the outermost layer of skin had formed, and some of the skin was becoming gray. Based on the putrid smell of the pus draining from one of the blisters, the rapid progression of the infection, and the visual appearance of the affected skin, the physician immediately began treatment for necrotizing fasciitis.

Travis was admitted to the intensive care unit and began intravenous administration of a broad-spectrum antibiotic to try to minimize further spread of the infection. Travis became confused and dizzy. Within a few hours of his hospital admission, his blood pressure dropped significantly and his breathing became shallower and more rapid. Skip to main content.

Mechanisms of Microbial Genetics. Search for:. What occurs to initiate the polymerization activity of RNA polymerase? Where does the signal to end transcription come from? Visualize how mRNA splicing happens by watching the process in action in this video. Think about It In eukaryotic cells, how is the RNA transcript from a gene for a protein modified after it is transcribed? Do exons or introns contain information for protein sequences? The mRNA formed in transcription is transported out of the nucleus, into the cytoplasm, to the ribosome the cell's protein synthesis factory.

Here, it directs protein synthesis. The ribosome is a very large complex of RNA and protein molecules. Each three-base stretch of mRNA triplet is known as a codon , and one codon contains the information for a specific amino acid.

This tRNA molecule carries an amino acid at its 3'-terminus, which is incorporated into the growing protein chain. The tRNA is then expelled from the ribosome. Figure 7 shows the steps involved in protein synthesis. Transfer RNA adopts a well defined tertiary structure which is normally represented in two dimensions as a cloverleaf shape, as in Figure 7. The structure of tRNA is shown in more detail in Figure 8. The reaction of esters with amines is generally favourable but the rate of reaction is increased greatly in the ribosome.

Each transfer RNA molecule has a well defined tertiary structure that is recognized by the enzyme aminoacyl tRNA synthetase, which adds the correct amino acid to the 3'-end of the uncharged tRNA. The presence of modified nucleosides is important in stabilizing the tRNA structure. Some of these modifications are shown in Figure The genetic code is almost universal.

It is the basis of the transmission of hereditary information by nucleic acids in all organisms. In theory only 22 codes are required: one for each of the 20 naturally occurring amino acids, with the addition of a start codon and a stop codon to indicate the beginning and end of a protein sequence. Many amino acids have several codes degeneracy , so that all 64 possible triplet codes are used.

For example Arg and Ser each have 6 codons whereas Trp and Met have only one. No two amino acids have the same code but amino acids whose side-chains have similar physical or chemical properties tend to have similar codon sequences, e. This means that if the incorrect tRNA is selected during translation owing to mispairing of a single base at the codon-anticodon interface the misincorporated amino acid will probably have similar properties to the intended tRNA molecule.

Although the resultant protein will have one incorrect amino acid it stands a high probability of being functional. Organisms show "codon bias" and use certain codons for a particular amino acid more than others. For example, the codon usage in humans is different from that in bacteria; it can sometimes be difficult to express a human protein in bacteria because the relevant tRNA might be present at too low a concentration. One strand of genomic DNA strand A, coding strand contains the following sequence reading from 5' to 3':.

The sequence of bases in the other strand of DNA strand B written 5' to 3' is therefore. Crick and Brenner proposed that a single tRNA molecule can recognize codons with different bases at the 3'-end owing to non-Watson-Crick base pair formation with the third base in the codon-anticodon interaction.

Not all combinations are possible; examples of "allowed" pairings are shown in Figure If not repaired by DNA repair enzymes, these mismatches can lead to genetic diseases and cancer. Show all chapters. Figure 1. Figure 2. Figure 3. Figure 4. Figure 5.

Figure 6. Alternative splicing Several different mechanisms of alternative splicing exist - a cassette exon can be either included in or excluded from the final RNA top , or two cassette exons may be mutually exclusive bottom.

Figure 7. Translation a and b tRNA molecules bind to the two binding sites of the ribosome, and by hydrogen bonding to the mRNA; c a peptide bond forms between the two amino acids to make a dipeptide, while the tRNA molecule is left uncharged; d the uncharged tRNA molecule leaves the ribosome, while the ribosome moves one codon to the right the dipeptide is translocated from one binding site to the other ; e another tRNA molecule binds; f a peptide bond forms between the two amino acids to make a tripeptide; g the uncharged tRNA molecule leaves the ribosome.

Figure 8. Figure 9. Protein synthesis Reaction of the growing polypeptide chain with the 3'-end of the charged tRNA. The amino acid is transferred from the tRNA molecule to the protein.

Figure Structures of wobble base pairs found in RNA.