Which arrow represents an amino acid being added to a growing polypeptide chain?
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Protein Synthesis Show
The Central Dogma of Molecular BiologyObjectives:
The dogma is the framework for understanding the transfer of information from DNA to RNA then to protein. DNA is "transcribed" into RNA (rewritten, like typing up your notes since DNA and RNA are such similar languages) and then "translated" into the different language of proteins. Most genes contain the information needed to make functional molecules called proteins. A few genes produce other molecules that help the cell assemble proteins. This journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps:
Together, transcription and translation are known as gene expression: the process by which information from a gene is used in the synthesis of a functional gene product. During transcription, the genetic information stored in DNA is transferred to a similar molecule called ribonucleic acid or RNA. Both RNA and DNA are chains of nucleotides; however, their nitrogen bases have slightly different chemical properties. The nitrogen base thymine (T) does not exist in RNA, so uracil (U) fills its place. See Table 1 for more information about the differences between DNA and RNA molecules. There are also three different types of RNA, each serving a different role in gene expression (see Table 2.) The type of RNA that contains the information for making a protein is called messenger RNA or mRNA because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm. During translation, the mRNA interacts with a specialized complex called a ribosome, partially composed of ribosomal RNA or rRNA which "reads" the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for one particular amino acid (see Table 3). Amino acids are the building blocks of proteins. A type of RNA called transfer RNA or tRNA brings the specifically coded-for amino acids to the ribosome, one at a time. Protein assembly continues until the ribosome encounters a "stop" codon that codes for no amino acid. Figure 1. Shows the central dogma of molecular biology. DNA is replicated to make more DNA (shown by the circular arrow at the top), transcribed into RNA (represented by the diagonal black arrow going from DNA-upper right to RNA-lower left) and then translated into a protein (represented by a black arrow going left to right from RNA to the protein). In special cases (represented by dashed lines), reverse transcription results in DNA from RNA, or RNA is replicated. Table 1. Differences between DNA and RNA
TranscriptionDNA transcription refers to the synthesis of RNA from a DNA template. This process is very similar to DNA replication. Many different proteins direct and regulate transcription. The most important enzyme is RNA polymerase, an enzyme that influences the synthesis of RNA from DNA template.
InitiationFor transcription to be initiated, RNA polymerase must be able to recognize the beginning sequence of a gene so that it knows where to start synthesizing an mRNA. It is directed to this initiation site by the ability of one of its subunits to recognize a specific DNA sequence found at the beginning of a gene, called the promoter sequence. The promoter sequence is a unidirectional sequence found on one strand of the DNA that instructs the RNA polymerase both where to start synthesis, and in which direction synthesis should continue. The RNA polymerase then unwinds the double helix at the identified point and begins synthesis of an RNA strand complementary to one of the strands of DNA. This strand is called the template strand, whereas the other strand is referred to as the coding strand. Synthesis can then proceed in a unidirectional manner, a process called elongation. ElongationElongation is the process of making the RNA. It proceeds along the template strand as the coding strand becomes longer, adding bases to the RNA as the synthesis continues. TerminationAlthough much is known about transcript processing, the signals and events that instruct RNA polymerase to stop transcribing and drop off the DNA template remain unclear. Experiments over the years have indicated that processed eukaryotic messages contain a poly(A) addition signal (AAUAAA) at their 3' end, followed by a string of adenines. This is referred to as the termination sequence. This poly(A) addition, also called the poly(A) site, contributes not only to the addition of the poly(A) tail but also to transcription termination and the release of RNA polymerase from the DNA template. However, transcription does not stop here. Rather, this site before it is aborted. It is either before or during this termination process that the transcript cut, at the poly (A) site, leading to the creation of two RNA molecules. The upstream portion of the newly formed, or nascent, RNA then undergoes further modifications, called post-transcriptional modification, and becomes mRNA. The downstream RNA becomes unstable and is rapidly degraded. A mature mRNA molecule is the result. TranslationTranslation is the process in which an RNA sequence is converted into a protein. As a part of this process, messenger RNA (mRNA) has been transcribed from a sequence of DNA and will travel out of the nucleus and into the cell's cytoplasm where it attracts the two subunit halves of ribosomes. The ribosome subunits bind to the mRNA and begin translating it to create a protein. Ribosomes translate mRNAs at a fairly constant rate, so the number of times an mRNA will be translated depends on how stable the molecule is. A very stable, long-lived mRNA will be translated many times while an unstable, short-lived mRNA will be translated fewer times. After the mRNA has been translated, it leaves the ribosome and, in most cases, is destroyed. Other mRNA transcripts code for proteins that belong in the endoplasmic reticulum (ER), Golgi complex or lysosomes. While even others code for proteins that will be secreted from the cell, or proteins that belong in the cell's plasma membrane. Like the mRNAs described above that code for cytoplasmic and other internal proteins, they attract subunits of ribosomes that begin translating the protein. Almost immediately, though, the emerging new protein stops the ribosome from continuing. The "paused" ribosome moves to the endoplasmic reticulum (ER), and docks on it. Docking makes the ribosome resume translating the mRNA. Depending on its final destination, the finished protein is either inserted into the ER membrane or released inside. Once inside or embedded in ER membrane, the protein is ready to be sent to other destinations for further modification. Figure 2. Organization of the major structures tied to mRNA translation. After DNA is transcribed into mRNAs in the nucleus shown in green (1), the mRNA molecules leave through nuclear pores (2) represented by the light green spots on the nucleus. In cytoplasm (yellow space around organelles) each mRNA molecule attracts the two subunits that make up 1 ribosome. If the mRNA codes for a protein destined for the nucleus, mitochondria, or cytoplasm, the entire process of translation occurs in the cytoplasm. If the mRNA codes for a protein that belongs in ER (3) shown in orange or Golgi (8) shown in purple, or a protein to be secreted from the cell, the ribosomes (5) represented by small purple spheres will bind to ER. The new protein is synthesized directly into the ER, and then transported in vesicles (7) shown as the pink sphere to the Golgi complex (8). CodonsRibosomes turn an mRNA molecule into a protein by reading the mRNA and converting the information it contains into an amino acid sequence. The ribosome reads the RNA in sets of three nucleotides, which are referred to as codons. Each codon specifies a given amino acid, although there are some repeats where more than one triplet of nucleotides codes for the same amino acid. The first codon on a strand of mRNA is usually AUG, which translates as a start. The ribosome will begin translating the mRNA sequence at this codon and then continue until it reaches one of three codons (UAG, UGA, UAA) that do not code for an amino acid. These codons are referred to as stop or termination codons. Since each codon specifies a certain amino acid, the way a ribosome will read a given mRNA can be predicted. For example, if an mRNA consisted of the nucleotide sequence: AUGUCUGGUAAAUAA then the ribosome would read the mRNA as AUG-UCU-GGU-AAA-UAA and generate the amino acid sequence: Methionine-Serine-Glycine-lysine. The last codon would merely result in the end of translation. The fact that the ribosome reads mRNA in triplets is significant because if single nucleotides coded for amino acids than proteins would only be a repeat of four amino acids and if sets of two nucleotides coded for amino acids than only a maximum of sixteen amino acids could be coded for by the mRNA.
Figure 3. Position of the mRNA in side a ribosome. The mRNA (shown in a long line at the bottom beginning in a khaki color and then breaking down into sections of orange, pink, and light blue) aligns inside of the ribosome (green) so that tRNAs (represented by the stacks of various blue discs) read the sequence 3 bases at a time. The 3-base groups are the codons. Each codon is translated when a tRNA binds. The tRNA is carrying an amino acid, which the ribosome adds to the growing polypeptide chain (shown as a chain of dark blue, orange, and red spheres). Table 3. Codons
Self-AssessmentTest your knowledge by completing the quiz below.
 GlossaryDogma: is the framework for understanding the transfer of information from DNA to RNA then to protein. Elongation: The RNA polymerase slides along the DNA, unwinding it as it goes. As this happens more, and more bases are added to the mRNA, elongating it. After RNA polymerase has passed, the DNA restores its double-stranded structure. Initiation: RNA synthesis begins after the RNA polymerase attaches to the DNA and unwinds it. RNA synthesis always occurs on the template strand. Messenger RNA (mRNA): Carries DNA transcript to ribosome. Ribosomal RNA (rRNA): Coils up to make part of the ribosome. Termination: When the mRNA is complete, the mRNA is released and the RNA polymerase releases from the DNA. Transfer RNA (tRNA): Transfers the necessary amino acids to the ribosome. Transcription: The process of rewriting the DNA code into RNA language which occurs in the nucleus. Translation: The process of using the RNA code as a template for translation into a protein which occurs in the cytoplasm. Grant and Other Information
Except where otherwise noted, this work by The Community College Consortium for Bioscience Credentials is licensed under a Creative Commons Attribution 4.0 International License. Text from BioBook licensed under CC BY NC SA and Boundless Biology Open Textbook licensed under CC BY SA. Other text from OpenStaxCollege licensed under CC BY 3.0. Modified by Alice Rudolph, M.A. for c3bc. Instructional Design by Courtney A. Harrington, Ph.D., Helen Dollyhite, M.A. and Caroline Smith, M.A. for c3bc Media by Brittany Clark and Antonio Davis for c3bc This product was funded by a grant awarded by the U.S. Department of Labor's Employment and Training Administration. The product was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership. What is the relationship DNADNA, RNA, and protein are all closely related. DNA contains the information necessary for encoding proteins, although it does not produce proteins directly. RNA carries the information from the DNA and transforms that information into proteins that perform most cellular functions.
What two processes ensure that the correct amino acid is added to a growing polypeptide chain?The two processes that ensure the correct amino acid is added to the polypeptide chain are as follows: Aminoacyl t-RNA synthetase recognizes the specific amino acid and binds it to its appropriate t-RNA. The charged t-RNA carrying its specific amino acid then binds to the appropriate mRNA codon.
What do the arrows indicate in DNA replication?» On the models, arrows on the bases indicate directionality.
Which arrow represents the process of transcription?A dashed arrow that represents transcription process is used to indicate RNA is transcribed from the sequence and RNA translated into protein (Fig. 9).
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