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    what is the role of rna? to provide the original blueprint for protein production to move information from the ribosomes to the nucleus for protein synthesis to break down proteins into amino acid monomers to move information from the nucleus to the ribosomes for protein synthesis

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    From RNA to Protein

    In the preceding section we have seen that the final product of some genes is an RNA molecule itself, such as those present in the snRNPs and in ribosomes. However, most genes in a cell produce mRNA molecules that serve as intermediaries on the pathway to proteins. In this section we examine how the cell converts the information carried in an mRNA molecule into a protein molecule. This feat of translation first attracted the attention of biologists in the late 1950s, when it was posed as the “coding problem”: how is the information in a linear sequence of nucleotides in RNA translated into the linear sequence of a chemically quite different set of subunits—the amino acids in proteins? This fascinating question stimulated great excitement among scientists at the time. Here was a cryptogram set up by nature that, after more than 3 billion years of evolution, could finally be solved by one of the products of evolution—human beings. And indeed, not only has the code been cracked step by step, but in the year 2000 the elaborate machinery by which cells read this code—the ribosome—was finally revealed in atomic detail.

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    Molecular Biology of the Cell. 4th edition.

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    From RNA to Protein

    In the preceding section we have seen that the final product of some genes is an RNA molecule itself, such as those present in the snRNPs and in ribosomes. However, most genes in a cell produce mRNA molecules that serve as intermediaries on the pathway to proteins. In this section we examine how the cell converts the information carried in an mRNA molecule into a protein molecule. This feat of translation first attracted the attention of biologists in the late 1950s, when it was posed as the “coding problem”: how is the information in a linear sequence of nucleotides in RNA translated into the linear sequence of a chemically quite different set of subunits—the amino acids in proteins? This fascinating question stimulated great excitement among scientists at the time. Here was a cryptogram set up by nature that, after more than 3 billion years of evolution, could finally be solved by one of the products of evolution—human beings. And indeed, not only has the code been cracked step by step, but in the year 2000 the elaborate machinery by which cells read this code—the ribosome—was finally revealed in atomic detail.

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    An mRNA Sequence Is Decoded in Sets of Three Nucleotides

    Once an mRNA has been produced, by transcription and processing the information present in its nucleotide sequence is used to synthesize a protein. Transcription is simple to understand as a means of information transfer: since DNA and RNA are chemically and structurally similar, the DNA can act as a direct template for the synthesis of RNA by complementary base-pairing. As the term signifies, it is as if a message written out by hand is being converted, say, into a typewritten text. The language itself and the form of the message do not change, and the symbols used are closely related.

    In contrast, the conversion of the information in RNA into protein represents a translation of the information into another language that uses quite different symbols. Moreover, since there are only four different nucleotides in mRNA and twenty different types of amino acids in a protein, this translation cannot be accounted for by a direct one-to-one correspondence between a nucleotide in RNA and an amino acid in protein. The nucleotide sequence of a gene, through the medium of mRNA, is translated into the amino acid sequence of a protein by rules that are known as the genetic code. This code was deciphered in the early 1960s.

    The sequence of nucleotides in the mRNA molecule is read consecutively in groups of three. RNA is a linear polymer of four different nucleotides, so there are 4 × 4 × 4 = 64 possible combinations of three nucleotides: the triplets AAA, AUA, AUG, and so on. However, only 20 different amino acids are commonly found in proteins. Either some nucleotide triplets are never used, or the code is redundant and some amino acids are specified by more than one triplet. The second possibility is, in fact, the correct one, as shown by the completely deciphered genetic code in Figure 6-50. Each group of three consecutive nucleotides in RNA is called a codon, and each codon specifies either one amino acid or a stop to the translation process.

    Figure 6-50

    The genetic code. The standard one-letter abbreviation for each amino acid is presented below its three-letter abbreviation (see Panel 3-1, pp. 132–133, for the full name of each amino acid and its structure). By convention, codons are always (more...)

    This genetic code is used universally in all present-day organisms. Although a few slight differences in the code have been found, these are chiefly in the DNA of mitochondria. Mitochondria have their own transcription and protein synthesis systems that operate quite independently from those of the rest of the cell, and it is understandable that their small genomes have been able to accommodate minor changes to the code (discussed in Chapter 14).

    In principle, an RNA sequence can be translated in any one of three different reading frames, depending on where the decoding process begins (Figure 6-51). However, only one of the three possible reading frames in an mRNA encodes the required protein. We see later how a special punctuation signal at the beginning of each RNA message sets the correct reading frame at the start of protein synthesis.

    Figure 6-51

    The three possible reading frames in protein synthesis. In the process of translating a nucleotide sequence into an amino acid sequence the sequence of nucleotides in an mRNA molecule is read from the 5′ to the 3′ end in (more...)

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    tRNA Molecules Match Amino Acids to Codons in mRNA

    The codons in an mRNA molecule do not directly recognize the amino acids they specify: the group of three nucleotides does not, for example, bind directly to the amino acid. Rather, the translation of mRNA into protein depends on adaptor molecules that can recognize and bind both to the codon and, at another site on their surface, to the amino acid. These adaptors consist of a set of small RNA molecules known as transfer RNAs (tRNAs), each about 80 nucleotides in length.

    Source : www.ncbi.nlm.nih.gov

    RNA and protein synthesis review (article)

    RNA and protein synthesis

    RNA and protein synthesis review

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    Key terms

    Term Meaning

    RNA (ribonucleic acid) Single-stranded nucleic acid that carries out the instructions coded in DNA

    Central dogma of biology The process by which the information in genes flows into proteins: DNA → RNA → protein

    Polypeptide A chain of amino acids

    Codon A sequence of three nucleotides that corresponds with a specific amino acid or start/stop signal during translation

    Transcription Process during which a DNA sequence of a gene is copied to make an RNA molecule

    Translation Process during which an mRNA molecule is used to assemble amino acids into polypeptide chains

    Mutation A change in a genetic sequence

    Structure of RNA

    DNA alone cannot account for the expression of genes. RNA is needed to help carry out the instructions in DNA.

    Like DNA, RNA is made up of nucleotide consisting of a 5-carbon sugar ribose, a phosphate group, and a nitrogenous base. However, there are three main differences between DNA and RNA:

    RNA uses the sugar ribose instead of deoxyribose.

    RNA is generally single-stranded instead of double-stranded.

    RNA contains uracil in place of thymine.

    These differences help enzymes in the cell to distinguish DNA from RNA.

    Image comparing the structure of single-stranded RNA with double-stranded DNA.

    Comparison of RNA and DNA molecules. Image modified from Wikimedia, CC BY-SA 3.0.

    Types of RNA

    Type Role

    Messenger RNA (mRNA) Carries information from DNA in the nucleus to ribosomes in the cytoplasm

    Ribosomal RNA (rRNA) Structural component of ribosomes

    Transfer RNA (tRNA) Carries amino acids to the ribosome during translation to help build an amino acid chain

    Central dogma of biology

    A gene that encodes a polypeptide is expressed in two steps. In this process, information flows from DNA

    \rightarrow → right arrow RNA \rightarrow → right arrow

    protein, a directional relationship known as the central dogma of molecular biology.

    The genetic code

    The first step in decoding genetic messages is transcription, during which a nucleotide sequence is copied from DNA to RNA. The next step is to join amino acids together to form a protein.

    The order in which amino acids are joined together determine the shape, properties, and function of a protein.

    The four bases of RNA form a language with just four nucleotide bases: adenine (A), cytosine (C), guanine (G), and uracil (U). The genetic code is read in three-base words called codons. Each codon corresponds to a single amino acid (or signals the starting and stopping points of a sequence).

    Genetic code table. Each three-letter sequence of mRNA nucleotides corresponds to a specific amino acid, or to a stop codon. UGA, UAA, and UAG are stop codons. AUG is the codon for methionine, and is also the start codon.

    Codon chart. Image from OpenStax, CC BY 3.0.

    [How do you read the codon table?]

    Transcription and translation

    Simplified schematic of central dogma, showing the sequences of the molecules involved.

    The two strands of DNA have the following sequences:

    5'-ATGATCTCGTAA-3' 3'-TACTAGAGCATT-5'

    Transcription of one of the strands of DNA produces an mRNA that nearly matches the other strand of DNA in sequence. However, due to a biochemical difference between DNA and RNA, the Ts of DNA are replaced with Us in the mRNA. The mRNA sequence is:

    5'-AUGAUCUCGUAA-5'

    Translation involves reading the mRNA nucleotides in groups of three, each of which specifies and amino acid (or provides a stop signal indicating that translation is finished).

    3'-AUG AUC UCG UAA-5'

    AUG \rightarrow → right arrow Methionine AUC \rightarrow → right arrow Isoleucine UCG \rightarrow → right arrow Serine UAA \rightarrow → right arrow "Stop"

    Polypeptide sequence: (N-terminus) Methionine-Isoleucine-Serine (C-terminus)

    In transcription, a DNA sequence is rewritten, or transcribed, into a similar RNA "alphabet." In eukaryotes, the RNA molecule must undergo processing to become a mature messenger RNA (mRNA).

    In translation, the sequence of the mRNA is decoded to specify the amino acid sequence of a polypeptide. The name translation reflects that the nucleotide sequence of the mRNA sequence must be translated into the completely different "language" of amino acids.

    Mutations

    Sometimes cells make mistakes in copying their genetic information, causing mutations. Mutations can be irrelevant, or they can affect the way proteins are made and genes are expressed.

    Substitutions

    A substitution changes a single base pair by replacing one base for another.

    Source : www.khanacademy.org

    How do genes direct the production of proteins?: MedlinePlus Genetics

    Genes make proteins through two steps: transcription and translation. This process is known as gene expression. Learn more about how this process works.

    How do genes direct the production of proteins?

    Most genes contain the information needed to make functional molecules called proteins. (A few genes produce regulatory molecules that help the cell assemble proteins.) The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression.

    During the process of transcription, the information stored in a gene's DNA is passed to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of building blocks called nucleotides, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.

    Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which "reads" the sequence of mRNA nucleotides. Each sequence of three nucleotides, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.) A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three nucleotides that does not code for an amino acid).

    The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the “central dogma.”

    Through the processes of transcription and translation, information from genes is used to make proteins.

    Credit: U.S. National Library of Medicine

    For more information about making proteins:

    The Genetic Science Learning Center at the University of Utah offers an interactive introduction to transcription and translation.

    The New Genetics, a publication of the National Institute of General Medical Sciences, includes discussions of transcription and translation.

    Biointeractive from the Howard Hughes Medical Institute illustrates the stages in the flow of information from DNA to RNA to protein. This tool also gives examples of how modern technologies that target the different stages are used to treat genetic diseases.

    Topics in the How Genes Work chapter

    What are proteins and what do they do?

    How do genes direct the production of proteins?

    Can genes be turned on and off in cells?

    What is epigenetics?

    How do cells divide?

    How do genes control the growth and division of cells?

    How do geneticists indicate the location of a gene?

    Other chapters in Help Me Understand Genetics

    The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.

    Source : medlineplus.gov

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