Conversion of the DNA Information - Essay Example

? About Genes About Genes DNA is the basic form in which instructions and information on the characteristics of any organism is stored in the cells. This information and instructions for the synthesis of the proteins is stored in DNA molecule in form of a sequence of nucleotides, including Adenine, Thymine, guanine and cytosine; commonly abbreviated as A, T, G and C respectively, in a given order. The sequence of these four nucleotide bases on a DNA molecule determine the sequence of amino acids on a given protein, that is, the genetic information on the DNA molecule provides the basis for the synthesis of proteins in a given organism. Starr and Taggart (2006) point out that all living organisms accomplish two key features for their survival, including manufacturing of their own energy and reproduction, through the biosynthesis of proteins using instructions and information encoded in their DNA molecules. Proteins play a central role in determining the architecture of a given cell, controlling cellular processes, catalyzing cellular reactions, as well as, providing membrane channels for cell to cell communication among other important roles. For the information and instructions for protein synthesis to be converted into proteins, there are two processes that are involved; transcription and translation. Clark and Pazdernik (2009) affirm that the genetic information contained in the DNA molecule, in form of a nucleotide sequence, is transformed into important biological use via two processes, transcription and translation. It is worth noting that the flow of genetic information from Deoxyribonucleic acid (DNA) to proteins via mRNA is unidirectional, according to the central dogma; the information is first transcribed into mRNA, which in turn is translated to from mRNA to polypeptides (proteins). Transcription Transcription entails conversion of the DNA information in a given section of the template strand into a complementary mRNA molecule using base pairing rules. This is the first step in the conversion of the instructions and information encoded in DNA into proteins. It involves the conversion of the DNA information in a given gene into its mRNA equivalent by the use of transcription machinery, under the rules of base-pairing, as indicated above. The base-pairing rules dictate that A pairs with T ( U, in RNA) and C pairs with G. In transcription, one strand of the gene, a region of DNA coding for a protein, functions as the template for the synthesis of a complementary mRNA molecule (Clark and Pazdernik, 2009). For this to be accomplished, the double helix DNA molecule is first opened by a protein complex (helicases), and a section of the DNA molecule (gene), is used as a template for the synthesis of a complementary mRNA molecule by RNA polymerase or transcription machinery. Starr and Taggart (2006) point out that the biosynthesis of mRNA involves first the uncoiling of the double helix DNA molecule, followed by the heating or melting of the two strands at beginning of the gene to be expressed and finally the making of the mRNA molecule, complementary to the gene sequence on the template DNA strand, by RNA polymerase. During the assembly of the mRNA molecule, RNA nucleotides are used, and not DNA nucleotides. mRNA biosynthesis takes place in the 5’ to 3’ direction. The process stops when the transcription machinery encounters the stop codon. On reaching the stop codon, signalling the end of the gene, the transcription machinery disintegrates realising the newly synthesized mRNA from DNA, which recoils back to its original form (Starr, Evers & Starr, 2009). Translation Translation involves the conversion of the instructions and information contained in the newly synthesized mRNA into amino acid sequence or proteins. The information contained in the newly synthesized mRNA is used by the translation machinery to synthesize proteins. Starr, Evers and Starr (2009) state that the nucleotide sequence in mRNA dictates the amino acid sequence in the final polypeptide. In translating the information contained in mRNA into protein, the translation machinery uses the genetic code. In the genetic code, a given combination of the three nucleotides (codon) represents a given amino acid, with the exception of the stop codons, UAG, UAA and UGA. The stop codon signals the termination of the translation process or end of a gene. Translation machinery stops translating on encountering the stop codon. AUG, which codes for Methionine, is the start codon and signals the start of a gene from where translation starts (Clark and Pazdernik, 2009). Since a specific combination of the three nucleotides represents a given amino acid, the mRNA is read in triplets or codons, from start codon, by the translation machinery. It is also worth noting that translation takes place in 5’ to 3’ direction (Clark and Pazdernik, 2009). Due to the fact that the base sequence of the template DNA determines the sequence of bases on the mRNA, which in turn is used by the translation machinery to construct the amino acid sequence of the resulting polypeptide, any change (mutation), in the DNA base sequence will indirectly affect the amino acid sequence in the final product, consequently affecting the structure or function of the resulting polypeptide. The level of mutation determines the effects on the final product. Example Original DNA sequence: 3’-TACCCTTTAGTAGCCACT-5’ Transcription: 5’-AUGGGAAAUCAUCGGUGA-3’ (mRNA) Translation: HN-Met-Gly-Asn-His-Arg-OH (amino acid sequence) Mutations 1. 3’-TACGCTTTAGTAGCCATT-5’ Transcription: 5’-AUGCGAAAUCAUCGGUAA-3’ (mRNA) Translation: HN-Met-Arg-Asn-His-Arg-OH (amino acid sequence) 2. 3’-TAACCTTTACTAGGCACT-5’ Transcription: 5’-AUUGGAAAUGAUCCGUGA-3’ (mRNA) Translation: HN-Met-Ile-Arg- (amino acid sequence) From the amino acid sequence above, it is evident that the second mutation affected the structure and function (type) of the resulting protein. This is due to the fact that it resulted in the change of the amino acid sequence in the final product. The first mutation resulted in the change of the second amino acid from Glycine to Arginine. Based on the varying properties of the two amino acids, the change has a profound impact on the structure and thus functional properties of the final protein product. Unlike Arginine has a large and a positively charged guanidinium group as its side chain, Glycine is small, has a hydrogen atom as its side chain and cannot be categorized as either polar or non-polar. Its small size due to lack of a genuine side chain enables Glycine to confer proteins structure with increased internal flexibility or rotation. Replacement of this Glycine with Arginine has this as one of the effects on the final product. Arginine also can form numerous hydrogen bonds because of its positively charged guanidinium group. This may dictate the final structure and function of the final protein (Starr and Taggart, 2006). As shown in diagram above, amino acids are linked through peptide bonds, which link the amino group of one amino acid to an acid group of another amino acid, to form a protein. As indicated earlier their sequence is predetermined by the DNA sequence of the gene coding for the given protein. One or many polypeptide chains folded into three dimensional shapes constitute a protein. The type (chemical properties) and sequence of amino acids in a polypeptide chain dictates its final shape. This is due to the fact that folding of the peptide chain or chains takes place in response to forces that repel or attract amino acids from each other or from water. In case of a single folded chain, the single chain may constitute a functional protein on its own or join other folded chains to form the final product. The final shape dictates a protein’s function. Detailed explanation Understanding the role played by amino acids in protein structure and function requires a thorough understanding of the characteristics of most important amino acids, how they form proteins, hence their role in the structure and function of proteins. Knowledge of transformation of simple polypeptides to functional proteins is also important. All amino acids are composed of a hydrogen atom bound to a carbon atom, an acid group, an amino group, which contains the nitrogen and a side chain. It is the side chain of the amino acids that gives them their inimitable characteristics or properties. The size and structure of the side chain vary from one amino acid to another. For instance, Glycine, the simplest amino acid, has a single hydrogen atom as its side chain. Other amino acids have relatively complex side chains; some hydrophilic and polar like Serine and Cysteine, while others hydrophobic and non-polar like Leucine. Polypeptides or proteins with predominantly hydrophilic amino acids, that is, amino acids with hydrophilic side chains, readily dissolve in aqueous solutions in living cells when compared to those with hydrophobic amino acids(Clark and Pazdernik, 2009). The number, sequence and combination of the different kinds of amino acids in a given protein give it a unique structure and function in living organisms. The arrangement and type amino acids determine a protein’s primary structure, which in turn determines its secondary structure. Once amino acids have been linked together, through peptide bonds, to form a polypeptide chain, hydrogen bonding and other forces between various parts of the chain cause the polypeptide chain or some of its regions to either fold into a bete-pleated structure or coil into an alpha-helix. These changes constitute the protein’s secondary structure. This secondary shape is sustained by regularly spaced hydrogen bonds each established between the –C=O group of one amino acid and the –N=H group of another amino acid in the various regions of the polypeptide chain. The secondary structure dictates the protein’s tertiary structure, which in turn determines its quaternary structure (Starr and Taggart, 2006). The four levels of a protein’s structure determine its three dimensional shape. The specific three-dimensional shape of a protein molecule dictates its function; every twist, indentation, coil and bump is vital. This three-dimensional shape is established and sustained by a variety of bonds, including disulphide bonds, ionic bonds, hydrophobic interactions and hydrogen bonds. A protein’s tertiary structure is dependent on its primary structure due to the fact that the bonds holding the tertiary structure together cannot be established without the precise arrangement of amino acids along the polypeptide chain. This means that for any protein to be able to carry out its given function, it must have the correct number and type of amino acids arranged in a specific order (Starr and Taggart, 2006). References Clark, D. P., & Pazdernik, N. J. (2009). Biotechnology: Applying the Genetic Revolution. London: Elsevier. Starr, C., & Taggart, R. (2006). Biology: The Unity And Diversity of Life, 11th Ed. Belmont, CA: Cengage Learning. Starr, C., Evers, C., & Starr, L. (2009). Biology Today and Tomorrow Without Physiology, 3rd . Belmont, CA: Cengage Learning. Read More