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The Universal Dogma of Genetics Introduction • The question is how does it get from a fragment of DNA (gene) to……………PTC tasting? • How is the information interpreted and then put into action as a real physical thing? • The answer to these questions is: Everything is written in the instructions. • Contrary to most students taking Bio101 at SCCC (who do not seem to read the instructions to anything very often) cells have developed mechanisms to be able to read the instructions written in code using the nitrogen basis (A,C,G and T) of the DNA and translate them into…proteins (enzymes) How does it work? • For example. We have studied that the inability to digest lactose is due to the lack of the enzyme lactase. The equation is as follows Glucose + Galactose Enzyme Energy Lactose H2O • What is the key to this equation??? Yes! The lack of the enzyme. • What causes that an individual does not have an enzyme? The lack of information (instructions or the recipe) to make the enzyme or we may have switched of the reading mechanism. From Genotype to Phenotype • The DNA (sequence of nitrogen bases) makes the genotype (genetic make up). The DNA is expressed as proteins (enzymes), which provide the molecular basis for phenotypic traits • • The information constituting an organism's genotype is carried in its sequence of DNA bases A particular gene—a linear sequence of many nucleotides—specifies a particular polypeptide. DNA is life…the rest is just translation • In eukaryotic organisms, DNA is stored in the nucleus where it is transcribed into mRNA; a process called transcription (In the nucleus) • mRNA translates the information that was copied or transcribed from DNA into the language of proteins (amino acids). It takes place in the cytoplasm (or to be more precise, in the ribosomes… we’ll come back to this); a process called translation The Central Dogma • Basically, a gene dictates production of a specific polypeptide. This is known as the one gene-one polypeptide hypothesis (Beadle and Tatum, 1953). • Based on this concept the Universal Dogma of Genetics has been stated. It reads as follows DNA Nucleotides transcription mRNA Nucleotides translation Protein Amino Acids DNA Transcription of DNA into RNA RNA Nucleus Cytoplasm Translation of RNA into Protein Protein How does it happen? Transcription • A typical gene consists of hundreds or thousands of nucleotides in a specific sequence. • The sequence (and number) of these nucleotides determines the protein produced by this gene. • The Genetic information written in the DNA is transcribed (or copied) to mDNA ( from nucleotide to nucleotide replacing T’s with U’s) • DNA must first be re-written (transcribed) as a LE 10-7a DNA molecule Gene 1 Gene 2 Gene 3 DNA strand A A A C C G G C A A A A U U U G G C C G U U U U Transcription RNA Codon Translation Polypeptide Amino acid RNA nucleotides RNA polymerase Direction of transcription Newly made RNA Template strand of DNA Messenger RNA • The type of RNA that encodes amino acid sequences is called messenger RNA (mRNA) • In eukaryotic cells, mRNA leaves the nucleus where it had been transcribed and enters the cytoplasm • Before mRNA can leave the nucleus, it is modified – A ‘tail’ and ‘cap’ are added – Introns are removed A cap and tail are added to protect the mRNA strand, facilitate its transport out of the nucleus and to help ribosomes bind to it Exon Intron Exon Intron Exon DNA Cap RNA transcript with cap and tail Transcription Addition of cap and tail Introns removed Tail Introns are intervening sequences of DNA which do not code for amino acids; must be removed Exons spliced together mRNA Coding sequence Nucleus Cytoplasm Exons are the coding regions, parts of the gene which remain and are translated into amino acids How does it happen? Translation • Translation: The Genetic information in the DNA “coding strand” is copied into mRNA (replacing T with U). • The mRNA is then translated using the code into amino acid sequences. • Each 3 mRNA bases make a CODON. • Each codon codes for an AA. DNA strand Transcription RNA Codon Translation Polypeptide Amino acid Genetic information written in codons is translated into amino acid sequences • Translation then converts the nucleic acid ‘language’ into the polypeptide (protein) ‘language’ • The sequence of RNA nucleotides dictates the sequence of amino acids of the polypeptide being produced • Thus, the RNA molecule acts as a messanger carrying genetic information from DNA Second base C U A UAU UCU UUU Phe U UGU Cys Tyr UAC UCC UUC G C UGC Ser U UUA UCA UAA Stop UGA Stop A UUG UCG UAG Stop UGG Trp G CUU CCU CAU CUC CCC CAC Leu U CGU His Leu C CUA CGU Pro CCA C Arg CAA CGA A CGG G Gln CUG CCG CAG AUU ACU AAU AUC lle ACC U AGU Ser Asn AAC AGC C ACA AAA AGA A ACG AAG AGG G GCU GAU GGU U Thr A AUA AUG Met or start GUU Arg Lys Asp GUC G GCC Val GUA GAC GCA C GGC Gly Ala GAA GGA A GGG G Glu GUG GCG GAG Genetic information written in codons is translated into amino acid sequences • In order for translation to proceed, the sequence of the 4 nucleotides in RNA (A,U, C,G) must somehow specify the 20 amino acids used to make up proteins • The flow of information from gene to protein is based on a triplet code; genetic instructions for the amino acid sequences of a polypeptide chain are written in DNA and RNA as a series of 3-base ‘words’, called codons The Genetic Code • The genetic code is a set of instructions indicating which codons are translated into which amino acid • The genetic code does not only specify which codons code for which amino acids, but also specify ‘start’ and ‘stop’ signals, which begin and end protein synthesis, respectively • For each of the 20 amino acids, there are 2-4 codons which code exclusively for them Third base First base Second base The Genetic Code • The genetic code is nearly universal; humans cells can translate bacterial RNA and vice versa Transfer RNA • In order to convert the 3-letter codons of nucleic acids into a single amino acid, a cell must employ a molecular interpreter, transfer RNA (tRNA) • tRNA recognizes the codons in the mRNA molecule and picks out the appropriate amino acids for incorporation into the growing polypeptide Transfer RNA • tRNA recognizes codons from mRNA via a special triplet of bases called an anticodon, which is complimentary to the codon on the mRNA • When the codon of mRNA complements the anticodon of tRNA, the appropriate amino acid is laid down at the other end of the tRNA molecule Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon Translation • Translation begins with a ‘start’ codon, and ends with a ‘stop’ codon • The amino acid methionine (Met) is always translated by the start codon (AUG) • What would the anticodon look like? • Stop codons (UAA, UAG, and UGA) do not code for amino acids but instead act only as signals to end translation DNA strand Transcription RNA Codon Translation Polypeptide Amino acid Review • Describe the differences between mRNA, tRNA and rRNA • What bases are found in DNA? In RNA? • Which molecule has codons? Anticodons? • What is transcription? Translation? Which happens first and where does each occur in the cell?