Transcription and Translation (Gene Expression) PDF

TRANSCRIPTION AND TRANSLATION  Central Dogma of Molecular Biology DNA mRNA Protein DNA DNA – Replication – nucleus DNA mRNA – Transcription – Nucleus mRNA Protein – Translation – Ribosome in the cytoplasm  DNA carries information that can be u...

TRANSCRIPTION AND TRANSLATION  Central Dogma of Molecular Biology DNA mRNA Protein DNA DNA – Replication – nucleus DNA mRNA – Transcription – Nucleus mRNA Protein – Translation – Ribosome in the cytoplasm  DNA carries information that can be used to construct the proteins which form structures and regulate the body’s activities.  Protein synthesis involves two processes: transcription and translation.  In transcription the DNA message is converted into an RNA molecule.  In translation the RNA message is used to assemble amino acids into a protein chain. 3 Central Dogma of Molecular Biology  DNA vs. RNA DNA RNA Nucleus only Nucleus and cytoplasm Double Stranded (ribosome) ATCG Single Stranded Deoxyribose Sugar AUCG Ribose sugar 3 main types mRNA - messenger tRNA - transfer rRNA - ribosomal RNA vs. DNA  First, let’s learn about RNA  RNA is also a nucleic acid, called ribonucleic acid  It only has one strand (DNA has two)  It contains the sugar ribose instead of deoxyribose  It has the nitrogenous base URACIL (U) instead of thymine (T) 7  There are THREE types of RNA  Messenger RNA (mRNA)  Long strands of RNA nucleotides that are formed complementary to one strand of DNA  Ribosomal RNA (rRNA)  Associates with proteins to form ribosomes in the cytoplasm  Transfer RNA (tRNA)  Smaller segments of RNA nucleotides that transport amino acids to the ribosome where proteins are made by adding 1 amino acid at a time 8 Messenger, ribosomal, transfer RNA 9  Triplet Code: Codons Universal code: Strong evidence for evolution 3 Nucleotides = 1 amino acid Codon Chart pg. 273 10  Transcription DNA  mRNA Copying of DNA’s message to mRNA Occurs in the nucleus Pre-mRNA is processed into mRNA and then leaves the nucleus for the cytoplasm (ribosome)  TRANSCRIPTION (the first step in protein synthesis)  Through transcription, the DNA code is transferred to mRNA in the nucleus.  DNA is unzipped in the nucleus and RNA polymerase binds to a specific section where a mRNA will be synthesized 12  Detailed Transcription Initiation: RNA polymerase binds to DNA at promoter region. – Promoter is before the gene that is to be transcribed (eukaryotes need transcription factors to help RNA polymerase bind to the TATA box) – Determines which strand of DNA to use Elongation - adds nucleotides to mRNA strand based on DNA strand in a 5’ 3’ direction (adding only to the 3’ end). Termination – RNA polymerase “falls off” the DNA strand when the termination sequence (terminator) is reached. – AAUAAA in eukaryotes, this is now pre-mRNA  Transcription Transcription Initiation Eukaryotes Transcription Process  Transcription does not happen all the time  Operon – the “switch” to turn on/off transcription  Promoter – DNA site that promotes RNA polymerase to bind  Repressor – molecule that binds to DNA to block transcription  Inducer – molecule that takes repressor away 15  RNA Processing Pre-mRNA RNA 5’ cap – guanine and phosphate cap on the 5’ end of mRNA 3’ poly-A tail – 50 to 250 Adenines are added to the 3’ end of the mRNA Both the 5’ cap and 3’ poly-A tail facilitate the export of mRNA from the nucleus Both protect the mRNA from degradation by hydrolytic enzymes in the cytoplasm Both help ribosomes attach to the 5’ ends of the mRNA strand RNA processing  Alternative RNA splicing Introns – non-coding sections of mRNA – Don’t leave the nucleus – only on pre-mRNA Exons – coding sections (expressed) of mRNA – Exit the nucleus – final mRNA Keeps the exons  gets rid of the introns snRNPs – cut the introns – Ribozyme – RNA that functions like an enzyme Spliceosomes – join remaining exons together to form final mRNA  Cleaning up the Message  When the genetic message is copied to make mRNA, the message contains unwanted base sequences.  The ‘junk’ sequences (called introns) are removed from the message and the remaining sequences (exons) are linked together to produce a sequence of codons that will translate into a polypeptide.  This process occurs before the message leaves the nucleus. 19  Changing nucleic acids into amino acids  The three-base code in DNA or mRNA is called a codon.  They are always coded in threes  Each triplet code corresponds with one amino acid  This is where TRANSLATION begins 21  TRANSLATION (the last step in protein synthesis)  Translation begins when mRNA binds to the RIBOSOME in the cell.  In translation, tRNA molecules act as the interpreters of the mRNA codon sequence.  At the middle of the folded strand, there is a three-base coding sequence in the tRNA called the anticodon.  Each anticodon is complementary to a codon on the mRNA. 22  Translation mRNA  protein Process of mRNA converting to a protein Occurs in the cytoplasm – ribosome 23  tRNA Translator of mRNA’s message is tRNA – transfer RNA – 80 nucleotides long – Hairpin shape – L shaped One end contains an anticodon which pairs with the codon on the mRNA – Codons determine which amino acid is coded for by the DNA The other end contains an amino acid attachment site – Aminoacyl-tRNA synthetase attaches the correct amino acid to the tRNA 24 tRNA 25  The Role of Ribosomes  The third type of RNA is ribosomal RNA (rRNA). Ribosomes are made of RNA and PROTEIN.  Ribosomes are the ‘decoding’ units of the cell. (Sites of protein synthesis)  Ribosomes consist of two major components — the small ribosomal subunit which reads the RNA, and the large subunit which joins amino acids to form a polypeptide chain.  Ribosomes have binding sites for both tRNA and mRNA molecules. 26  Ribosomes Pair codons on mRNA with anticodons on tRNA to form polypeptides Made of large and small subunits – rRNA – ribosomal RNA – Made in the nucleolus Contain multiple binding sites – mRNA binding site – P site – peptidyl – tRNA site – A site – aminoacyl – tRNA site – E site – exit site 27 Ribosomes 28  Making a protein Initiation – Small subunit binds to mRNA – Start codon AUG – methionine at P site Elongation – A site recognizes codon and pairs with correct tRNA – Peptide bond forms between the carboxyl end of the polypeptide at the P site and amino acid at the A site – Amino acid in the A site translocates to the P site Termination – Stop codon is reached at the A site UAA, UAG, UGA – Release factors free the polypeptide from the ribosome 29  Making a Protein TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Amino Polypeptide acids tRNA with amino acid Ribosome attached Gly tRNA Anticodon A A A U G G U U U G G C 5 Codons 3 mRNA Figure 17.13  Proteins Fold spontaneously into primary, secondary, and tertiary structures. Chaperone proteins assist in folding. Some polypeptides become quaternary with multiple subunits Signal peptide – directs proteins through the endomembrane system 31 Targeting Polypeptides to locations 32  Prokaryotes vs. Eukaryotes Eukaryotes Prokaryotes Locations: No nucleus – Free cytosol Ribosomes: Stay in the – Tanscription and cell somewhere (free floating, translation same location mitochondria, etc) – Rough ER Ribosomes: SRP sends Smaller ribosomes ribosome to go to ER, becomes protein in endomembrane system, or cell membrane protein or is secreted from cell (all in vesicles) Nucleus – Pre-mRNA  mRNA – transcription Larger ribosomes 33 – translation  Mutations Point Mutations – one base altered – Base-pair substitution Silent mutation – no effect Missense mutation – changes an amino acid Nonsense mutation – creates a stop codon – Insertion – extra base – Deletion – removal of a base Frameshift mutations – nonfunctional 34 proteins