Topic 4 - Transcription + Translation 1f070431784446d094efb63dbcef01b1.pdf

🍗 Topic 4 - Transcription + Translation TRANSCRIPTION Nucleic Acid Structure Pyrimidines vs Purines Topic 4 - Transcription + Translation 1  Deoxyribose vs Ribose...

🍗 Topic 4 - Transcription + Translation TRANSCRIPTION Nucleic Acid Structure Pyrimidines vs Purines Topic 4 - Transcription + Translation 1  Deoxyribose vs Ribose Overall concept of Transcription ▪ Transcription = RNA Synthesis ▪catalysed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides ▪ RNA polymerase does not need any primer ▪ same base-pairing rules as DNA, except that uracil substitutes for thymine Define “Promoter, Terminator, Transcription Unit” Topic 4 - Transcription + Translation 2  Explain the Initiation stage of Transcription RNA polymerase binds to the promoter, opens up DNA strand TATA Box is sequence polymerase binds to RNA transcript is complimentary to DNA, formed from 5’ to 3’ Explain the Elongation stage of Transcription RNA Polymerase untwists double helix Transcription occurs at a rate of 40 nucleotides per second in eukaryotes DNA is read 3’ to 5’, RNA transcript formed is 5’ to 3’ Topic 4 - Transcription + Translation 3  Explain the termination stage of transcription (particularly Eukaryotes) In bacteria, polymerase stops transcription at the terminator In eukaryotes, RNA Modification occurs → 5’ cap, 3’ poly A tail In Eukaryotes → RNA P II transcribes polyadenylation (poly A) sequence Polyadenylation = poly A tail of RNA transcript that only had adenine bases on the 3’ end - increases stability of molecule 5’ end of mRNA receives a modified nucleotide (5; cap) what is the importance of RNA modification pre-mRNA recieves a 5’ cap, 3’ end gets a poly-A tail Modifications are important for: Protect mRNA from hydrolytic enzymes Help riboosme attach to the 5’ end Facilitate export If cap + tail not present → RNA hydrolysed, loses function RNA Polymerase I Ribosomal RNA (rRNA) are synthesises by RNA P1 In eukaryotes, rRNA has 4 components: 28S, 5.8S, 5S, 18S Large Subunit = 28s, 5.8s, 5s Small Subunit = 18s In Prokaryotes, rRNA has 3 components 5s, 23s, 16s Topic 4 - Transcription + Translation 4  Large Subunit = 5s, 23s Small Subunit - 16s RNA polymerase II synthesises mRNA synthesises small nucleolar RNA (snRNA) = rRNA, tRNA synthesises micro RNAs (miRNA) - small non coding RNA involved in RNA silencing + post transcriptional regulation of genes What 2 things do RNA Polymerase III synthesise Synthesises tRNA’s Synthesises 5s rRNA RNA Splicing Introns - non-coding regions Exons - coding regions RNA Splicing removes introns by splicing by spliceosomes enzyme at splice sites joining exons by dna ligase Bacterial cells have no introns - no splicing required if introns are included → alternative splicing → different protein formed Ribozymes Topic 4 - Transcription + Translation 5  Ribozymes are enzyme that can splice RNA TRANSLATION Codons Genetic information from gene to protein is based on a triplet code (three non- overlapping nucleotides) Triplet code is transcribed onto mRNA where it is then read and translated to amino acids, forming a polypeptide translation codons are read in the 5' to 3' direction Genetic code is redundant (more than one codon may specify a particular amino acid) tRNA structure Transfer RNA (tRNA) is used to help translate mRNA tRNA transfer amino acids to the growing polypeptide in a ribosome Each tRNA has an anticodon which base-pairs with a complementary codon on mRNA tRNA is roughly L-shaped with the 5' and 3' ends both located near one end of the structure - 3' end acts as an attachment site for an amino acid Topic 4 - Transcription + Translation 6  How is there an accurate match between tRNA + anticodon Accurate translation has two steps: Correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase Correct match between the tRNA anticodon and an mRNA codon Ribosome structure Topic 4 - Transcription + Translation 7  Initiation of translation The start codon (AUG) signals the start of translation ▪ First, a small ribosomal subunit binds with mRNA and an initiator tRNA (carrying methionine) ▪ the small subunit moves along the mRNA until it reaches the start codon ▪ initiation factors proteins bring in the large subunit that completes the “translation initiation complex” Topic 4 - Transcription + Translation 8  Elongation of the polypeptide chain During elongation, amino acids are added one by one to the C-terminus of the growing chain (C-terminus has an exposed carboxyl group) ▪ Each addition involves proteins called elongation factors ▪ Elongation occurs in three steps: codon recognition, peptide bond formation, and translocation Codon recognition - tRNA is positioned on the A site of ribosome Topic 4 - Transcription + Translation 9  Peptide bond formation between adjacent amino acids in polypeptide chains. the polypeptide chain shifts from the P site to the amino acid on the A site, which requires energy from GTP Translocation - shifting of mRNA by one codon along the ribosome, energy from GTP allows tRNA in the A position to shift to the P position, and the tRNA in the P position to exit through the E site. Termination Elongation continues until a stop codon in the mRNA reaches the A site of the ribosome ▪ The A site accepts a protein called a release factor, which causes the addition of a water molecule (hydrolysis) instead of an amino acid ▪ This reaction releases the polypeptide, and the ribosomal subunits dissociate. Post translational modifications What do free ribosomes synthesise. Topic 4 - Transcription + Translation 10  mostly synthesize proteins that function in the cytosol what do bound ribosomes (attached to the ER) synthesise. Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell Targeting Polypeptides to Specific Locations. Polypeptides destined for the ER or for secretion are marked by a signal peptide ▪ A signal-recognition particle (SRP) binds to the signal peptide ▪ The SRP escorts the ribosome to a receptor protein built into the ER membrane What are polyribosome/polysome Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) ▪ Polyribosomes enable a cell to make many copies of a polypeptide quickly Topic 4 - Transcription + Translation 11