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Protein Synthesis Protein Synthesis The instructions contained in the DNA must be copied into mRNA. This is transcription. Transcription takes place in the nucleolus. mRNA copies are then transported to the ribosomes for translation or protein synthesis. Ribosomes are in the cytoplasm. 2 During transcription, the portion of DNA that is to be copied unzips: hydrogen bonds are broken by a helicase enzyme exposes the bases of the complementary DNA strands Only one of the strands is copied – the transcribing strand. Activated RNA nucleotides line up opposite their exposed complimentary bases: U with A A with T G with C C with G 3 The adjacent nucleotides are then joined together by a RNA polymerase, which: uses the energy released when the two extra phosphate groups are removed from the activated nucleotides Note: the mRNA has a base sequence that is complementary to that of the transcribing strand the triplets of bases that code for an amino acid in the mRNA are called codons 4 1. Helicase enzyme breaks the hydrogen bonds 3. The nontranscribing strand of DNA 4. Polymerase enzyme bonds the nucleotides 2. The transcribing strand acts as the template A T G G T A C A T C T T A C T A U G G U A C A U C U U A C U T A C C A T G T A G A A T G A 6. mRNA leaves the nucleus via the nuclear pore 5. Complementary nucleotides line up (AT, CG, UA, GC) Note: once produced, the mRNA molecule is released and leaves the nucleus via the nuclear pores. It travels to the ribosome for translation, i.e. reading5 of the message and production of protein. As described, the process of transcription has similarities to DNA replication: The DNA molecule unwinds and unzips. It acts as the template for synthesis of the RNA and complementary base pairing ensures that the message is transcribed accurately. 6 Q. How is this different from DNA replication? You may find it helpful to consider: exposure of the DNA to act as a template; which strands are copied; the nucleotides involved and any differences in their composition, e.g. pentose sugar involved; complementary pairing; other features, e.g. nature of the resulting polynucleotide. Only one strand, the transcribing strand, is copied. DNA nucleotides are composed of deoxyribose, phosphate and adenine, guanine, thymine or cytosine. RNA nucleotides are composed of ribose, phosphate and adenine, guanine, uracil or cytosine. DNA is double stranded whilst RNA is single stranded. 7 Translation Translation takes place in the cytoplasm at the ribosomes. Ribosomes are made in the nucleolus from protein and ribosomal RNA (rRNA). There are two subunits: a large subunit and a small subunit with a groove between them in which the mRNA fits 8 tRNA Transfer RNA (tRNA) transports amino acids to the ribosome for incorporation into protein. Each tRNA is a single polynucleotide that is folded into a clover-leaf shape. Hydrogen bonds between complementary bases stabilise the shape. tRNA has: an anticodon that is complementary to the codon on the mRNA three unpaired bases to which a specific amino acid can be attached This ensures that the right amino acid gets to the right place. 9 U A C U A C Anticodon. The bases are complementary to those on the mRNA. Three bases for the attachment of a specific amino acid. 10 During translation, mRNA enters the groove between the small and large subunit. The ribosome is large enough for two codons to be side by side. The very first codon is always the same: initiation or start codon AUG codes for methionine The anticodon of a tRNA carrying methionine hydrogen bonds to the first codon. 11 A second tRNA, carrying the second amino acid coded for, hydrogen bonds to the second codon. A peptide bond is formed between the two amino acids and the ribosome moves three bases (one codon) along the mRNA so that another codon is exposed. A third tRNA brings another amino acid which is added to the dipeptide and so the process: continues until a termination or stop codon (UAA, UAG or UGA) is reached Add descriptions/explanations to the diagrams showing translation. 12 A U G U A C G U A C A U C A U C U U A C U G A A U G A G U A Leu Met Val His Thr • mRNA lies in the groove of the ribosome, which has places for two codons (triplets of bases) coding for specific amino acids. • The start codon always encodes for methionine. • In the cytoplasm, tRNA transferases load tRNA with its amino acid. • tRNA (complementary anticodon) aligns itself with its codon.13 A U G U A C Met G U A C A U Val C A U G U His C U U A G A Leu A C U A U G A Thr • A second tRNA binds to the next codon. • Again, complementary base pairing ensures that only the correct tRNA binds. 14 A U G U A C Met G U A C A U Val C A C U G U His U U A G A Leu A C U A U G A Thr • The two amino acids are held very close to each other. • A peptide bond is produced between the carboxylic acid group and amino group of the two amino acids. • Condensation reaction occurs – removal of water. • Ribosome has enzyme that does this. 15 A U A U C G G U C A Val Met A U C A U G U A His C U G U A Leu A A U C U G A Thr • Ribosome moves along, exposing the next codon on the mRNA. • Complementary base pairing between the codon and anticodon of the next tRNA happens. • Peptide bond is formed between, in this case, valine and histidine. 16 A U A U G C C G U A U A C A U G U A C G U A U A A C U U Met Val His G A Leu Thr • Ribosome moves along, exposing the next codon on the mRNA. • Complementary base pairing occurs between the newly exposed codon and the anticodon. • Process continues until a stop codon, e.g. UGA is reached. • No tRNA for this, so the protein and mRNA are released. 17 The protein may need further processing before it is active: insulin requires considerable processing first some enzymes require activation by cyclic AMP See activation of enzymes. 18 The process of transcription and translation are summarised on the next slide. Key elements: unzipping of DNA by helicase complementary mRNA synthesised using an RNA polymerase mRNA leaves nucleus and goes to ribosome spaces for two codons complementary base pairing between codon and anticodon of tRNA peptide bond inserted between amino acids peptide grows by one amino acid at a time, until stop codon reached 19 Single-stranded messenger RNA leaves nucleus and travels to the ribosome for translation A T G G T A C A T C T T A C T A U G G U A C A U C U U A C U T A C C A T G T A G A A T G A A U G GU A C A U C U U A C U UA C CA U GU AG A A U GA Met Val His Leu Thr A UG G U A C A U C U U A C U GU A G A A UA C CA U U GA Met Val His Amino acids transferred to ribosome by transfer RNA Leu Thr Complementary base pairing allows transcription and correct sequencing of amino acids at the ribosome 20