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PROTEIN SYNTHESIS -Transcription & Translation – ‘Patterns of Life’ pg 121-133 Introduction • DNA contains the code to make protein • DNA is in the nucleus • Protein is made in the ribosome • The information must get from the nucleus to the ribosome - mRNA Nuclear envelope • Continuous with pores • rough endoplasmic reticulum and ribosomes. Key Pieces of Equipment • DNA • RNA – 3 types » Messenger » Transfer » Ribosomal (mRNA) (tRNA) (rRNA) • RNA polymerase (protein) RNA vs DNA • • • • Single stranded Thyamine (T) is replaced by Uracil(U) A lot smaller No oxygen molecule RNA • Linear polymer • Bases: Purines (adenine & guanine) Pyrimidines (cytosine & uracil) • Most are single stranded, therefore does not form a double helix. Although can fold and twist into itself. Messenger RNA • Codon a sequence of three nucleotides that code for a single amino acid • mRNA carries the code from the nucleus to the ribosome Transfer RNA • Transfer RNA (tRNA): decodes the information • tRNA has an anti-codon which matches a specific codon of mRNA • Each tRNA attaches to a specific amino acid that compliments its anti-codon • There are 20 different tRNA types (one for each type of amino acid) Ribosomal RNA • Ribosomal RNA (rRNA): makes up approximately 50% of a ribosome. • The assembly place of protein synthesis. Ribosome • 2 subunits – heavy and light The Genetic Code • 20 different amino acids used to produce proteins. • The code for a specific amino acid is controlled by the 4 different nucleotides (T,C,A,G) in the DNA. • This code goes in sequences of three (Triplets) • A triplet codes for a single amino acid • WHY? • These triplets produce codons in mRNA • A codon codes for a specific amino acid • Some combinations act as STOP (UAA, UGA, UAG) and START (AUG) signals. • One codon is the start codon - AUG Genetic Code cont. Protein Synthesis-the process • Two parts to protein synthesis: – Translation of mRNA from DNA » Occors in the nucleus – Transcription of mRNA into a polypeptide chain » Occurs at the ribosome Transcription 1. One section of DNA unwinds. One side acts as a template for the mRNA to be synthesised. • The RNA polymerase reads the DNA from 3’ end of the DNA. • RNA polymerase then binds complementary bases to form the mRNA. • Codes on the DNA act as punctuation, indicating beginning and end of protein and transcription. • Once termination sequence reached, transcription ceases. The mRNA is then completed and it travels out of the nucleus to the ribosome Translation • mRNA becomes bound to a ribosome. • The mRNA’s strand is placed in the ‘P’ site (the first position on a ribosome) • Next to the ‘P’ site is the ‘A’ site. • Both sites hold one tRNA molecule. • Once the start codon is placed into the ‘P’ site, translation occurs. • Next a tRNA fits into the ‘P’ site based upon the codon on the mRNA lined up on the ‘P’ site. • Another tRNA fits into the ‘A’ site also based on the codon on the mRNA at the ‘A’ site. • When both sites have a tRNA present, the amino acids on the tRNA bond together to form the first link of a protein chain. • i. ii. Once the two amino acids have linked together: The tRNA in the P site is released The mRNA moves codon so the tRNA in the ‘A’ site is shifted into the ‘P’ site. iii. iv. v. The ‘A’ site is now clear so a new tRNA molecule moves in corresponding to the mRNA This amino acid links to the protein chain. This process continues until a stop codon is read at the ‘A’ site. Termination • Once a stop codon is read at the ‘A’ site, the ribosome has finished synthesising the protein. • A new tRNA molecule is not added to the protein chain. • The two ribosome units separate. • Protein chain released from ribosome. Protein synthesis animation • http://highered.mcgrawhill.com/sites/0072437316/student_view0/ chapter15/animations.html#