Protein Synthesis - Translation (PDF)

1/7/25 Protein Synthesis Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information TRANSCRIPTION DNA mRNA Ribosome...

1/7/25 Protein Synthesis Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information TRANSCRIPTION DNA mRNA Ribosome TRANSLATION (a) Prokaryotic cell. In a cell lacking a nucleus, mRNA Polypeptide produced by transcription is immediately translated without additional processing. Nuclear envelope TRANSCRIPTION DNA RNA PROCESSING Pre-mRNA mRNA Ribosome TRANSLATION (b) Eukaryotic cell. The nucleus provides a separate Polypeptide compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. 1  1/7/25 Protein Synthesis Prokaryotes Eukaryotes – no nucleus to separate - transcription occurs in transcription and nucleus and translation translation in cytoplasm – Naked DNA - Nucleosome/ chromatin – translation can begin - pre-mRNA produced, immediately while then processed to mRNA is being produce mature mRNA transcribed - Mature mRNA exits nucleus into the cytoplasm to be translated. ANIMATIONS: TRANSLATION: https://www.youtube.com/watch?v=VZP1wo37AX8 2  1/7/25 PROTEIN BIOSYNTHESIS - TRANSLATION Central Dogma of Molecular Biology Transcription Translation Translation requires a genetic code through which the information contained in the nucleic acid sequence is expressed in the form of specific amino acid sequence. Genetic code Genetic code is a dictionary that gives the correspondence between sequence of bases & sequence of amino acids. Codons Individual words in the code (codons) are each composed of three nucleotide bases. Codons are found in the messenger RNA. The sequence of a codon is always read from its 5’end to its 3’ end. Since there are four nucleotide bases (adenine, guanine, cytosine & uracil) to produce a three-base codon, therefore, 43= 64 different combinations of bases. 3  1/7/25 Genetic code Protein Synthesis: The Genetic Code (on RNA) 4  1/7/25 Protein Synthesis: The Genetic Code (on DNA) Genetic code Genetic Code dictionary is useful in determining which amino acid is coded by which codon of mRNA. e.g. codon 5’- CAU-3’ codes for His whereas 5’-AUG-3’ codes for Met. AUG – start codon (also code for amino acid methionine). Directs the ribosome where to start synthesizing a polypeptide. Three codons (UAG, UGA & UAA) do not code for any amino acids but rather are termination codons (signalling completion of protein chain synthesis). 5  1/7/25 Characteristic Genetic code Specific: A specific codon always codes for a single amino acid. Universal: Specificity of the genetic code (with few exceptions) has been conserved from very early stages of evolution. Degenerate (Redundant): Although each triplet corresponds to a single amino acid, a given amino acid may have more than one triplet coding for it. e.g. Arg is specified by six different codons. Characteristic Genetic code Non-overlapping & Commaless: The code is read from a fixed starting point as a continuous sequence of bases, taken three at a time. e.g. ABCDEFGHIJKL…… is read as ABC/ DEF/ GHI/ JKL …. without any “punctuation” between the codons. If one or two nucleotides are either deleted or added to the interior of a message sequence, the reading frame will be altered & the amino acid sequence will be different from that point. 6  1/7/25 Characteristic Genetic code −−UCA UCC UAU GGC U−− Ser TyrSer Gly………… Ser Ser Tyr Gly Addition of ‘U’ 5’−−UCACCUAUGGCU−−3’ Ser Pro Met Ala Deletion of ‘C’ −−UCA CUA UGG CU−− Ser Leu Trp Frame shift mutation can alter reading frame of mRNA PROKARYOTES 7  1/7/25 EUKARYOTES PROTEIN BIOSYNTHESIS - TRANSLATION Components required for a Translational System 1. Amino acid 2. tRNA 3. mRNA 4. Aminoacyl tRNA synthetases 5. Ribosomes 6. Protein factors 7. ATP & GTP 8  1/7/25 PROTEIN BIOSYNTHESIS - TRANSLATION Involved all 3 types of RNAs: – messenger RNA (mRNA) – ribosomal RNA (rRNA) – transfer RNA (tRNA) PROTEIN BIOSYNTHESIS - TRANSLATION Types of RNAs :- i. Messenger RNA (mRNA) Ø single-stranded molecule Ø carries genetic information transcribed from DNA in the nucleus into the cytoplasm Ø attaches to ribosome which translates the triplet codes into a sequence of amino acids in a polypeptide chain 9  1/7/25 5’ UTR 3’ UTR PROTEIN BIOSYNTHESIS - TRANSLATION ii. Ribosomal RNA (rRNA) Ø Ribosomes contains rRNA and proteins Ø Consist of 2 subunits - size according to sedimentation unit Svedberg (S). Eukaryotes – 80S (60S and 40S) Prokaryotes – 70S (50S and 30S) Ø Provide attachment sites for mRNA and tRNA interaction during protein synthesis. 10