From Gene to Protein
Gene Expression Process by which DNA directs the synthesis of a protein 2 stages transcription translation All organisms One gene one protein
1. Transcription of DNA Gene Composed of DNA 1000s of base pairs long Sequence of G, A, T, C is a code ACATACATGTAGCAACGAAACCTGAACATGCGAACATTGACACAACAACAAUGUAGCCA A portion of the insulin gene
A gene is transcribed into mrna One DNA strand is a template Follow base complementation using G, A, U, C Which strand is the template strand?
This mrna is referred to as a transcript Single stranded Uracil nucleotide (not thymine) ONLY gene of interest is transcribed mrna will leave the nucleus for translation step
mrna (transcript) leaves nucleus Translated to protein in cytoplasm What organelle translates mrna to protein? How many amino acids? How many amino acids long is a protein? How could 10 identical protein molecules be translated from 1 mrna? What happens to mrna when the cell has finished with it? Can the cell transcribe more than one gene at a time?
2. Translation of mrna Nucleotides (G, A, U, C) translated to amino acids Ribosomes assemble amino acids to form polypeptide What is the difference between a polypeptide and a protein? What is the length difference between a chromosome and an mrna transcript? Why do researchers use frog embryos to study translation?
Differences between prokaryotes and Proks No nucleus eukaryotes Translation can begin while transcription is still in progress
Euks Nuclear envelope Transcript is modified before leaving nucleus Primary transcript mrna mrna Translation in cytoplasm
Central Dogma DNA mrna protein (F. Crick 1956)
Triplet code 4 3 = The genetic code # combinations of 3 using G, A, T, C Can code for all 20 aa Why not a doublet code? Start list of the 64. Each triplet codes for a certain amino acid
Codon table (1960 s Nirenberg and Khorana) 3 stop codons 1 start codon Redundant Nearly universal
Which strand of DNA is the template? Identify the codons (triplets) Code is non-overlapping Compare the mrna to the non-template strand How many codons are needed for a 20 amino acid long protein? How many nucleotides?
The reading frame Amino acids? AUG UGG GUU GGC UCA met trp- phe-gly-ser
Transcription in more detail 1. Initiation of transcription A. RNA polymerase binds to template strand of DNA upstream ----------------------------------------------downstream
The promoter is upstream from the start site for transcription
B. Transcription factors (proteins) are required for RNA pol to bind
C. mrna begins to be synthesized (RNA pol does not need a primer)
2. Elongation of transcript RNA pol synthesizes 5 3 More than one transcript made
3. Termination RNA pol detaches
Translation summary
Translation in more detail Cytoplasm The players: Transfer RNA trna Ribosomes mrna Amino acids
1. trna 2 o structure is a cloverleaf CONCEPT CHECK How many bases is a trna? How many bases is an anticodon? Where is the amino acid site with respect to the anticodon? Why does the trna look to be double stranded? What does the t mean in trna?
A charged trna carries an amino acid Contains an anticodon Complementary to a codon Note the CCA
2. Ribosome rrna + protein Large and small subunits
3 binding sites on ribosome P site holds trna A site for trna with next amino acid E site allows trna to leave Note: large and small subunits
Ribosomes adds each aa from trna to growing polypeptide chain up to 100,000s per cell Activity overview of protein synthesis
Stages of translation 1. Initiation Ribosome scans mrna for AUG start trna brings met
2. Elongation Amino acids added to chain Peptide bonds between amino acids
3. Termination stop codon Polypeptide released
Polyribosomes One mrna translated by many ribosomes
Wobble hypothesis How many sense codons? Stop codons? BUT only 45 trnas Some trnas recognize more than one codon
3 rd position in mrna codon wobbles Us and Cs can be matched with G in anticodon Rules of base pairing relaxed in the 3 rd position
After translation ER Golgi Post translational modification