The Three T’s 1. Transcription 2. Translation 3. Termination Protein Synthesis The Three T’s 1. Transcription 2. Translation 3. Termination
Transcription in Eukaryotes ie: Animal cells
Protein Synthesis in Prokaryotes ie: single celled Bacteria
Where does All this Happen
Protein Synthesis Nucleic Acids (DNA & RNA) carry the hereditary information. This information is contained in codons. What are Codons? A codon is a set of three base pairs (A, T, C, G) that directs or codes for amino acids. (ex. GCU – Alanine)
How Many Amino acids? There are 20 essential amino acids, however they can be combined in any order, just like the four nucleotides. This permits the production of the many different proteins which let organisms grow and function.
Transcription This process occurs in the nucleus. A section of DNA called a gene is unwound and unzipped.
A RNA copy of one of the DNA strands is made A RNA copy of one of the DNA strands is made. This strand is made complimentary to the nucleotides on the the DNA Strand. DNA Bases: Adenine, Thymine, Cytosine, Guanine RNA Bases: Adenine, Uracil, Cytosine, Guanine
DNA serves as a template for the synthesis of RNA. A regulatory protein binds to the promoter sequence. An enzyme (RNA Polymerase) binds to the promoter. Together they open the DNA double helix. RNA Polymerase proceeds down one strand moving in the 3’ to 5’ direction, as it does it assembles a complementary strand of RNA. Each ribonucleotide is inserted into the growing RNA strand following the rules of base pairing. Transcription stops when the termination sequence is reached. The completed RNA copy is now called messenger RNA or mRNA and carries the coded message to the ribosomes in the cytoplasm.
DNA Strand 1: AGCTATCGAGCAT DNA Strand 2: TCGATAGCTCGTA RNA copy: AGCUAUCGAGCAU
Transcription RNA polymerase (enzyme) attaches to the promoter region of the DNA
Translation The small subunit of the ribosome binds to a site on the 5’ end of the start of the message strand The ribosome moves downstream (53’) until it encounters the start codon AUG At this time, the large subunit joins and the “initiator t-RNA” enters the scene After this, the tRNA binds to the p site on the ribosome The first amino acid that starts this sequence of peptide bonds is Methionine (one of the 20 a.a.’s) Eucaryotes: methionine Bacteria: f Met (modified)
T RNA
1. Ribosome attaches to mRNA and reads first codon. 2 1. Ribosome attaches to mRNA and reads first codon. 2. A transfer RNA molecule (tRNA for short) brings the correct amino acid to the ribosome & drops it off. ex: AUG (codes for methionine) tRNA brings methionine to ribosome.
3. The ribosome moves on to the next codon and another tRNA brings the next amino acid. 4. The amino acids in the growing chain are linked together by a peptide bond. The growing chain is called a polypeptide or protein molecule. 5. When the ribosome reaches a STOP codon the polypeptide is released and the mRNA falls off.
Elongation more specifically! As another tRNA arrives with its associated a.a. and binds to the next codon site, it becomes covalently linked to the incoming a.a. with a peptide bond The initiator tRNA is released from the P site The ribosome moves downstream (53’) The more recently-arrived tRNA w/ its peptide moves to P site and opens the A site for the arrival of a NEW tRNA and its corresponding a.a.
Termination End of the message is a STOP codon STOP codons: UAA, UAG, UGA Protein release factor sees these codons at the A site and tells the polypeptide chain to release from the ribosome site Ribosome splits into its subunits until it is time to make more protein!
Termination
Check out this link for an animation on Protein Synthesis http://www.lewport.wnyric.org/jwanamaker/animations/Protein%20Synthesis%20-%20long.html