RNA and Protein Synthesis

RNA Structure n Like DNA- Nucleic acid- composed of a long chain of nucleotides (5-carbon sugar + phosphate group + 4 different nitrogenous bases) n 3 Main differences between DNA/RNA: –RNA sugar is Ribose (vs. Deoxyribose) –RNA is single stranded (vs. Double strand) –RNA contains Uracil (vs. Thymine)

RNA Structure n BASICALLY: Each RNA molecule is a working copy of a single gene; A single DNA sequence is copied to form a RNA sequence n THEN, the RNA sequence is used for ONE JOB: PROTEIN SYNTHESIS n Assembly of amino acids into proteins controlled by RNA

RNA Sequence n 3 types of RNA: –Messenger RNA (mRNA) –Transfer RNA (tRNA) –Ribosomal RNA (rRNA) –Messenger RNA = Carry copies of the “instructions” or “messages” to assemble amino acids into proteins

RNA Sequence n mRNA carries the “message” from the DNA (found in the nucleus) to the Ribosomes (within the cytoplasm) for protein synthesis to occur n Ribosomes are composed of several proteins along with a form of RNA called rRNA n tRNA = transfers amino acids to ribosomes

Transcription n Transcription- the process involved in copying DNA to produce an RNA “transcript;” In other words, the nucleotide sequence of DNA is copied to produce a complementary RNA sequence n RNA polymerase- binds to the DNA sequence, “reads” the template to produce the RNA “transcript”

Transcription n How does RNA polymerase “know” where to start and stop making an RNA copy of the DNA sequence? n RNA polymerase binds to PROMOTER regions of DNA (signals on DNA to indicate where the enzyme should bind to produce the RNA transcript) n Similar sequences show where enzyme should stop copying DNA sequence

RNA Editing n RNA sequences need to be “edited” or “fine tuned” prior to its readiness during protein synthesis n rRNA molecules are produced from larger RNA molecules that are cut and trimmed to their final sizes n Introns- pieces of RNA sequence removed during the editing process

RNA Editing n Exons- remaining portion of RNA sequences are “expressed sequences”; exons are spliced together to produce final RNA sequence n Purpose? Some RNA molecules are cut and spliced in different ways in different tissues making it possible for single gene to produce several different RNA forms

The Genetic Code n Proteins are produced from the linkage of amino acids together n Together these strings of amino acids are called POLYPEPTIDES n Protein properties are determined by the order of the different amino acids joined together in polypeptide production

The Genetic Code n The Genetic code = the “language” of mRNA instructions n mRNA made up of 4 “letters” -A, G, C and U; n CRITICAL THINKING QUESTION: –a. There are a total of 20 amino acids –b. How many “letters” define the “word” for each amino acid sequence?

The Genetic Code n Codon- three consecutive nucleotides that specify a single amino acid that is to be added to the growing polypeptide sequence n Given there are four different bases, HOW MANY 3-base codons are possible? n Therefore, there is REDUNDANCY among the 20 amino acids

The Genetic Code n AUG = Methionine = Start Codon n UGA, UAA and UAG ALL = Stop Codon n Stop Codon = the period at the end of a sentence or signify end of polypeptide sequence

Translation n Translation = “decoding” of mRNA message into polypeptide chain (protein) n Ribosome = site of Protein synthesis n Process involves: –1. DNA (within nucleus) transcribed into single strand mRNA sequence –2. mRNA sequence shipped out to cytoplasm

Translation –3. In cytoplasm, mRNA attaches to ribosome –4. Each “codon” of mRNA is matched up with “anticodon” (complimentary) sequence on tRNA (NOTE: attached to each tRNA is the appropriate amino acid that matches the codon-anticodon recognition sequence)

Translation –5. Like an assembly line, the AUG codon on the mRNA/ribosome is matched up (via a peptide bond) with the UAC anticodon sequence on the tRNA (also containing methionine); the amino acid is added to the growing peptide sequence and, once this occurs, the tRNA is discarded to make room on the ribosome for the next tRNA/anticodon/attached amino acid

Translation n 6. This codon/anticodon recognition continues one amino acid at a time (again, tRNA binds to ribosome at recognition sequence, adds appropriate amino acid and then discarded) until stop codon reached indicating the completion of the polypeptide sequence