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Protein Synthesis

Protein Synthesis. DNA  RNA  Protein. 2 Major Steps. Transcription – DNA is transcribed (copied!) into single stranded mRNA (DNA code transcribed into RNA code) Translation – mRNA is translated into protein (amino acids). Transcription. Occurs in nucleus (where the DNA is located!)

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Protein Synthesis

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  1. Protein Synthesis

  2. DNA  RNA  Protein

  3. 2 Major Steps • Transcription – DNA is transcribed (copied!) into single stranded mRNA (DNA code transcribed into RNA code) • Translation – mRNA is translated into protein (amino acids)

  4. Transcription • Occurs in nucleus (where the DNA is located!) • Generally divided into three steps – initiation, elongation, termination

  5. A. Initiation • RNA polymerase (enzyme) attaches to promoter regions of DNA at TATA box • RNAP requires TATA box and other transcription factors to be present

  6. B. Elongation • RNAP can only add nucleotides to the 3’ of the growing mRNA strand; thus mRNA is synthesized in the 5’ 3’ direction

  7. C. Termination • Transcription ends when mRNA polymerase reaches a specific STOP sequence: • ATT • ATC • ACT

  8. mRNA processing • After transcription, mRNA must be modified so that it can exit the nucleus 1. GTP cap added to 5’ end of mRNA (stability & attachment point for ribosome down the road) 2. Poly-A tail added to 3’ end of mRNA (stability & guidance so mRNA goes from nucleus to ribosome) 3. Removal of introns

  9. Introns vs. Exons • Exons – sequences that contain the code for a polypeptide (protein); exons are expressed • Introns – non-coding sequences of mRNA

  10. Original, unprocessed mRNA contains both introns and exons • Before mRNA exits nucleus for translation at ribosome, snRNPs(small nuclear ribonucleoproteins) remove introns and splice exons together

  11. Translation • Occurs in cytoplasm where processed mRNA meets ribosome and is translated into protein • Steps: • Initiation • Elongation • Termination

  12. For translation to occur, you need… • Activated tRNA -tRNA in cytoplasm finds the correct amino acid -Attachment (activation) requires 1 ATP and enzyme

  13. 2. Ribosomal subunits • Ribosomes consist of 2 subunits (small and large) • Each subunit is made of rRNA and protein • Each is built separately in nucleolus • The two subunits exit nucleolus and form the ribosome in the cytoplasm when translation starts

  14. The Genetic Code • Codon – triplets of bases • mRNA makes a template strand (from DNA) that is translated into protein via a triplet code • Every three base pairs codes for a particular aa

  15. 1. Initiation • Brings together mRNA (from nucleus), tRNA (in cytoplasm), and ribosomal subunits (from nucleolus) • Small ribosomal subunit binds to mRNA and a specific tRNA (methionine) • Small subunit scans mRNA until it finds AUG (start codon) • Large subunit attaches the complex

  16. 2. Elongation • Begins with the next tRNA arriving at the P site of the ribosome

  17. Polyribosome

  18. 3. Termination • Occurs when the ribosome encounters one of three STOP codons located on mRNA • At this point, polypeptide synthesis is complete and the ribosome detaches from the polypeptide

  19. Point Mutations • Chemical changes in just one base pair of a gene • 2 general types: • Base pair substitutions • Base pair insertions or deletions

  20. 1. Base pair substitutions • Substitution of one base pair with another • Often silent mutationsbecause they don’t have an effect on encoded protein due to redundancy of genetic code: • DNA: CCG  CCA • mRNA: GGC  GGU • AA: gly  gly

  21. Example: Sickle Cell Anemia • Results from a single point mutation in a gene that codes for one of the polypeptide chains that form hemoglobin • Fatigue, paleness, rapid heart rate, shortness of breath, jaundice

  22. 2. Insertions & Deletions • Additions or deletions of nucleotide base pairs • BAD NEWS… these are often called frameshift mutations because they literally shift the reading frame of the mRNA  protein

  23. Example: Tay-Sachs Disease • Inherited frameshift mutation on chromosome 15 • Mutation results in malformation of hexoaminidase A, a protein that breaks down a particular chemical in gangliosides (nerve tissue) • Deafness, blindness, dementia, paralysis, slow growth, mental retardation etc

  24. Mutagens • Spontaneous mutations – errors that result from cellular machinery malfunction • Environmental mutations – damage to DNA caused by environment • Mutagens – chemical agents that interact with DNA and cause problems • X-rays, UV light, various drugs (seriously)

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