Protein Synthesis-Transcription

1. Protein Synthesis-Transcription

2. Why are proteins so important? • Nearly every function of a living thing is carried out by proteins… -DNA replication -Structural proteins (skin, muscles, etc.) -Transport proteins (hemoglobin, etc.) -Fight infection (antibodies) -Enzymes (digest food, copy DNA, etc.)

4. STEP 1: TRANSCRIPTION: ·DNA has the information (“blueprints”) to make proteins, BUT…it can’t leave the nucleus (too big!) ·So, it needs a MESSENGER to carry the blueprints from the nucleus to the protein-making factories…the RIBOSOMES! ·the “messenger” is “messenger RNA” (or mRNA)…..RNA is different from DNA?

5. ·RNA (ribonucleic acid) is different from DNA in 3 ways: 1)RNA is single stranded 2)The sugar in RNA is ribose (instead of deoxyribose) 3)Instead of thymine (T), RNA contains uracil (U) (base pairing rules still apply: C with G; A with U)

7. Enzymes “unzip” the DNA • RNA nucleotides link to the DNA bases, forming a mRNA strand

8. ·  For each gene, only 1 of the 2 strands is transcribed (the Antisense strand); the antisense strand acts as a template! ·  the mRNA therefore, has the sequence of the SENSEstrand of DNA (the coding strand) Anti-sense (Template) Sense Strand

9. ·  Transcription of mRNA from template DNA is catalyzed by RNA polymerases which separate the 2 DNA strands and link RNA nucleotides as they base-pair along the DNA template

10. ·  RNA nucleotides are added only to the 3’ end of growing RNA strand thus • mRNA is synthesized in the 5’ 3’ direction

11. 3 Stages of Transcription

12. Initiation: RNA polymerases bind to DNA at regions called PROMOTERS --(specific sequence of DNA- TATA Box)(eukaryotes). --This binding site is where transcription begins (initiation site)---

13. Direction of transcription “downstream” start TAC on DNA downstream from TATA box What does this provide?

14. 2.Elongation ·  During transcription elongation, mRNA grows about 30-60 nucleotides per second • as the mRNA strand elongates, it peels away from the DNA template • 2 strands of DNA double helix are reunited (bonds reformed)

15. 3. Termination --In prokaryotes, Transcription proceeds until RNA polymerase reaches a termination site on the DNA --In eukaryotes, the mechanism for cleaving the pre-mRNA from DNA is more complicated & not completely understood --Transcription ends when RNA polymerase “falls off” the DNA

17. Protein Synthesis in Prokaryotes vs. Eukaryotes • In bacteria, transcription and translation happen in the same location and often simultaneously! • In eukaryotic cells, the nuclear envelope separates transcription from translation…this provides time for RNA processing (this is an extra step between transcription and translation) • does not occur in prokaryotes

18. Post-transcription modification of mRNA in eukaryotes: 1) Alteration of mRNA ends: *the 5’ end is “capped” with a modified form of guanine (G) (5’ CAP) -protects the mRNA from hydrolytic enzymes -serves as an “attach here” signal for small ribosomal subunits

19. Post-transcription modification of mRNA in eukaryotes: *at the 3’ end, an enzyme adds a poly-A tail (30-200 adenine nucleotides) -inhibits degradation of the mRNA -may facilitate the export of mRNA from the nucleus to the cytoplasm

20. Post-transcription modification of mRNA in eukaryotes: 2) RNA Splicing *INTRONS = noncoding segments of DNA are cut out of the mRNA *EXONS = coding regions of DNA; exons are eventually expressed—the remaining exons are spliced together

22. For ALL life! strongest support for a common origin for all life Code has duplicates several codons for each amino acid mutation insurance! The mRNA code (for nDNA) • Start codon • AUG • methionine • Stop codons • UGA, UAA, UAG