Protein Synthesis-Transcription

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.)

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?

· 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)

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

· 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 SENSE strand of DNA (the coding strand) Anti-sense (Template) Sense Strand

· 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

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

3 Stages of Transcription

1.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)---

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

2. Elongation · During transcription elongation, mRNA grows about 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)

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

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

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

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

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

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