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PROTEIN SYNTHESIS

PROTEIN SYNTHESIS. Involves 3 steps :- Transcription RNA processing Translation. transcription. Involves 3 steps :- Initiation Elongation Termination. INITIATION. DNA unwinds using helicase . This produces a transcription bubble . Only one strand of DNA is copied.

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PROTEIN SYNTHESIS

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  1. PROTEIN SYNTHESIS • Involves 3 steps :- • Transcription • RNA processing • Translation

  2. transcription • Involves 3 steps :- • Initiation • Elongation • Termination

  3. INITIATION • DNA unwinds using helicase. • This produces a transcription bubble. • Only one strand of DNA is copied. • The strand that serves as the template is known as the sense strand. • The strand that lies dormant is the antisense strand.

  4. INITIATION • RNA synthesis is catalyzed by RNA polymerase • The DNA sequence where RNA polymerase attaches is called the promoter. Promoter RNA polymerase

  5. Promoter-rich in A’s &T’s ….. “TATA Box” Transcription Factors- proteins That bind to Promoter so RNA Polymerase knows where to begin Promoter Eukaryotic promoters 5¢ 3¢ 3¢ 5¢ TATA box Start point Template DNA strand Several transcription factors Transcription factors 5¢ 3¢ 3¢ 5¢ Additional transcription factors Transcription factors RNA polymerase II 5¢ 3¢ 5¢ 3¢ 5¢ RNA transcript Transcription initiation complex

  6. ELONGATION • RNA polymerase brings free RNA nucleotides to the DNA strand and forms a molecule of mRNA in the 5’ to 3’ direction.

  7. Elongation Anti-sense strand of DNA RNA nucleotides RNA Polymerase II 3¢ 5¢ 3¢ end 5¢ 3¢ Direction of Transcription “Downstream” Sense strand of DNA 5¢ Newly made RNA

  8. TERMINATION • When the RNA polymerase reaches a termination point, it separates from the DNA template. • The terminator sequence is rich in A’s. This polyadenylation signal is the end of the gene

  9. Processing RNA • Enzymes modify pre-mRNA before the message leaves the nucleus

  10. Start codon Stop codon Cap 5¢ Poly-A tail Alteration of the Ends • The 5 end receives a modified nucleotide (G) cap • The 3 end gets a poly-A tail

  11. Functions of these mods: • facilitate export of mRNA • protect mRNA from hydrolytic enzymes • help ribosomes attach to the 5’ end

  12. Exon Intron Exon Intron Exon 3¢ Pre-mRNA 5¢Cap Poly-A tail Introns cut out and exons spliced together Coding segment Poly-A tail 5¢Cap Splicing • Introns=noncoding sections • Exons=coding sections • Spliceosomes (snRNPs) remove introns and join exons, creating a continuous coding sequence

  13. RNA transcript (pre-mRNA) 5¢ Exon 1 Intron Exon 2 Protein Other proteins snRNA snRNPs LE 17-11 Spliceosome 5¢ Spliceosome components Cut-out intron mRNA 5¢ Exon 1 Exon 2

  14. Now that the RNA has been processed, it leaves the nucleus via the nuclear pores and enters the cytoplasm.

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