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The Genetic Code and Transcription

Understand the genetic code and transcription in eukaryotes, from DNA to protein synthesis. Learn about mRNA, codons, RNA polymerase, promoters, introns, and exons. Explore the central dogma of molecular genetics and the role of RNA in gene expression.

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The Genetic Code and Transcription

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  1. The Genetic Code and Transcription Chapter 12 Honors Genetics Ms. Susan Chabot

  2. Molecular Genetics - From DNA to Protein

  3. Characteristics of the Genetic Code • mRNA is written in linear form using DNA as a template for synthesis. • Each “word” in the mRNA strand is composed of a 3-letter sequence called a CODON. • Each CODON specifies a SINGLE Amino Acid. • There is 1 start codon for initiation of protein synthesis and 3 stop codons for ending protein synthesis for a specific protein. • A given amino acid can have more than one codon sequence.

  4. The “Central Dogma” of Molecular Genetics

  5. There are Different RNAs with Distinct Functions

  6. Transcription is a Key Step in Gene Expression • Transcription makes an mRNA copy of DNA. • This mRNA copy is complementary to the gene sequence found on one strand of DNA. • DNA directs the synthesis of RNA in the nucleus.

  7. RNA Review RNA is a nucleic acid polymer that uses a slightly different sugar than DNA and the base uracil in place of thymine.

  8. RNA is Single-Stranded

  9. Transcription in Eukaryotes • Page 252 for more detail #1: Transcription in eukaryotes occurs in the nucleus under the direction of 3 different forms of RNA polymerase. #2: Eukaryotic arrangement of DNA must be uncoiled around histones. #3: In addition to PROMOTERS, ENHANCERS assist in locating correct strand for replication to begin (cis and trans acting factors). #4: Processing or “capping” the 5’ and 3’ ends of the mRNA transcript upon completion.

  10. RNA Polymerase in Eukaryotes

  11. Role of RNA Polymerase • Enzyme capable of directing the synthesis of a mRNA copy from a strand of DNA. • Substrate nucleotides contain RIBOSE sugars and the base URACIL in place of thymine. • NO PRIMER is required in synthesis of mRNA as in complementary strands of DNA during replication. • Locates 3’ to 5’ directionality in DNA strand so that mRNA can be constructed in 5’ to 3’ direction.

  12. Promoters • Step #1 in the production of a mRNA sequence is TEMPLATE BINDING. • Requires recognition of specific DNA sequences called PROMOTERS. • PROMOTERS are recognized by RNA polymerase. • Once the promoter is recognized, the double helix denatures in that region = TRANSCRIPTION START SITE. • Promoters govern the efficiency of mRNA production, mutations in the promoter region result in less transcription with dire consequences.

  13. Promoter Sequences • TATA box = sequences rich in A and T; TATAAT • Roughly 30 nucleotide pairs upstream from the start of transcription. • Additional promoter elements regulate the efficiency of transcription in response to cell needs: • Enhancers increase transcription levels • Silencers decrease transcription levels • Considered cis-acting elements

  14. RNA polymerase acts here Transcription

  15. A gene

  16. Coding Regions of Eukaryotic Genes are Interrupted by Intervening Sequences • Discovered in 1977 • Discovery of DNA sequences not present in the final mRNA transcript. • Intervening sequences – INTRONS • Expressed sequences – EXONS • Splicing involves removing the INTRONS and rejoining the EXONS into a final mRNA transcript.

  17. Eukaryotic Genes are Segmented Introns are removed from the primary transcript and exons are spliced together to make mRNA. In some genes, more than 90% of the pre-mRNA is destroyed, never to appear in the mRNA.

  18. ASSIGNMENT

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