The Genetic Code and Transcription

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

10. 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.

11. RNA Polymerase in Eukaryotes

12. 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.

13. 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.

14. Promoter Sequences • TATA box = sequences rich in A and T; TATAAT • Considered cis-acting elements • In organic chemistry cis means “next to” or “on the same side as” • Additional sequences recognized in E. coli is TTGACA

15. RNA polymerase acts here Transcription The enzyme RNA polymerase opens the DNA strands and synthesizes an RNA complement to only one of the DNA strands.

16. A gene The most important step in the control of gene expression is WHEN to transcribe a gene.

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. The Genetic Language Uses 4 Letters Written Into 3-Letter Words

19. What the Genetic Code Specifies: AMINO ACIDS Two examples

20. What Translation Accomplishes The sequence of amino acids determines the structure, and therefore the function, of a protein. During translation, information present in the mRNA is read by the ribosome to synthesize a polypeptide.

21. Many antibiotics block steps in translation within bacterial cells. Translation requires: ribosomes mRNA tRNA amino acids

24. Ribosomes are Complicated Protein Synthesizing Machines

25. Translation Is a Cyclic, Multistep Process

27. A tobacco plant expressing the firefly luciferase gene. Basic Genetic Mechanisms are Universal The storage of genetic information in DNA, the use of an RNA intermediate that is read in three letter words, and the mechanism of protein synthesis are essentially the same in all organisms.