Gene Activity: How Genes Work

1. Gene Activity: How Genes Work Mader: Biology 8th Ed.

2. Outline • Function of Genes • One Gene-One Enzyme Hypothesis • Genetic Code • Transcription • Processing Messenger RNA • Translation • Transfer RNA • Ribosomal RNA Mader: Biology 8th Ed.

3. Function of Genes • Genes Specify Enzymes • Beadle and Tatum performed experiments on Neurospora crassa and proposed each gene specifies the synthesis of one enzyme. • One gene-one enzyme hypothesis Mader: Biology 8th Ed.

4. Beadle and Tatum Experiment Mader: Biology 8th Ed.

5. Function of Genes • Genes Specify a Polypeptide • The one gene-one enzyme hypothesis suggests a genetic mutation causes a change in the structure of a protein. • A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide of a protein. Mader: Biology 8th Ed.

6. From DNA to RNA to Protein • Central Dogma of Molecular Biology: • Genes code for the sequence of nucleotides in RNA molecules. • RNA brings about formation of a protein coded for by DNA of a particular gene. Mader: Biology 8th Ed.

7. RNA • Uracil (U) replaces thymine (T) of DNA. • Types of RNA • Messenger (mRNA) - Takes message from DNA in nucleus to ribosomes in cytoplasm. • Transfer (tRNA) - Transfers amino acids to ribosomes. • Ribosomal (rRNA) - Help make up ribosomes. Mader: Biology 8th Ed.

8. Structure of RNA Mader: Biology 8th Ed.

9. Steps of Gene Expression • During transcription, DNA serves as a template for RNA formation. • During translation, an mRNA transcript directs the sequence of amino acids in a polypeptide. Mader: Biology 8th Ed.

10. Gene Expression Mader: Biology 8th Ed.

11. The Genetic Code • There is a genetic code for each of the 20 amino acids found in proteins. • Genetic code is a triplet code, with each codon consisting of three nucleotide bases. • Code Properties • Universal • Degenerate • Unambiguous • Contains start and stop signals Mader: Biology 8th Ed.

12. Transcription • During transcription, an mRNA molecule is formed with a sequence of bases complementary to a portion of one DNA strand. • RNA polymerase joins the nucleotides together in 5’  3’ direction. • Promoter defines start of a gene, the direction of transcription, and the strand to be transcribed. • Elongation continues until polymerase comes to a DNA terminator sequence. Mader: Biology 8th Ed.

13. Transcription Mader: Biology 8th Ed.

14. Processing Messenger RNA • Newly formed mRNA molecule, primary mRNA transcript, is modified before it leaves the eukaryotic nucleus. • Cap on 5’ end, and poly-A tail on 3’ end. • Introns are removed during RNA processing, leaving only exons. • Mature mRNA • In most cases, mRNA splicing is done by a spliceosome. Mader: Biology 8th Ed.

15. Transcription Mader: Biology 8th Ed.

16. Translation • Transfer RNA molecules transfer amino acids to the ribosomes. • tRNA molecules bind with one particular amino acid, and bear an anticodon complementary to the codon for that amino acid. • tRNA synthetases attach correct amino acid to the correct tRNA molecule. Mader: Biology 8th Ed.

17. Transfer RNA Mader: Biology 8th Ed.

18. Role of Ribosomal RNA • Ribosomal RNA (rRNA) is produced off a DNA template in the nucleolus of a nucleus. • Packaged with proteins into ribosomal subunits, one larger than the other. • Contain binding sites to facilitate complementary base pairing between tRNA anticodons and mRNA codons. Mader: Biology 8th Ed.

19. Ribosome Structure and Function Mader: Biology 8th Ed.

20. Translation Mader: Biology 8th Ed.

21. Review • Function of Genes • One Gene-One Enzyme Hypothesis • Genetic Code • Transcription • Processing Messenger RNA • Translation • Transfer RNA • Ribosomal RNA Mader: Biology 8th Ed.

22. Mader: Biology 8th Ed.