Gene Activity

1. Gene Activity How Genes Work

2. Gene expression • DNA’s sequence of nucleotides -> sequences of amino acids -> specific enzymes ->structures in organisms

3. Experiments • 1900’s – Archibald Garrod – • inborn error of metabolism – relationship between inheritance and metabolic diseases • Suggested a link between genes and proteins

4. Experiment • 1940’s - Beadle and Tatum – Neurospora crassa, red bread mold • Spores grown on minimal medium because it produces all enzymes it needs. • Introduced mutations in spores by x-ray. • Found molds had defective gene which leads to one defective enzyme. • One gene-one enzyme hypothesis

5. Experiment • 1949 - Linus Pauling and Harvey Itano • Studied structure of protein Hemoglobin in sickle cell anemia patients to see if differ than normal rbc’s • Looked at charge difference in amino acids • Demonstrated that a mutation leads to a change in the structure of a protein • In sickle cell, there are 2 polypeptide chains involved but one is affected • One gene – one polypeptide hypothesis

6. DNA to RNA to Proteins • Proteins are link between genotype and phenotype • Gene – segment of DNA that specifies the amino acid sequence of a protein.

7. RNA – compare to DNA • Sugar ribose, not deoxyribose • Uracil not thymine • Single stranded, not double • No helix

8. 3 major classes of RNA • Messenger RNA (mRNA) – takes message from DNA to ribosome in nucleus • Transfer (tRNA) – transfers amino acids to ribosomes • Ribsomal RNA – (rRNA) make up part of ribosome with proteins

9. Gene expression • Protein synthesis takes place by 2 steps: • Transcription – DNA to mRNA • Translation – mRNA to amino acid • Results in polypeptide

10. Genetic code • Codon – triplet code of nucleotides = 64 combinations, more than enough to code for 20 amino acids. • 1961 Nirenberg and Matthei translated 3 nucleotides at a time to assign an amino acid to each mRNA codon.

11. Properties of genetic code • Degenerate – most amino acids have more than 1 codon, helps in reducing # of mutations • Unambiguous – each codon has one meaning • Start and stop codons (1,3) • Universal to all living things (exception in mitochondria and chloroplasts) • Strong evidence of the sharing of common ancestors

12. transcription • DNA strand unwinds, RNA nucleotides pair with DNA nucleotides only in 5’ – 3’ direction. • RNA polymerase begins at region of DNA called promoter and ends at DNA stop sequence.

13. Transcription • Primary RNA strand is modified before it leaves nucleus • Cap (guanine) added to 5’ • Tail (chain of adenine) added to 3’ – poly A tail • Exons – expressed DNA • Introns – removed from strand, RNA splicing by spliceosomes • = mature RNA • Ribozymes – RNA with enzymatic function, RNA could have preceded DNA in evolution of cells!!

14. Introns • Junk DNA • Up to 95% of protein coding gene = intron • Possible that presence of introns allows exons to be put together in various sequences so that different mRNA and proteins can result from a single gene…???

15. Translation • Sequence of mRNA translated to amino acids in a polypeptide • tRNA transfers amino acids to ribosome on 3’ end • Anticodon on opposite end of tRNA

16. Structure of ribosome • rRNA made in nucleolus, packaged into subunits, move out of nucleus and combine during translation. • Polyribosome – several ribosomes are often attached to and translating at the same time on the same mRNA

17. 3 steps of translation • Initiation – components of translation come together • Elongation – polypeptide increases in length, P sites, A sites, E sites • Termination – polypeptide and components are separated from each other.