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.