Phenotypes: physical traits of an organism

How do genes determine physical traits?: Sir Archibald Garrod --> Alkaptunuria --> black urine --> hypothesize that gene producing enzyme that breaks down alkapton is defective

One gene-one enzyme hypothesis: 1941, George Beadle & Edward Tatum experimented with bread mold Neurospora

One gene-one polypeptide hypothesis: 1 enzyme (protein) --> many polypeptides --> 1 polypeptide --> by 1 gene

DNA: deoxyribonucleic acid --> structure: double helix --> many nucleotides --> 5-carbon sugars + 1 phosphate group + 1 nitrogenous base --> Purines: Adenine, Guanie --> Pyrimides: Thymine, Cytosine

One gene controls production of one polypeptide --> one gene must control the sequence order of the linking of amino acids within the polypeptide --> one amino acid corresponds to one codon (a group of three adjacent nucleotides) on that gene along the DNA

Eukaryotic cell: DNA within nucleus --> polypeptide outside nucleus & within the cytoplasm --> RNA forms linkage between the two separated entities

Differences of DNA (A-T, deoxy ribose sugars, double-stranded helix structure) & RNA (no T with Uracil, ribose sugars, single-stranded)

DNA-RNA-proteins processes: 1) Transcription: process by which DNA code for polypeptide is copied by RNA --> 2) Translation: process by which message transcripted by RNA is converted into polypeptide

Transcription: RNA polymerase enzyme "unzips" helix structures strands of DNA --> form template for copying --> RNA forms & grows until "stop" codon is reached --> RNA breaks away --> this RNA is messenger RNA (mRNA) --> it carries DNA instructions from nucleus to cytoplasm where polypeptide is translated

mRNA: in eukaryotic cells --> amino acid codons (exons) are separated by non-amino acid codons (introns) --> processing of mRNA before leaving nucleus --> head of mRNA is added with CAP (PPP) --> tail of mRNA is AAAAA.... --> enzymes cut out all introns --> all exons are spliced together to form actual mRNA --> transcription finished --> sent for translation

tRNA: transfer RNA with ~80 nucleotides --> head: sequence of three bases - anticodon --> tail: one amino acid

rRNA: ribosomal RNA --> like mRNA & tRNA --> transcripted from DNA within nucleus --> within nucleolus, rRNA combines with various proteins to form ribosome --> unequal units: small subunit & large subunit --> ribosome: site of protein synthesis

Translation: mRNA, tRNA & ribosome travel to cytoplasm (amino acids exist) through nuclear pores --> amino acids attach to tRNA tails --> ribo subunit attaches to AUG codon --> "start" codon AUG: initiator of mRNA --> tRNA with anitcodon attaches to initiator codon --> large ribosomal subunit (with two sites for two tRNA - P site, A site) attaches to mRNA --> next tRNA attches to the A site of ribosome --> peptide bond forms between the amino acids of the two attached tRNA's: dipeptide --> elongation: P site tRNA separates its amino acid which is bonded to the other attached amino acid --> 1st tRNA separates --> ribosome moves 1 codon down the mRNA --> present tRNA is moved into P site --> new tRNA specified by mRNA codon is attached to form new amino acid --> 1 codon = 1 amino acid --> each mRNA codon is matched by exact matching tRNA anticodon (unique copying) --> Termination: "stop" codon UAA --> special tRNA with corresponding anticodon attaches --> no new amino acid attached --> it cuts off the old tRNA-amino acid bond --> old tRNA separates --> ribosome & tRNA separates from mRNA --> protein formed is coiled & performs its functions

Mystery: some genes are turned on (expressed), while others are turned off --> how??