Chapter 17

 1902 – Archibald Garrod  Suggested genes dictate phenotype through enzymes of reactions ▪ Alkaptonuria – black urine ▪ Contains alkapton (darkens upon exposure to air) ▪ Garrod: Most people must have an enzyme that breaks down alkapton  Early theories thought is was one gene-one enzyme

 Genes are instructions for proteins  Requires two major steps  Transcription – DNA to RNA (mRNA) ▪ Takes place in the nucleus  Translation – mRNA to polypeptide ▪ Takes place at ribosomes  Central dogma ▪ DNA RNA Protein

 4 nucleotides specify 20 amino acids  Codons – instructions for polypeptide formation in 3 base sequences  When translating for polypeptides, only one strand of the DNA is used – template strand ▪ For a particular gene, same strand used every time

 mRNA is complimentary- not identical  Same rules EXCEPT - U instead of T  Codons match the non- template strand with the one exception  Codons are read in the 5’ – 3” direction – creates a reading frame  1 codon for start  3 codons for stop

 Code is universal  Same code in all living organsims  Same code for myosin in yeast as it is in animals  While the code may be the same there are some differences in the translation mechanisms

 mRNA polymerase splits the DNA strands joins complimentary RNA nucleotides  Only 5’ – 3’  Can start their own strand ▪ In bacteria attaches to a promoter and ends at terminator

 Start of transcription begins upstream at the promoter  RNA polymerase binds  In eukaryotes – RNA polymerase binds to transcription factors – group of proteins  Transcription Initiation Complex ▪ TATA box

 RNA polymerase untwists and exposes about nucleotides for pairing  Transcription takes place at about 40 nucleotides per second in eukaryotes  Single gene can be transcribed by multiple RNA polymerase molecules simultaneously

 Bacteria have a terminator sequence of nucleotides  Eukaryotes have a polyadenylation signal  AAUAA – nucleotides after this sequence will cause the pre-mRNA to break free  Pre-mRNA then is altered ▪ 5’ cap ▪ Poly-A tail

 Large portions of RNA are removed before leaving the nucleus  Introns –portion of the DNA that do not code for a protein  Exons – portion of the DNA that is expressed  RNA Splicing  snRNPs

 mRNA to protein production  Uses tRNA  Brings the amino acid to ribosome  Contains an anticodon that matches the mRNA codon  tRNA is synthesized from DNA templates in nucleus

 Binding of tRNA and mRNA involves enzymes – aminoacyl tRNA synthetases  Catalyzes the transfer of amino acids to tRNA  Pairing occurs at ribosomes  A large and small subunit

 Ribosomes have three binding sites  P site – holds the tRNA carrying the growing polypeptide  A site – holds the next tRNA in sequence  E site – exit site for tRNA  As polypeptide grows it leaves via the exit tunnel of the ribosome

 Polypeptide production divided into three phases  Initiation – bringing together mRNA, tRNA, and ribosomal subunits together  Elongation – amino acids are added to the C terminus one by one  Termination – elongation ends when a stop codon is reached

 Polypeptide begins to coil/fold as it exits  Gene determines the primary structure which in turn determines shape  Some proteins undergo further modification ▪ Cutting, binding to another polypeptide

 Two types  Free – floating in the cytosol ▪ Produce proteins that will stay in the cytosol  Bound – attached to endoplasmic reticulum ▪ Produce proteins that will be used by the endomembrane system (Golgi, lysosomes, vacuoles, plasma membrane)

 Any change in the genetic sequence  Point mutations – one or a few nucleotide pairs changed ▪ Substitution – replacement of a single nucleotide  Insertions/Deletions – addition or losses of nucleotides ▪ Results in a frameshift mutation