Gene Activity How Genes Work

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

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

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

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

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.

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

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

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

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.

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

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.

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!!

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…???

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

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

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.