GENE EXPRESSION What is a gene? Mendel –Unit of inheritance conferring a phenotype Modern definition –Unit of DNA directing the synthesis of a polypeptide or functional RNA molecule

GENE EXPRESSION How is a gene expressed? DNA  RNA  protein DNA  RNA –Transcription RNA  protein –Translation

TRANSCRIPTION Synthesis of RNA from a DNA template Occurs in the nucleus of a eukaryotic cell Similar to DNA synthesis Some differences

TRANSCRIPTION How is it different from DNA synthesis? Start at promoter Unidirectional –Single strand transcribed Continuous (not discontinuous) Catalyzed by RNA polymerase Termination sequence No mismatch repair or ligation Thymine is replaced with Uracil

TRANSCRIPTION

RNA PROCESSING RNA transcripts must be modified prior to transport from the nucleus to the cytoplasm

RNA PROCESSING Splicing –Removal of introns (junk) Capping –Backwards G-residue at 5’ end Polyadenylation –Addition of A-residues to 3’ end

TRANSLATION Synthesis of a protein from an mRNA template Occurs in the cytoplasm of eukaryotic cells Requires ribosomes, tRNAs, and various other factors Utilizes genetic code

TRANSLATION Messenger RNA mRNA Temporary copy of gene Information deciphered using the genetic code Functional unit is codon –Three consecutive nucleotides –“Triplet”

TRANSLATION mRNA codons 3 consecutive nucleotides –64 different codons (4 3 ) 61 encode specific amino acids 61 > 20; some duplication

TRANSLATION mRNA codons 3 consecutive nucleotides –One of these (AUG) is a start codon –3 are stop codons (UAG, UGA, UAA) Terminate translation Recognized by tRNAs

TRANSLATION Transfer RNA (tRNA) Short segments of RNA –~80 nucleotides long Two important regions –Anticodon Nucleotide triplet Complementary to codon of mRNA –Amino acid attachment site Amino acid corresponding to codon is covalently attached to the tRNA

TRANSLATION Ribosomes Cellular organelles Workhorse Site of translation Facilitate interaction between mRNA and tRNA Facilitate polymerization of amino acids

TRANSLATION

MUTATION Mutation Heritable –Change in DNA sequence Relatively rare Typically occurs during DNA replication or repair May affect gene expression Spontaneous or mutagen

MUTATION Mutation Different effects –Harmful –Neutral –Beneficial Most are neutral or harmful A minority are beneficial Ultimate source of genetic variation –Natural selection acts upon variation

MUTATION Single gene mutations How do mutations affect gene expression? A change in DNA can give rise to a change in RNA codons, which can encode different amino acids

MUTATION Base substitutions mutations Affect only a single base “Point mutation” –Affects a single base pair One base replaced by another, does not change “downstream” triplet

MUTATION Base Substitution mutations Can cause change in amino acids e.g., GAA  GTA (glu  val) Phenotype may be affected

MUTATION Frameshift mutations Insertion or deletion of base(s) –e.g., GGA  GAGA (gly  glu) Alter reading frame –All downstream amino acids altered Protein function affected –Effect similar to nonsense mutation

Frameshift