Genes and How They Work

Chapter 15

Gene Expression

•Transcription

–DNA transcribed into RNA.

•RNA polymerase binds to promoter site of DNA.

–Moves along DNA strand and adds corresponding complementary RNA nucleotide.

–Makes mRNA (m = messenger)

•Translation

–mRNA is translated into amino acids of polypeptide.

•rRNA of ribosome binds to “start” of mRNA.

–Disengages at stop signal.

Genetic Code

•Information of RNA is written in “3-letter words” = codons (AUC, GUA, CCC, UGC)

–Each codes for one specific amino acid.

–Genetic code is nearly universal in all organisms.

»Slight variation in different lineages

»Mitochondria

»Chloroplasts

Transcription

•RNA Polymerase

–Only one of two DNA strands (template) is transcribed.

–Non-transcribed strand has same sequence as mRNA (sense strand).

–Polymerase adds nucleotides to the growing 3’ end of an RNA chain.

•Synthesis proceeds in 5’à3’ direction.

Transcription

•Promoter

–Transcription starts by RNA polymerase binding to sites called promoters on DNA template strand.

•Initiation

–Other factors bind & assemble transcription complex.

•unwinds DNA helix.

Transcription

•Elongation

–Transcription bubble moves down DNA leaving growing RNA strand protruding from the bubble (multiple bubbles on any given strand of DNA)

•Termination

–Stop sequences at end of gene cause dissociation of RNA polymerase, DNA, mRNA.

Translation

•Start codon of mRNA molecule binds to rRNA in a ribosome.

–tRNA with complimentary anticodon, binds to exposed codon on mRNA.

•Some tRNA molecules recognize more than one codon (3^rd letter in Codon can usually vary)

Formation of Initiation Complex

Translation

•Activating Enzymes

–activating enzymes attach amino acids to specific tRNA molecules

Translation

•Start and Stop Signals

–Start signal coded by AUG codon.

–Stop signal coded by one of three codons: UAA - UAG - UGA.

•Initiation

–Polypeptide synthesis begins with formation of initiation complex.

•Initiation Factors

Formation of Initiation Complex

Translation

•Elongation

–After initiation complex forms, large ribosome subunit binds

–Exposes next codon on mRNA

–another tRNA w/ amino acid attached

Translation

•Ribosome moves along mRNA molecule.

–Amino acids linked together to form polypeptide chain

Translation

•Termination

–Nonsense (ie. stop) codons recognized by release factors

•polypeptide released & complex disassembled

Introns

•DNA sequence specifying a protein is found in segments (exons) scattered among noncoding segments (introns).

–Not all genes have introns

•RNA transcript is produced for the entire gene.

–Spliceosomes – remove introns and join exons to form mature RNA.

–Mature mRNA is translated

Introns and Exons