Lecture 15 - Gene Expression

flow of genetic information

• proposed by Francis Crick

• DNA → RNA → protein - unidirectional

RNA/ribonucleic acid

• sugar - ribose (additional -OH)

• bases - A, U, C, G

• one strand

• 3 main kinds:

  • mRNA

  • rRNA

  • tRNA

codons/genetic code

• sequence of 3 bases produces 1 amino acid

• only 20 different amino acids

  • umnambiguous & redundant

wobble hypothesis

• proposed by Francis Crick

• explains why multiple codons can code for a single amino acid

• one tRNA molecule can recognize and bind to more than one codon, due to less precise base pairing that can arise between the 3rd base of the codon and the base at the 1st position on the anticodon

transcription

• occurs in the nucleus

• first part of gene expression

• synthesis of RNA complementary to DNA

• there stages:

  • initiation

  • elongation

  • termination

transcription initiation

• one DNA strand serves as template for RNA synthesis (other strand not transcribed)

• DNA read in 3’-5’ direction & mRNA synthesized in 5’-3’ direction (just like DNA replication)

• components:

  • promotor - specific DNA sequence that indicates transcription start site (promote not transcribed)

  • RNA polymerase - enzyme that makes RNA strand; binds promotor, unwinds helix, begins transcription

transcription elongation

• RNA synthesis

• antiparallel to template strand (5’-3’)

• just like DNA replication but uses RNA triphosphate

transcription termination

• specific DNA sequence (NOT stop codon) causes RNA polymerase to release DNA

• mRNA transcript release

mRNA modification

• occurs in nucleus and only in eukaryotes

• mRNA after transcription is not ready for protein synthesis

• 5’ cap and 3’ poły-A tail is added to the mRNA

  • tail - protect mRNA from being broken down

  • cap - allows to move out of nucleus

• strand is exported out of the nucleus, ribosome recruitment and protection

RNA splicing

• occurs in the nucleus

• introns are removed & exons are connected

• results in mature mRNA

introns

regions that do not code amino acids

exons

regions that code for amino acids

translation

• mRNA → polypeptide

• occurs in ribosomes (cytoplasm or surface of ER)

• three stages:

  • initation

  • elongation

  • termination

transfer DNA (tRNA)

• single-stranded

• ~80 nucleotides

• in translation

• many variants and carries specific amino acids into ribosome

anticodon

• 3 bases on loops of tRNA

• complementary to codon in mRNA

• how each codon codes for one specific amino acid

ribosomal RNA (rRNA)

• catalyzes peptide bond formation

• acts as a ribozyme

• makes up the ribosome with protein

translation initiation

• mRNA binds to small ribosomal unit

• first tRNA binds (complementary to start codon)

• large subunit associates

translation elongation

• cyclical process

  • codon recognition

  • peptide bond formation - endergonic, requires E

  • translocation - work, requires E

• each cycle adds 1 amino acid to polypeptide

translation termination

• stop codon recognized by release factors (proteins)

• cannot form new peptide bond → components dissociate