Chapter 8: Gene Expression and the flow of information from DNA to RNA to protein

gene expression

the flow of genetic information from DNA to RNA to protein

a codon is the equivalent of…

3 nucleotides, or one amino acid

template strand

is complimentary to mRNA during transcription

RNA-like strand

has the same polarity and sequence as mRNA. The 5’ to 3’ in the mRNA corresponds to N to C terminus in the polypeptide

summary of genetic code

- genetic code has triplet codons

- codons are non-overlapping

- three nonsense codons don’t encode an amino acid: UAA, UAG, and UGA

- genetic code is ambiguous

- reading frame is established from a fixed starting point codon for translation initiation is AUG

- mRNAs and polypeptides have corresponding polarities

- mutations can be created in 3 ways; frameshift, missense, and nonsense

where else is exceptional genetic code found?

cilliates, mitochondria, and some prokaryotes

transcription in prokaryotes

- Initiation: RNA polymerase binds to promoter sequence located near beginning of gene, and DNA is unwound to form an open promoter complex. Phosphodiester bonds form between the first two nucleotides

- elongation: constructing RNA copy of the gene. The sigma factor separates from RNA polymerase (core enzyme)

- termination: the end of transcription in prokaryotes. terminators are sequences that signal the end of transcription, and there are two types in bacteria: extrinsic (requires rho factor) and intrinsic (don’t require additional factors). Usually form hairpin loops due to intramolecular H-bonding)

promoters

DNA sequences that provide the signal to RNA polymerase for starting transcription

RNA polymerase adds nucleotides in a…

5’ to 3’ direction. Formation of phosphodiester bonds occurs using ribonucleotide triphosphates, and the energy for transcription is provided by the hydrolysis of bonds in NTPs

Terminators

sequences that provide the signal to RNA polymerase for stopping transcription

sigma factor

binds to RNA polymerase (holoenzyme) in prokaryote transcription

enhancers

found only in eukaryotes. They can be thousands of base pairs away from the promoter and are required for efficient transcription

structure of eukaryotic mRNAs

- 5’ methylated cap

- 3’ poly-A tail

- Introns

exons

sequences found in a gene’s DNA and mature mRNA (expressed regions)

Introns

sequences found in DNA but not in mRNA (intervening regions)

removing an intron

spliceosome

catalyzes splicing

ribosomes

where translation takes place. These coordinate movement of transfer RNAs (tRNAs) carrying specific amino acids

anticodon

each tRNA has one that is complementary to an mRNA codon

charged tRNA

a specific tRNA is covalently coupled to a specific amino acid.

aminoacyl-tRNA synthetase

catalyzes attachment of tRNAs to specific amino acids.

wobble

some tRNAs recognize more than one codon

a ribosome has two subunits composed of RNA and protein (know this)

different parts of a ribosome

initiation stage

start codon is AUG at 5’ end of mRNA

elongation stage

amino acids are added to growing polypeptide

termination stage

polypeptide synthesis stops at the 3’ end of the reading frame

Translation of mRNAs on ribosomes