RNA and The Genetic Code

Initiation RNA code

AUG

termination RNA code

UAA, UGA, UAG

silent mutations caused by point mutation

no effect on protein synthesis

Nonsense (truncation) mutations caused by point mutations

produce a premature stop codon

Missense mutations

produce a codon that codes for a different amino acid

Frameshift mutations caused by point mutations

result from nucleotide addition or deletion and change the reading frame of subsequent codons

Messenger RNA (mRNA)

carries the message from DNA in the nucleus via transcription of the gene; travels into the cytoplasm to be translated

Transfer RNA (tRNA)

brings in amino acids; recognizes the codon on the mRNA using its anticodon

Ribosomal RNA (rRNA)

makes up much of the ribosome; enzymatically active

steps of transcription (DNA to mRNA)

1. Helicase and topoisomerase unwind DNA double helix. 2. RNA polymerase II binds to TATA box within promoter region of gene (25 base pairs upstream from first transcribed base). 3. hnRNA synthesized from DNA template (antisense) strand.

posttranscriptional modifications

• 7-methylguanylate triphosphate cap added to 5' end • Polyadenosyl (poly-A) tail added to 3' end • Splicing done by spliceosome; introns removed and exons ligated together. Alternative splicing combines different exons to acquire different gene products

Translation

occurs at the ribosome, three stages: initiation, elongation, termination

Posttranslational modifications

• Folding by chaperones • Formation of quaternary structure • Cleavage of proteins or signal sequences • Covalent addition of other biomolecules (phosphorylation, carboxylation, glycosylation, prenylation)

Operons

are inducible or repressible clusters of genes transcribed as a single mRNA. control gene expression in prokaryotes

Transcription factors

search for promoter and enhancer regions in the DNA

Promoters

within 25 base pairs of the transcription start site.

Enhancers

more than 25 base pairs away from the transcription start site.