Biochemistry - RNA and the Genetic Code

Central Dogma

DNA -\> RNA -\> Protein

Initiation

AUG (methionine)

Termination

UAA, UGA, UAG

Wobble

allow mutations to occur without affecting the protein

Point mutations

silent, missense, nonsense

Silent mutations

have no effect on the amino acid produced by a codon because of redundancy in the genetic code

Nonsense mutations

mutations which produce a premature stop codon

Missense mutations

produce a codon that codes for a different amino acid

Frameshift mutations

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

RNA vs DNA

ribose vs. deoxyribose, single stranded vs. double stranded, uracil vs. thymine

Three major types of RNA

mRNA, tRNA, rRNA

messenger RNA (mRNA)

Carries the message from DNA in the nucleus via transcription of the gene; it 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

Transcription steps

-helicase and topsisomerase unwind DNA double helix

-RNA polymerase II binds to TATA box within promoter region of gene

-hnRNA synthesized from DNA template strand

hnRNA

Heterogeneous nuclear RNA; the primary transcript made in eukaryotes before splicing.

posttranslational modification

-7-methylguanylate triphosphate cap added to 5' end

-poly-A tail added to the 3' end

-splicing

Splicing

the process of removing introns and reconnecting exons in a pre-mRNA

Alternative splicing

Splicing of introns in a pre-mRNA that occurs in different ways, leading to different mRNAs that code for different proteins or protein isoforms. Increases the diversity of proteins.

Where does translation occur?

at the ribosome

Three stages of translation

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

inducible or repressible clusters of genes transcribed as a single mRNA

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