Biochemistry Ch 7. RNA and the Genetic Code Flashcards
Central dogma
States that DNA is transcribed to RNA which is translated to protein
Degenerate code
Allows multiple codons to encode for the same amino acid
Initiation codon
AUG
Termination codon
UAA, UGA, UAG
Mutations without effects
Redundancy and wobble allows mutations to occur without effects in the protein
Wobble
Third base in the codon
Point mutation types
Silent mutations, nonsense mutations, and missense mutations
Silent mutations
Have no effect on protein synthesis
Nonsense mutations
aka truncation, mutations that produce a premature stop codon
Missense mutations
Produce a codon that codes for a different amino acid
Frameshift mutations
Result from nucleotide addition or deletion and chance the reading frame of subsequent codons
RNA vs DNA
RNA substitutes a ribose sugar for deoxyribose, substitution of uracil for thymine, single stranded instead of double stranded
Three types of RNA
Messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)
Messenger RNA
Carries the messages from DNA in the nucleus via transcription of the gene, it travels into the cytoplasm to be translated
Transfer RNA
Brings in amino acids and recognizes the codon on the mRNA using its anticodon
Ribosomal RNA
Makes up the ribosome and is enzymatically active
Helicase
Unwinds the DNA helix
RNA polymerase II
Binds to the TATA box within the promoter region of the gene(25 base pairs upstream from first transcribed base)
TATA box
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Prometer region
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hnRNA
Synthesized from the DNA template (antisense) strand
Posttranscriptional modifications include
A 7-methylguanylate triphosphate cap is added to the 5’ end
A polyadenosyl (poly-A) tail is added to the 3’ end
Splicing is done by snRNA and snRNPs in the spliceosome, introns are removed in a lariat structure and exons are ligated together
Prokaryotic cells can increase variability of gene products through polycistronic genes while eukaryotic cells use alternative splicing
Polycistronic genes
Starting transcription in different sites within the gene leads to different gene products
Alternative splicing
Combining different eons in a modular fashion to acquire different gene products
Ribosomes
Factories where translation (protein synthesis) occurs
Stages of translation
Initiation, elongation, and termination
Initiation in prokaryotes
Occurs with 30S ribosome attaches to the Shine-Dalgarno sequence and scans for a start codon, it lays down N-formylmethionine in the P site of the ribosome
Shine-Dalgarno sequence
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Initiation in eukaryotes
Occurs when the 40S ribosome attaches to the 5’ cap and scans for a start codon, it lays down methionine in the P site of the ribosome
Elongation
Involves the addition of new aminoacyl-tRNA into the A site of the ribosome and transfer of the growing polypeptide chain from the tRNA in the P site of the tRNA in the A site, the now uncharged tRNA pauses in the E site before exiting the ribosome
Termination
Occurs when the codon in the A site is a stop codon, uses a release factor to release the protein
Release factor
Places a water molecule on the polypeptide chain and thus releases the protein
Posttranslational modifications
Folding by chaperones
Formation of quaternary structure
Cleavage of proteins or signal sequences
Covalent addition of other biomolecules (phosphorylation, carboylatvon, glycosylation, prenylation)
Jacob-Monod model
Model of repressors and actors that explains how operons work
Operons
Inducible or repressible clusters of genes transcribed as a single mRNA
Inducible systems
Bonded to a repressor under normal conditions, they can be turned on by an inducer pulled the repressor from the operator site, example is the lac operon
Chaperones
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Repressor
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Operator site
=
Inducer
-
Repressible systems
Are transcribed under normal conditions, can be turned off by a corepressor coupling with the repressor and the binding of this complex to the operator site, example is the trp operon
Transcription factors
Search for promoter and enhancer regions in the DNA
Promoters
Within 25 base pairs of the transcription start site
Enhancers
Are more than 25 base Paris from the transcription start site
Modification of chromatin structure
Affects the ability of transcriptional enzymes to access the DNA through histone acetylation (increased accessibility) or DNA methylation (decreased accessibility)
