RNA

Question 1

Types of RNA

Answer 1

• mRNA: Carries genetic information from DNA to ribosome for translation; eukaryotic mRNA is monocistronic (each mRNA molecule translates into only one protein product), while prokaryotic mRNA is polycistronic (starting process of translation at different locations in mRNA can result in different proteins).
• tRNA: Carries amino acids to ribosome during protein synthesis; must be charged/activated with amino acid by aminoacyl-tRNA synthetase; mature tRNA found in cytoplasm.
• rRNA: Synthesized in nucleolus and part of ribosomal machinery; can function as ribozymes.

Question 2

Start and Stop Codons

Answer 2

• Start Codon: AUG (codes for methionine).

* Stop Codons: UGA, UAG, UAA.

Question 3

Degeneracy and Wobble

Answer 3

• Degeneracy: More than one codon can specify for a single amino acid.
• Wobble: Variable third base in codon can still specify the same amino acid.
• Both help protect against mutations in coding regions of DNA.

Question 4

Mutations

Answer 4

• Point Mutation: Single nucleotide in codon is replaced; can be silent (codon codes for same amino acid), missense (codon codes for a different amino acid), nonsense (codon changed to stop codon).
• Frameshift Mutation: Insertion or deletion of nucleotides in mRNA sequence that shifts reading frame.

Question 5

Transcription

Answer 5

• Occurs in nucleus.
• Helicase unwinds dsDNA and topoisomerase prevents formation of supercoils.
• RNA polymerase II (eukaryotes) searches for and binds to TATA box in promoter region in DNA with the help of transcription factors; does not require primer to start generating transcript and does not proofread transcript; produces heterogenous nuclear RNA (hnRNA).
• hnRNA is produced from template (antisense) strand and is identical to coding (sense) strand.

Question 6

Posttranscriptional Modification

Answer 6

• Converts hnRNA to mRNA.
• Intron/Exon Splicing: Introns are removed by spliceosome (snRNA/snRNP complex) and exons are ligated together.
• 5’ Cap: 7-methylguanylate triphosphate cap added to 5’ end of hnRNA during transcription to aid ribosome binding and to prevent degradation.
• 3’ Poly-A Tail: Polyadenosyl tail added to 3’ end of mRNA transcript to prevent against rapid degradation.
• Alternate splicing of eukaryotic hnRNA can produce different proteins.

Question 7

Translation

Answer 7

• mRNA exits nucleus through nuclear pours and finds ribosome to begin translation in cytoplasm; GTP required for all three steps of translation.
• Initiation: In eukaryotes, small ribosomal subunit binds to 5’ cap of mRNA and initiator Met-tRNA binds AUG start codon in P site; in prokaryotes, small ribosomal subunit binds to 5’ untranslated region of mRNA and initiator fMet-tRNA binds AUG start codon in P site. Large subunit binds to small subunit to form initiation complex.
• Elongation: Ribosome reads mRNA in 5’ to 3’ direction and synthesizes proteins from its amino (N-) to carboxyl (C-) terminus. A site holds incoming aminoacyl-tRNA. P site holds tRNA that carries growing polypeptide chain; peptidyl transferase catalyzes peptide bond formation. E site holds uncharged tRNA before they exit ribosome.
• Termination: When stop codon moves into A site, release factor causes water molecule to be added to polypeptide chain, allowing peptidyl transferase to hydrolyze completed polypeptide chain from final tRNA.
• Eukaryotic 80S ribsome has 60S subunit and 40S subunit; prokaryotic 70S ribsome has 50S subunit and 30S subunit.

Question 8

Posttranslational Processing

Answer 8

• Folding of polypeptide chain by chaperone proteins.
• May need to be cleaved from larger, inactive peptide.
• May need to associate with other subunits to form quaternary structure.
• Phosphorylation, carboxylation, glycosylation, prenylation.

Question 9

Prokaryote Gene Expression Control

Answer 9

• Inducible systems (such as the lac operon) are bonded to a repressor under normal conditions; they can be turned on by an inducer pulling the repressor from the operator site.
• Repressible systems (such as the trp operon) are transcribed under normal conditions; they can be turned off by a corepressor coupling with the repressor and the binding of this complex to the operator site.

Question 10

Eukaryote Gene Expression Control

Answer 10

• Transcription factors search for promoter and enhancer regions in the DNA.
• Gene amplification accomplished through enhancers and gene duplication.
• Modification of chromatin structure affects the ability of transcriptional
enzymes to access the DNA; histone acetylation increases accessibility and transcription while DNA methylation decreases accessibility and transcription.