Chapter 9 Study Guide.docx

Chapter 9 Study Guide 

Suppressor: mutation that reverses an earlier mutation - Amber mutation induces early STOP (UAG) - Help elucidate amino acid codes - Cosuppressors helps suppressors suppress a mutation - Reoressors inhibits expression - Derepressors inhibits repression

Codons   

AUG (Start codon) Stop codons are considered nonsense because they do not code for anything. Stop codons are: - UAG (Amber) - UAA (Ochre) - UGA (Opal)

Structure of tRNA

slide 14,15,16

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5’ position is less confined - Accommodates non-stranded base parings Degeneracy accounts for multiple codes for the same AA - Same tRNA binds to multiple AAs Conversion of adenine forms inosine

Key Sites of interaction in Bacterial Ribosomes:   

A site: entry of charged tRNA P site: of growing polypeptide - + binding site for AUG E site: exit for unchanged stem of tRNA

Translation’s 3 steps: Step1: Initiation 

Small ribosomes bind to AUG of mRNA - IF is an initiating factor - Assisted by IF that stops mRNA movement and initial complex to P site

Step2: Elongation 

Amino acid corresponding to the mRNA - mRNA codon is dropped to A site - hooks to predecessors by peptide bonds using ratchet mechanism  assisted by EF (elongation factors)

Step3: Termination 

  

nonsense codons halts synthesis. - Addition of RF (release factors) catalyzes hydrolysis reaction  Water molecule pushes peptide free UAA, UAG, UGA are NONSENSE codes

RF1 → UAA & UAG

RF2 → UAA & UGA

RF3 removes RF1 & RF2

Ribosome recycling factor + EF-G = dissociated ribosome

Reading frame: dividing the sequence of nucleotides in a nucleic acid (DNA or RNA) molecule into a set of consecutive, non-overlapping triplets. Open reading frame: sequence of DNA that starts with start codon “ATG” (not always) and ends with any of the three termination codons (TAA, TAG, TGA). Frameshift: genetic mutation caused by indels (insertions or deletions) of many nucleotides in a DNA sequence that is not divisible by three.

Name three features that induce a globular protein shape. Name three things that can dramatically alter that shape. 

Amino acid sequence, folding pattern, functions induce a globular protein shape.

Commented [SS1]:

What is a ribosome? Article consisting of RNA and associated proteins, found in large numbers in the cytoplasm of living cells. They bind messenger RNA and transfer RNA to synthesize polypeptides and proteins