Bio Ch 17

  1. One gene, one protein concept
    • Transcription
    • -Synthesis of mRNA from DNA template (Both nucleic acids use nearly the same code)
    • -mRNA transcript of gene's protein instructions (mRNA=messenger RNA)
    • Translation
    • -Synthesis of polypeptid from mRNA transcript
    • -Nucleotides translated into amino acid order
  2. Triplet code
    • mRNA base triplets=codons
    • -ex. Codon UGG codes for trytophan
    • The code is redundant
    • -Reduncancy=1+ triplet codes per amino acid.
    • Code is NOT ambiguous
    • -Each triplet code is for one amino acid only
    • Some triplets do not code for amino acids
    • -Stop codons (no amino acid)-termination (end point of translation)
    • Start codon (& amino acid methionine)
    • initiation point of translation
  3. Reading Frame
    • starting point of reading nucleotides crucial
    • ex:
    • There are no spaces in genetic code
    • Incorrect starting point results in different polypetptide
  4. Evolution of Code
    • Genetic Code is universal
    • Code must have evoled very early
    • --Shared by all organisms
    • --Very few codon differences
    • Universal properties allow gene transfer
    • ---insertion of foreign genes in an organism
    • ---Biotechnology applications
  5. Transcription Initiation
    • Transcription factors help binding
    • -proteins that help start transcription
    • Transcription initiation complex
    • -Transcription factors and RNA polymerase
    • Complex starts transcription at promoter
    • -TATA box: specific sequence in promoter
    • Two DNA strands unwound
    • Transcription is initated.
  6. Transcription Elongation
    • RNA polymerase moves along DNA
    • Exposes 10-20 DNA bases for transcription
    • RNA polymerase moves on, helix reforms
    • Transcription rate ≈ 60 bases / second
    • A Gene can be transcribed simultaneously
    • –Multiple RNA polymerases on same gene
    • –Multiple transcripts produced
    • Increases production of a single protein
  7. DNA termination
    • RNA polymerase transcribes terminator
    • sequence (AAUAAA)
    • Then pre-mRNA is cut free from enzyme
    • RNA is further modified after transcription
    • –Both ends of the transcript are modified
    • Starting point receives a cap molecule
    • Cleavage site receives tail sequence
  8. Modification of pre-mRNA
    • Guanosine triphosphate (G-PPP) cap
    • -Protects mRNA from degradation
    • -Indicates location for ribosome attachment
    • Poly A tail (...AAAAAA...) sequence
    • -protective
    • -aids in export from nucleus
    • Image Upload 1
  9. mRNA splicing
    • Not all pre-mRNA transcribed is use
    • Sections are cut & pasted = RNA splicing
    • Most genes have noncoding nucleotides
    • –Exons: coding regions Introns: noncoding
    • Primary transcript has both introns & exons
    • mRNA that leaves nucleus is reduced
    • –Introns removed and exons spliced together
    • Done by spliceosomes
  10. tRNA
    • •anticodons bring amino acids
    • –tRNA = transfer RNA
    • –tRNA transfers amino acids to ribosome
    • •From amino acid pool in cytoplasm
    • –Ribosome adds amino acids to polypeptide
  11. Ribosomes
    • •Facilitate coupling of tRNA with mRNA
    • –Allow matching of anticodons to codons
    • •Ribosome consists of two subunits
    • –Large subunit, small subunit
    • –Subunits formed in nucleoli (nucleus)
    • –Exported to cytoplasm via nuclear pores
    • •Constructed of rRNA + proteins
    • –rRNA = ribosomal RNA
  12. Initiation
    • •Small ribosomal subunit binds to RNAs
    • –mRNA and initiator tRNA
    • –Attaches to mRNA at 5’ end (leader)
    • •Initiator tRNA binds with mRNA codon
    • –mRNA initiation codon AUG
    • –Initiator tRNA carries methionine
    • •Large ribosomal subunit joins complex
  13. Elongation
    • 3 Steps:
    • 1)Codon recognition (mRNA in site A binds with incoming tRNA)
    • 2) Peptide bond formation (New amino acid joined to polypeptide. Catalyzed by rRNA [acts as ribozhyme] Poly peptide chain moved to newly arrived tRNA)
    • 3) Translocation- Ribosome moves newest tRNA from A to P site. Previous tRNA from P site moves to E site. mRNA is moved along with the tRNA

    • elongation continues to stop until codon in A site. Release factor protein binds to stop codon. Hydrolysis of polypeptide from tRNA
  14. Signal peptides
    • •Ribosomes can be free or bound
    • –Free in cytosol, bound on ER
    • –Synthesis of all proteins begins in cytosol
    • •Signal peptide is cue for attachment to ER
    • •Near leading end of polypeptide
    • –Signal recognition particle (SRP)
    • •Brings ribosome to receptor on ER
    • –Polypeptide released into cisterna of ER
  15. Base pair substutions, insertions, deletions (mutations)
    • –Replacement of one nucleotide pair
    • –Can be silent mutations
    • •Redundancy of genetic code
    • –Could still possibly code for same amino acid
    • –Ex. GGC & GGU both code for glycine
    • •Or, different amino acid could act similarly
    • –Significant if mutation affects protein function
    • •Ex. changes active site of enzyme protein
    • •Insertions and Deletions
    • –Addition or loss of base pairs
    • •Change overall length of gene (vs. substitutions)
    • –Usually affect gene product much more
    • –Creates a frameshift
    • •Affects all triplet codons downstream
    • •Extensive missense (or nonsense) effect
    • •Frameshift usually makes gene nonfunctional
    • –Unless frameshift near very end of gene
Card Set
Bio Ch 17