AP Biology Ch.17 Notes
Terms in this set (29)
Explain the reasoning that led Archibald Garrod to first suggest that genes dictate phenotypes through enzymes.
Garrod was an MD
Focused on rare and unusual birth defects; inborn errors of metabolism
Urine turns black; contains alkapton
Hypothesized a missing enzyme for brealing down alkapton
Describe Beadle and Tatum's experiments with Neurospora and explain the contribution they made to our understanding of how genes control metabolism.
Neurospora can grow on a minimal medium (MM): sucrose, salts, + biotin
Can synthesize all other biomolecules: amino acids, nitrogen bases, etc.
Biomolecules are made stepwise by a series of enzymes
Beadle and Tatum collected mutants that required arginine in their MM
Intermediates in the pathway were known
Mutants can be grouped by which intermediate cures them
Concluded that each group of mutants lack a different enzyme in the pathway
Distinguish between the "one gene-one enzyme" hypothesis and the "one gene-one polypeptide" hypothesis and explain why the original hypothesis was changed.
One gene-one enzyme
Each gene makes an enzyme
But some proteins are not enzymes
Counter examples: insulin, keratin, hemoglobin
One gene-one polypeptide
Counter examples: RNA genes, regulatory sequences
Explain how RNA differs from DNA.
Uracil instead of thymine
Used to express traits rather than store and transmit traits
Briefly explain how information flows from gene to protein.
In DNA, order of bases = information
Copied by base-pairing rule into mRNA = transcription
Ribosome clamps onto mRNA, moves along three bases at a time
Each mRNA codon matches and anticodon on a specific tRNA
tRNA carries a specific anticodon at one end, amino acid matching that anticodon at the other end
Ribosomes align proper tRNAs and form the peptide bonds
Distinguish between transcription and translation.
Transcription = DNA bases → RNA bases by complementarity
Translation = RNA bases → amino acid sequence
Compare where transcription and translation occur in prokaryotes and in eukaryotes.
Transcription and translation both in the cytoplasm
Transcription in the nucleus; pre-mRNA processed to mRNA
Translation in the cytoplasm (with possible exceptions)
Define "codon" and explain the relationship between the linear sequence of codons on mRNA and the linear sequence of amino acids in a polypeptide.
Codon = three base sequence signifying the position of an amino acid
Order of codons gives the order of amino acids in protein
In general, conserved; but with taxonomic variations, and mutant forms
Explain the significance of the reading frame during translation.
mRNAs are read first three, next three, next three, etc.
Shifting the start one base changes the triplets downstream
Explain the early techniques used to identify what amino acids are specified by the triplets UUU, AAA, GGG, and CCC.
Synthesized poly-uracil to serve as an artificial mRNA
In vitro protein synthesis made only poly-phenylalanine
Explain why polypeptides begin with methionine when they are synthesized.
Don't always, see code variation link above
Codon for met often signifies the starting place for the ribosome
Often removed after translation
Explain in what way the genetic code is redundant and unambiguous.
Redundant: more than one codon for most amino acids
Unambiguous: in an organism, each codon codes for a specific amino acid
Explain the evolutionary significance of a nearly universal genetic code.
Lateral transfer (species-to-species) of genetic material is fruitful
Homologous structures signify a common ancestor with the structure
Universal code indicates a single common ancestor for all living systems
Explain how RNA polymerase recognizes where transcription should begin. Describe the promoter, the terminator, and the transcription unit.
RNA polymerase binds at the promoter
Promoter includes transcription start point
Determines template strand
Protein transcription factors bind to the promoter first + RNA polymerase = transcription initiation complex
In prokaryotes, proteins bound to RNA polymerase specify binding
Transcription unit extends from within the promoter past the terminator
Palindromic terminator sequence may form a hairpin loop
Explain the general process of transcription, including the three major steps of initiation, elongation, and termination.
Eukayotes: half of promoters have TATAAAA; binds transcription factor
Prokaryotes: TATAAAT is a common RNA polymerase binding site
Elongation of the RNA Strand
RNA polymerase adds nucleotides to 3' end
No proofreading function
Termination of Transcription
Terminator sequence is transcribed
Transcribed sequence forms hairpin in prokaryotes
AAUAAA termination sequence in eukaryotes reveals 3' processing site
Explain how RNA ends are modified after transcription in eukaryotic cells.
Methyl-G and PPP added
Protects from degradation and allows ribosome binding
3' poly(A) tail
Pre-mRNA cut downstream from AAUAAA; poly(A) added
Same functions as 5' cap
Describe the segmental arrangement of the mRNA codons and the splicing mechanism.
Exon = expressed sequence; intron = interspersed sequence
Precise, complex excision involving spliceosomes
Spliceosomes are protein + small nuclear RNA
Define and explain the role of ribozymes.
RNA-only enzymes involved in RNA processing
Introns may also catalyze their own excision
Describe the functional and evolutionary significance of introns.
Introns may control gene activity
Alternate splicing can give >1 protein from the same gene
Exons code for functional domains of proteins
Widely spaced functional components allows recombination → new proteins
Describe the structure and functions of tRNA.
Short, about 80 nucleotides
Folded with four base-paired regions
Amino acid attachment site at 3' end
Anticodon at the other end of folded molecule
Modified after transcription
Transfer amino acids to ribosome
Anticodon matches amino acid
Anticodon hydrogen bonds to mRNA codon
Wobble allows fewer than 61 tRNAs
Amino acids added by aminoacyl-tRNA synthetases
Only 20, not one to recognize each anticodon
Recognize the end with the amino acid attachment site
Describe the structure and functions of ribosomes.
Large with 60% rRNA and 40% protein
Two subunits found separately unless actively translating
Groove for mRNA
P-, A-, and E- sites for tRNA binding
Clamp onto mRNA at start codon
Form peptide bonds
Move codon-by-codon along mRNA
Describe the process of translation (including initiation, elongation, and termination) and explain which enzymes, protein factors, and energy sources are needed for each stage.
Ribosome Association and Initiation of Translation
Small subunit binds to mRNA leader or 5' cap and tRNAmet in P-site
Large subunit then binds
Protein initiation factors and energy from GTP are required
Elongation of the Polypeptide Chain
Next tRNA binds codon under A-site; uses elongation factors and GTP
Peptide bond formed
Growing polypeptide chain bound to tRNA in A-site
Translocation = ribosome shifts over one codon
Termination of Translation
Release factor binds stop codon (UAA, UAG, or UGA) under A-site
Water molecule added (hydrolysis)
New polypeptide released from last tRNA
Describe the significance of polyribosomes.
Ribosomes can sequentially attach to mRNAs
Allows many copies of a protein to be made using the same message
Explain what determines the primary structure of a protein and describe how a polypeptide must be modified before it becomes fully functional.
Primary structure = amino acid order dictated by mRNA
Chaperone proteins enable folding
Sugars, lipids, or phosphates may be added
Pieces may be removed
More than one chain may be bound together
Describe what determines whether a ribosome will be free in the cytosol or attached to the rough endoplasmic reticulum.
Membrane-bound ribosomes make endomembrane and secreted proteins
Amino-terminal signal peptide is bound by signal-peptide recognition particle
Complex binds to rough ER
Signal peptide is removed from finished protein
Define "point mutations." Distinguish between base-pair substitutions and base-pair insertions. Give examples of each and note the significance of such changes.
Point mutations change one or a few base pairs
Change a single base, maybe a single amino acid (missense)
Don't affect codons downstream
Usually have less drastic effects
Example: sickle-cell; one amino acid changed, altered protein shape
Insertions and Deletions
Change the reading frame = frameshift mutations
Change all codons downstream often to a stop codon (nonsense)
May change all amino acids downstream and can truncate
Describe several examples of mutagens and explain how they cause mutations.
Radiation: gamma, X, UV, cosmic rays
Chemical: nitrous acid, sodium azide, bromine, arsenic, base analogs
Radiation can break bonds holding DNA together
Base analogs are incorporated but base-pair incorrectly
Reactive chemical mutagens react with bases in DNA
Compare protein synthesis in prokaryotes and eukaryotes.
Eukaryotes have 3, prokaryotes have 1
Initiation factors bind to promoter first in eukaryotes
Different termination signals and ribosome sizes
Prokaryotes can have simultaneous transcription and translation
Describe the historical evolution of the concept of a gene.
Mendel: discrete unit of inheritance that affects phenotype
Morgan: place on a chromosome, locus
Region of a specific nucleotide sequence
DNA sequence coding for a protein
Any sequence that change be changed to alter phenotype