Chapter 17: Gene Expression: From Gene to Protein
Concept 17.1 Genes Specify Proteins Via Transcription and Translation
Gene Expression
Process where DNA directs synthesis of proteins
2 Stages
Translation
Transcription
Evidence from Studying Metabolic Defects
Garrod - suggested genes dictate phenotypes through enzymes, proteins that catalyze specific chem reactions in the cell
"inborn errors of metabolism"
One gene-One enzyme hypothesis
Garrod's hypothesis that a gene dictates the production of a specific enzyme
Cells synthesize and degrade most organic molecules thru metabolic pathways
Drosophila
each mutation affecting eye color blocks pigment synthesis at a specific step preventing enzyme production
Nutritional Mutants in Neurospora: Scientific Inquiry
change in a mutants phenotype
X rays where known to cause mutations
wild type neurospora has modest food requirements
Minimal medium - wild type mold cells use their metabolic pathways to produce moleules for growth
Basic Principles of Transcription and Translation
genes give instructions for making specific proteins
gene DOES NOT build a protein directly
bridge bt DNA and protein synthesis is the nucleic acid RNA
DNA - A, G, C, T
RNA - A, G, C, U
In DNA/RNA, monomers are the 4 types of nucleotides that differ in nitrogenous bases
Transcription
synthesis of RNA using info in the DNA
Messenger RNA (mRNA)
carries genetic message from DNA to protein synthesis machinery of the cell
Translation
synthesis of a polypeptide using the info in mRNA
change in language
cell must translate nucleotide sequence of mRNA into amino acid sequence of a polypeptide
Ribosomes
sites of translation
molecular complexes that facilitate the orderly linkage of amino acids in to polypeptide chains
Primary Transcript
initial RNA transcript form any gene, even specifying RNA that isn't translated into protein
DNA ---> RNA ---> PROTEIN
The Genetic Code
There are only 4 nucleotide bases to specify 20 amino acids
can't be a language
Codons: Triplets of Nucleotides
smallest units of uniform length that can code foe all amino acids
verified that the flow of info from gene to protein is based on a triplet code.
Triplet Code
genetic instructions fora . polypeptide chain are written in the DNA asa series of non overlapping , 3 nucleotide words
Coding Code
non template DNA strand
used when genes sequence is reported
Concept 17.2 Transcription is the DNA-directed Synthesis of RNA: A Closer Look
Molecular Components of Transcriptions
RNA Polymerase
an enzyme that prise the 2 strands of DNA apart & joins RNA nucleotides complementary to the DNA template strand, elongating the RNA polynucleotide
Promoter
the DNA sequence where RNA polymerase attaches and initiates transcription
in bacteria, the sequence that signals the end of transcription is called terminator
"Downstream" & "Upstream"
Transcription Unit
The Stretch of DNA downstream from the promoter that's transcribed into an RNA molecule
Bacteria have a single type of RNA polymerase
synthesizes mRNA & other types of RNA that function in protein synthesis
EX: Ribosomal RNA
The promoter of a gene includes within it the transcription start point-- the nucleotide where RNA polymerase actually begins synthesis of the mRNA -- & extends nucleotide pairs upstream form the start point
RNA Polymerase Binding and Initiation of Transcription
Transcription Factors
in eukaryotes, a collection of proteins
mediate the binding of RNA polymerase and the initiation of transcription
Transcription Initiation Complex
the whole complex of of transcription factors and RNA polymerase II bound to the promoter
TATA Box
DNA sequence in forming the initiation complex at a eukaryotic promoter
Elongation of the RNA Strand
Termination of Transcription
bacteria and eukaryotes differ in the way they terminate transcription
Bacteria
transcription proceeds thru a terminator sequence in DNA
Eukaryotes
RNA polymerase II transcribes a sequence on the DNA called polyadenylation signal sequence
Concept 17.3 Eukaryotic Cells Modify RNA After Transcription
enzymes in eukaryotic nucleus modify pre-mRNA before the genetic message is dispatched to the cytoplasm
RNA Processing
both ends of the primary transcript are altered
produce an mRNA molecule ready for translation
Alteration of mRNA Ends
Split Genes and RNA Splicing
RNA Splicing
a remarkable stage of RNA processing in the eukaryotic nucleus
where large portions of the RNA molecules are removed and the remaining portions are reconnected
click to edit
Introns
non coding segments of nucleic acid that lie bt coding regions
Exons
the other regions, translated into amino acid sequences
Spliceosome
the removal of introns is accomplished by a large complex made of proteins and small RNAs
Alternative RNA Splicing
many genes are known to give rise to 2 or more diff polypeptides, depending of which segments are treated as exons during RNA processing
Domains
proteins often have a modular architecture consisting of discrete structural and functional regions
Concept 17.4 Translation is the RNA-Directed Synthesis of a Polypeptide
Transfer RNA (tRNA)
the message is a series of codons along an mRNA molecule, & the translator is called tRNA
process of translations "reads" a genetic message and builds a polypeptide accordingly
Structure & Function of tRNA
the key is the fact that each tRNA molecule enables translation of a given mRNA codon to a certain amino acid
Anticodon
the loop extending from the other end of the L includes the anticodon
the particular nucleotide triplet that base-pairs to a specific mRNA codon
Aminoacyl-tRNA Synthesis
the correct matching up of tRNA and amino acid is carried out by a family of related enzymes
Wobble
the flexible base pairing at this condon position
Structure & Function of Ribosomes
Ribosomal RNAs (rRNAs)
a ribosome consists of a large subunit and a small subunit, each made up of proteins and one or more rRNAs
in eukaryotes, the subunits are made in the nucleus
P Site
Peptideyl-tRNA - holds the tRNA carrying the growing polypeptide chain
A Site
Aminoacyl-tRNA - holds the tRNA carrying the next amino acid to be added to the chain
E Site
Exit site - where the discharged tRNAs leave the ribosome from
Polyribosomes
Concept 17.5 Mutations of One or a Few Nucleotides ca Affect Protein Structure and Function
Mutations
changes to the genetic info of a cell
responsible for the huge diversity of genes found among organisms bc mutations are the ultimate source of new genes
Point Mutations
small scale mutations of one or a few nucleotide pairs including point mutations
changes in a single nucleotide pair of a gene
Nucleotide pair sub
the replacement of one nucleotide and its partner w another pair of nucleotides
silent mutation
missense mutations
no observable effect on the phenotype
substitutions that change one amino acid to another one
Inertions & Disertions
additions or losses of nucleotide pairs in a gene
Frameshift Mutation
happens when the number of nucleotides inserted or deleted isn't a multiple of 3
Mutagents
a number of physical and chemical agents
interact w DNA in ways that cause mutations