chapter 10: gene expression Flashcards
why diseases occur
mutant phenotype arises from a change in the protein’s amino acid sequence
archibald garrod
an early 1900s physician who applied mendelian genetics to patients and proposed that one gene codes to one enzyme
“inborn error of metabolism” (genetically determined biochem disease)
one gene - one polypeptide hypoth
one gene codes for one polypeptide (although they dont have to)
(modification of te one gene on protein hypoth)
molecular bio
the study of nucleic acids and proteins, focusing on gene expressiob
to translate genes into proteins
- transcription: the info in a dna sequence is copied into a complementary rna seq
- translation: rna sequence creates amino acid seq of a polypeptide
messenger rna
travels from nucleus to cytoplasm to be translated into a polypeptide
ribosomal rna
catalyzes peptide bond formation between amino acids fo form a polypeptide
transfer rna
mediates between mrna and protein; can 1. bind to a specific amino acid and 2. recognize a specific seq of nucleotides in mrna through complementary base pairing; recognizes which base should be added next to polypeptide chain
requirements for transcription
- dna template
- nucleotide triphoephates (atp, gtp, ctp, utp)
- an rna polymerase enzyme
promoter
dna sequence that initiates transcription by binding to the rna polymerase. they tell rna polymerase:
- where to start
- which two dna strands to transcribe
transcription initiation site
on the promoter; where transcription begins
elongation
rna polymerase unwinds 13 base pairs of dna and begins ro add new nucleotides to the growing strand
termination
base sequences that specify where to terminate transcription
coding regions
expressed as proteins; usually continuous
introns
noncoding sequences in genes that interrupt the coding region
exons
the regions between introns on genes; they are still expressed
nucleic acid hybridization
- dna denatured by heat to break hydrogen bonds and separate the strands
- a probe (single stranded nucleic acid from another source) is incubated with the dna. if complementary, the strands will join through hydorgen bonding
RESULT: a hybrid
rna splicing
removes introns and splices exons together
consensus sequences
- short stretches of dna that appear with little variation, despite the gene
- at boundaries of introns/exons
- bound by small nuclear ribonucleoprotein particles (snRNPS), ehich have complementary bases to the consensus sequence
spliceosome
rna-protein complex that cuts the pre-mRNA, releases the introns, and joins the exons
RESULT: mature mRNA
alternative splicing hypothesis
- not all exons are inclided in every mRNA; some are spliced out
- results in different mRNAs and diff polypeptides from a single gene
5’ cap
- added to the 5’ end of the pre-mRNA as it is transcribed
- facilitates the binding of mRNA to the ribosome for translation and protects mRNA from being digested by ribonucleases (enzymes)
poly A tail
- added to the 3’ end of pre-mRNA
- helps exit from nucleus
- mRNA stability
redundant code
many diff codons for each amino acid
ambiguous code
***genetic code is NOT ambiguous
would mean that a single codon codes for diff amino acids
silent mutation
- genetic code is redundant
- no change in amino acid sequence
missense mutations
change in amino acid sequence
nonsense mutation
result in premature stop codon
frame shift mutation
result in insertion or deletion
how to ensure right protein is made
- transfer rnas (tRNAs) must chemically read each mRNA codon correctly
- tRNA must deliver the amino acid that corresponds with the codon
tRNAs
- specific to each of the 20 amino acids (when carrying it, the trna is “charged”)
- bind to mRNA: rhe anticodon on trna is complementary to the mrna codon for the amino acid
- trnas interact with ribosomes
aminoacyl-tRNA synthetases
family of enzymes specific to amino acid/corresponding trna
ribosome sites where trna can bind
- a (amino acid) site: charged trna anticodon binds to mrna codon
- p (polypeptide) site: trna adds its amino acid to the polypeptide chain
- e (exit) site: trna rests before picking up another amino acid and restarting
fidelity function
- ensures charged trna with correct anticodon binds to right codon on mRNA
- hydrogen bonds will form
initiation complex
- charged trna and small ribosomal subunit bind to mRNA
- small subunit moves slong mRNA until it reaches start codon
- anticodon binds to start codon
peptidyl transferase activity
- large subunit breaks bond between methionine and its tRNA in P site
- large subunit catalyzes the formation of oeotide bond between methionine and anino acid
release factor
protein that allows hydrolysis of bond between polypeptide chain and tRNA in p site
stops elongation cycle
polyribosome/polysome
an assemblags of a strand of mRNA and its beadlike ribosomes + their growinf peptide chains
posttranslational aspects of protein synthesis
proteins can be modified after translation (adding new chem groups that contribute to fuction of mature protein)
signal sequence (aka signal peptide)
short stretch of amino scids thst indicates where in the cell the polypeptide belongs
(diff signals depending on where proteins are destined to go)
proteolysis
- modification of protein post translation
- cutting of the polypeptide chain
- some proteins are actually polyproteins (long polypeptides of distinct proteins) and can be cut into proteases
glycoslation
- modification of protein post translation
- addition of carbohydrates to proteins –> form glycoproteins
phosphorylation
- modification of protein post translation
- addition of phosphate groups
- change the conformstion of the protein, exposing binding site for another protein or active site for an enzyme
- caralyzed by protein kinases: important in cell signalling
