Ch. 17: From Gene to Protein Mendel’s work revealed that proteins are the link between genotype and phenotype Tall vs. dwarf height in pea plants was due to a growth hormone synthesized or not; due to a presence of an enzyme!! DNA directs synthesis of proteins: Transcription Translation
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Ch. 17: From Gene to Protein Mendel’s work revealed that proteins are the link between genotype and phenotype Tall vs. dwarf height in pea plants was due.
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Ch. 17: From Gene to ProteinMendel’s work revealed that proteins
are the link between genotype and phenotypeTall vs. dwarf height in pea plants was due
to a growth hormone synthesized or not; due to a presence of an enzyme!!
DNA directs synthesis of proteins:Transcription Translation
Beadle and TatumWorked with breadmold; x-rayed and
examined mutant growthDiscovered the function of a gene is to
dictate the production of a specific enzymeRestated hypothesis as one gene – one
polypeptideHowever, keep in mind…some genes code for
RNA that have important functions but are not translated into protein
GenesThe DNA provides the instructions to make
the protein RNA is the link between gene and proteinDNA codes for RNA and RNA codes for the
protein….known as the “central dogma” of biology
Transcription/TranslationThe DNA and RNA molecules are composed
of nucleotide monomers.When converting from DNA to RNA you are
simply transcribing the code from the language of DNA nucleotides to RNA nucleotides
Proteins are “written” in the language of amino acids.
When converting from RNA to protein we are translating from the nucleotide language to amino acid language
RNAIn what ways are RNA molecules different
from DNA?RNA is single strandedIn RNA Uracil replaces ThymineNucleotides have ribose instead of
deoxyribose.In eukaryotes RNA leaves the nucleus
What are the functions of these 4 different types of RNA?
4 types of RNAmRNA takes DNA’s message out to the
ribosome for protein synthesistRNA brings amino acids to the ribosome
for protein synthesisrRNA structural component of ribosomessnRNA involved in RNA splicing
The Genetic CodemRNA strand is complementary and
antiparallel to DNA templateRNA consists of four “letters” A, U, G,
and CProteins consist of 20 “letters” the
amino acidsIf 1 RNA base codes for 1 amino acid,
then only 4 amino acids can be coded for.
How many different amino acids can be coded for if 2 RNA’s code for 1 amino acid?
42 = 16 : Not enough!How many different amino acids can be
coded for if 3 RNA’s code for 1 amino acid?43 = 64: More than enough for the 20
different amino acids….
CodonsmRNA base triplets are called codonsCodons are read in the 5’3’ direction# of nucleotides making up the genetic
message is 3x the # of amino acids64 codons deciphered by mid 1960’s Stop codons: UAA, UGA, UAGStart signal and methionine: AUG
There is redundancy…2 codons for one a.a., but not ambiguity…1 codon doesn’t code for 2+a.a.!
The Code is UniversalThe code is shared by almost all organismsCCG codes for what amino acid?Proline. This holds true for all species of
living organisms.Bacteria, therefore can be programmed to
synthesize human proteins by inserting human DNA
Eukaryotic Transcription3 steps:
InitiationElongationTermination
RNA polymerases are usedRNA pol. II used for mRNA synthesisRNA pol. I and III used for all other RNA (not
coded into protein)Direction of transcription downstream
(5’3”)
InitiationSignaled by a promoter
DNA sequence is TATAAAA, called a “TATA” box
ElongationRNA pol. moves along DNA and untwists it
10-20 bases at a timeRNA nucleotides are added to 3’ end (about
60/sec in eukaryotes)DNA double helix reforms as new RNA peels
away
TerminationProkaryotes: terminator sequence on DNA
causing RNA pol. to detach and mRNA to be released
Eukaryotes: premRNA is cleaved due to a particular DNA sequence but needs to be processed into mRNA! 1. 5’end cap is added2. 3’ tail called a poly-A tail is added
In prokaryotes, RNA is directly translated into the polypeptide
RNA in eukaryotes is processed before translation
The function of the cap is:prevent mRNA degradation by hydrolytic
enzymeshelps attach to the ribosome
Function of the 3’ tail:same functions as the 5’capalso helps facilitate export of mRNA from nucleus
RNA splicingRemoves non-coding regions (introns)snRNP (short nuclear ribonucleoproteins)
recognize the splicing signals that are at the ends of introns
The RNA in the snRNP is called snRNA (small nuclear RNA)
snRNP + protein = spliceosomeThe spliceosome cuts and releases the
introns, and then joins exons together
Evolutionary role of intronsIntrons may play regulatory roleDifferent intron removal may lead to different
proteinsIntrons may enhance crossing over between
homologous regions by increasing the distance between exons
TranslationmRNA delivers the message in the
“nucleotide language”tRNA translates the message into the
“amino acid language”End of tRNA molecule is an anticodon…
triplet, complementary to mRNAEx. mRNA UUU; tRNA AAA +
phenylalanine
Structure and function of tRNATranscribed from template DNA strand in
nucleusUsed repeatedlyAbout 80 nucleotides long, single stranded
with H-bonds causing a 3D structure
Recognition steps to translation1. Amino acid joined to correct tRNA by
aminoacyl-tRNA synthetase…20 of those (each specific to an individual amino acid)
This step is catalyzed ATP The tRNA with the amino acid is known as aminoacyl
tRNA
2. Correct match between tRNA anticodon and mRNA codon
Wobble relaxation in the base pairing rules with 3rd base at the 3’ end of mRNA
tRNA Assembly
Ribosomes2 subunits (large and small)Constructed of protein and rRNAOnly functional when attached to mRNA2/3 of ribosomal mass is rRNA (most
abundant type of RNA)
Ribosomal binding sitesP site peptidyl tRNA site; holds the tRNA
carrying the growing polypeptide chainA site aminoacyl tRNA site; holds the tRNA
carrying the next amino acidE site exit site; site where tRNAs leave the
ribosome
Building a polypeptide3 stages of translation
InitiationElongationTermination
InitiationSmall ribosomal subunit binds to mRNA and
initiator tRNA carrying methionine Small subunit scans downstream along mRNA
until it reaches start codon … AUG, establishing the “reading frame”.
Initiator tRNA H-bonds to start codonmRNA + initiator tRNA + small ribosomal
subunit + large subunit = translation initiation complex … requires proteins called initiation factors and energy in the form of GTP
Proteins synthesized from N-terminus C-terminus
Elongation Proteins called elongation factors are
required to add new amino acids to preceding ones
GTP required Ribosomes moves along mRNA in the 5’
3’ direction 3 steps to elongation
1. Codon recognition2. Peptide bond formation3. Translocation (moving along A, P, E sites)
TerminationProtein called release factor binds to stop
codon in the A site bringing in a water molecule instead of an amino acid
Polypeptide is released through the exit tunnel of the ribosome’s large subunit