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

Translation assembly comes apart