chapter 15-ii translation genes and how they work

Chapter 15-II Translation

Genes and How They Work

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tRNA and Ribosomes

• tRNA molecules carry amino acids to the ribosome for incorporation into a polypeptide– Aminoacyl-tRNA synthetases add amino acids to the

acceptor stem of tRNA– Anticodon loop contains 3 nucleotides complementary

to mRNA codons

3

4

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c: Created by John Beaver using ProteinWorkshop, a product of the RCSB PDB, and built using the Molecular Biology Toolkit developed by John Moreland and Apostol Gramada (mbt.sdsc.edu). The MBT is fi nanced by grant GM63208

2D “Cloverleaf” Model

Acceptor end

Anticodonloop

׳3׳5

3D Ribbon-like Model Acceptor end

Anticodon loop

3D Space-filled Model

Anticodon loop

Acceptor end

Icon

Anticodon end

Acceptor end

tRNA charging reaction

• Each aminoacyl-tRNA synthetase recognizes only 1 amino acid but several tRNAs

• Charged tRNA – has an amino acid added using the energy from ATP– Can undergo peptide bond formation without addition

al energy

• Ribosomes do not verify amino acid attached to tRNA

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6


tRNA

PiPi

NH3+

O–

C OC

O CO

OH

OAMP O

OH

AMP

Aminogroup

Carboxylgroup

TrpNH

3+ NH

3+

Trp Trp

ATPAminoacid site

Acceptingsite

Anticodonspecific to tryptophan

Aminoacyl-tRNAsynthetase

tRNAsite

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tRNA

PiPi

NH3+

O–

C OC OC

O CO

CO

OH

OAMP O

OH

AMP

AMP

O

O

Charged tRNA travels to ribosomeAminogroup

Carboxylgroup

TrpNH

3+ NH

3+

NH3

+

NH3+Trp Trp

TrpATP

Aminoacid site

Acceptingsite

Anticodonspecific to tryptophan

Aminoacyl-tRNAsynthetase

tRNAsite

ChargedtRNA

dissociates

Trp

9

• The ribosome has

– multiple tRNA binding sites• P site – binds the tRNA attached to the growing peptide chain• A site – binds the tRNA carrying the next amino acid• E site – binds the tRNA that carried the last amino acid

– two primary functions 1. Decode the mRNA 2. Form peptide bonds

Peptidyl transferase

mRNA

3´

3´

Largesubunit

Smallsubunit

Enzymatic component of the ribosome --Forms peptide bonds between amino acids

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Translation

• In prokaryotes, initiation complex includes– Initiator tRNA charged with N-formylmethionine– Small ribosomal subunit– mRNA strand

• Ribosome binding sequence (RBS) of mRNA positions small subunit correctly

• Large subunit now added• Initiator tRNA bound to P site with A site e

mpty

• Initiations in eukaryotes similar except– Initiating amino acid is methionine– More complicated initiation complex– Lack of an RBS – small subunit binds to 5′ cap of mRNA

• Elongation adds amino acids– 2nd charged tRNA can bind to empty A site– Requires elongation factor called EF-Tu to bind to tR

NA and GTP– Peptide bond can then form– Addition of successive amino acids occurs as a cycle

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• There are fewer tRNAs than codons• Wobble pairing allows less stringent pairing betw

een the 3′ base of the codon and the 5′ base of the anticodon

• This allows fewer tRNAs to accommodate all codons

• Termination– Elongation continues until the ribosome encounters a

stop codon– Stop codons are recognized by release factors which

release the polypeptide from the ribosome

Wobble pairing

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Protein targeting

• In eukaryotes, translation may occur in the cytoplasm or the rough endoplasmic reticulum (RER)

• Signal sequences at the beginning of the polypeptide sequence bind to the signal recognition particle (SRP)

• The signal sequence and SRP are recognized by RER receptor proteins

• Docking holds ribosome to RER• Beginning of the protein-trafficking pathway

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Mutation: Altered Genes

Missense mutation

Silent mutation Nonsense mutations

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Polar

Normal HBB Sequence

Abormal HBB Sequence

Nonpolar (hydrophobic)

Amino acids

Nucleotides

Amino acids

Nucleotides

Leu

C C C CGT T TA GG A GAA

Thr Pro Glu Glu

CT TGAA

Lys Ser

Leu

C C C CGT T TA GG GAA

Thr Pro val Glu

CTT TGAA

Lys Ser

1

1

NormalDeoxygenated

Tetramer

AbnormalDeoxygenated

Tetramer

Tetramers form long chainswhen deoxygenated. Thisdistorts the normal red bloodcell shape into a sickle shape.

Hemoglobintetramer

"Sticky" non-polar sites

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2

1

1

2

2

Frameshift mutationsAddition or deletion of a single baseMuch more profound consequences

Alter reading frame downstreamTriplet repeat expansion mutation

Huntington diseaseRepeat unit is expanded in the disease allele relati

ve to the normal

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Chromosomal mutations

• Change the structure of a chromosome

– Deletions – part of chromosome is lost– Duplication – part of chromosome is copied– Inversion – part of chromosome in reverse order– Translocation – part of chromosome is moved to a

new location

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--Too much change, however, is harmful to the individual with a greatly altered genome

--Balance must exist between amount of new variation and health of species

Mutations are the starting point for evolution

chapter 15-ii translation genes and how they work

Documents

trna travels

initiator trna

trp slide

trna pipi pipi nh

ribosomes trna molecules

amino acids

amino acid e site

trp atp amino acid site