Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN The DNA genotype is

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN

• The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits

• The information constituting an organism’s genotype

– Is carried in its sequence of its DNA bases

• A particular gene, a linear sequence of many nucleotides

– Specifies a polypeptide


• The DNA of the gene is transcribed into RNA

– Which is translated into the polypeptide

Figure 10.6A

DNA

Transcription

RNA

Protein

Translation


Genetic information written in codons is translated into amino acid sequences

• The “words” of the DNA “language”

– Are triplets of bases called codons

• The codons in a gene

– Specify the amino acid sequence of a polypeptide


DNA strand

Transcription

Translation

Polypeptide

RNA

Amino acid

Codon

A A A C C G G C A A A A

U U U G G C C G U U U U

Gene 1

Gene 2

Gene 3

DNA molecule

Figure 10.7


The genetic code is the Rosetta stone of life

• Nearly all organisms

– Use exactly the same genetic code

Figure 10.8A

UUC

UGUUGC

UGA Stop

Met or start

Phe

Leu

Leu

Ile

Val Ala

Thr

Pro

Ser

Asn

Lys

His

Gln

Asp

Glu

Ser

Arg

Arg

Gly

CysTyr

G

A

C

U

U C A G

Th

ird

bas

e

Second base

Fir

st b

ase

UUA

UUU

CUC

CUU

CUG

CUA

AUC

AUU

AUG

AUA

GUC

GUU

GUG

GUA

UCC

UCU

UCG

UCA

CCC

CCU

CCG

CCA

ACC

ACU

ACC

ACA

GCC

GCU

GCG

GCA

UAC

UAU

UAG Stop

UAA Stop

CAC

CAU

CAGCAA

AAC

AAU

AAG

AAA

GAC

GAU

GAG

GAA

UGG Trp

CGC

CGU

CGGCGA

AGCAGU

AGG

AGA

GGC

GGU

GGG

GGA

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

UUG


RNA codon table, with corresponding amino acids


• An exercise in translating the genetic code

Figure 10.8B

T A C T T C A A A A T C

A T G A A G T T T T A G

A U G A A G U U U U A G

Transcription

Translation

RNA

DNA

Met Lys PhePolypeptide

Startcondon

Stopcondon

Strand to be transcribed


Transcription produces genetic messages in the form of RNA

• A close-up view of transcription

RNApolymerase

RNA nucleotides

Direction of transcription

Template Strand of DNA

Newly made RNA

TC

AT C C A A T

T

GG

CC

AATTGGAT

G

U

C A U C C AA

U

Figure 10.9A


• In the nucleus, the DNA helix unzips

– And RNA nucleotides line up along one strand of the DNA, following the base pairing rules

• As the single-stranded messenger RNA (mRNA) peels away from the gene

– The DNA strands rejoin


• Transcription of a gene RNA polymerase

DNA of gene

PromoterDNA Terminator

DNA

Area shownIn Figure 10.9A

GrowingRNA

Completed RNARNApolymerase

Figure 10.9B

1 Initiation

2 Elongation

3 Termination


Eukaryotic RNA is processed before leaving the nucleus

• Noncoding segments called introns are spliced out

– And a cap and a tail are added to the endsExon Intron Exon Intron Exon

DNA

Cap TranscriptionAddition of cap and tail

RNAtranscript with capand tail

Introns removedTail

Exons spliced together

mRNA

Coding sequence Nucleus

Cytoplasm

Figure 10.10


Transfer RNA molecules serve as interpreters during translation

• Translation

– Takes place in the cytoplasm


• A ribosome attaches to the mRNA

– And translates its message into a specific polypeptide aided by transfer RNAs (tRNAs)

Amino acid attachment site

Hydrogen bond

RNA polynucleotide chain

AnticodonFigure 10.11A


• Each tRNA molecule

– Is a folded molecule bearing a base triplet called an anticodon on one end

• A specific amino acid

– Is attached to the other endAmino acidattachment site

AnticodonFigure 10.11B, C


Ribosomes build polypeptides

• A ribosome consists of two subunits

– Each made up of proteins and a kind of RNA called ribosomal RNA

tRNAmolecules

mRNA Small subunit

Growingpolypeptide

Largesubunit

Figure 10.12A


• The subunits of a ribosome

– Hold the tRNA and mRNA close together during translation

Largesubunit

mRNA-binding site

Smallsubunit

tRNA-binding sites

Growing polypeptide

Next amino acid to be added to polypeptide

mRNA

tRNA

Codons

Figure 10.12B, C


An initiation codon marks the start of an mRNA message

Start of genetic message

End

Figure 10.13A


• mRNA, a specific tRNA, and the ribosome subunits

– Assemble during initiation

Met Met

Initiator tRNA

1 2mRNA Small ribosomal

subunit

Startcodon

Large ribosomalsubunit

A siteU A CAU C

A U G A U G

P site

Figure 10.13B


Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation

• Once initiation is complete

– Amino acids are added one by one to the first amino acid


• Each addition of an amino acid

– Occurs in a three-step elongation process

Polypeptide

P site

mRNA Codons

mRNAmovement

Stopcodon

NewPeptidebond

Anticodon

Aminoacid

A site

Figure 10.14

1 Codon recognition

2 Peptide bondformation

3 Translocation


• The mRNA moves a codon at a time

– And a tRNA with a complementary anticodon pairs with each codon, adding its amino acid to the peptide chain


• Elongation continues

– Until a stop codon reaches the ribosome’s A site, terminating translation


Review: The flow of genetic information in the cell is DNARNAprotein

• The sequence of codons in DNA, via the sequence of codons

– Spells out the primary structure of a polypeptide


Polypeptide

TranscriptionDNA

mRNA

RNApolymerase

Amino acid Translation

tRNA

Enzyme

Anticodon

ATP

InitiatortRNA

Largeribosomalsubunit

Start Codon

Codons

mRNA

Stop codon

Smallribosomalsubunit

Growingpolypeptide

New peptidebond forming

mRNA

Figure 10.15

• Summary of transcription and translation

mRNA is transcribed from a DNA template.1

Each amino acidattaches to its propertRNA with the help of aspecific enzyme and ATP.

2

Initiation ofpolypeptide synthesis

The mRNA, the first tRNA,and the ribosomal subunits come together.

3

Elongation4A succession of tRNAsadd their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time.

5

The ribosome recognizes a stop codon. The poly-peptide is terminated and released.

Termination


Mutations can change the meaning of genes

• Mutations are changes in the DNA base sequence

– Caused by errors in DNA replication or recombination, or by mutagens

C T T C A T

Normal hemoglobin

Mutant hemoglobin DNA

G A A G U A

Sickle-cell hemoglobin

Normal hemoglobin DNA

Glu Val

mRNA mRNA

Figure 10.16A


• Substituting, inserting, or deleting nucleotides alters a gene

– With varying effects on the organismNormal gene

mRNA

Base substitution

Base deletion Missing

Met Lys Phe Gly Ala

Met Lys Phe Ser Ala

Met Lys Leu Ala His

A U G A A G U U U G G C G C A

A U G A A G U U U A G C G C A

A U G A A G U U G G C G C A U

U

Protein

Figure 10.16B

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Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN The DNA genotype is