THE FLOW OF GENETIC INFORMATION FROM DNA TO

RNA TO PROTEIN

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• describe how transcription and translation regulate the flow of cellular information• identify how the language of nucleic acid is translated into the language of proteins• use the genetic code to predict what amino acid will be added based on a sequence of DNA• describe how transcription leads to the creation of mRNA

• differentiate between introns and exons

Learning Objectives

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

§ DNA specifies traits by dictating protein synthesis.

§ The molecular chain of command is from– DNA in the nucleus to RNA and

– RNA in the cytoplasm to protein.

§ Transcription is the synthesis of RNA under the direction of DNA.

§ Translation is the synthesis of proteins under the direction of RNA.

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Figure 10.6A_s1

DNA

NUCLEUS

CYTOPLASM

Figure 10.6A_s2

DNA

NUCLEUS

CYTOPLASM

RNA

Transcription

Figure 10.6A_s3

DNA

NUCLEUS

CYTOPLASM

RNA

Transcription

Translation

Protein

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

§ The connections between genes and proteins– The initial one gene–one enzyme hypothesis was

based on studies of inherited metabolic diseases.

– The one gene–one enzyme hypothesis was expanded to include all proteins.

– Most recently, the one gene–one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides.

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10.7 Genetic information written in codons is translated into amino acid sequences

§ The sequence of nucleotides in DNA provides a code for constructing a protein.

– Protein construction requires a conversion of a nucleotide sequence to an amino acid sequence.

– Transcription rewrites the DNA code into RNA, using the same nucleotide “language.”

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10.7 Genetic information written in codons is translated into amino acid sequences

– The flow of information from gene to protein is based on a triplet code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping three-base “words” called codons.

– Translation involves switching from the nucleotide “language” to the amino acid “language.”

– Each amino acid is specified by a codon.– 64 codons are possible.– Some amino acids have more than one possible codon.

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Figure 10.7

DNAmolecule

Gene 1

Gene 2

Gene 3

ATranscription

RNA

Translation Codon

Polypeptide

Aminoacid

A A C C G G C A A A A

U U G G C C G U U U U

DNA

U

Figure 10.7_1

A

Transcription

RNA

Translation Codon

PolypeptideAminoacid

A A C C G G C A A A A

U U G G C C G U UU U

DNA

U

10.8 The genetic code dictates how codons are translated into amino acids

§ Characteristics of the genetic code

– Three nucleotides specify one amino acid.– 61 codons correspond to amino acids.

– AUG codes for methionine and signals the start of transcription.

– 3 “stop” codons signal the end of translation.

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10.8 The genetic code dictates how codons are translated into amino acids

§ The genetic code is

– redundant, with more than one codon for some amino acids,

– unambiguous in that any codon for one amino acid does not code for any other amino acid,

– nearly universal—the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals, and

– without punctuation in that codons are adjacent to each other with no gaps in between.

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Figure 10.8ASecond base

Third

bas

e

Firs

t bas

e

Figure 10.8B_s1

T

Strand to be transcribed

A C T T C AA

A A A T DNA

AA T C

T T T T G A G G

Figure 10.8B_s2

T

Strand to be transcribed

A C T T C AA

A A A T DNA

AA T C

T T T T G A G G

RNA

Transcription

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

Figure 10.8B_s3

T

Strand to be transcribed

A C T T C AA

A A A T DNA

AA T C

T T T T G A G G

RNA

Transcription

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

Translation

Polypeptide Met Lys Phe

Stopcodon

Startcodon

Figure 10.8C

10.9 Transcription produces genetic messages in the form of RNA

§ Overview of transcription

– An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication.

– RNA nucleotides are linked by the transcription enzyme RNA polymerase.

– Specific sequences of nucleotides along the DNA mark where transcription begins and ends.

– The “start transcribing” signal is a nucleotide sequence called a promoter.

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10.9 Transcription produces genetic messages in the form of RNA

– Transcription begins with initiation, as the RNA polymerase attaches to the promoter.

– During the second phase, elongation, the RNA grows longer.

– As the RNA peels away, the DNA strands rejoin.

– Finally, in the third phase, termination, the RNA polymerase reaches a sequence of bases in the DNA template called a terminator, which signals the end of the gene.

– The polymerase molecule now detaches from the RNA molecule and the gene.

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Figure 10.9A

RNApolymerase

Free RNAnucleotides

Templatestrand of DNA

Newly made RNA

Direction oftranscription

TG

AG G

A

A

U C C AC

T TA

AC

CG

GU

T UTAACCTA

TC

Figure 10.9B

RNA polymeraseDNA of gene

PromoterDNA

Initiation1

2

TerminatorDNA

3

Elongation Area shownin Figure 10.9A

TerminationGrowingRNA

RNApolymerase

CompletedRNA

Figure 10.9B_1

RNA polymerase

DNA of gene

PromoterDNA

Initiation1

TerminatorDNA

Figure 10.9B_2

2 Elongation Area shownin Figure 10.9A

GrowingRNA

Figure 10.9B_3

Termination

RNApolymerase

CompletedRNA

3GrowingRNA

10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA

§ Messenger RNA (mRNA)– encodes amino acid sequences and

– conveys genetic messages from DNA to the translation machinery of the cell, which in

– prokaryotes, occurs in the same place that mRNA is made, but in

– eukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasm.

– Eukaryotic mRNA has– introns, interrupting sequences that separate– exons, the coding regions.

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10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA

§ Eukaryotic mRNA undergoes processing before leaving the nucleus.– RNA splicing removes introns and joins exons to

produce a continuous coding sequence.

– A cap and tail of extra nucleotides are added to the ends of the mRNA to

– facilitate the export of the mRNA from the nucleus, – protect the mRNA from attack by cellular enzymes, and

– help ribosomes bind to the mRNA.

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Figure 10.10

DNACap

Exon Intron Exon

RNAtranscriptwith capand tail

ExonIntron

TranscriptionAddition of cap and tail

Introns removed Tail

Exons spliced together

Coding sequenceNUCLEUS

CYTOPLASM

mRNA

10.11 Transfer RNA molecules serve as interpreters during translation

§ Transfer RNA (tRNA) molecules function as a language interpreter,– converting the genetic message of mRNA

– into the language of proteins.

§ Transfer RNA molecules perform this interpreter task by– picking up the appropriate amino acid and

– using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the mRNA.

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Figure 10.11AAmino acid

attachment site

Hydrogen bond

RNA polynucleotidechain

Anticodon

A simplifiedschematic of a tRNA

A tRNA molecule, showingits polynucleotide strandand hydrogen bonding

Figure 10.11BEnzyme

tRNA

ATP

10.12 Ribosomes build polypeptides

§ Translation occurs on the surface of the ribosome.– Ribosomes coordinate the functioning of mRNA and

tRNA and, ultimately, the synthesis of polypeptides.

– Ribosomes have two subunits: small and large.

– Each subunit is composed of ribosomal RNAs and proteins.

– Ribosomal subunits come together during translation.

– Ribosomes have binding sites for mRNA and tRNAs.

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Figure 10.12A

tRNAmolecules

Growingpolypeptide

Largesubunit

Smallsubunit

mRNA

Figure 10.12B

tRNA binding sites

mRNA binding site

Large subunit

Small subunit

Psite

Asite

Figure 10.12C

mRNA

Codons

tRNA

Growingpolypeptide

The next aminoacid to be addedto the polypeptide

10.13 An initiation codon marks the start of an mRNA message

§ Translation can be divided into the same three phases as transcription:1. initiation,

2. elongation, and

3. termination.

§ Initiation brings together– mRNA,

– a tRNA bearing the first amino acid, and

– the two subunits of a ribosome.

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10.13 An initiation codon marks the start of an mRNA message

§ Initiation establishes where translation will begin.

§ Initiation occurs in two steps.1. An mRNA molecule binds to a small ribosomal subunit and

the first tRNA binds to mRNA at the start codon.– The start codon reads AUG and codes for methionine.

– The first tRNA has the anticodon UAC.

2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function.

– The first tRNA occupies the P site, which will hold the growing peptide chain.

– The A site is available to receive the next tRNA.

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Figure 10.13A

Start of genetic message

Cap

End

Tail

Figure 10.13B

InitiatortRNA

mRNA

Start codonSmallribosomalsubunit

Largeribosomalsubunit

Psite

Asite

Met

A U G

U A C

2

A U G

U A C

1

Met

10.14 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.

§ Elongation is the addition of amino acids to the polypeptide chain.

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§ Each cycle of elongation has three steps.1. Codon recognition: The anticodon of an incoming

tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome.

2. Peptide bond formation: The new amino acid is joined to the chain.

3. Translocation: tRNA is released from the P site and the ribosome moves tRNA from the A site into the P site.

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

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§ Elongation continues until the termination stage of translation, when– the ribosome reaches a stop codon,– the completed polypeptide is freed from the last tRNA,

and– the ribosome splits back into its separate subunits.

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

© 2012 Pearson Education, Inc.

Figure 10.14_s1

Polypeptide

mRNA

Codon recognition

Anticodon

Aminoacid

Codons

Psite

Asite

1

Figure 10.14_s2

Polypeptide

mRNA

Codon recognition

Anticodon

Aminoacid

Codons

Psite

Asite

1

Peptide bond2formation

Figure 10.14_s3

Polypeptide

mRNA

Codon recognition

Anticodon

Aminoacid

Codons

Psite

Asite

1

Peptide bond2formation

Translocation3

Newpeptidebond

Figure 10.14_s4

Polypeptide

mRNA

Codon recognition

Anticodon

Aminoacid

Codons

Psite

Asite

1

Peptide bond2formation

Translocation3

Newpeptidebond

Stopcodon

mRNAmovement

10.15 Review: The flow of genetic information in the cell is DNA ® RNA ® protein

§ Transcription is the synthesis of RNA from a DNA template. In eukaryotic cells,– transcription occurs in the nucleus and

– the mRNA must travel from the nucleus to the cytoplasm.

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10.15 Review: The flow of genetic information in the cell is DNA ® RNA ® protein

§ Translation can be divided into four steps, all of which occur in the cytoplasm:1. amino acid attachment,

2. initiation of polypeptide synthesis,

3. elongation, and

4. termination.

© 2012 Pearson Education, Inc.

Figure 10.15DNA

Transcription

mRNARNApolymerase

Transcription

Translation

Amino acid

Enzyme

CYTOPLASM

Amino acidattachment2

1

3

4

tRNAATP

Anticodon

Initiation ofpolypeptide synthesis

Elongation

Largeribosomalsubunit

InitiatortRNA

Start CodonmRNA

Growingpolypeptide

Smallribosomalsubunit

New peptidebond forming

CodonsmRNA

Polypeptide

Termination5

Stop codon