102 Mollecular Bio_of Gene

BIOLOGYCONCEPTS & CONNECTIONS

Fourth Edition

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor

From PowerPoint® Lectures for Biology: Concepts & Connections

CHAPTER 10Molecular Biology of the Gene

Modules 10.6 – 10.16


• The information constituting an organism’s genotype is carried in its sequence of bases

THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN

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


• A specific gene specifies a polypeptide

– The DNA is transcribed into RNA, which is translated into the polypeptide

Figure 10.6A

DNA

DNA

Protein

TRANSCRIPTION

TRANSLATION


• Studies of inherited metabolic disorders first suggested that phenotype is expressed through proteins• Studies of the bread mold Neurospora crassa led to the one gene-one polypeptide hypothesis

Figure 10.6B


• 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

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


Figure 10.7

DNA molecule

Gene 1

Gene 2

Gene 3

DNA strand

TRANSCRIPTION

RNA

Polypeptide

TRANSLATIONCodon

Amino acid


• Virtually all organisms share the same genetic code

10.8 The genetic code is the Rosetta stone of life

Figure 10.8A


• An exercise in translating the genetic code

Figure 10.8B

Startcodon

RNA

Transcribed strand

StopcodonTranslation

Transcription

DNA

Polypeptide


10.9 Transcription produces genetic messages in the form of RNA

Figure 10.9A

RNApolymerase

RNA nucleotide

Direction oftranscription

Newly made RNA

Templatestrand of DNA


• In transcription, the DNA helix unzips– RNA nucleotides

line up along one strand of the DNA following the base-pairing rules

– The single-stranded messenger RNA peels away and the DNA strands rejoin

RNA polymerase

DNA of gene

PromoterDNA Terminator

DNAInitiation

Elongation

Termination

Area shownin Figure 10.9A

GrowingRNA

RNApolymerase

Completed RNA

Figure 10.9B


• Noncoding segments called introns are spliced out

• A cap and a tail are added to the ends

10.10 Eukaryotic RNA is processed before leaving the nucleus

Figure 10.10

DNA

RNAtranscriptwith capand tail

mRNA

Exon Intron IntronExon Exon

TranscriptionAddition of cap and tail

Introns removed

Exons spliced together

Coding sequence

NUCLEUS

CYTOPLASM

Tail

Cap


• In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide

• The process is aided by transfer RNAs

10.11 Transfer RNA molecules serve as interpreters during translation

Figure 10.11A

Hydrogen bond

Amino acid attachment site

RNA polynucleotide chain

Anticodon


• Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other

Figure 10.11B, C

Anticodon

Amino acidattachment site


10.12 Ribosomes build polypeptides

Figure 10.12A-C

Codons

tRNAmolecules

mRNA

Growingpolypeptide

Largesubunit

Smallsubunit

mRNA

mRNAbindingsite

P site A site

P A

Growingpolypeptide

tRNA

Next amino acidto be added topolypeptide


10.13 An initiation codon marks the start of an mRNA message

Figure 10.13A

End

Start of genetic message


• mRNA, a specific tRNA, and the ribosome subunits assemble during initiation

Figure 10.13B

1

Initiator tRNA

mRNA

Startcodon Small ribosomal

subunit

2

P site

Largeribosomalsubunit

A site


• The mRNA moves a codon at a time relative to the ribosome

– A tRNA pairs with each codon, adding an amino acid to the growing polypeptide

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


Figure 10.14

1 Codon recognition

Amino acid

Anticodon

AsiteP site

Polypeptide

2 Peptide bond formation

3 Translocation

Newpeptidebond

mRNAmovement

mRNA

Stopcodon


• The sequence of codons in DNA spells out the primary structure of a polypeptide

– Polypeptides form proteins that cells and organisms use

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


• Summary of transcription and translation

Figure 10.15

1Stage mRNA istranscribed from aDNA template.

Anticodon

DNA

mRNARNApolymerase

TRANSLATION

Enzyme

Amino acid

tRNA

InitiatortRNA

Largeribosomalsubunit

Smallribosomalsubunit

mRNA

Start Codon

2Stage Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP.

3Stage Initiation of polypeptide synthesis

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

TRANSCRIPTION


Figure 10.15 (continued)

4Stage ElongationGrowingpolypeptide

Codons

5Stage Termination

mRNA

Newpeptidebondforming

Stop Codon

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

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

Polypeptide


• Mutations are changes in the DNA base sequence

– These are caused by errors in DNA replication or by mutagens

– The change of a single DNA nucleotide causes sickle-cell disease

10.16 Mutations can change the meaning of genes


Figure 10.16A

Normal hemoglobin DNA

mRNA

Normal hemoglobin

Glu

Mutant hemoglobin DNA

mRNA

Sickle-cell hemoglobin

Val


• Types of mutations

Figure 10.16B

mRNA

NORMAL GENE

BASE SUBSTITUTION

BASE DELETION

Protein Met Lys Phe Gly Ala

Met Lys Phe Ser Ala

Met Lys Leu Ala His

Missing

102 Mollecular Bio_of Gene

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