chapter 10 protein synthesis

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 10Protein Synthesis


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

• The flow of information is from DNA to RNA to protein

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


• A specific gene specifies a polypeptide

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

Figure 10.6A

DNA

RNA

Protein

TRANSCRIPTION

TRANSLATION

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• 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


Mutate wild type fungus

*Supply all mutant isolates with complete media

*Grow purified mutants with minimal media

to find nutritional mutants

*Determine what is the nutritional limitation find mutation


There for the gene used to produce an enzyme that helps cells manufacture Arginine amino acid

was mutated in that fungal strain


Transcription produces genetic messages in the form of RNA

Figure 10.9A

RNApolymerase

RNA nucleotide

Direction oftranscription

Newly made RNA

Templatestrand of DNA


RNA Transcription

• Process in which the genetic information on DNA is transferred to RNA

• During transcription only 1 DNA stand serves as the template or pattern from which RNA is formed.


• 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 10.9Bhttp://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf


RNA Transcription

1. Initiation

• The enzyme RNA polymerase attaches to the promoter site on the DNA

• Promoter – a sequence of nucleotides that is found on one of the DNA strands

– tells RNA polymerase to start transcription and which of the two DNA strands to transcribe


RNA Transcription

2. Elongation

• RNA nucleotides attach to the free DNA nucleotides by hydrogen bonds one at a time

• As RNA synthesis continues the growing RNA strand peels away from the DNA and the DNA strands rejoin


RNA Transcription

3. Termination

• RNA polymerase reaches the terminator.

• Terminator – a sequence of bases on DNA that signals the end of the gene

• The RNA polymerase detaches from the DNA and the RNA molecule is complete


• Noncoding segments called introns are spliced out

• The coding segments called exons are joined together

• 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

http://www.four-h.purdue.edu/apple_genomics/flash/movie3.swf

http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter14/animation_quiz_3.html


• 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

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

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


Translation

• The process in which a polypeptide is synthesized using the genetic information encoded on an mRNA molecule

• The following are needed for translation to occur

1. mRNA

- Contains the instructions for the assembly of proteins

- Codon – a sequence of 3 bases on mRNA that specifies a specific amino acid that will be added to the polypeptide chain


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

• The process is aided by transfer RNAs

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


Translation

2. tRNA (transfer RNA)

• Carries an amino acid to the ribosome

• A tRNA molecule is composed of

– A single strand of RNA (about 80 nucleotides)

– A loop at one end that contains the anticodon

– Anticodon – a sequence of 3 bases on tRNA that are complementary to the bases on mRNA

– At the opposite end of the loop is a site where an amino acit can attach


Translation

3. Amino acids

• Located in the cytoplasm

• Synthesized from other chemicals or obtained from food


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


Translation

4. Ribosomes

• Organelles where protein synthesis occurs

• Consists of 2 subunits each made up of proteins and ribosomal RNA (rRNA)

– Small subunit – has binding site for mRNA

– Large subunit – has binding site for tRNA


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


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


• 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


Steps of Translation

1. Initiation

• mRNA binds to the ribosome

• The start codon (AUG) is reached

• The first amino acid (methionine) is brought to the ribosome by the tRNA

2. Elongation

• Amino acids are added one by one to a growing polypeptide chain


Steps of Translation

3. Termination

• The stop codon is reached

• The completed polypeptide is released


Modification of the polypeptide

Endoplasmic reticulum

• Collects proteins made by the ribosomes

• Packages them into vesicles which move to the Golgi apparatus

Golgi apparatus

• Proteins are altered, packaged into vesicles, and transported to different parts of the cell or exported out of the cell


• 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


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

– Polypeptides form proteins that cells and organisms use

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


• 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

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

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• 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


Types of Mutations

There are 2 general categories of mutations:

1. Base substitution

• The replacement of one nucleotide with another

• Can result in no change in the protein

• An insignificant change

– The altered amino acid has no effect on the function of the protein


Types of Mutations

• A change that is crucial to life of the organism

– The altered amino acid has an effect on the function of the protein

2. Base insertions or deletions

• One or more bases are added or deleted from the DNA

• Often have disastrous effects

– The nucleotide sequence following the change alters the genetic message (reading frame)


Mutations are Useful

Mutations are useful because they

1. Provide diversity that allows evolution by natural selection to occur

2. Essential tool for geneticists

• Create different alleles needed for genetic research

chapter 10 protein synthesis

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