Protein Synthesis 06-07

BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition

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

CHAPTER 10 Protein Synthesis

From PowerPoint® Lectures for Biology: Concepts & Connections Copyright © 2003 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 the sequence of bases in DNA • The flow of information is from DNA to RNA to protein

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

• A specific gene specifies a polypeptide – The DNA is transcribed into RNA, which is translated into the polypeptide http://www.wiley.com/legacy/college/boyer/0470003790/animations/central_dogma/central_dogma.swf DNA

TRANSCRIPTION

RNA

TRANSLATION

Protein Figure 10.6A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

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

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

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

There for the gene used to produce an enzyme that helps cells manufacture Arginine amino acid was mutated in that fungal strain Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Transcription produces genetic messages in the form of RNA

RNA polymerase

RNA nucleotide

Direction of transcription Template strand of DNA Figure 10.9A

Newly made RNA

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

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.

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

RNA polymerase

• 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

DNA of gene

Promoter DNA Initiation

Elongation

Terminator DNA

Area shown in Figure 10.9A

Termination Growing RNA

Completed RNA

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf Figure 10.9B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

RNA polymerase

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

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

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

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

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

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

10.10 Eukaryotic RNA is processed before leaving http://www.four-h.purdue.edu/apple_genomics/flash/movie3.swf the nucleus

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

• Noncoding segments called introns are spliced out

Exon Intron

Intron

Exon

DNA

Cap RNA transcript with cap and tail

• The coding segments called exons are joined together • A cap and a tail are added to the ends

Exon

Transcription Addition of cap and tail

Introns removed

Tail

Exons spliced together mRNA Coding sequence NUCLEUS

CYTOPLASM

Figure 10.10

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

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

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

Gene 1

Gene 3

DNA molecule

Gene 2

DNA strand

TRANSCRIPTION

RNA Codon TRANSLATION

Polypeptide Figure 10.7

Amino acid

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

The genetic code is the Rosetta stone of life • Virtually all organisms share the same genetic code

Figure 10.8A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• An exercise in translating the genetic code Transcribed strand

DNA

Transcription

RNA

Start codon

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

Translation

Stop codon

Figure 10.8B

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

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

Transfer RNA molecules serve as interpreters during translation • In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide • The process is aided by transfer RNAs

Amino acid attachment site

Hydrogen bond

RNA polynucleotide chain

Anticodon Figure 10.11A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

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

Anticodon Figure 10.11B, C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

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

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

Translation

3. Amino acids • Located in the cytoplasm • Synthesized from other chemicals or obtained from food

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

10.12 Ribosomes build polypeptides

Next amino acid to be added to polypeptide

Growing polypeptide tRNA molecules

P site

A site Growing polypeptide

Large subunit

tRNA

P

A mRNA

mRNA binding site Codons

mRNA

Small subunit

Figure 10.12A-C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

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

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

An initiation codon marks the start of an mRNA message

Start of genetic message

End

Figure 10.13A

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

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

Large ribosomal subunit

Initiator tRNA P site

A site

Start codon

mRNA

Small ribosomal subunit

1 Figure 10.13B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

2

New peptide bond forming

Growing polypeptide

Codons

Stage 4 Elongation 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.

mRNA Polypeptide

Stop Codon

Figure 10.15 (continued)

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

Stage 5 Termination The ribosome recognizes a stop codon. The polypeptide is terminated and released.

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

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

Amino acid Polypeptide A site

P site

Anticodon

mRNA 1

Codon recognition

mRNA movement

Stop codon New peptide bond

3

Translocation

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

2

Peptide bond formation

Figure 10.14

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

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

Steps of Translation 3. Termination

•

The stop codon is reached

•

The completed polypeptide is released

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

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

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

• Summary of transcription and translation

TRANSCRIPTION

DNA

mRNA RNA polymerase

Stage 1 mRNA is transcribed from a DNA template.

Amino acid TRANSLATION Enzyme

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

tRNA

Initiator tRNA

mRNA

Figure 10.15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Anticodon Large ribosomal subunit Start Codon

Small ribosomal subunit

Stage 3 Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together.

Review: The flow of genetic information in the cell is DNARNAprotein • The sequence of codons in DNA spells out the primary structure of a polypeptide – Polypeptides form proteins that cells and organisms use

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

Mutations can change the meaning of genes • 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

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

Normal hemoglobin DNA

mRNA

Mutant hemoglobin DNA

mRNA

Normal hemoglobin

Sickle-cell hemoglobin

Glu

Val

http://www.cleanvideosearch.com/media/action/yt/watch?v=1fN7rOwDyMQ&safety_mode=true&persist_safety_mode=1&safe=active

Figure 10.16A

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

• Types of mutations NORMAL GENE

mRNA Protein

Met

Lys

Phe

Gly

Ala

Lys

Phe

Ser

Ala

BASE SUBSTITUTION

Met

Missing

BASE DELETION

Met

Lys

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

Leu

Ala

His

Figure 10.16B

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

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

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)

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

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

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