Chapter 14Chapter 14

From DNA to ProteinFrom DNA to Protein

Ricin – comes from the castor oil plant Ricin – comes from the castor oil plant

Inactivates ribosomes, the protein-building Inactivates ribosomes, the protein-building machinery of all cellsmachinery of all cells

Impacts, Issues: Impacts, Issues: Ricin and Your Ricin and Your RibosomesRibosomes

Marvelous Mussel AdhesiveMarvelous Mussel Adhesive

Mussel binds itself to rocks with threads Mussel binds itself to rocks with threads

coated with the protein bysusscoated with the protein bysuss

Gene for bysuss has been put into yeastGene for bysuss has been put into yeast

Yeast synthesize the protein based on the Yeast synthesize the protein based on the

instructions in the mussel DNAinstructions in the mussel DNA

WHY IS THIS POSSIBLE?WHY IS THIS POSSIBLE?

Central DogmaCentral Dogma

DNA RNA PROTEIN

TRANSCRIPTION TRANSLATION

Same two steps produce all proteins:Same two steps produce all proteins:

1) DNA is transcribed to form RNA1) DNA is transcribed to form RNA Occurs in the nucleusOccurs in the nucleus RNA moves into cytoplasmRNA moves into cytoplasm

2) RNA is translated to form polypeptide 2) RNA is translated to form polypeptide chains, which fold to form proteinschains, which fold to form proteins

Steps from DNA to ProteinsSteps from DNA to Proteins

Three Classes of RNAsThree Classes of RNAs

Messenger RNAMessenger RNA Carries protein-building instructionCarries protein-building instruction

Ribosomal RNARibosomal RNA Major component of ribosomesMajor component of ribosomes

Transfer RNATransfer RNA Delivers amino acids to ribosomes Delivers amino acids to ribosomes

A Nucleotide Subunit of RNAA Nucleotide Subunit of RNA

phosphate group

sugar (ribose)

uracil (base)

Fig. 14-2, p. 220

base pairing during transcription

DNA

RNA

DNA

DNA

base pairing during DNAreplication

Fig. 14-2c, p.220

Base Pairing during

Transcription

Transcription & DNA ReplicationTranscription & DNA Replication

Like DNA replicationLike DNA replication Nucleotides added in 5’ to 3’ directionNucleotides added in 5’ to 3’ direction

Unlike DNA replicationUnlike DNA replication Only small stretch is templateOnly small stretch is template RNA polymerase catalyzes nucleotide RNA polymerase catalyzes nucleotide

additionaddition Product is a single strand of RNAProduct is a single strand of RNA

PromoterPromoter

A base sequence in the DNA that A base sequence in the DNA that signals the start of a genesignals the start of a gene

For transcription to occur, RNA For transcription to occur, RNA polymerase must first bind to a polymerase must first bind to a promoter (Initiation)promoter (Initiation)

InitiationInitiation

promoter region

RNA polymerase, the enzyme that catalyzes transcription

a RNA polymerase initiates transcription at a promoter region in DNA. It recognizes a base sequence located next to the promoter as a template. It will link the nucleotides adenine, cytosine, guanine, and uracil into a strand of RNA, in the order specified by DNA.

Fig. 14-3a, p.220

newly forming RNA transcript

DNA template unwindingDNA template winding up

DNA template at selected transcription site

b All through transcription, the DNA double helix becomes unwound in front of the RNA polymerase. Short lengths of the newly forming RNA strand briefly wind up with its DNA template strand. New stretches of RNA unwind from the template (and the two DNA strands wind up again).

Fig. 14-3b, p.220

Elongation

direction of transcription3´

growing RNA transcript

5´

5´ 3´

c What happened at the assembly site? RNA polymerase catalyzed the assembly of ribonucleotides, one after another, into an RNA strand, using exposed bases on the DNA as a template. Many other proteins assist this process.

Fig. 14-3c, p.221

Adding Nucleotides

d At the end of the gene region, the last stretch of the new transcript is unwound and released from the DNA template. Shown below is a model for a transcribed strand of RNA.

Fig. 14-3d, p.221

Termination

unit of transcription in a DNA strand

exon intron

mature mRNA transcript

poly-A tail

snipped out snipped out

exon exonintron

cap

transcription into pre-mRNA

Fig. 14-4, p.221

Transcript Modification

mRNA ModificationmRNA Modification

5' cap – modified guanine cap5' cap – modified guanine cap 3' poly-A tail3' poly-A tail Introns = interuptionsIntrons = interuptions Exits = exitExits = exit Alternative splicingAlternative splicing

mature mRNA transcript

5’ 3’

3’

poly-A tail

5’

snipped out snipped out

cap

transcription into pre-mRNA

unit of transcription in a DNA strand

exon intron exon exonintron3’ 5’

Stepped Art

Fig. 14-4, p.221

Conference AssignmentConference Assignment

Response paper:Response paper: Find an article that relates to the conference.Find an article that relates to the conference.

• You will turn a copy of this in.You will turn a copy of this in. Basically a five paragraph essay:Basically a five paragraph essay:

• Summarize the articleSummarize the article• How does this topic relate to the conferenceHow does this topic relate to the conference• Your response to this topic (why is it important, Your response to this topic (why is it important,

etc)etc)

Genetic CodeGenetic Code Set of 64 base Set of 64 base

tripletstriplets CodonsCodons 61 specify amino 61 specify amino

acidsacids 3 stop translation3 stop translation High conservedHigh conserved

Fig. 14-6, p.222

DNA

mRNA

mRNAcodons

threonine proline glutamate glutamate lysine

amino acids

Fig. 14-5, p.222

Genetic Code

tRNA StructuretRNA Structure

codon in mRNA

anticodon

amino acid OH

amino-acidattachment site

Figure 14.7Page 223

tRNA StructuretRNA Structure

aa attachment siteaa attachment site Attached by activating enzymesAttached by activating enzymes

There are 45 tRNA, not 64 as the genetic There are 45 tRNA, not 64 as the genetic code would suggest, WHY?code would suggest, WHY? Wobble effectWobble effect

funnel

small ribosomal subunit

large ribosomal subunit

intact ribosome+Fig. 14-8, p.223

Ribosomes

2 units of rRNA and protein2 units of rRNA and protein Large and small units converge when Large and small units converge when

translatingtranslating Built in nucleus (nucleosome)Built in nucleus (nucleosome)

Three Stages of TranslationThree Stages of TranslationOROR

Gum Drop Protein SynthesisGum Drop Protein Synthesis

InitiationInitiation

ElongationElongation

TerminationTermination

OverviewOverview

Transcription

Translation

mRNA rRNA tRNA

Mature mRNA transcripts

ribosomal subunits

mature tRNA

InitiationInitiation

Initiator tRNA binds to small Initiator tRNA binds to small ribosomal subunitribosomal subunit

Small subunit/tRNA Small subunit/tRNA complex attaches to mRNA complex attaches to mRNA and moves along it to an and moves along it to an AUG “start” codonAUG “start” codon

Large ribosomal subunit Large ribosomal subunit joins complexjoins complex

Binding Sites Binding Sites

binding site for mRNA

P (first binding site for tRNA)

A (second binding site for tRNA)

ElongationElongation

mRNA passes through ribosomal subunits mRNA passes through ribosomal subunits tRNAs deliver amino acids to the tRNAs deliver amino acids to the

ribosomal binding site in the order ribosomal binding site in the order specified by the mRNAspecified by the mRNA

Peptide bonds form between the amino Peptide bonds form between the amino acids and the polypeptide chain growsacids and the polypeptide chain grows

ElongationElongation

TerminationTermination

Stop codon into placeStop codon into place No tRNA with anticodonNo tRNA with anticodon Release factors bind to Release factors bind to

the ribosomethe ribosome mRNA and polypeptide mRNA and polypeptide

are releasedare released

new polypeptide chain

mRNA

What Happens to the What Happens to the New Polypeptides?New Polypeptides?

Some just enter the cytoplasmSome just enter the cytoplasm

Many enter the endoplasmic reticulum and Many enter the endoplasmic reticulum and move through the cytomembrane system move through the cytomembrane system where they are modifiedwhere they are modified

Gene MutationsGene Mutations

Base-Pair Substitutions or Point Base-Pair Substitutions or Point

MutationsMutations InsertionsInsertions DeletionsDeletions

Base-Pair SubstitutionBase-Pair Substitution

original base triplet in a DNA strand

During replication, proofreading enzymes make a substitution

a base substitution within the triplet (red)

original, unmutated sequence

a gene mutation

possible outcomes:

or

Frameshift MutationsFrameshift Mutations

InsertionInsertion Extra base added into gene regionExtra base added into gene region

DeletionDeletion Base removed from gene regionBase removed from gene region

Both shift the reading frameBoth shift the reading frame

Result in many wrong amino acids Result in many wrong amino acids

THREONINE PROLINE GLUTAMATE GLUTAMATE LYSINE

THREONINE PROLINE VALINE GLUTAMATE LYSINE

THREONINE PROLINE GLYCINE ARGININE

part of DNA template

mRNA transcribed from DNA

resulting amino acid sequence

base substitution in DNA

altered mRNA

altered amino acid sequence

deletion in DNA

altered mRNA

altered amino acid sequence

Fig. 14-10, p.226

Frameshift Mutation

TransposonsTransposons

DNA segments that move DNA segments that move spontaneously about the genomespontaneously about the genome

When they insert into a gene region, When they insert into a gene region, they usually inactivate that genethey usually inactivate that gene

TransposonsTransposons

Barbara McClintockBarbara McClintock

Nonuniform coloration of kernels in strains Nonuniform coloration of kernels in strains of indian cornof indian corn

Fig. 14-11, p.227

Mutation Rates Mutation Rates

Each gene has a characteristic mutation Each gene has a characteristic mutation rate rate Can be used as a molecular clock for Can be used as a molecular clock for

evolutionevolution Average rate for eukaryotes is between Average rate for eukaryotes is between

1010-4-4 and 10 and 10-6-6 per gene per generation per gene per generation Only mutations that arise in germ cells can Only mutations that arise in germ cells can

be passed on to next generationbe passed on to next generation

MutagensMutagens

Ionizing radiation (X Ionizing radiation (X

rays)rays)

Nonionizing radiation Nonionizing radiation

(UV)(UV)

Natural and synthetic Natural and synthetic

chemicalschemicals

How can you explain the occurrence of How can you explain the occurrence of birth defects (caused by altered genes) in birth defects (caused by altered genes) in children and grandchildren of the victims of children and grandchildren of the victims of the atomic bombs that destroyed Hiroshima the atomic bombs that destroyed Hiroshima and Nagasaki, Japan, when the victims and Nagasaki, Japan, when the victims themselves were only mildly affected?themselves were only mildly affected?

Euk vs Prok Protein SynthesisEuk vs Prok Protein Synthesis

Most have intronsMost have introns mRNA must be mRNA must be

completely formed completely formed and pass thru and pass thru nuclear membranenuclear membrane

mRNA modified mRNA modified before translationbefore translation

Larger ribosomesLarger ribosomes

None (w/ None (w/ exceptions)exceptions)

Often start Often start translation before translation before transcription is donetranscription is done

Begin at AUG but Begin at AUG but proceeded by a proceeded by a special sequencespecial sequence

Small ribosomesSmall ribosomes