Chapter 14Chapter 14

From DNA to ProteinFrom DNA to Protein

ObjectivesObjectives 1. Know how the structure and behavior of DNA

determines the structure and behavior of the three forms of RNA during transcription.

2. Understand the role the three forms of RNA play in determining the primary structure of polypeptide chains during translation.

3. Understand the types of mutations and their role in genetic variation.

Ricin – comes from the castor oil plant, Ricin – comes from the castor oil plant, and can be used as a bioweapon. and can be used as a bioweapon.

Ricin has deadly effects – it inactivates Ricin has deadly effects – it inactivates ribosomes, the protein-building machinery ribosomes, the protein-building machinery of all cells.of all cells.

Protein synthesis stops, cells die, the Protein synthesis stops, cells die, the individual dies, there is no antidote.individual dies, there is no antidote.

14.0: 14.0: Ricin and Your RibosomesRicin and Your Ribosomes

Life depends on enzymesLife depends on enzymes

Enzymes are proteins. All proteins consist Enzymes are proteins. All proteins consist of polypeptide chains.of polypeptide chains.

The chains are sequences of amino acids The chains are sequences of amino acids that correspond to genes – sequences of that correspond to genes – sequences of nucleotide bases in DNA.nucleotide bases in DNA.

Enzymes may selectively unwind certain Enzymes may selectively unwind certain regions of DNA to encode information to regions of DNA to encode information to make proteins.make proteins.

The path from genes to proteinsThe path from genes to proteins

DNADNA RNA RNA Protein Protein

The two steps include:The two steps include: Transcription Transcription

Translation 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

14.1 How is RNA Transcribed 14.1 How is RNA Transcribed from DNAfrom DNA

The first step in protein synthesis, is The first step in protein synthesis, is transcription, a sequence of nucleotide transcription, a sequence of nucleotide bases is exposed in an unwound region of bases is exposed in an unwound region of a DNA strand.a DNA strand.

That sequence is the template upon which That sequence is the template upon which a single strand of RNA is assembled from a single strand of RNA is assembled from adenine, cytosine, guanine, and uracil adenine, cytosine, guanine, and uracil subunits.subunits.

Three Classes of RNAsThree Classes of RNAs

Messenger RNA (mRNA)Messenger RNA (mRNA) Carries protein-building instruction or codeCarries protein-building instruction or code

Ribosomal RNA (rRNA)Ribosomal RNA (rRNA) Combines with proteins to form ribosomesCombines with proteins to form ribosomes

Transfer RNA (tRNA)Transfer RNA (tRNA) Delivers the correct amino acids to ribosomes Delivers the correct amino acids to ribosomes

RNA moleculeRNA molecule

One phosphate groupOne phosphate group A five carbon sugar – riboseA five carbon sugar – ribose Four bases:Four bases:

AdenineAdenine CytosineCytosine GuanineGuanine Uracil – instead of ThymineUracil – instead of Thymine

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 Base Pairing during during

TranscriptionTranscription

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 of DNA is templateOnly small stretch of DNA 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 The promoter is a base sequence in The promoter is a base sequence in

the DNA that signals the start of a the DNA that signals the start of a gene.gene.

For transcription to occur, RNA For transcription to occur, RNA polymerase must first bind to a polymerase must first bind to a promoter, and then move along to the promoter, and then move along to the end of the gene. end of the gene.

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

PromoterPromoter

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

Gene TranscriptionGene Transcription

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 NucleotidesAdding Nucleotides

unit of transcription in a DNA strand

exon intron

mature mRNA transcript

3’ end poly-A tail

snipped out non-coding introns

snipped out non- coding introns

exon exonintron

5’ end Cap

transcription into pre-mRNA

Fig. 14-4, p.221

Transcript Modification of mRNATranscript Modification of mRNA

Modifications of mRNAModifications of mRNA

In nucleusIn nucleus 5’ end is capped with guanine – “start” signal 5’ end is capped with guanine – “start” signal

for translations. Cap will also help bind the for translations. Cap will also help bind the mRNA to a ribosomemRNA to a ribosome

3’ end a “poly-A-tail” is added. 100 – 200 3’ end a “poly-A-tail” is added. 100 – 200 molecules of adenine is addedmolecules of adenine is added

Introns (non-coding) are snipped out, exons Introns (non-coding) are snipped out, exons (coding portions) are spliced together(coding portions) are spliced together

Alternative splicing results in different proteinsAlternative splicing results in different proteins

14.2 Genetic 14.2 Genetic CodeCode

Set of 64 base Set of 64 base tripletstriplets

CodonsCodons 61 specify amino 61 specify amino

acidsacids 3 stop translation3 stop translation

Fig. 14-6, p.222

The Genetic CodeThe Genetic Code

Every three bases in mRNA (triplet) code Every three bases in mRNA (triplet) code of an amino acid.of an amino acid.

Both DNA and its RNA transcript are Both DNA and its RNA transcript are linear sequences of nucleotides carrying linear sequences of nucleotides carrying the hereditary code.the hereditary code.

The genetic code consists of 61 triplets The genetic code consists of 61 triplets that specify amino acids, AUG – “start” that specify amino acids, AUG – “start” codon Methionine, and three “stops”codon Methionine, and three “stops”

DNA

mRNA

mRNAcodons

threonine proline glutamate glutamate lysine

amino acids

Fig. 14-5, p.222

Genetic CodeGenetic Code

14.3 The other RNA’s - tRNA14.3 The other RNA’s - tRNA

codon in mRNA

anticodon

amino acid OH

amino-acidattachment site

Figure 14.7Figure 14.7Page 223Page 223

tRNAtRNA

Each kind of tRNA has an Each kind of tRNA has an anticodonanticodon that that is complementary to an mRNA codonis complementary to an mRNA codon

Each tRNA also carries one specific amino Each tRNA also carries one specific amino acid.acid.

After the mRNA arrives in the cytoplasm After the mRNA arrives in the cytoplasm an anticodon on a tRNA bonds to the an anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct codon on the mRNA, and thus a correct amino acid is brought into place.amino acid is brought into place.

RibosomesRibosomes

A ribosome has two subunits Each ribosome is composed of rRNA and

structural proteins) that come together only during translation.

Polypeptide chains are built on ribosomes.

funnel

small ribosomal subunit

large ribosomal subunit

intact ribosome+

Fig. 14-8, p.223

RibosomesRibosomes

14.4 Three Stages of 14.4 Three Stages of TranslationTranslation

InitiationInitiation

ElongationElongation

TerminationTermination

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 released into the are released into the cytoplasm.cytoplasm.

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

Polysome- several ribosomes moving Polysome- several ribosomes moving along mRNA at the same time.along mRNA at the same time.

OverviewOverview

Transcription

Translation

mRNA rRNA tRNA

Mature mRNA transcripts

ribosomal subunits

mature tRNA

binding site for mRNA

P (first binding site for tRNA)

A (second binding site for tRNA)

initiation

elongation

AminoAcid

1

Amino

Acid2

AminoAcid

1AminoAcid

2

a A mature mRNA transcript leaves the nucleus through a pore in the nuclear envelope.

c Initiation ends when a large and small ribosomal subunit converge and bind together.

b Initiation, the first stage of translating mRNA, will start when an initiator tRNA binds to a small ribosomal subunit.

d The initiator tRNA binds to the ribosome.

e One of the rRNA molecules

Fig. 14-9a-e, p.224

f The first tRNA is released

g A third tRNA binds with the next codon

h Steps f and g are repeated

termination

i A STOP codon moves into the area where the chain is being built.

j The new polypeptide chain is released from the ribosome.

k The two ribosomal subunits now separate, also.

Fig. 14-9f-k, p.224

14.5 Gene Mutations14.5 Gene Mutations

A gene mutation is a change in one to several bases in the

nucleotide sequence of DNA, which can result in a

change in the protein synthesized.

Base-Pair SubstitutionsBase-Pair Substitutions

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 MutationFrameshift 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

See the non-uniform coloration of See the non-uniform coloration of kernels in strains of Indian Cornkernels in strains of Indian Corn

Mutation Rates Mutation Rates

Mutations occur spontaneously while DNA is Mutations occur spontaneously while DNA is being replicated, but fortunately special being replicated, but fortunately special enzymes correct most of the mistakes.enzymes correct most of the mistakes.

Each gene has a characteristic mutation rateEach gene has a characteristic mutation rate Average rate for eukaryotes is between 10Average rate for eukaryotes is between 10 -4-4 and and

1010-6-6 per gene per generation per gene per generation Only mutations that arise in germ cells can be Only mutations that arise in germ cells can be

passed on to next generationpassed on to next generation

MutagensMutagens

Ionizing radiation (X rays)Ionizing radiation (X rays)

Nonionizing radiation (UV)Nonionizing radiation (UV) Natural and synthetic chemicals - Natural and synthetic chemicals -

alkylating agents – add methyl or alkylating agents – add methyl or ethyl groups to DNA making it more ethyl groups to DNA making it more vulnerable to mutations. (smoking)vulnerable to mutations. (smoking)

MutationsMutations

If a mutation arises in a somatic cell, it will affect only that cell and will not be passed on to offspring.

If, however, the mutation arises in a gamete, it may be passed on and thus enter the evolutionary arena.

Either kind of mutation may prove to be harmful, beneficial, or neutral in its effects.