From DNA to Protein

From DNA to From DNA to ProteinProtein

Chapter 14Chapter 14

Ricin and your Ricin and your RibosomesRibosomes

DNA and RNA

DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring.

RNAs have various functions. Some serve as disposable copies of DNA’s genetic message; others are catalytic.

DNA template

mRNA

protein stop Gly Glu

Amino acid Amino acid Amino acid

Transcription

Translation

Converting a Gene to an Converting a Gene to an RNARNA

TranscriptionTranscription Enzymes use the nucleotide sequence of Enzymes use the nucleotide sequence of

a gene to synthesize a complementary a gene to synthesize a complementary strand of RNAstrand of RNA

Occurs in the nucleusOccurs in the nucleus

Converting mRNA to Converting mRNA to ProteinProtein

TranslationTranslation The information carried by mRNA is The information carried by mRNA is

decoded into a sequence of amino decoded into a sequence of amino acids, resulting in a polypeptide chainacids, resulting in a polypeptide chain

RNA in Protein SynthesisRNA in Protein Synthesis3 Different Kinds of RNA:3 Different Kinds of RNA:

Messenger RNA (mRNA)Messenger RNA (mRNA)Contains information transcribed from DNAContains information transcribed from DNA

Ribosomal RNA (rRNA)Ribosomal RNA (rRNA)Main component of ribosomes, where Main component of ribosomes, where

polypeptide chains are builtpolypeptide chains are built

Transfer RNA (tRNA)Transfer RNA (tRNA)Delivers amino acids to ribosomesDelivers amino acids to ribosomes

Gene ExpressionGene Expression

Gene expressionGene expressionA multistep process in which genetic A multistep process in which genetic

information encoded by a gene is information encoded by a gene is converted into a structural or functional converted into a structural or functional part of a cell or bodypart of a cell or body

Transcription: DNA to Transcription: DNA to RNARNA

RNA polymeraseRNA polymerase assembles RNA by assembles RNA by linking RNA nucleotides into a chainlinking RNA nucleotides into a chain

A new RNA strand is complementary A new RNA strand is complementary in sequence to the DNA strand from in sequence to the DNA strand from which it was transcribedwhich it was transcribed

DNA template

mRNA

Transcription

Transcription: DNA to RNA

Base-Pairing in DNA Synthesis and Transcription

The Process of The Process of TranscriptionTranscription

RNA polymerase and regulatory proteins RNA polymerase and regulatory proteins attach to a attach to a promoterpromoter (a specific binding (a specific binding site in DNA close to the start of a gene)site in DNA close to the start of a gene)

RNA polymerase moves over the gene in RNA polymerase moves over the gene in a 5' to 3' direction, unwinds the DNA a 5' to 3' direction, unwinds the DNA helix, reads the base sequence, and joins helix, reads the base sequence, and joins free RNA nucleotides into a free RNA nucleotides into a complementary strand of mRNAcomplementary strand of mRNA

(gene)

Fig. 14-5a, p. 218

Fig. 14-7, p. 220

gene

exon

intron

exon

intron

exonDNA

transcription into RNAcap poly-A

tail

mRNA

5’ 3’

snipped out

snipped out

mRNA

Pre-

Post-Transcriptional Modifications

mRNA – The MessengermRNA – The Messenger

CodonCodon--A sequence of three mRNA A sequence of three mRNA nucleotides that codes for a specific nucleotides that codes for a specific amino acidamino acid The order of codons in mRNA The order of codons in mRNA

determines the order of amino acids in a determines the order of amino acids in a polypeptide chainpolypeptide chain

Genetic Information

• From DNA to mRNA to amino acid sequence

codon

Codons of the Genetic Codons of the Genetic CodeCode

rRNA and tRNA – The rRNA and tRNA – The TranslatorsTranslators

tRNAs deliver amino acids to tRNAs deliver amino acids to ribosomes ribosomes tRNA has an tRNA has an anticodonanticodon

RibosomesRibosomes

2 subunits Ribosomesmade of rRNA & proteins

Translation: RNA to Translation: RNA to ProteinProtein

Translation Translation converts genetic converts genetic information carried by an mRNA into information carried by an mRNA into a new polypeptide chain a new polypeptide chain

TranslationTranslation

Translation occurs in the cytoplasm Translation occurs in the cytoplasm of cellsof cells

Translation occurs in three stagesTranslation occurs in three stages1.1. InitiationInitiation

2.2. ElongationElongation

3.3. TerminationTermination

Elongation

Fig. 14-12c, p. 223

Elongation

C An initiator tRNA carries the amino acid methionine, so the first amino acid of the new polypeptide chain will be methionine. A second tRNA binds the second codon of the mRNA (here, that codon is GUG, so the tRNA that binds carries the amino acid valine).

A peptide bond forms between the first two amino acids (here, methionine and valine).

Fig. 14-12d, p. 223

D The first tRNA is released and the ribosome moves to the next codon in the mRNA. A third tRNA binds to the third codon of the mRNA (here, that codon is UUA, so the tRNA carries the amino acid leucine).

A peptide bond forms between the second and third amino acids (here, valine and leucine).

Fig. 14-12e, p. 223

E The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth mRNA codon (here, that codon is GGG, so the tRNA carries the amino acid glycine).

A peptide bond forms between the third and fourth amino acids (here, leucine and glycine).

TerminationTermination When the ribosome encounters a When the ribosome encounters a

stop codon, polypeptide synthesis stop codon, polypeptide synthesis endsends Release factors bind to the ribosomeRelease factors bind to the ribosome Enzymes detach the mRNA and Enzymes detach the mRNA and

polypeptide chain from the ribosomepolypeptide chain from the ribosome

Mutated Genes Mutated Genes & Their Protein Products& Their Protein Products

If the nucleotide sequence of a gene If the nucleotide sequence of a gene changes, it may result in an altered changes, it may result in an altered gene product, with harmful effectsgene product, with harmful effects

MutationsMutations Small-scale changes in the nucleotide Small-scale changes in the nucleotide

sequence of a cell’s DNA that alter the sequence of a cell’s DNA that alter the genetic codegenetic code

Common MutationsCommon Mutations

Base-pair-substitutionBase-pair-substitution DeletionDeletion or or insertioninsertion

Fig. 14-13, p. 224

THREONINE PROLINE GLUTAMATEGLUTAMATE LYSINE

THREONINE PROLINE VALINE GLUTAMATE LYSINE

THREONINE PROLINE GLYCINE ARGININE

c. Frame shift

b. Base pair substitution

a. Normal Hemoglobin chain

What Causes Mutations?What Causes Mutations?

1.1. Spontaneous mutationsSpontaneous mutations Uncorrected errors in DNA replicationUncorrected errors in DNA replication

Harmful environmental agentsHarmful environmental agents

2. UV radiation2. UV radiation

3. chemicals3. chemicals

Mutations Caused by Mutations Caused by RadiationRadiation

Ionizing radiation damages Ionizing radiation damages chromosomes, nonionizing (UV) chromosomes, nonionizing (UV) radiation forms thymine dimersradiation forms thymine dimers

Inherited MutationsInherited Mutations

Mutations in somatic cells of sexually Mutations in somatic cells of sexually reproducing species are not inheritedreproducing species are not inherited

Mutations in a germ cell or gamete Mutations in a germ cell or gamete may be inherited, with evolutionary may be inherited, with evolutionary consequencesconsequences

Fig. 14-16, p. 226

Transcription Assembly of RNA on unwound regions of DNA molecule

mRNA rRNA tRNAmRNA processing

proteinsmature mRNA transcripts

ribosomal subunits

mature tRNA

Convergence of RNAsTranslation cytoplasmic

pools of amino acids,

ribosomal subunits, and

tRNAsAt an intact ribosome, synthesis of a polypeptide chain at the binding sites for mRNA and tRNAs

Protein