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
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