3. Gene ExpressionGene Expression
Three Classes of RNAThree Classes of RNA
– Messenger RNA (mRNA)Messenger RNA (mRNA)
Takes a message from DNA to the ribosomesTakes a message from DNA to the ribosomes
CodonCodon
TranscriptionTranscription
– Ribosomal RNA (rRNA)Ribosomal RNA (rRNA)
Makes up ribosomes (along with proteins)Makes up ribosomes (along with proteins)
– Transfer RNA (tRNA)Transfer RNA (tRNA)
Transfers amino acids to ribosomesTransfers amino acids to ribosomes
AnticodonAnticodon
TranslationTranslation
4. Gene ExpressionGene Expression
Gene Expression Requires Two Steps:Gene Expression Requires Two Steps:
– TranscriptionTranscription
– TranslationTranslation
5. Overview: the roles of transcription and translation in the flow of genetic informationOverview: the roles of transcription and translation in the flow of genetic information
(Layer 1)(Layer 1)
6. Overview: the roles of transcription and translation in the flow of genetic informationOverview: the roles of transcription and translation in the flow of genetic information
(Layer 2)(Layer 2)
7. Overview: the roles of transcription and translation in the flow of genetic informationOverview: the roles of transcription and translation in the flow of genetic information
(Layer 3)(Layer 3)
8. Overview: the roles of transcription and translation in the flow of genetic informationOverview: the roles of transcription and translation in the flow of genetic information
(Layer 4)(Layer 4)
9. Overview: the roles of transcription and translation in the flow of genetic informationOverview: the roles of transcription and translation in the flow of genetic information
(Layer 5)(Layer 5)
14. Gene ExpressionGene Expression
TranscriptionTranscription
– During transcription, a segment of the DNADuring transcription, a segment of the DNA
serves as a template for the production of anserves as a template for the production of an
RNA moleculeRNA molecule
– Formation of Messenger RNA (mRNA)Formation of Messenger RNA (mRNA)
DNA helix is opened so complementary baseDNA helix is opened so complementary base
pairing can occur – HELICASE!pairing can occur – HELICASE!
RNA polymeraseRNA polymerase binds to a promoterbinds to a promoter
RNA polymerase joins new RNA nucleotidesRNA polymerase joins new RNA nucleotides
in a sequence complementary to that on thein a sequence complementary to that on the
DNADNA
– Messenger RNA formed - CODONMessenger RNA formed - CODON
Strand carries code for proteinStrand carries code for protein
Is complementary to the section of DNAIs complementary to the section of DNA
19. Gene ExpressionGene Expression
Transfer RNA - ANTICODONTransfer RNA - ANTICODON
– tRNA transports amino acids to the ribosomestRNA transports amino acids to the ribosomes
– Single stranded nucleic acid that correlates aSingle stranded nucleic acid that correlates a
specific nucleotide sequence with a specificspecific nucleotide sequence with a specific
amino acidamino acid
– Amino acid binds to one end, the opposite endAmino acid binds to one end, the opposite end
has an anticodonhas an anticodon
– the order of mRNA codons determines thethe order of mRNA codons determines the
order in which tRNA brings in amino acidsorder in which tRNA brings in amino acids
21. Gene ExpressionGene Expression
Translation (overview)Translation (overview)
– In cytoplasm/ribosomeIn cytoplasm/ribosome
– mRNA - Triplet codon: AUG initiates translationmRNA - Triplet codon: AUG initiates translation
– tRNA – attached to certain amino acids –tRNA – attached to certain amino acids –
corresponds to anticodon --- complementary tocorresponds to anticodon --- complementary to
codoncodon
– mRNA bonds to tRNA – H bondsmRNA bonds to tRNA – H bonds
– 20 different enzymes for each amino acid20 different enzymes for each amino acid
– Needs ATPNeeds ATP
– 2 amino acids make peptide bond – continues2 amino acids make peptide bond – continues
– Stop codon: UAA, UAG, UGA – nonsense codonStop codon: UAA, UAG, UGA – nonsense codon
– mRNA fragment – returns to nucleusmRNA fragment – returns to nucleus
27. Genetic CodeGenetic Code
-- The Genetic CodeThe Genetic Code
Triplet code- each 3-nucleotide unit of a mRNATriplet code- each 3-nucleotide unit of a mRNA
molecule is called a CODONmolecule is called a CODON
There are 64 different mRNA codonsThere are 64 different mRNA codons
– 61 code for particular amino acids61 code for particular amino acids
Redundant code-some amino acids have numerousRedundant code-some amino acids have numerous
code wordscode words
Provides some protection against mutationsProvides some protection against mutations
– 1 start codon – AUG – also codes for a.a. - MET1 start codon – AUG – also codes for a.a. - MET
– 3 are stop codons signal polypeptide termination3 are stop codons signal polypeptide termination
30. Gene ExpressionGene Expression
Degenerate – two or more codonsDegenerate – two or more codons
code for the same amino acidcode for the same amino acid
Universal – all living organisms use theUniversal – all living organisms use the
same code (including viruses)same code (including viruses)
(Transcribe and Translate a Gene)(Transcribe and Translate a Gene)
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37. One Gene, One PolypeptideOne Gene, One Polypeptide
One gene will always make one enzyme.One gene will always make one enzyme.
Became – One Gene, One Polypeptide HypothesisBecame – One Gene, One Polypeptide Hypothesis
(Beadle and Tatum)(Beadle and Tatum)
One Allele, One Protein (many different versions of a gene)One Allele, One Protein (many different versions of a gene)
– ExceptionsExceptions
Some genes do not produce polypeptidesSome genes do not produce polypeptides
Some genes code for RNA which does not produce aSome genes code for RNA which does not produce a
polypeptide. - (no start codon)polypeptide. - (no start codon)
