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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Chapter 9Gene Expression:
From Genes to Proteins
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
DNA Codes for Protein
The information required to produce proteins is encoded in the nucleotide sequence of DNA
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Relationship Between Genes, Proteins, and Phenotype
• Archibald Garrod (early 1900s)– Inborn errors of metabolism
• Alkaptonuria altered metabolism of alkapton (homogentisic acid)
• Urine turns black • Using pedigree analysis he determined it is an autosomal recessive trait
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Metabolic Pathways
Fig. 9.1
• Chains of chemical reactions
• In a pathway, an enzyme is required to convert one compound into another
• Each reaction is controlled by a different enzyme
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
One Gene, One Enzyme
• Beadle and Tatum (1930s)
• Experiments on Neurospora, a common bread mold
Fig. 9.2
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
DNA Stores Genetic Information
• Phenotype is the result of protein function
• When the function is altered or absent, the result is a mutant phenotype
• Proteins are products of genes• Genes are made of DNA• Changes in DNA may change protein functions
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Genetic Code
• Information is stored in the sequence of nucleotides in the DNA
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
The Genetic Code
Fig. 9.3
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
About the genetic code:
AAGCTTCAATTC
Lys-Leu-Gln-Phe
AAGCTTCAATTCSer-Phe-Asn
AAGCTTCAATTC
Ala-Ser-Ile
open reading frames
1. Triplet code: 3 bases encode an amino acid.
2. Continuous code.
3. Non-overlapping
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
About the genetic code cont’d.
5. Code is degenerate.
4. Code is universal.
Exceptions: AUG - Met UGG - Trp
7. Nonsense or STOP codons
6. Wobble hypothesis.
61 sense codons3 nonsense codons
UAAUGAUAG
STOP
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Skin cell Liver cell
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
Protein 2
Protein 4
Protein 5
Protein 1
Protein 3
Protein 4
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
RNA
• Messenger (mRNA) – single-stranded, complementary copy of DNA
• Transfer RNA (tRNA) contains a binding site for mRNA codon and a binding site for a specific amino acid
• Ribosomal RNA (rRNA) – component of the ribosome
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Protein Synthesis
• Linear sequence of nucleotides of DNA is transferred to a linear sequence of amino acids of the protein
Two steps• Transcription
– DNA to mRNA• Translation
– mRNA to protein
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Fig. 9.4
The Flow of Genetic Information
Central dogma
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Transcription
• Occurs in the nucleus• One strand is used as a template to produce mRNA
• Three stages –Initiation–Elongation–Termination
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning Fig. 9.5
Transcription
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Transcription: Initiation
Fig. 9.5a,b
• RNA polymerase binds to the promoter region
• DNA unwinds
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Transcription: Elongation
• 30–50 nucleotides/second• RNA nucleotides pair with DNA template A–U, T–A, G–C, and C–G
Fig. 9.5c
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Transcription: Termination
• RNA polymerase reaches the terminator and mRNA is released
Fig. 9.5d
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Product of Transcription: Pre-mRNA
• Large pre-mRNA molecules are composed of exons and introns
• Exons are nucleotide sequences that are transcribed and translated
• Introns are nucleotide sequences that are transcribed but not translated– 0–75 per gene– Size ranges from 100–100,000 nucleotides
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Organization of an Eukaryotic Gene
Fig. 9.6
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Processing Pre-mRNA
• Introns are removed• Exons are spliced together• Cap is added to the 5’ end
– The cap helps attach mRNA to the ribosome
• Poly-A tail is added to the 3’ end• The mRNA is transported out of the nucleus to the ribosome for translation
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Processing Pre-mRNA
Fig. 9.7
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Nobel Prize in Medicine, 1993
Introns
Phil Sharp Rich Roberts
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Proteins
• Proteins are composed of amino acids joined together by peptide bonds
• 20 different amino acids can be used to produce proteins
• Amino acids have a Carboxyl (acid) group, an amino group, and an unique R group
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Fig. 9.8
Amino Acids Are the Subunits of Proteins
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Domains - can bring residues far apart in primary sequence in contact with each other.
Primary sequence - sequence of amino acidsin a protein.
Secondary structure - depends on primary sequence. Beta sheets and alpha helices.Can be usually predicted from sequence.
Tertiary structure - 3-D structure of how the proteinfolds.Cannot be always predicted.
Quaternary structure - multimers. Many subunits eachof which is a polypeptide.
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Tertiary structure Quaternary structure
Hemoglobin
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Diseases arising from defects in protein folding
• May cause changes in protein folding• Examples
– A form of Alzheimer disease– MPS VI– Cystic fibrosis– Prion diseases
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Prions
Fig. 9.13
Stanley Pruisiner won the Nobel Prize in 1997 for his work with prions and their role in disease
BSE
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Ribosomes• The site of protein synthesis • May be free in the cytoplasm or attached to endoplasmic reticulum
• Contains two binding sites– A (Amino) and P (peptide) site
• Composed of – Two subunits– rRNA and protein
Fig. 9.9
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Transfer RNA (tRNA)
Fig. 9.10
• Adapters that bond to an amino acid and recognize specific mRNA codons
• Small, single- stranded RNA that folds back, forming a cloverleaf shape
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
HIV infection
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Nobel Prize in Medicine, 1975
Retroviral mechanisms
Renato DulbeccoDavid Baltimore Howard Temin
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Nucleotide analogs used as anti-AIDS drugs
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Translation
• Converts mRNA sequence to amino acid sequence
• Occurs within the ribosomes• Requires various factors: enzymes, amino acids, energy, mRNA, rRNA and tRNA
• Three steps– Initiation– Elongation– Termination
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Steps in Translation: Initiation
• Small ribosomal subunit binds to mRNA
• tRNA carrying methionine binds to the start codon (AUG) with in P site
• Forms initiation complex• Large ribosomal subunit binds to small subunit
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Fig. 9.9a-d
Initiation
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Steps in Translation: Elongation
• tRNA for the second amino acid binds to the mRNA within the second ribosomal binding site (A site)
• Peptide bonds forms between methionine and second amino acid
• Ribosome moves down mRNA• tRNA brings in the third amino acid
into the A site• Peptide bond forms
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Termination
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Steps in Translation: Termination
• Elongation continues until the ribosomes reaches a stop codon
• No tRNA binds • Synthesis is completed and mRNA, tRNA, are released from the ribosome
• Polypeptide is folded into 3-dimensional shape
• Many antibiotics interfere with steps in protein synthesis
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Mechanism of antibiotic action
Specifically target protein translation in bacteria.
Since prokaryotes and eukaryotes use different proteins for translation, antibiotics can specificallyaffect bacterial, but not cellular translation.
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Puromycin prevents elongation of polypeptide. Causes premature termination.
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Streptomycin
Tetracycline
Chloramphenicol
Erythromycin
Binds to 30s subunit and prevents initiation of protein synthesis
Prevents elongation by preventing charged tRNA frombinding to A site.
Binds 50s subunit and blocks peptidyl transferase reaction.
Prevents translocation
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Chapter 9 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Proteome
• Set of proteins present in a cell under specific environmental conditions
• It is estimated that humans can make over 100,000 proteins
• This is greater than the number of genes in the human genome
• How is this possible?