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Genes as DNA:
How Genes Encode Proteins
Chapter 5
Central Points
 Genes made of DNA that encodes proteins
 Transcription: DNA copied into mRNA
 Translation: information transferred to protein
 Mutations: changes in DNA
 Changes in DNA produce changes in proteins
5.1 How Do Genes Control Traits?
 Individuals carry two copies of each gene
 One from each parent
 Different forms are alleles
 Genes contain information to produce proteins
 Proteins contribute to the observable traits or
phenotype
What Is a Protein?
 Provide structure
 Be enzymes
 Be chemical messengers
 Act as receptors
 Be carrier molecules
Protein Subunits: Amino Acids
 20 found in the body
 amino acids have different chemical groups
 All contain both a carboxyl group and an amino
group
 Billions of combinations possible
Chemical Structure of Amino Acids
Essential Amino Acids
Amino acids
Fig. 5-3, p. 86
How Does DNA Carry Information?
 DNA carries four nucleotides: A, T, G, and C
• Three nucleotide codon in messenger RNA
(mRNA) specifies one amino acid
 Order of DNA bases determine the order of
amino acids but not all DNA codes for proteins
Gene to Protein
 Transcription: DNA  mRNA
 Translation: mRNA Protein
Animation: Overview of transcription
5.2 What Happens in Transcription?
 First step of information transfer
 Information in DNA sequence gene is copied
into sequence of bases in mRNA
RNA polymerase
DNA to be
transcribed
Initiation
1
Promoter
Terminator
Elongation
2
mRNA transcript
Termination
3
mRNA
4
RNA polymerase
Completed pre-mRNA
p. 88
Transcription
 RNA polymerase binds to promoter, DNA is
template to produce mRNA
 mRNA is a complementary copy of DNA
 Bases pair, except T, is replaced by U
 End of the gene, marked termination sequence
 mRNA processed before leaving nucleus
Animation: From DNA to proteins (gene
transcription)
5.3 What Happens in Translation?
 Second step, processed mRNA to the ribosome
 Protein produced from information on mRNA
 Each mRNA codon codes for an AA
 Transfer RNA (tRNA) acts as an adaptor
Transfer RNA
 Recognizes and binds to one amino acid
 Recognizes the mRNA codon for that amino acid
 At one end binds a specific amino acid
 Other end has a 3 nucleotide anticodon that
pairs with mRNA codon for specific amino acid
Ribosome
mRNA
tRNA
Growing protein
p. 88
Translation (1)
 Synthesis of protein from mRNA
 Occurs within ribosomes
 AUG (start codon) encodes for methionine
 Second AA is in position, an enzyme forms a
peptide bond between the two AA
 tRNA for the first AA is released
Translation (2)
 Ribosome to next codon and repeats adding AA
to growing AA chain
 Stop codons (UAA, UAG, and UGA) do not
code for AA and ribosome detaches from mRNA
 AA chain released, folds into a 3-D protein
TRANSCRIPTION
DNA
tRNA
mRNA
tRNA
rRNA
Nucleus
Cytoplasm
mRNA
Ribosomes
TRANSLATION
Protein
p. 89
Animation: The 4 steps of translation
Genetic Code for Amino Acids
5.4 Turning Genes On and Off
 Only 5–10% genes active
 Gene regulation turns genes on and off
 Promoter controls expression
 Also, cells receive signals
 Enhancers increase protein production
Controlling Gene Expression
5.5 Mutations
 Changes in DNA
 Produce:
• Nonfunctional protein
• Partially functional protein
• No protein
 Affect the timing and level of gene expression
 Some no change
Mutagens
 Increase chance of mutation
 Mistakes during DNA replication
 By-product of normal cell functions
 Include:
• Environmental factors
• Radiation
• Chemicals
Animation: Mutations and translation
Animation: From DNA to proteins (base
substitution)
5.6 Cause of Genetic Disorders
 Change in DNA alters mRNA
 Single nucleotide change can alter codon and
possibly amino acid
 Change in amino acid sequence causes changes in
• 3-D structure of protein
• Defective protein folding
• Protein function
Genetic Code
Disorders from Altered 3-D Shape
 Cystic fibrosis
 Form of Alzheimer disease
 Mad cow disease
 Cruzfelt-Jacob disease (CJD)
Sickle Cell Anemia
 Mutation in the hemoglobin gene
 Hemoglobin (HbA) is composed of two proteins:
• Alpha globin
• Beta globin
 Single nucleotide point mutation alters one of
146 AA, affects the beta globin
 Causes hemoglobin molecules to stick together
Hemoglobin Molecule
Red Blood Cells
Point Mutation in Sickle Cell Anemia
Valine
Histidine
Leucine Threonine Proline Glutamic Glutamate
acid
Valine
Histidine Leucine Threonine Proline
Valine
Glutamate
Fig. 5-9, p. 93
5.7 Other Single-Gene Defects
 Cystic fibrosis (CF)
• Misfolded protein
• Protein destroyed
 Huntington disease (HD)
• Trinucleotide repeats
• Multiple CAG repeats