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Active Lecture PowerPoint® Presentation for
Essentials of Genetics
Seventh Edition
Klug, Cummings, Spencer, Palladino
Chapter 13
Translation and Proteins
Copyright © 2010 Pearson Education, Inc.
Outline
• Overview of translation
• Components of translation machinery
• The process of translation
• Eukaryotic translation
• Relationship between genes and proteins
• Protein structure
• Summary (animation)
Gene
Expression
Step 1
Transcription
Step 2
Translation
mRNA
Components of Translation
Machinery
• mRNA transcribed from DNA
• Amino acids
• tRNAs
• Ribosomes
Translation - Components
Codon
Ribosome
• Location of protein synthesis in cells
• Composed of RNA and proteins
• Composed of a large and a small
subunit
–Prokaryotes 50S and 30S
–Eukaryotes 60S and 40S
tRNA
(Clover leaf
model)
Charging
tRNA
by
Aminoacyl tRNA
Synthetase
Translation of mRNA Can Be
Divided into Three Steps
• Initiation
• Elongation
• Termination
Various Proteins Assist
Translation
Ribosome Binding Sites
The ribosome has three binding sites for
tRNA.
A site: Acceptor site
tRNA bringing amino acid binds
P site: Peptidyl site
tRNA carrying the peptide chain
binds
E site: Exit site
Translation Initiation
• Small subunit of the ribosome binds to the
start codon of mRNA. Initiation factors help.
• The first amino acid attached to an initiator
tRNA, binds to mRNA
• The first amino acid
– Prokaryotes: Formyl-methionine (f-Met)
– Eukaryotes: methionine
• The large subunit of the ribosome binds
Translation Initiation
Translation Elongation
• Ribosome advances one codon at a time
• tRNA in A site is translocated to the P site so
that the A site is available for the next
charged tRNA to bind
• A peptide bond is formed between amino
acid in A site and the amino acid in the P site
by peptidyl transferase, a ribozyme
composed of RNA in the large subunit
Translation Termination
• Ribosome binds to the stop codon of
mRNA.
• Ribosome releases mRNA & the polypeptide
chain synthesized
• Release factors assist this
• Ribosome subunits dissociate
Translation is More Complex in
Eukaryotes
In eukaryotes:
• the ribosomes are larger than in bacteria
• transcription and translation are spatially and
temporally separated
• ribosomes scan for the initiator tRNA that is
in the proper context, as identified by the
Kozak sequence
• translation generally requires more factors for
initiation, elongation, and termination than
translation in bacteria does
Relationship between Genes &
Proteins
• George Beadle (1933) provided evidence
that genes are directly responsible for
enzymes.
• Work by Beadle & Edward Tatum on
Neurospora led to one gene:one enzyme
hypothesis
• Studies on human hemoglobin established
one gene: one polypeptide hypothesis
Experiments of
Beadle & Tatum
Biochemical Pathways
A
Enzyme
1
D
C
B
Enzyme
2
Enzyme
3
Defective enzyme is a result of mutation in the
gene
Hypothesis: One Gene - One Enzyme
Sickle-cell Anemia
Sickle-cell Anemia
• Patients suffer from lack of oxygen to tissue
and severe damage
• Inherited as a recessive Mendelian trait
Affected individuals are homozygous for Hbs
• Heterozygots (HbAHbS) are carriers but not
largely affected
• HbS hemogloblin has a different rate of
electrophoretic migration than HbA
hemoglobin
Protein Structure (self study)
Four levels of protein structure
– Primary structure (amino acid chain)
– Secondary structure (α-helices & β-sheets)
– Tertiary structure (a single subunit)
– Quarternary structure (group of subunits)
Tertiary Level Protein
Respiratory
pigment
myoglobin
Quarternary Level Protein
Hemoglobin
Diversity in Protein Function
• Proteins play diverse roles in the body
• Examples: Hemoglobin, Collagen,
Keratin, Actin, Myosin, Immunoglobulins,
Transport proteins, some hormones and
their receptors, histones, enzymes
Important features of proteins
• Some proteins may be posttranslationally
modified
• Protein function is directly related to the
structure
• Proteins consist of functional domains
Genetics, Technology & Society
p 297
Prions (protenaceous infectious agents)
• Mad cow
• Creutzfeldt –Jakob disease
• Kuru disease
• Scrapie in sheep
• BSE in cattle