Transcript Table of Contents - Milan Area Schools

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One Gene, One Polypeptide
• In the 1940s, Beadle and Tatum showed that when
an altered gene resulted in an altered phenotype,
that altered phenotype always showed up as an
altered enzyme.
• This lead to the one-gene, one-enzyme hypothesis.
Figure 12.1 One Gene, One Enzyme (Part 1)
Figure 12.1 One Gene, One Enzyme (Part 2)
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One Gene, One Polypeptide
• The gene–enzyme connection has undergone
several modifications. Some enzymes are
composed of different subunits coded for by
separate genes.
• This suggests, instead of the one-gene, one
enzyme hypothesis, a one-gene, onepolypeptide relationship.
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DNA, RNA, and the Flow of Information
• The expression of a gene takes place in two
steps:
 Transcription makes a single-stranded RNA
copy of a segment of the DNA.
 Translation uses information encoded in the
RNA to make a polypeptide.
Figure 12.2 The Central Dogma
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DNA, RNA, and the Flow of Information
• Messenger RNA, or mRNA moves from the
nucleus of eukaryotic cells into the cytoplasm,
where it serves as a template for protein synthesis.
• Transfer RNA, or tRNA, is the link between the
code of the mRNA and the amino acids of the
polypeptide, specifying the correct amino acid
sequence in a protein.
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DNA, RNA, and the Flow of Information
• Certain viruses use RNA rather than DNA as their
information molecule during transmission.
• HIV and certain tumor viruses (called retroviruses)
have RNA as their infectious information
molecule; they convert it to a DNA copy inside the
host cell and then use it to make more RNA.
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Transcription: DNA-Directed RNA Synthesis
• In normal prokaryotic and eukaryotic cells,
transcription requires the following:
 A DNA template for complementary base
pairing
 The appropriate ribonucleoside triphosphates
(ATP, GTP, CTP, and UTP)
 The enzyme RNA polymerase
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Transcription: DNA-Directed RNA Synthesis
• Just one DNA strand (the template strand) is used
to make the RNA.
• For different genes in the same DNA molecule,
the roles of these strands may be reversed.
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Transcription: DNA-Directed RNA Synthesis
• Initiation
 Begins at promoter.
• Elongation
 Transcript is antiparellel
 More mistakes than replication
• Termination.
 Fall off or helped off.
Figure 12.4 (Part 1) DNA is Transcribed in RNA
Figure 12.4 (Part 2) DNA is Transcribed in RNA
Figure 12.4 (Part 3) DNA is Transcribed in RNA
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The Genetic Code
• Genetic code relates genes (DNA) to mRNA and
mRNA to the amino acids of proteins.
• mRNA is read in three-base segments called
codons.
• The 64 possible codons code for only 20 amino
acids and the start and stop signals.
 Redundancy
 Wobble
Figure 12.5 The Universal Genetic Code
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• The codon in mRNA and the amino acid in a
protein are related by way of an adapter—a
specific tRNA molecule.
• tRNA has three functions:
 It carries an amino acid.
 It associates with mRNA molecules.
 It interacts with ribosomes.
Figure 12.7 Transfer RNA
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• Midpoint in the sequence are three bases called the
anticodon.
 contact point between the tRNA and the mRNA.
 complementary (and antiparallel) to the mRNA
codon.
 codon and anticodon unite by complementary
base pairing.
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• Each ribosome has two subunits: a large one and
a small one.
 Each made of rRNA and protein.
• When they are not translating, the two subunits
are separate.
Figure 12.9 Ribosome Structure
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Translation: RNA-Directed Polypeptide Synthesis
• Translation begins with an initiation complex: a
charged tRNA with its amino acid and a small
subunit, both bound to the mRNA.
• The start codon (AUG) designates the first amino
acid in all proteins.
• The large subunit then joins the complex.
Figure 12.10 The Initiation of Translation
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Translation: RNA-Directed Polypeptide Synthesis
• Ribosomes move in the 5-to-3 direction on the
mRNA.
• The peptide forms in the N–to–C direction.
Figure 12.11 Translation: The Elongation Stage
Figure 12.12 The Termination of Translation
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Regulation of Translation
• Many antibiotics are considered magic bullets
because they will affect the ribosomes of bacteria
and have no effect on our ribosomes.
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Post translational Events
• Two post translational events can occur after the
polypeptide has been synthesized:
 The polypeptide may be moved to another
location in the cell, or secreted.
 The polypeptide may be modified by the
addition of chemical groups, folding, or
trimming.
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Post translational Events
• As the polypeptide chain forms, it folds into its 3-D
shape.
• The amino acid sequence also contains an
“address label” indicating where in the cell the
polypeptide belongs. It gives one of two sets of
instructions:
 Finish translation and be released to the
cytoplasm.
 Stall translation, go to the ER, and finish
synthesis at the ER surface.
Figure 12.15 A Signal Sequence Moves a Polypeptide into the ER (Part 1)
Figure 12.15 A Signal Sequence Moves a Polypeptide into the ER (Part 2)
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Post translational Events
• Most proteins are modified after translation.
• Three types of modifications:
 Proteolysis (cleaving)
 Glycosylation (adding sugars)
 Phosphorylation (adding phosphate groups)
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Mutations: Heritable Changes in Genes
• Mutations are heritable changes in DNA—
changes that are passed on to daughter cells.
• Multicellular organisms have two types of
mutations:
 Somatic mutations are passed on during
mitosis, but not to subsequent generations.
 Germ-line mutations are mutations that occur
in cells that give rise to gametes.
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Mutations: Heritable Changes in Genes
• All mutations are alterations of the DNA nucleotide
sequence and are of two types:
 Point mutations are mutations of single genes.
 Chromosomal mutations are changes in the
arrangements of chromosomal DNA segments.
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Mutations: Heritable Changes in Genes
• Point mutations result from the addition or
subtraction of a base or the substitution of one base
for another.
 Can happen spontaneously or due to mutagens.
 Many are silent
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Mutations: Heritable Changes in Genes
• Some mutations, called missense mutations,
cause an amino acid substitution.
• An example in humans is sickle-cell anemia, a
defect in the -globin subunits of hemoglobin.
• The -globin in sickle-cell differs from the normal
by only one amino acid.
Figure 12.17 Sickled and Normal Red Blood Cells
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Mutations: Heritable Changes in Genes
• Nonsense mutations are base substitutions that
substitute a stop codon.
• The shortened proteins are usually not functional.
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Mutations: Heritable Changes in Genes
• A frame-shift mutation consists of the insertion
or deletion of a single base in a gene.
• This type of mutation shifts the code, changing
many of the codons to different codons.
• These shifts almost always lead to the production
of nonfunctional proteins.
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Mutations: Heritable Changes in Genes
• DNA molecules can break and re-form, causing four
different types of mutations:
 Deletions are a loss of a chromosomal segment.
 Duplications are a repeat of a segment.
 Inversions result from breaking and rejoining
when segments get reattached in the opposite
orientation.
 Translocations result when a portion of one
chromosome attaches to another.
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Mutations: Heritable Changes in Genes
• Mutations have both benefits and costs.
• Germ line mutations provide genetic diversity for
evolution, but usually produce an organism that
does poorly in its environment.
• Somatic mutations do not affect offspring, but can
cause cancer.