Dr. Marchette
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Key Concepts
Life depends on enzymes and other
proteins.
All proteins consist of polypeptide chains.
The chains are sequences of amino acids
that corresponds to genes—sequences of
nucleotide bases in DNA.
The path leading from genes to proteins has
two steps. Transcription and translation.
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During transcription, the two stands of
DNA double helix are unwound in a
gene region.
Exposed chains of one strand become the
template for assembling a single strand
of RNA.
Only one type of RNA transcript encodes
the message that gets translated into
protein.
It is called messenger RNA (mRNA).
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In transcription, the first step in protein
synthesis, a sequence of nucleotide bases
is exposed in a unwound region of a
DNA strand.
That sequence is the template upon
which a single stand of RNA is
assembled from adenine, cytosine,
quanine and uracil subunits.
DNA
A-T
C-G
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RNA
A-U
C-G
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Messenger RNA (mRNA)
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Ribosomal RNA (rRNA)
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Carries protein-building instruction
Major component of ribosomes
Transfer RNA (tRNA)
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Delivers amino acids to ribosomes
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uracil (base)
phosphate
group
sugar
(ribose)
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Same two steps produce all proteins:
1) Transcription- DNA is transcribed to form
mRNA
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Occurs in the nucleus
mRNA moves into cytoplasm
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Like DNA replication
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Nucleotides added in 5’ to 3’ direction
Unlike DNA replication
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Only small stretch is template
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RNA polymerase catalyzes nucleotide
addition
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Product is a single strand of RNA
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A base sequence in the DNA
that signals the start of a gene
For transcription to occur, RNA
polymerase must first bind to a
promoter
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transcribed DNA winds
up again
DNA to be
transcribed unwinds
mRNA
transcript
RNA polymerase
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5’
growing RNA transcript
3’
5’
3’
direction of transcription
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http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/rna_modifi
cations.html
unit of transcription in a DNA strand
3’
exon
intron
exon
transcription
intron
5’
exon
into pre-mRNA
poly-A
tail
3’
cap
5’
snipped
out
snipped
out
5’
3’
mature mRNA transcript
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DNA
G
C
A
T
RNA
G
C
A U
DNA
C
G
T
A
DNA
C
G
T A
base pairing in DNA replication
base pairing in transcription
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codon in mRNA
anticodon
amino-acid
attachment site
amino
acid
OH
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tunnel
small ribosomal subunit large ribosomal subunit
intact ribosome
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In gene, transcription, a sequence of exposed
bases on one of the two strands of the DNA
molecule serves as a template for synthesizing
a complementary strand of mRNA.
RNA polymerase assemble the RNA from four
kinds of ribonucelotides that differ I their
bases: A,U,C,G.
Before leaving the nucleus, each new mRNA
transcript, or pre-mRNA, undergoes
modification into final form.
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Key Concepts
The nucleotide sequence in DNA is read three
bases at a time.
Sixty four base triplets correspond to specific
amino acids and represent the genetic code.
The code words have been highly conserved
through time.
Only in a few eukaryotes, prokaryotes, and
prokaryote derived organelles have slight
variation on the code.
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The correspondence between genes and
proteins is encoded in protein-building
“words” in mRNA transcripts.
Three nucleotide bases make up each three
letter code.
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Set of 64 base triplets
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Codons
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61 specify amino
acids
3 stop translation
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The gentic code is a set of 64 different codons,
which are nucleotide bases in mRNA that are
“read” in sets of three.
Different codons (base triplets) specify
different amino acids.
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Key Concepts
During translation, amino acids are bonded
together in a polypeptide chain in a sequence
specified by the base triplets in mRNA.
Transfer RNA delivers amino acids one at a
time to the ribosome.
An RNA component of ribosome catalyzes the
chain reaction.
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The codons in mRNA transcript are the words
in protein building messages.
Without translators, words that originated
from DNA mean nothing: it take the other two
classes of RNA to synthesize the proteins.
2) Translation- mRNA with tRNA and rRNA at a
ribosome is translated to form polypeptide
chains of amino acids, which fold to form
proteins, occurs in cytoplasm
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Only mRNA carries DNA’s protein building
instructions from the nucleus to the cytoplasm.
Transfer RNA (tRNA) deliver amino acids to
ribosome.
Their anticodons base pair with codons in the
order specified by mRNA.
Polypeptide chains are built on ribosomes, each
consisting of a large and a small subunit made
of tRNA and proteins.
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Key Concepts
An mRNA transcript encodes DNA’s
information about a protein enters an intact
ribosome.
There, its codons are translated into
polypeptide chain – a protein primary
structure.
Translation of the protein building message
proceeds through three continuous stages
called: initiation, elongation and termination.
Initiation
Elongation
Termination
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http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animatio
ns/ribosome.html
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Initiator tRNA binds to
small ribosomal subunit
Small subunit/tRNA
complex attaches to
mRNA and moves along it
to an AUG “start” codon
Large ribosomal subunit
joins complex
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binding site for mRNA
P (first
binding site
for tRNA)
A (second
binding site
for tRNA)
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mRNA passes through ribosomal subunits
tRNAs deliver amino acids to the ribosomal
binding site in the order specified by the
mRNA
Peptide bonds form between the amino acids
and the polypeptide chain grows
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Stop codon into place
No tRNA with anticodon
Release factors bind to the
ribosome
mRNA and polypeptide
are released
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mRNA
new
polypeptide
chain
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Translation is initiated when a small ribosomal
subunit and an initiator tRNA arrive at a mRNA
transcript's start codon, and a large ribosomal
subunit binds to them.
tRNA delivers amino acids to a ribosome in the
order dictated by the linear sequence of mRNA
codons.
A polypeptide chain lengthens as peptide bonds
from between amino acids.
Translation ends when a stop codon triggers
events that cause the polypeptide chain and the
mRNA to detach from the ribosome.
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Key Concepts
Gene mutations introduce changes in protein
structure, protein function, or both.
The changes may lead to small variation in the
shared traits that characterize individuals of a
population.
When a cell taps its genetic code, it is making
proteins with precise structural and functional
roles that keep it alive.
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If a gene changes, the mRNA transcribed from
it may change a specify an altered protein.
If the protein has a critical role, the out come
will be a dead or abnormal cell.
Base-Pair Substitutions
Insertions
Deletions
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http://learning.mgccc.cc.ms.us/jd/science/carter/chapter14/animations/base_pair_su
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a base substitution
within the triplet (red)
original base triplet
in a DNA strand
During replication, proofreading
enzymes make a substitution
possible outcomes:
or
original, unmutated
sequence
a gene mutation
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Insertion
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Extra base added into gene region
Deletion
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Base removed from gene region
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Both shift the reading frame
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Result in many wrong amino acids
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mRNA
parental DNA
arginine
glycine
tyrosine
tryptophan asparagine amino acids
altered mRNA
arginine
glycine
leucine
leucine
glutamate
DNA with
base insertion
altered aminoacid sequence
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DNA segments that move
spontaneously about the genome
When they insert into a gene region,
they usually inactivate that gene
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Each gene has a characteristic mutation rate
Average rate for eukaryotes is between 10-4 and
10-6 per gene per generation
Only mutations that arise in germ cells can be
passed on to next generation
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Ionizing radiation (X rays)
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Nonionizing radiation (UV)
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Natural and synthetic chemicals
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A gene mutation is permanent in one or more
bases in the nucleotide sequence of DNA.
The most common types are base pair
substitution, deletion, and insertion .
Exposure to harmful radiation and chemicals in
the environment can cause mutations in DNA.
A protein specified by a mutated gene may be
harmful, neutral, or beneficial effect on the
individual’s capacity to function in the
environment.
Fig. 14-12, p.227
Fig. 14-14, p.229