Transcript Chapter 25: Molecular Basis of Inheritance

PROTEIN
SYNTHESIS
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DNA
and
Genes
DNA
• DNA contains genes,
sequences of nucleotide
bases
• These Genes code for
polypeptides (proteins)
• Proteins are used to build
cells and do much of the
work inside cells
Genes & Proteins
 Proteins are made of
amino acids linked
together by peptide
bonds
 20 different amino acids
exist
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Amino Acid Structure
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Polypeptides
• Amino acid
chains are
called
polypeptides
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DNA Begins the Process
• DNA is found inside the
nucleus
• Proteins, however, are made
in the cytosol of cells by
organelles called ribosomes
• Ribosomes may be free in the
cytosol or attached to the
surface of rough ER
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Starting with DNA
• DNA ‘s code must be copied
and taken to the cytosol
• In the cytosol, this code
must be read so amino acids
can be assembled to make
polypeptides (proteins)
• This process is called
PROTEIN SYNTHESIS
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RNA
Roles of RNA and DNA
• DNA is the MASTER
PLAN
• RNA is the
BLUEPRINT of the
Master Plan
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RNA Differs from DNA
• RNA has a sugar ribose
DNA has a sugar deoxyribose
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•
•
Other Differences
RNA contains the base
uracil (U)
DNA has thymine (T)
RNA molecule is
single-stranded
DNA is
double-stranded
DNA
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Structure of RNA
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Three Types of RNA
.
•
•
•
•
•
•
•
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Messenger RNA (mRNA)
copies DNA’s code
carries genetic info to ribosomes
Ribosomal RNA (rRNA)
along with protein,
makes up ribosomes
Transfer RNA (tRNA)
transfers amino acids to ribosomes
where proteins are synthesized
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Messenger RNA
• Long Straight chain of
Nucleotides
• Made in the Nucleus
• Copies DNA
• leaves through nuclear pores
• Contains the Nitrogen Bases
A, G, C, U
• ( no T )
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Messenger RNA (mRNA)
• Carries the information for a
specific protein
• Made of 500 - 1000 basepairs
• Sequence of 3 bases called
codon
• AUG – methionine start codon
• UAA, UAG, or UGA –
– stop codons
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Ribosomal RNA (rRNA)
• rRNA is a single strand
– 100 to 3000 nucleotides long
• Globular in shape
• Made inside nucleus of a cell
• Associates with proteins
– form ribosomes
• Site of protein Synthesis
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The Genetic Code
• A codon designates an amino acid
• An amino acid may have
– more than one codon
• There are 64 possible codons
• Only 20 amino acids,
– but Some codons tell the ribosome
– to stop translating
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The Genetic Code
•Use the code by reading from
•center to the outside
•Example:
•AUG = Methionine
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Name the Amino Acids
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•
•
•
•
GGG?
UCA?
CAU?
GCA?
AAA?
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Remember the
Complementary Bases
On DNA:
A-T
C-G
On RNA:
A-U
C-G
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Transfer RNA (tRNA)
• Clover-leaf shape
• Single stranded molecule
– attachment site at one end
for an amino acid
• Opposite end has 3 nucleotide
bases called the anticodon
amino acid
attachment site
UA C
anticodon
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Transfer RNA
amino acid
attachment site
U A C
anticodon
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Codons and Anticodons
• The 3 bases of an anticodon are
complementary to 3 bases of a codon
• Example: Codon ACU
Anticodon UGA
UGA
ACU
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Transcription
and
Translation
Pathway to Making a
Protein
DNA
mRNA
tRNA (ribosomes)
Protein
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DNA  RNA  Protein
Nuclear
membrane
DNA
Transcription
mRNA
Eukaryotic
Cell
Ribosome
Translation
Protein
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Protein Synthesis
 The production or synthesis of
polypeptide chains (proteins)
 Two phases:
Transcription & Translation
 mRNA must be processed before
it leaves the nucleus of
eukaryotic cells
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Transcription
• The process of copying
the sequence of one
strand of DNA, the
template strand
• mRNA copies the template
strand
• Requires the enzyme RNA
Polymerase
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Template Strand
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Question:
 What would be the
complementary RNA strand
for the following DNA
sequence?
DNA 5’-GCGTATG-3’
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Answer:
• DNA 5’-GCGTATG-3’
• RNA 3’-CGCAUAC-5’
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Transcription
• During transcription, RNA
polymerase binds to DNA and
separates the DNA strands
• RNA Polymerase then uses
one strand of DNA as a
template to assemble
nucleotides into RNA
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RNA Polymerase
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Result of Transcription
CAP
New Transcript
Tail
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mRNA Transcript
•mRNA leaves the nucleus
through its pores and goes to
the ribosomes
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Translation
• Translation is the process
of of decoding the mRNA
into a polypeptide chain
• Ribosomes read mRNA
three bases or 1 codon at
a time and construct the
proteins
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Transcription
Translation
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Ribosomes
• Made of a large and small
subunit
• Composed of rRNA (40%)
and proteins (60%)
• Have two sites for tRNA
attachment --- P and A
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Step 1- Initiation
• mRNA transcript
start codon AUG
attaches to the
small ribosomal
subunit
• Small subunit
attaches to large
ribosomal subunit
mRNA transcript
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Ribosomes
Large
subunit
P
Site
A
Site
mRNA
Small
subunit
A U G
C U A C U U C G
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Step 2 - Elongation
• As ribosome moves, two tRNA with
their amino acids move into site A and
P of the ribosome
• Peptide bonds join the amino acids
Initiation
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
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Elongation
peptide bond
aa1
aa3
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
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aa1
peptide bond
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa4
aa2
aa3
4-tRNA
2-tRNA
A U G
3-tRNA
G C U
G A U G A A
C U A C U U C G A A C U
mRNA
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aa1
peptide bonds
aa4
aa2
aa3
2-tRNA
4-tRNA
G A U
(leaves)
3-tRNA
A U G
G C U
G A A
C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
4-tRNA
G A A G C U
G C U A C U U C G A A C U
mRNA
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peptide bonds
aa1
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
G A A
4-tRNA
G C U
G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa4
aa5
Termination
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
terminator
or stop
codon
C A U G U U U A G
mRNA
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End Product –The Protein!
• The end products of protein
synthesis is a primary structure
of a protein
• A sequence of amino acid
bonded together by peptide
bonds
aa2
aa1
aa3
aa4
aa5
aa199
aa200
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Messenger RNA
(mRNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein methionine
codon 2
codon 3
glycine
serine
codon 4
isoleucine
codon 5
codon 6
glycine
alanine
codon 7
stop
codon
Primary structure of a protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6
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Transcription
• Promoters are regions on
DNA that show where RNA
Polymerase must bind to
begin the Transcription of
RNA
• Called the TATA box
• Specific base sequences act
as signals to stop
• Called the termination signal
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mRNA Processing
• After the DNA is
transcribed into RNA,
editing must be done to
the nucleotide chain to
make the RNA functional
• Introns, non-functional
segments of DNA are
snipped out of the chain
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mRNA Editing
• Exons, segments of DNA that
code for proteins, are then
rejoined by the enzyme ligase
• A guanine triphosphate cap is
added to the 5” end of the
newly copied mRNA
• A poly A tail is added to the 3’
end of the RNA
• The newly processed mRNA can
then leave the nucleus
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