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Transcript
PROTEIN
SYNTHESIS
copyright cmassengale
1
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
copyright cmassengale
2
Amino Acid Structure
copyright cmassengale
3
Polypeptides
• Amino acid
chains are
called
polypeptides
copyright cmassengale
4
Starting with DNA
• DNA ‘s code must be copied
and taken to the cytosol
• In the cytoplasm, this code
must be read so amino acids
can be assembled to make
polypeptides (proteins)
• This process is called
PROTEIN SYNTHESIS
copyright cmassengale
5
Pathway to Making a
Protein
DNA
mRNA
Read by ribosomes
Protein
copyright cmassengale
6
DNA  RNA  Protein
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
copyright cmassengale
7
RNA
copyright cmassengale
8
RNA Differs from DNA
1. RNA
DNA
2. RNA
DNA
3. RNA
DNA
has a sugar ribose
has a sugar deoxyribose
contains the base uracil (U)
has thymine (T)
molecule is single-stranded
is double-stranded
9
Structure of RNA
copyright cmassengale
10
.
Three Types of RNA
• Ribosomal RNA (rRNA), along
with protein, makes up the
ribosomes
• Messenger RNA (mRNA) copies
DNA’s code & carries the
genetic information to the
ribosomes
• Transfer RNA (tRNA) transfers
amino acids to the ribosomes
where proteins are synthesized
copyright cmassengale
11
Ribosomal RNA (rRNA)
• rRNA is a single strand 100 to 3000
nucleotides long
• Globular in shape
• Made inside the nucleus of a cell and
exported to cytoplasm
• Associates with
proteins to form
ribosomes
• Site of protein
Synthesis
copyright cmassengale
12
Messenger RNA
• Long Straight chain of
Nucleotides
• Copies DNA and carries
the information for a
specific protein
• Made up of 500 to 1000+
nucleotides long, organized
into 3-base codons
copyright cmassengale
13
Remember the
Complementary Bases
On DNA:
A-T
C-G
On RNA:
A-U
C-G
copyright cmassengale
14
Genetic Code
 DNA contains a triplet code
 Every three bases on DNA stands
for ONE amino acid
 Each three-letter unit on mRNA is
called a codon
 Most amino acids have more than
one codon!
 20 amino acids: 64 different triplets
 ALL organisms use the SAME code
15
The Genetic Code
•Example:
AUG codes
for
Methionine
16
copyright cmassengale
17
Transfer RNA (tRNA)
• Clover-leaf shape
• Single stranded molecule with
attachment site at one end
for an amino acid
• Opposite end has three
nucleotide bases called the
anticodon
copyright cmassengale
18
Transfer RNA
amino acid
attachment site
U A C
anticodon
copyright cmassengale
19
Codons and Anticodons
• The 3 bases of an
anticodon are
complementary to
the 3 bases of a
codon
• Example: Codon ACU
Anticodon UGA
copyright cmassengale
UGA
ACU
20
Two Parts of Protein
Synthesis
 Transcription makes an RNA
molecule complementary to a
portion of DNA
 Translation occurs when the
sequence of bases of mRNA
DIRECTS the sequence of amino
acids in a polypeptide
21
Transcription
Translation
22
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
copyright cmassengale
23
Template Strand
copyright cmassengale
24
Question:
 What would be the
complementary RNA strand
for the following DNA
sequence?
DNA 5’-GCGTATG-3’
copyright cmassengale
25
Answer:
• DNA 5’-GCGTATG-3’
• RNA 3’-CGCAUAC-5’
copyright cmassengale
26
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
copyright cmassengale
27
RNA Polymerase
copyright cmassengale
28
mRNA Transcript
•mRNA leaves the nucleus
through its pores and goes to
the ribosomes
copyright cmassengale
29
Translation
• Translation is the process
of decoding the mRNA
into a polypeptide chain
• Ribosomes read mRNA
three bases or 1 codon at
a time and construct the
proteins
copyright cmassengale
30
Ribosomes
• Made of a large and small
subunit
• Composed of rRNA (40%)
and proteins (60%)
• Have two sites for tRNA
attachment --- P and A
copyright cmassengale
31
Translation
• Three steps:
1. initiation: mRNA binds to
ribosome at start codon (AUG)
2. elongation: amino acids linked
3. termination: stop codon
(UAG, UAA, or UGA) signals
ribosome to release mRNA
32
Step 1- Initiation
• mRNA transcript
attaches to the
small ribosomal
subunit
• Small subunit
attaches to large
ribosomal subunit
mRNA transcript
copyright cmassengale
33
Ribosomes
Large
subunit
P
Site
A
Site
mRNA
Small
subunit
A U G
C U A C U U C G
copyright cmassengale
34
Initiation
aa2
aa1
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
copyright cmassengale
35
Step 2 - Elongation
• As ribosome moves, tRNA with their
amino acids move into the ribosome
• Peptide bonds join the amino acids
copyright cmassengale
36
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
copyright cmassengale
37
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
copyright cmassengale
Ribosomes move over one codon
38
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
copyright cmassengale
39
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
copyright cmassengale
Ribosomes move over one codon
40
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
copyright cmassengale
41
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
copyright cmassengale
Ribosomes move over one codon
42
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
copyright cmassengale
43
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
copyright cmassengale
aa200
44