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Protein Synthesis
• DNA  RNA  Protein
Transcription Translation
DNA
 Basis for heredity from
parents to offspring. Located
in nucleus of cell.
 Regulates synthesis of
proteins. Tells what
particular polypeptide chain
to make.
*Proteins are important in
structure of organelles,
plasma membrane, enzymes
and hormones.
Genetic Code
 Contains DNA which is a
long twisted (double helix)
molecule composed of
nucleotides. DNA never
leaves the nucleus.
• Nucleotides have a sugar
group, phosphate group
(PO4-2), and 1 of 4 nitrogen
groups. These are adenine,
thymine (uracil in RNA),
cytosine, or guanine.
 The sugar and
phosphate group make
up the uprights and the
nitrogen bases make up
the rungs. (see diagram)
• Nitrogen groups pair up.
Adenine with thymine
and cytosine with
guanine. This is called a
nitrogen-base pair.
DNA
Genetic Code
 When three base pairs are
together, they form a triplet
code which makes an amino
acid. There are 20 amino
acids altogether. (see
diagram on board).
 When 10 or more amino
acids are formed, these form
a gene. A gene is a section of
DNA that contains all the
triplet base pairs to code for
one complete polypeptide
chain.
 The genetic code is the
message the gene
provides to make a
polypeptide chain.
 When 50 amino acids
are formed, these form a
protein. Examples are
alanine, valine, cysteine
and glycine.
Genetic Code
Protein Synthesis
• 2 processes of
protein
synthesis
1.Transcription
2.Translation
1. Transcription
 Process by which
RNA is formed.
• DNA regulates protein
synthesis and
ribosomes are where
proteins are
manufactured.
 DNA needs amino acids so
we need breakdown of
protein to get amino acids.
So go to where proteins are
manufactured which is the
ribosome.
• During transcription, triplet
code of DNA is transcribed
(copied) onto an RNA
molecule.
Transcription
Transcription
1 of 3 types of RNA is formed
• A. messenger RNA (mRNA)
• B. transfer RNA (tRNA)
• C. ribosomal RNA (rRNA)
A. mRNA
• Exits nucleus
through pores.
Passes into
cytoplasm and
associates with
ribosomes.
B. tRNA
Different structure
then mRNA. It is a
folded structure.
Exits nucleus and
passes into
cytoplasm
• Transports free
amino acids to
ribosomes
C. rRNA
Still a different structure. It is spherical.
Combines with proteins in nucleolus to form a
ribosome.
Exits into cytoplasm and remains as free
ribosome or anchors to endoplasmic reticulum.
• Look at p. 68 fig. 3-17
Transcription
DNA
DNA
Cytosine
Thymine
Adenine
Guanine
Adenine
Thymine
Each triplet is an amino acid. This is a double helix.
The mRNA comes in and “unzips” the double
helix.Then you get the following.
Transcription
DNA
Cytosine
Thymine
Adenine
RNA
Guanine
Adenine
Uracil
DNA
Guanine
Adenine
Thymine
RNA
Cytosine
Uracil
Adenine
The triplet formed by the mRNA is
called a codon.
2. Translation
• The interpretation
of the codon
message in mRNA
to a specific
sequence of amino
acids. Entirely done
in the cytoplasm on
the ribosomes.
Translation
A. mRNA attaches to
ribosomes
B. tRNA brings free
amino acids. Only 2
tRNA at a time on
mRNA. tRNA has triplet
base called anticodon.
C. Anticodon combines
with complementary
codon on mRNA
D.Continues on down
ribosome and peptide
bonds are formed
E.Sequence is stopped
when stop codon is
reached
F.New protein is then
formed
Look at p.69 fig. 3-18
Questions 136156