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Chapter 3
Cell Structures
and Their
Functions
Dividing Cells
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Protein Synthesis
• DNA serves as master blueprint for protein
synthesis
• DNA controls enzyme production and cell
activity is regulated by enzymes (Proteins)
• Genes are segments of DNA carrying
instructions for a polypeptide chain
• Triplets of nucleotide bases form the genetic
library
• Each triplet specifies coding for an amino acid
Protein Synthesis
• Two step process
– Transcription
• cell makes a copy of gene necessary to make a
particular protein: messenger RNA (mRNA)
• mRNA then travels from nucleus to ribosomes
where information is translated into a protein
– Translation
• requires both mRNA and transfer RNA (tRNA)
• tRNA brings amino acids necessary to synthesize
the protein to the ribosome
Overview of Protein Synthesis
1. DNA contains the information
necessary to produce proteins
2. Transcription of one DNA
strand results in mRNA, which
is a complementary copy of
the information in the DNA
strand needed to make a
protein
3. The mRNA leaves the nucleus
and goes to a ribosome
4. Amino acids, the building
blocks of proteins, are carried
to the ribosome by tRNAs
5. In the process of translation,
the information contained in
mRNA is used to determine
the number, kinds, and
arrangement of amino acids in
the polypeptide chain
Fig. 3.23
Overview of Protein Synthesis
1. DNA contains the information
necessary to produce proteins
2. Transcription of one DNA
strand results in mRNA, which
is a complementary copy of
the information in the DNA
strand needed to make a
protein
3. The mRNA leaves the nucleus
and goes to a ribosome
4. Amino acids, the building
blocks of proteins, are carried
to the ribosome by tRNAs
5. In the process of translation,
the information contained in
mRNA is used to determine
the number, kinds, and
arrangement of amino acids in
the polypeptide chain
Fig. 3.23
Overview of Protein Synthesis
1. DNA contains the information
necessary to produce proteins
2. Transcription of one DNA
strand results in mRNA, which
is a complementary copy of
the information in the DNA
strand needed to make a
protein
3. The mRNA leaves the nucleus
and goes to a ribosome
4. Amino acids, the building
blocks of proteins, are carried
to the ribosome by tRNAs
5. In the process of translation,
the information contained in
mRNA is used to determine
the number, kinds, and
arrangement of amino acids in
the polypeptide chain
Fig. 3.23
Overview of Protein Synthesis
1. DNA contains the information
necessary to produce proteins
2. Transcription of one DNA
strand results in mRNA, which
is a complementary copy of
the information in the DNA
strand needed to make a
protein
3. The mRNA leaves the nucleus
and goes to a ribosome
4. Amino acids, the building
blocks of proteins, are carried
to the ribosome by tRNAs
5. In the process of translation,
the information contained in
mRNA is used to determine
the number, kinds, and
arrangement of amino acids in
the polypeptide chain
Fig. 3.23
Overview of Protein Synthesis
1. DNA contains the information
necessary to produce proteins
2. Transcription of one DNA
strand results in mRNA, which
is a complementary copy of
the information in the DNA
strand needed to make a
protein
3. The mRNA leaves the nucleus
and goes to a ribosome
4. Amino acids, the building
blocks of proteins, are carried
to the ribosome by tRNAs
5. In the process of translation,
the information contained in
mRNA is used to determine
the number, kinds, and
arrangement of amino acids in
the polypeptide chain
Fig. 3.23
Transcription
• Synthesis of mRNA, tRNA, and rRNA
based on the nucleotide sequence in DNA
– Messenger RNA (mRNA) – carries genetic
information from DNA in nucleus to ribosomes
in the cytoplasm
– Transfer RNAs (tRNAs) – bound to amino
acids base pair with codons of mRNA at
ribosome to begin process of protein
synthesis
– Ribosomal RNA (rRNA) – a structural
component of ribosomes
Transcription
1. The strands of the DNA molecule
separate from each other. One
DNA strand serves as a template
for mRNA synthesis
2. Nucleotides that will form mRNA
pair with DNA nucleotides
according to the base-pair
combinations shown in the key at
the top of the figure. Thus, the
sequence of nucleotides in the
template DNA strand (purple)
determines the sequence of
nucleotides in mRNA (grey). RNA
polymerase (the enzyme is not
shown) joins the nucleotides of
mRNA together
3. As nucleotides are added, an
mRNA molecule is formed
Fig. 3.24
Transcription: RNA Polymerase
• An enzyme that oversees synthesis of
RNA
• Unwinds the DNA template
• Adds complementary ribonucleoside
triphosphates on the DNA template
• Joins these RNA nucleotides together
• Encodes a termination signal to stop
transcription
Transcription
• Posttranscriptional processing modifies
mRNA before it leaves nucleus by
removing introns (non-coding) and then
splicing exons (coding) together with
enzymes called spliceosomes
– Functional mRNA consists only of exons
• Alternative splicing produces different
combination of exons, allowing one gene
to produce more than one type of protein
Translation
• Synthesis of proteins in response to codons of
mRNA
– Codon: set of 3 nucleotides, codes for 1 amino acid
during translation
– Anticodon: part of tRNA; consists of 3 nucleotides and
is complementary to a particular codon of mRNA
• mRNA moves through nuclear pores to
ribosomes
• tRNA, which carries amino acids, interacts at
ribosome with mRNA.
• anticodons of tRNA bind to codons of mRNA, amino acids are
joined to form a protein
Translation
1. To start protein synthesis, a ribosome
binds to mRNA. The ribosome has
two binding sites for tRNA with its
amino acid. Note that the first codon
to associate with a tRNA is AUG, the
start codon, which codes for
methionine. The codon of mRNA and
the anitcodon of tRNA are aligned and
joined. The other tRNA binding site is
open
Fig. 3.25
Translation
1. To start protein synthesis, a ribosome
binds to mRNA. The ribosome has two
binding sites for tRNA with its amino
acid. Note that the first codon to
associate with a tRNA is AUG, the start
codon, which codes for methionine.
The codon of mRNA and the anitcodon
of tRNA are aligned and joined. The
other tRNA binding site is open
2. By occupying the open tRNA binding
site, the next tRNA is properly aligned
with mRNA and with the other tRNA
Fig. 3.25
Translation
1.
2.
3.
To start protein synthesis, a ribosome
binds to mRNA. The ribosome has
two binding sites for tRNA with its
amino acid. Note that the first codon
to associate with a tRNA is AUG, the
start codon, which codes for
methionine. The codon of mRNA and
the anitcodon of tRNA are aligned and
joined. The other tRNA binding site is
open
By occupying the open tRNA binding
site, the next tRNA is properly aligned
with mRNA and with the other tRNA
An enzyme within the ribosome
catalyzes a synthesis reaction to form
a peptide bond between the amino
acids. Note that the amino acids are
now associated with only one of the
tRNAs
Fig. 3.25
Translation
3. An enzyme within the ribosome catalyzes a
synthesis reaction to form a peptide bond
between the amino acids. Note that the
amino acids are now associated with only
one of the tRNAs
4. The ribosome shifts position by three
nucleotides. The tRNA without the amino
acid is released from the ribosome, and the
tRNA with the amino acids takes its position.
A tRNA binding site is left open by the shift.
Additional amino acids can be added by
repeating steps 2 through 4
Fig. 3.25
Translation
3. An enzyme within the ribosome catalyzes a
synthesis reaction to form a peptide bond
between the amino acids. Note that the
amino acids are now associated with only
one of the tRNAs
4. The ribosome shifts position by three
nucleotides. The tRNA without the amino
acid is released from the ribosome, and the
tRNA with the amino acids takes its position.
A tRNA binding site is left open by the shift.
Additional amino acids can be added by
repeating steps 2 through 4
5. Eventually a stop codon in the mRNA, such
as UAA, ends the process of translation. At
this point, the mRNA and polypeptide chain
are released from the ribosome.
6. Multiple ribosomes attach to a single mRNA
to form a polyribosome. As the ribosomes
move down the mRNA, proteins attached to
the ribosomes lengthen and eventually
detach from the mRNA
Fig. 3.25
Translation
1. To start protein synthesis, a ribosome binds to mRNA.
The ribosome has two binding sites for tRNA with its
amino acid. Note that the first codon to associate with a
tRNA is AUG, the start codon, which codes for
methionine. The codon of mRNA and the anitcodon of
tRNA are aligned and joined. The other tRNA binding
site is open
2. By occupying the open tRNA binding site, the next tRNA
is properly aligned with mRNA and with the other tRNA
3. An enzyme within the ribosome catalyzes a synthesis
reaction to form a peptide bond between the amino
acids. Note that the amino acids are now associated with
only one of the tRNAs
4. The ribosome shifts position by three nucleotides. The
tRNA without the amino acid is released from the
ribosome, and the tRNA with the amino acids takes its
position. A tRNA binding site is left open by the shift.
Additional amino acids can be added by repeating steps
2 through 4
5. Eventually a stop codon in the mRNA, such as UAA,
ends the process of translation. At this point, the mRNA
and polypeptide chain are released from the ribosome.
6. Multiple ribosomes attach to a single mRNA to form a
polyribosome. As the ribosomes move down the mRNA,
proteins attached to the ribosomes lengthen and
eventually detach from the mRNA
Fig. 3.25
Information Transfer from DNA to RNA
• DNA triplets are transcribed into mRNA
codons by RNA polymerase
• Codons base pair with tRNA anticodons at
the ribosomes
• Amino acids are peptide bonded at the
ribosomes to form polypeptide chains
• Start and stop codons are used in initiating
and ending translation