Transcript Cell Biology # 4

Cell Cycle
• Defines changes from formation of cell
until it reproduces
• Includes:
– Interphase
• Cell grows and carries out functions
– Cell division (mitotic phase)
• Divides into two cells
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Interphase
• Period from cell formation to cell division
• Nuclear material called chromatin
• Three subphases:
– G1 (gap 1)—vigorous growth and metabolism
• Cells that permanently cease dividing said to be in
G0 phase
– S (synthetic)—DNA replication occurs
– G2 (gap 2)—preparation for division
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Figure 3.31 The cell cycle.
G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
M
G2
Growth and final
preparations for
division
G2 checkpoint
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter
nuclei. (1 of 6)
Interphase
Centrosomes (each
has 2 centrioles)
Plasma
membrane
Nucleolus
Chromatin
Nuclear
envelope
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DNA Replication
• Prior to division cell makes copy of DNA
• DNA helices separated into replication
bubbles with replication forks at each end
– Each strand acts as template for
complementary strand
• DNA polymerase begins adding
nucleotides at RNA primer
• DNA polymerase continues from primer
– Synthesizes one leading, one lagging strand
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DNA Replication
• DNA polymerase only works in one
direction
– Leading strand synthesized continuously
– Lagging strand synthesized discontinuously
into segments
– DNA ligase splices short segments of
discontinuous strand together
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DNA Replication
• End result: two identical DNA molecules
formed from original
– During mitotic cell division one complete copy
given to new cell; one retained in original cell
• Process is called semiconservative
replication
– Each DNA composed of one old and one new
strand
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Figure 3.32 Replication of DNA: summary.
Free nucleotides DNA polymerase
Chromosome
Old (parental) strand acts as a
template for synthesis of new
strand
Leading
strand
Two new strands (leading
and lagging) synthesized
in opposite directions
Old
DNA
Replication
bubble
Lagging
strand
Enzymes unwind
Replication
the double helix and
fork
expose the bases
Adenine
Thymine
Cytosine
Guanine
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DNA
polymerase
Old (template)
strand
DNA Replication
PLAY
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Animation: DNA Replication
Cell Division
• Meiosis - cell division producing gametes
• Mitotic cell division - produces clones
– Essential for body growth and tissue repair
– Occurs continuously in some cells
• Skin; intestinal lining
– None in most mature cells of nervous tissue,
skeletal muscle, and cardiac muscle
• Repairs with fibrous tissue
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Events Of Cell Division
• Mitosis—division of nucleus
– Four stages ensure each cell receives copy of
replicated DNA
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
– Cytokinesis—division of cytoplasm-by
cleavage furrow
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Figure 3.31 The cell cycle.
G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
M
G2
Growth and final
preparations for
division
G2 checkpoint
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Cell Division
PLAY
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Animation: Mitosis
Prophase
• Chromosomes become visible, each with
two chromatids joined at centromere
• Centrosomes separate and migrate
toward opposite poles
• Mitotic spindles and asters form
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Prophase
• Nuclear envelope fragments
• Kinetochore microtubules attach to
kinetochore of centromeres and draw
them toward equator of cell
• Polar microtubules assist in forcing poles
apart
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter
nuclei. (2 of 6)
Early Prophase
Early mitotic
spindle
Aster
Chromosome
consisting of two
sister chromatids
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Centromere
Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter
nuclei. (3 of 6)
Late Prophase
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Spindle pole
Polar microtubule
Fragments
of nuclear
envelope
Kinetochore
Kinetochore
microtubule
Metaphase
• Centromeres of chromosomes aligned at
equator
• Plane midway between poles called
metaphase plate
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter
nuclei. (4 of 6)
Metaphase
Spindle
Metaphase
plate
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Anaphase
• Shortest phase
• Centromeres of chromosomes split
simultaneously—each chromatid becomes
a chromosome
• Chromosomes (V shaped) pulled toward
poles by motor proteins of kinetochores
• Polar microtubules continue forcing poles
apart
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter
nuclei. (5 of 6)
Anaphase
Daughter
chromosomes
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Telophase
• Begins when chromosome movement
stops
• Two sets of chromosomes uncoil to form
chromatin
• New nuclear membrane forms around
each chromatin mass
• Nucleoli reappear
• Spindle disappears
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Cytokinesis
• Begins during late anaphase
• Ring of actin microfilaments contracts to
form cleavage furrow
• Two daughter cells pinched apart, each
containing nucleus identical to original
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Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter
nuclei. (6 of 6)
Telophase
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Cytokinesis
Nuclear
envelope
forming
Nucleolus forming
Contractile
ring at
cleavage
furrow
Control of Cell Division
• "Go" signals:
– Critical volume of cell when area of
membrane inadequate for exchange
– Chemicals (e.g., growth factors, hormones)
– Availability of space–contact inhibition
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Control of Cell Division
• To replicate DNA and enter mitosis
requires
– Cyclins–regulatory proteins
• Accumulate during interphase
– Cdks (Cyclin-dependent kinases)–bind to
cyclins  activated
• Enzyme cascades prepare cell for division
– Cyclins destroyed after mitotic cell division
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Control of Cell Division
• "Go" signals
– G1 checkpoints (restriction point) most
important
• If doesn't pass  G0–no further division
– Late in G2 MPF (M-phase promoting factor)
required to enter M phase
• "Other Controls" signals
– Repressor genes inhibit cell division
• E.g., P53 gene
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Protein Synthesis
• DNA is master blueprint for protein
synthesis
• Gene - segment of DNA with blueprint for
one polypeptide
• Triplets (three sequential DNA nitrogen
bases) form genetic library
– Bases in DNA are A, G, T, and C
– Each triplet specifies coding for number, kind,
and order of amino acids in polypeptide
PLAY
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Animation: DNA and RNA
Protein Synthesis
• Genes composed of exons and introns
– Exons code for amino acids
– Introns–noncoding segments
• Role of RNA
– DNA decoding mechanism and messenger
– Three types–all formed on DNA in nucleus
• Messenger RNA (mRNA); ribosomal RNA
(rRNA); transfer RNA (tRNA)
• RNA differs from DNA
– Uracil is substituted for thymine
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Roles of the Three Main Types of RNA
• Messenger RNA (mRNA)
– Carries instructions for building a polypeptide,
from gene in DNA to ribosomes in cytoplasm
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Roles of the Three Main Types of RNA
• Ribosomal RNA (rRNA)
– Structural component of ribosomes that, along
with tRNA, helps translate message from
mRNA
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Roles of the Three Main Types of RNA
• Transfer RNAs (tRNAs)
– Bind to amino acids and pair with bases of
codons of mRNA at ribosome to begin
process of protein synthesis
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Figure 3.34 Simplified scheme of information flow from the DNA gene to mRNA to protein structure during
transcription and translation.
Nuclear
envelope
Transcription
RNA Processing
DNA
Pre-mRNA
mRNA
Nuclear
pores
Ribosome
Translation
Polypeptide
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Protein Synthesis
• Occurs in two steps
– Transcription
• DNA information coded in mRNA
– Translation
• mRNA decoded to assemble polypeptides
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Transcription
• Transfers DNA gene base sequence to
complementary base sequence of mRNA
• Transcription factors–gene activators
– Loosen histones from DNA in area to be
transcribed
– Bind to promoter-DNA sequence specifying
start site of gene on template strand
– Mediate binding of RNA polymerase (enzyme
synthesizing mRNA) to promoter
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Transcription
• Three phases
– Initiation
• RNA polymerase separates DNA strands
– Elongation
• RNA polymerase adds complementary nucleotides
– Termination
• Termination signal indicates "stop"
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Processing of mRNA
• mRNA edited and processed before
translation
– Introns removed by spliceosomes
– mRNA complex proteins associate to guide
export, ensure accuracy for translation
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Figure 3.35 Overview of stages of transcription.
Slide 1
RNA polymerase
Coding strand
DNA
Promoter
region
Termination
signal
Template strand
1 Initiation: With the help of transcription factors, RNA polymerase
binds to the promoter, pries apart the two DNA strands, and initiates
mRNA synthesis at the start point on the template strand.
mRNA
Template strand
Coding strand of DNA
2 Elongation: As the RNA polymerase moves along the template
strand, elongating the mRNA transcript one base at a time, it unwinds
the DNA double helix before it and rewinds the double helix behind it.
Rewinding
of DNA
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
mRNA transcript
mRNA
3 Termination: mRNA synthesis ends when the termination signal
is reached. RNA polymerase and the completed mRNA transcript are
released.
Completed mRNA transcript
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RNA polymerase
DNA-RNA hybrid region
Template
strand
RNA
polymerase
The DNA-RNA hybrid: At any given moment, 16–18 base pairs of
DNA are unwound and the most recently made RNA is still bound to
DNA. This small region is called the DNA-RNA hybrid.
Figure 3.35 Overview of stages of transcription.
Slide 2
RNA polymerase
Coding strand
DNA
Promoter
region
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Template strand
Termination
signal
Figure 3.35 Overview of stages of transcription.
Slide 3
RNA polymerase
Coding strand
DNA
Promoter
region
Template strand
Termination
signal
1 Initiation: With the help of transcription factors, RNA polymerase
binds to the promoter, pries apart the two DNA strands, and initiates
mRNA synthesis at the start point on the template strand.
mRNA
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Template strand
Figure 3.35 Overview of stages of transcription.
Slide 4
RNA polymerase
Coding strand
DNA
Promoter
region
Template strand
Termination
signal
1 Initiation: With the help of transcription factors, RNA polymerase
binds to the promoter, pries apart the two DNA strands, and initiates
mRNA synthesis at the start point on the template strand.
mRNA
Template strand
Coding strand of DNA
2 Elongation: As the RNA polymerase moves along the template
strand, elongating the mRNA transcript one base at a time, it unwinds
the DNA double helix before it and rewinds the double helix behind it.
Rewinding
of DNA
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
mRNA transcript
mRNA
DNA-RNA hybrid region
Template
strand
RNA
polymerase
The DNA-RNA hybrid: At any given moment, 16–18 base pairs of
DNA are unwound and the most recently made RNA is still bound to
DNA. This small region is called the DNA-RNA hybrid.
© 2013 Pearson Education, Inc.
Figure 3.35 Overview of stages of transcription.
Slide 5
RNA polymerase
Coding strand
DNA
Promoter
region
Termination
signal
Template strand
1 Initiation: With the help of transcription factors, RNA polymerase
binds to the promoter, pries apart the two DNA strands, and initiates
mRNA synthesis at the start point on the template strand.
mRNA
Template strand
Coding strand of DNA
2 Elongation: As the RNA polymerase moves along the template
strand, elongating the mRNA transcript one base at a time, it unwinds
the DNA double helix before it and rewinds the double helix behind it.
Rewinding
of DNA
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
mRNA transcript
mRNA
3 Termination: mRNA synthesis ends when the termination signal
is reached. RNA polymerase and the completed mRNA transcript are
released.
Completed mRNA transcript
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RNA polymerase
DNA-RNA hybrid region
Template
strand
RNA
polymerase
The DNA-RNA hybrid: At any given moment, 16–18 base pairs of
DNA are unwound and the most recently made RNA is still bound to
DNA. This small region is called the DNA-RNA hybrid.
Translation
• Converts base sequence of nucleic acids
into amino acid sequence of proteins
• Involves mRNAs, tRNAs, and rRNAs
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Genetic Code
• Each three-base sequence on DNA
(triplet) represented by codon
– Codon—complementary three-base
sequence on mRNA
– Some amino acids represented by more than
one codon
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Figure 3.36 The genetic code.
SECOND BASE
C
UUU
A
UCU
UAU
Phe
U
G
U
UGU
Tyr
Cys
UAC
UGC
C
UCA
UAA Stop
UGA Stop
A
UUG
UCG
UAG Stop
UGG
Trp G
CUU
CCU
CAU
CGU
U
CUC
CCC
CAC
UUC
UCC
UUA
Ser
Leu
His
C
Leu
FIRST BASE
CUA
CCA
C
CGC
Pro
Arg
CAA
CGA
A
CGG
G
Gln
CUG
CCG
CAG
AUU
ACU
AAU
A
AUC
lle
ACC
U
AGU
Ser
Asn
AAC
AGC
Thr
AUA
AUG
ACA
Met or
Start
GUU
AAA
AGA
Arg
Lys
ACG
AAG
GCU
GAU
GUC
GCC
Val
GAC
G
GGU
U
Gly
GCA
GAA
GUG
GCG
GAG
GGA
A
GGG
G
Glu
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C
GGC
Ala
GUA
A
AGG
Asp
G
C
THIRD BASE
U
Role of tRNA
• 45 different types
• Binds specific amino acid at one end
(stem)
• Anticodon at other end (head) binds
mRNA codon at ribosome by hydrogen
bonds
– E.g., if codon = AUA, anticodon = UAU
• Ribosome coordinates coupling of mRNA
and tRNA; contains three sites
– Aminoacyl site; peptidyl site; exit site
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Sequence of Events in Translation
• Three phases that require ATP, protein
factors, and enzymes
– Initiation
– Elongation
– Termination
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Translation: Initiation
• Small ribosomal subunit binds to initiator
tRNA and mRNA to be decoded; scans for
start codon
• Large and small ribosomal units attach,
forming functional ribosome
• At end of initiation
– tRNA in P site; A site vacant
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Translation: Elongation
• Three steps
– Codon recognition
• tRNA binds complementary codon in A site
– Peptide bond formation
• Amino acid of tRNA in P site bonded to amino acid
of tRNA in A site
– Translocation
• tRNAs move one position–A  P; P  E
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Translation: Elongation
• New amino acids added by other tRNAs
as ribosome moves along mRNA
• Initial portion of mRNA can be "read" by
additional ribosomes
– Polyribosome
• multiple ribosome-mRNA complex
– Produces multiple copies of same protein
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Figure 3.38 Polyribosome arrays.
Growing polypeptides
Completed
polypeptide
Incoming
ribosomal
subunits
Ribosomes
Polyribosome
Start of
mRNA
mRNA
End of
mRNA
Each polyribosome consists of one strand of mRNA
being read by several ribosomes simultaneously. In this
diagram, the mRNA is moving to the left and the “oldest”
functional ribosome is farthest to the right.
© 2013 Pearson Education, Inc.
This transmission electron micrograph shows
a large polyribosome (400,0003).
Translation: Termination
• When stop codon (UGA, UAA, UAG)
enters A site
– Signals end of translation
– Protein release factor binds to stop codon 
water added to chain  release of
polypeptide chain; separation of ribosome
subunits; degradation of mRNA
– Protein processed into functional 3-D
structure
© 2013 Pearson Education, Inc.
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
Slide 1
2 Elongation. Amino acids are added one at
a time to the growing peptide chain via a
process that has three repeating steps.
Template
strand of
DNA
Amino acid
Met
corresponding
to anticodon
Pre-mRNA
tRNA
P A
GGC
AUACCGCUA
mRNA
Met
1 Initiation. Initiation occurs
when four components combine:
• A small ribosomal subunit
• An initiator tRNA that carries
the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next
phase, elongation, begins.
Cytosol (site
of translation)
Met
P
site
Large
ribosomal
subunit
E
site
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Aminoacyl-tRNA
synthetase
Initiator tRNA
bearing anticodon
Newly made (and
edited) mRNA
leaves nucleus and
travels to a free or
attached ribosome
for decoding.
A
site
Start
codon
Ile
Pro
E
Nucleus (site
of transcription)
Methionine
(amino acid)
Amino acid
corresponding
to anticodon
The correct amino
acid is attached
to each species
of tRNA by a
synthetase enzyme.
Small
ribosomal
subunit
Polypeptide
tRNA
anticodon
New peptide
Ile
bond
Pro
Leu
Released
tRNA
Ile
Pro
Leu
P A
E
Complementary
GGC GAU
mRNA codon
AUACCG CUA
2a Codon recognition.
P A
E
The anticodon of an
GAU
incoming tRNA binds with
CCGCUA CUC
2b Peptide bond
the complementary mRNA
codon (A to U and C to G)
formation. The growing
Direction of
in the A site of the
polypeptide bound to the
ribosome movement
ribosome.
tRNA at the P site is
2c Translocation. As the
transferred to the amino
entire ribosome translocates, it
acid carried by the tRNA
shifts by one codon along the
mRNA:
in the A site, and a new
• The unloaded tRNA in the P
peptide bond is formed.
site is moved to the E site
and then released.
• The tRNA in the A site moves
to the P site.
• The next codon to be
translated is now in the empty
A site ready for step 2a again.
P
E
Release factor
CCU
Polypeptide
CUGGGA UGA
Stop codon
3 Termination. When a stop codon (UGA,
UAA, or UAG) arrives at the A site, elongation
ends. Release of the newly made polypeptide
is triggered by a release factor and the
ribosomal subunits separate, releasing the
mRNA.
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
Template
strand of
DNA
Pre-mRNA
Met
Amino acid
corresponding
to anticodon
The correct amino
acid is attached
to each species
of tRNA by a
synthetase enzyme.
tRNA
mRNA
Nucleus (site
of transcription)
Methionine
Newly made
(amino acid)
(and edited)
Met
mRNA leaves
nucleus and
travels to a free
or attached
ribosome for
decoding.
Aminoacyl-tRNA
synthetase
Initiator tRNA
bearing anticodon
Cytosol (site
of translation)
Met
P
site
Large
ribosomal
subunit
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E
site
A
site
Start
codon
Slide 2
Small
ribosomal
subunit
1 Initiation. Initiation occurs
when four components combine:
• A small ribosomal subunit
• An initiator tRNA that carries
the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished,
the next phase, elongation,
begins.
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
2 Elongation. Amino acids are
added one at a time to the
growing peptide chain via a process
that has three repeating steps.
Amino acid
corresponding
to anticodon
lle
Pro
E
P A
GGC
AUA CCG CUA
tRNA
anticodon
Complementary
mRNA codon
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2a Codon recognition.
The anticodon of an
incoming tRNA binds with
the complementary mRNA
codon (A to U and C to G) in
the A site of the ribosome.
Slide 3
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
Polypeptide
New peptide
Ile
Pro
bond
Leu
E
P
A
GG C G A U
A UA CCG C UA
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2b Peptide bond
formation. The growing
polypeptide bound to the
tRNA at the P site is
transferred to the amino acid
carried by the tRNA in the A
site, and a new peptide bond
is formed.
Slide 4
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
Slide 5
Released
tRNA
Ile
Pro
Leu
E
P
A
G AU
C CG C U A C UC
Direction of ribosome movement
2c Translocation. As the entire ribosome translocates, it shifts by one codon along the mRNA:
• The unloaded tRNA in the P site is moved to the E site and then released.
• The tRNA in the A site moves to the P site.
• The next codon to be translated is now in the empty A site ready for step 2a again.
© 2013 Pearson Education, Inc.
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
E
P
CC U
CUGGGA UGA
Release factor
Stop codon
3 Termination. When a stop codon (UGA,
UAA, or UAG) arrives at the A site, elongation ends.
Release of the newly made polypeptide is triggered
by a release factor and the ribosomal subunits
separate, releasing the mRNA.
© 2013 Pearson Education, Inc.
Slide 6
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
E
P
CC U
CUGGGA UGA
Release factor
Polypeptide
Stop codon
3 Termination. When a stop codon (UGA,
UAA, or UAG) arrives at the A site, elongation
ends. Release of the newly made polypeptide
is triggered by a release factor and the
ribosomal subunits separate, releasing the
mRNA.
© 2013 Pearson Education, Inc.
Slide 7
Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the
ribosome to form a particular polypeptide.
Slide 8
2 Elongation. Amino acids are added one at
a time to the growing peptide chain via a
process that has three repeating steps.
Template
strand of
DNA
Amino acid
Met
corresponding
to anticodon
Pre-mRNA
tRNA
P A
GGC
AUACCGCUA
mRNA
Met
1 Initiation. Initiation occurs
when four components combine:
• A small ribosomal subunit
• An initiator tRNA that carries
the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next
phase, elongation, begins.
Cytosol (site
of translation)
Met
P
site
Large
ribosomal
subunit
E
site
© 2013 Pearson Education, Inc.
Aminoacyl-tRNA
synthetase
Initiator tRNA
bearing anticodon
Newly made (and
edited) mRNA
leaves nucleus and
travels to a free or
attached ribosome
for decoding.
A
site
Start
codon
Ile
Pro
E
Nucleus (site
of transcription)
Methionine
(amino acid)
Amino acid
corresponding
to anticodon
The correct amino
acid is attached
to each species
of tRNA by a
synthetase enzyme.
Small
ribosomal
subunit
Polypeptide
tRNA
anticodon
New peptide
Ile
bond
Pro
Leu
Released
tRNA
Ile
Pro
Leu
P A
E
Complementary
GGC GAU
mRNA codon
AUACCG CUA
2a Codon recognition.
P A
E
The anticodon of an
GAU
incoming tRNA binds with
CCGCUA CUC
2b Peptide bond
the complementary mRNA
codon (A to U and C to G)
formation. The growing
Direction of
in the A site of the
polypeptide bound to the
ribosome movement
ribosome.
tRNA at the P site is
2c Translocation. As the
transferred to the amino
entire ribosome translocates, it
acid carried by the tRNA
shifts by one codon along the
mRNA:
in the A site, and a new
• The unloaded tRNA in the P
peptide bond is formed.
site is moved to the E site
and then released.
• The tRNA in the A site moves
to the P site.
• The next codon to be
translated is now in the empty
A site ready for step 2a again.
P
E
Release factor
CCU
Polypeptide
CUGGGA UGA
Stop codon
3 Termination. When a stop codon (UGA,
UAA, or UAG) arrives at the A site, elongation
ends. Release of the newly made polypeptide
is triggered by a release factor and the
ribosomal subunits separate, releasing the
mRNA.
Role of Rough ER in Protein Synthesis
• mRNA–ribosome complex directed to
rough ER by signal-recognition particle
(SRP)
• Forming protein enters ER
• Sugar groups may be added to protein,
and its shape may be altered
• Protein enclosed in vesicle for transport to
Golgi apparatus
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Figure 3.39 Rough ER processing of proteins.
1 The SRP directs the
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
ER signal
sequence
Slide 1
2 Once attached to the ER, the SRP is
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
3 An enzyme clips off the signal
sequence. As protein synthesis
continues, sugar groups may be
added to the protein.
Ribosome
mRNA
Signal
Signal
recognition
sequence
particle
removed
(SRP)
Receptor site Growing
polypeptide
4 In this example, the completed protein
is released from the ribosome and folds
into its 3-D conformation, a process aided
by molecular chaperones.
Sugar
group
Released
protein
5 The protein is enclosed within a
protein coated transport vesicle. The
transport vesicles make their way to
the Golgi apparatus, where further
processing of the proteins occurs
(see Figure 3.19).
Rough ER cistern
Cytosol
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Transport vesicle
pinching off
Protein-coated
transport vesicle
Figure 3.39 Rough ER processing of proteins.
1 The SRP directs the
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
ER signal
sequence
Ribosome
mRNA
Signal
recognition
particle
(SRP)
Receptor site
Rough ER cistern
Cytosol
© 2013 Pearson Education, Inc.
Slide 2
Figure 3.39 Rough ER processing of proteins.
1 The SRP directs the
U
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
ER signal
sequence
Ribosome
mRNA
Signal
recognition
particle
(SRP)
Receptor site Growing
polypeptide
Rough ER cistern
Cytosol
© 2013 Pearson Education, Inc.
2 Once attached to the ER, the SRP is
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
Slide 3
Figure 3.39 Rough ER processing of proteins.
1 The SRP directs the
U
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
ER signal
sequence
Slide 4
2 Once attached to the ER, the SRP is
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
3 An enzyme clips off the signal
sequence. As protein synthesis
continues, sugar groups may be
added to the protein.
Ribosome
mRNA
Signal
Signal
recognition
sequence
particle
removed
(SRP)
Receptor site Growing
polypeptide
Rough ER cistern
Cytosol
© 2013 Pearson Education, Inc.
Sugar
group
Figure 3.39 Rough ER processing of proteins.
1 The SRP directs the
U
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
ER signal
sequence
Slide 5
2 Once attached to the ER, the SRP is
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
3 An enzyme clips off the signal
sequence. As protein synthesis
continues, sugar groups may be
added to the protein.
Ribosome
mRNA
Signal
Signal
recognition
sequence
particle
removed
(SRP)
Receptor site Growing
polypeptide
4 In this example, the completed protein
is released from the ribosome and folds
into its 3-D conformation, a process aided
by molecular chaperones.
Sugar
group
Released
protein
Rough ER cistern
Cytosol
© 2013 Pearson Education, Inc.
Figure 3.39 Rough ER processing of proteins.
1 The SRP directs the
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
ER signal
sequence
Slide 6
2 Once attached to the ER, the SRP is
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
3 An enzyme clips off the signal
sequence. As protein synthesis
continues, sugar groups may be
added to the protein.
Ribosome
mRNA
Signal
Signal
recognition
sequence
particle
removed
(SRP)
Receptor site Growing
polypeptide
4 In this example, the completed protein
is released from the ribosome and folds
into its 3-D conformation, a process aided
by molecular chaperones.
Sugar
group
Released
protein
5 The protein is enclosed within a
protein coated transport vesicle. The
transport vesicles make their way to
the Golgi apparatus, where further
processing of the proteins occurs
(see Figure 3.19).
Rough ER cistern
Cytosol
© 2013 Pearson Education, Inc.
Transport vesicle
pinching off
Protein-coated
transport vesicle
Summary: From DNA to Proteins
• Complementary base pairing directs
transfer of genetic information in DNA into
amino acid sequence of protein
– DNA triplets  mRNA codons
– Complementary base pairing of mRNA
codons with tRNA anticodons ensures correct
amino acid sequence
– Anticodon sequence identical to DNA
sequence except uracil substituted for
thymine
© 2013 Pearson Education, Inc.
Figure 3.40 Information transfer from DNA to RNA to polypeptide.
DNA
molecule
Gene 2
Gene 1
Gene 4
DNA: DNA base sequence
(triplets) of the gene codes
for synthesis of a
Particular polypeptide
chain
mRNA: Base sequence
(codons) of the
transcribed mRNA
tRNA: Consecutive
base sequences of tRNA
anticodons recognize the
mRNA codons calling for
the amino acids they
transport
Polypeptide: Amino acid
sequence of the
polypeptide chain
Triplets
1
2
3
4
5
6
7
8
9
Codons
1
2
3
4
5
6
7
8
9
Anticodon
tRNA
Start
translation
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Stop;
detach
Other Roles of DNA
• Intron ("junk") regions of DNA code for
other types of RNA:
– Antisense RNA
• Prevents protein-coding RNA from being translated
– MicroRNA
• Small RNAs that silence mRNAs made by certain
exons
– Riboswitches
• Folded RNAs that act as switches regulating
protein synthesis in response to environmental
conditions
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Cytosolic Protein Degradation
• Autophagy
– Cytoplasmic bits and nonfunctional organelles
put into autophagosomes; degraded by
lysosomes
• Ubiquitins
– Tag damaged or unneeded soluble proteins in
cytosol
– Digested by soluble enzymes or proteasomes
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Extracellular Materials
• Body fluids-interstitial fluid, blood plasma,
and cerebrospinal fluid
• Cellular secretions-intestinal and gastric
fluids, saliva, mucus, and serous fluids
• Extracellular matrix–most abundant
extracellular material
– Jellylike mesh of proteins and
polysaccharides secreted by cells; acts as
"glue" to hold cells together
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Developmental Aspects of Cells
• All cells of body contain same DNA but
cells not identical
• Chemical signals in embryo channel cells
into specific developmental pathways by
turning some genes on and others off
• Development of specific and distinctive
features in cells called cell differentiation
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Apoptosis and Modified Rates of Cell
Division
• During development more cells than
needed produced (e.g., in nervous
system)
• Eliminated later by programmed cell death
(apoptosis)
– Mitochondrial membranes leak chemicals that
activate caspases  DNA, cytoskeleton
degradation  cell death
– Dead cell shrinks and is phagocytized
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Apoptosis and Modified Rates of Cell
Division
• Organs well formed and functional before
birth
• Cell division in adults to replace short-lived
cells and repair wounds
• Hyperplasia increases cell numbers when
needed
• Atrophy (decreased size) results from
loss of stimulation or use
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Theories of Cell Aging
• Wear and tear theory-Little chemical insults and
free radicals have cumulative effects
• Mitochondrial theory of aging–free radicals in
mitochondria diminish energy production
• Immune system disorders-autoimmune
responses; progressive weakening of immune
response; C-reactive protein of acute
inflammation causes cell aging
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Theories of Cell Aging
• Most widely accepted theory
– Genetic theory-cessation of mitosis and cell
aging programmed into genes
• Telomeres (strings of nucleotides protecting ends
of chromosomes) may determine number of times
a cell can divide
• Telomerase lengthens telomeres
– Found in germ cells; ~ absent in adult cells
© 2013 Pearson Education, Inc.