Transcript Chapter 3 PowerPoint

Chapter 3 Part D
Cells:
The Living
Units
© Annie Leibovitz/Contact Press Images
© 2016 Pearson Education, Inc.
PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
3.10 Cell Cycle
• Series of changes a cell undergoes from the
time it is formed until it reproduces
• Two major periods of cell cycle:
– Interphase
• Cell grows and carries on its usual activities
– Cell division (mitotic phase)
• Cell divides into two
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Interphase
• Period from cell formation to cell division, when
cell carries out its routine activities and prepares
for cell division
• During interphase, nuclear material is in
uncondensed chromatin state
• Interphase consists of subphases, which
include the process of DNA replication
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Interphase (cont.)
• Subphases
– Interphase broken into three subphases:
• G1 (gap 1): vigorous growth and metabolism
– Cells that permanently cease dividing are said to be in
G0 phase
• S (synthetic): DNA replication occurs
• G2 (gap 2): preparation for division
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Figure 3.28 The cell cycle.
G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
G2
Growth and final
preparations for
division
G2/M checkpoint
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Interphase (cont.)
• DNA replication
– Prior to division, the cell makes a copy of DNA
– Double-stranded DNA helices unwind and unzip
• Replication fork: point where strands separate
• Replication bubble: active area of replication
• Each strand acts as a template for a new
complementary strand
– RNA starts replication by laying down short
strand that acts as a primer
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Interphase (cont.)
– DNA polymerase attaches to primer and begins
adding nucleotides to form new strand
• DNA polymerase synthesizes both new strands at one
time (one leading and one lagging strand)
– DNA polymerase works only in one direction, so
leading strand is synthesized continuously;
however, because lagging strand is “backwards,”
it is synthesized discontinuously into segments
– Another enzyme, DNA ligase, then splices short
segments of discontinuous lagging strand
together
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Interphase (cont.)
• End result: two identical “daughter” DNA
molecules are formed from the original
• During mitotic cell division, one complete copy
will be given to new cell while one is retained in
original cell
• Process is called semiconservative
replication because each new double-stranded
DNA is composed of one old strand and one
new strand
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Figure 3.29 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
the double helix and
expose the bases
Adenine
Thymine
Cytosine
Guanine
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Replication
fork
(area where
hydrogen bonds
between base
pairs are broken
and DNA strands
separate)
DNA polymerase
Old (template) strand
Cell Division
• Most cells need to replicate continuously for growth
and repair purposes
– Skeletal, cardiac, and nerve cells do not divide efficiently;
damaged cells are replaced with scar tissue
• M (mitotic) phase of cell cycle is phase in which
division occurs; consists of 2 distinct events:
– Mitosis
– Cytokinesis
• Control of cell division is crucial, so cells divide
when necessary, but do not divide unnecessarily
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Figure 3.28 The cell cycle.
G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
G2
Growth and final
preparations for
division
G2/M checkpoint
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Cell Division (cont.)
• M phase
– Mitosis is the division of nucleus, in which the
duplicated DNA is distributed to new daughter
cells
• Four stages of mitosis ensure each cell receives a full
copy of replicated DNA
– Prophase
– Metaphase
– Anaphase
– Telophase
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Cell Division (cont.)
• Prophase can be broken into two parts:
1. Early prophase
• Chromatin condenses, forming visible chromosomes
• Each chromosome and its duplicate (called sister
chromatids) are held together by a centromere
• Centrosome and its duplicate begin synthesizing
microtubules that push each centrosome to opposite
poles of cell
– Called the mitotic spindle
– Other microtubules called asters radiate from
centrosome
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Cell Division (cont.)
• Prophase (cont.)
2. Late prophase
• Nuclear envelope breaks up
• Special microtubules attach to specific area on
centromeres called kinetochore and serve to pull
chromosomes to center (equator) of cell
• Remaining nonkinetochore microtubules push against
each other, causing poles of cell to move farther apart
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Focus Figure 3.3-1b Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it
produces two identical daughter cells.
Early Prophase
Early mitotic
spindle
Aster
Centromere
Chromosome
consisting of two
sister chromatids
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Focus Figure 3.3-1c Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it
produces two identical daughter cells.
Late Prophase
Spindle
pole
Nonkinetochore
microtubule
Fragments
of nuclear
envelope
Kinetochore
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Kinetochore
microtubule
Cell Division (cont.)
• Metaphase
– Centromeres of chromosomes are precisely
aligned at cell’s equator
– The imaginary plane midway between poles is
called metaphase plate
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Focus Figure 3.3-2a Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it
produces two identical daughter cells.
Metaphase
Spindle
Metaphase
plate
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Cell Division (cont.)
• Anaphase
– Shortest of all phases
– Centromeres of chromosomes split
simultaneously—each sister chromatid now
becomes a separate chromosome
– Chromosomes are pulled toward their respective
poles by motor proteins of kinetochores
• One chromosome of each original pair goes to
opposite poles
– Nonkinetochore microtubules continue forcing
poles apart
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Focus Figure 3.3-2b Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it
produces two identical daughter cells.
Anaphase
Daughter
chromosomes
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Cell Division (cont.)
• Telophase
– Begins when chromosome movement stops
– Each set of chromosomes (at opposite ends of
cell) uncoils to form chromatin
– New nuclear membranes form around each
chromatin mass
– Nucleoli reappear
– Spindle disappears
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Cell Division (cont.)
• Cytokinesis
– Begins during late anaphase and continues
through mitosis
– Ring of actin microfilaments contracts to form
cleavage furrow
– Two daughter cells are pinched apart
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Focus Figure 3.3-2c Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it
produces two identical daughter cells.
Telophase Cytokinesis
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Nuclear
envelope
forming
Nucleolus
forming
Contractile
ring at
cleavage
furrow
Cell Division (cont.)
• Control of cell division
– “Go” and “Stop” signals direct when a cell should
and should not divide
• Go signals include:
– Critical surface-to-volume ratio of cell, when area of
membrane becomes inadequate for exchange
– Chemicals (example: growth factors, hormones)
• Stop signals include:
– Availability of space; normal cells stop dividing when
they come into contact with other cells
» Referred to as contact inhibition
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Cell Division (cont.)
• Two groups of proteins are crucial to cell’s
ability to accomplish S phase and enter mitosis:
– Cyclins: regulatory proteins that accumulate
during interphase
– Cdks (Cyclin-dependent kinases) that activate
cyclins when they bind to them
– Cyclin-Cdk complex in turn activates enzyme
cascades that prepare cell for division
– Cyclins are destroyed after mitotic cell division,
and process begins again
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Cell Division (cont.)
• Checkpoints are key events in the cell cycle
where cell division processes are checked and,
if faulty, stopped until repairs are made
– G1 checkpoint (restriction point) is the most
important of the three major checkpoints
– If cell does not pass, it enters G0, in which no
further division occurs
© 2016 Pearson Education, Inc.
Figure 3.28 The cell cycle.
G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
G2
Growth and final
preparations for
division
G2/M checkpoint
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3.11 Protein Synthesis
• DNA is master blueprint that holds the code for
protein synthesis
– DNA directs the order of amino acids in a
polypeptide
• A segment of DNA that holds the code for one
polypeptide is referred to as a gene
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3.11 Protein Synthesis
• The code is determined by the specific order of
nitrogen bases (Adenine, Guanine, Thymine,
and Cytosine) in the gene
– Code consists of three sequential bases (triplet
code)
• Example: GGC codes for amino acid proline, whereas
GCC codes for arginine
– Each triplet specifies the code for a particular
amino acid
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3.11 Protein Synthesis
• Genes are composed of exons and introns
– Exons are part of gene that actually codes for
amino acids
– Introns are noncoding segments interspersed
amongst exons
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The Role of RNA
• RNA is the “go-between” molecule that links DNA
to proteins
– RNA copies the DNA code in nucleus, then carries it into
cytoplasm to ribosomes
• All RNA is formed in nucleus
• RNA differs from DNA
– Uracil is substituted for thymine in RNA
– RNA has ribose instead of deoxyribose sugar
• Three types of RNA:
– Messenger RNA (mRNA)
– Ribosomal RNA (rRNA)
– Transfer RNA (tRNA)
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The Role of RNA (cont.)
• Messenger RNA (mRNA)
– Single stranded
– Code from DNA template strand is copied with
complementary base pairs, resulting in a strand
of mRNA
• Process is referred to as transcription
– mRNA maintains the triplet code (codon) from
DNA
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The Role of RNA (cont.)
• Ribosomal RNA (rRNA)
– Structural component of ribosomes, the
organelle where protein synthesis occurs
– Along with tRNA, helps to translate message
from mRNA into polypeptide
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The Role of RNA (cont.)
• Transfer RNA (tRNAs)
– Carrier of amino acid
– Have special areas that contain a specific triplet
code (anticodon) that allows each tRNA to carry
only a specific amino acid
– Anticodon of tRNA will complementary base-pair
with codon of mRNA at ribosome, adding its
specific amino acid to growing polypeptide chain
• Process is referred to as translation
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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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Figure 3.30 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
Start
translation
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Stop;
detach
Transcription
• Process of transferring code held in DNA gene
base sequence to complementary base
sequence of mRNA
• Transcription factors (protein complex)
activate transcription by:
– Loosening histones from DNA in area to be
transcribed so DNA segment can be exposed
– Binding to special sequence of gene to be
transcribed, called promoter (starting point)
• Occurs only on DNA template strand
– Mediating binding of RNA polymerase, enzyme
that synthesizes mRNA, to promoter region
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Transcription (cont.)
• Transcription is broken down into three phases:
1. Initiation
• RNA polymerase separates DNA strands
2. Elongation
• RNA polymerase adds complementary nucleotides to
growing mRNA matching sequence of based on DNA
template strand
– Short, 12-base-pair segment where DNA and mRNA
are temporarily bonded is referred to as DNA-RNA
hybrid
3. Termination
• Transcription stops when RNA polymerase reaches
special termination signal code
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Slide 1
Figure 3.31 Overview of stages of transcription.
RNA polymerase
Coding strand of gene
DNA
Promoter
Template strand of gene
region containing
the start point
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
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.
mRNA
Completed mRNA
RNA polymerase
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
mRNA
3 Termination: mRNA synthesis ends when the termination
signal is reached. RNA polymerase and the completed MRNA
transcript are released.
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Rewinding
of DNA
Coding strand of DNA
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.
Slide 2
Figure 3.31 Overview of stages of transcription.
RNA polymerase
Coding strand of gene
DNA
Promoter
Template strand of gene
region containing
the start point
© 2016 Pearson Education, Inc.
Termination
signal
Slide 3
Figure 3.31 Overview of stages of transcription.
RNA polymerase
Coding strand of gene
DNA
Promoter
Template strand of gene
region containing
the start point
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
© 2016 Pearson Education, Inc.
Template strand
Slide 4
Figure 3.31 Overview of stages of transcription.
RNA polymerase
Coding strand of gene
DNA
Promoter
Template strand of gene
region containing
the start point
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
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.
mRNA
Rewinding
of DNA
Coding strand of DNA
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
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.
© 2016 Pearson Education, Inc.
Slide 5
Figure 3.31 Overview of stages of transcription.
RNA polymerase
Coding strand of gene
DNA
Promoter
Template strand of gene
region containing
the start point
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
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.
mRNA
Completed mRNA
RNA polymerase
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
mRNA
3 Termination: mRNA synthesis ends when the termination
signal is reached. RNA polymerase and the completed MRNA
transcript are released.
© 2016 Pearson Education, Inc.
Rewinding
of DNA
Coding strand of DNA
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.
Transcription (cont.)
• Processing of mRNA
– Newly formed mRNA is then edited and
processed before translation can begin
• Before processing, it is referred to as pre-mRNA
– Introns are removed by special proteins called
spliceosomes, leaving only exon coding regions
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Translation
• Step of protein synthesis where the language of
nucleic acids (base sequence) is translated into
the language of proteins (amino acid sequence)
• Process involves:
– mRNA
– Genetic code
– tRNA and ribosomes
– Translating events
– and sometimes the rough ER
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Translation (cont.)
• Genetic code
– Each three-base sequence on DNA (triplet code)
is represented by a complementary three-base
sequence on mRNA called codon
– There are 64 possible codons
• 4 bases (A, U, C, G) and 3 places, so 43  64
– There are 3 “stop” codons but rest are codons
for amino acids
– There are only 20 possible amino acids, so this
means that some amino acids are represented
by more than one codon
• Redundancy helps protect against transcription errors
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Translation (cont.)
• Role of tRNA
– tRNA binds a specific amino
acid at one end (stem); once
amino acid is loaded onto
tRNA, molecule is now called
an aminoacyl-tRNA
– Anticodon at other end (head) is triplet code
that determines which amino acid will be bound
at stem
• Example: tRNA with anticodon UAU will only be able
to load a methionine amino acid to its stem region
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Translation (cont.)
– Anticodon of tRNA will bind only to codon on
mRNA that is complementary
• Example: if codon is AUA, only a tRNA with anticodon
UAU will be able to bond
– Ribosomes coordinate coupling of mRNA and
tRNA
– Ribosomes contain one binding site for mRNA
and three binding sites for tRNA:
• Aminoacyl site for incoming aminoacyl-tRNA
• Peptidyl site for tRNA linked to growing polypeptide
chain
• Exit site for outgoing tRNA
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Translation (cont.)
• Sequence of events in translation
– Translation occurs in three phases that require
ATP, protein factors, and enzymes
1. Initiation
2. Elongation
3. Termination
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Translation (cont.)
1. Initiation
– Small ribosomal subunit binds to a special
initiator tRNA (methionine) and then to the
mRNA to be decoded
• Ribosome scans mRNA looking for first methionine
codon, which is referred to as the start codon
– When anticodon of initiator tRNA binds to start
codon, large ribosomal unit can then attach to
small ribosomal unit forming a functional
ribosome
– At end of initiation, initiator tRNA is in P site of
ribosome, and A and E sites are empty
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Translation (cont.)
2. Elongation: involves three steps:
2a. Codon recognition: tRNA binds complementary
codon in A site of ribosome
2b. Peptide bond formation: Ribosomal enzymes
transfer and attach growing polypeptide chain from
tRNA in P site over to amino acid of tRNA in A site
2c. Translocation: ribosome shifts down three bases
of mRNA, displacing tRNAs by one position
tRNA in A site moves into P site
tRNA in P site moves into E site
tRNA in E site is ejected from ribosome
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Translation (cont.)
2. Elongation (cont.)
– Once A site is empty, a new tRNA can enter,
bringing its amino acid cargo, and whole process
starts over
– After a portion of mRNA is “read,” additional
ribosomes may attach to already read part and
start another round of translation of same mRNA
• Polyribosome is a multiple ribosome-mRNA complex
that produces multiple copies of same protein
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Figure 3.32 Polyribosome arrays.
Growing polypeptides
Completed
polypeptide
Incoming
ribosomal
subunits
Start of
mRNA
Polyribosome
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.
Ribosomes
mRNA
This transmission electron micrograph shows
a large polyribosome (400,000).
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Translation (cont.)
3. Termination
– When one of three stop codons (UGA, UAA,
UAG) on mRNA enters A site, translation ends
– Protein release factor binds to stop codon,
causing water to be added to chain instead of
another tRNA
– Causes release of polypeptide chain as well as
separation of ribosome subunits and degradation
of mRNA
– Final polypeptide product will be further
processed by other cell structures into functional
3-D protein
© 2016 Pearson Education, Inc.
Slide 1
Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a
particular polypeptide.
1
Getting Ready
• Making mRNA (transcription)
• Attaching amino acid to tRNA
• tRNAs diffuse to ribosome
2 Elongation:
Amino acids are added one at a time to
the growing peptide chain via a process
that has three repeating steps: 2a, 2b, 2c.
Initiation:
Initiation occurs when four components combine at the P site:
• A small ribosomal subunit
• An initiator tRNA carrying the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next phase, elongation, begins.
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.
Methionine
(amino acid)
Amino
acid that
corresponds
to anticodon
Leu
P
E
GGC
tRNA
anticodon
Amino acid
corresponding
to anticodon
A
A U A C C G CU A
Complementary
mRNA codon
2b Peptide bond formation:
Initiator tRNA
bearing
anticodon
Growing
polypeptide chain
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. A new peptide bond is formed.
tRNA
E
New peptide
bond
A
P
GG C G A U
A U A C CG C U A
A
site
The correct amino acid
is attached to each species of
tRNA by a synthetase enzyme
(aminoacyl-tRNA synthetase).
p
site
Large
ribosomal
subunit
E
site
2c Translocation:
The entire ribosome translocates,
shifting its position one codon
along the mRNA.
Start
codon
Released
tRNA
The unloaded tRNA from the
P site is now in the E site.
It is released.
E
P
G AU
Small
ribosomal
subunit
Pre-mRNA
3
When a stop codon
(UGA, UAA, or UAG)
arrives at the A site,
elongation ends.
P
Release
factor
CCU
C U G G G A UG A
Nucleus (site
of transcription)
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Newly made (and edited)
mRNA leaves nucleus and
travels to a ribosome for
decoding.
Stop codon
Cytosol (site
of translation)
The next codon to be
translated is now in the
empty A site, ready for
step 2a again.
Polypeptide
chain
Termination:
E
Template
strand of
DNA
A
CCG C U A C U C
Direction of ribosome
movement
mRNA
The tRNA that was in the
A site is now in the P site.
Release factor triggers the ribosomal subunits to
separate, releasing the mRNA and new polypeptide.
Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a
particular polypeptide.
1
Getting Ready
• Making mRNA (transcription)
• Attaching amino acid to tRNA
• tRNAs diffuse to ribosome
Initiation:
Initiation occurs when four components combine at the P site:
• A small ribosomal subunit
• An initiator tRNA carrying the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next phase, elongation, begins.
Methionine
(amino acid)
Amino
acid that
corresponds
to anticodon
Initiator tRNA
bearing
anticodon
tRNA
A
site
The correct amino acid
is attached to each species of
tRNA by a synthetase enzyme
(aminoacyl-tRNA synthetase).
p
site
Large
ribosomal
subunit
E
site
Start
codon
Small
ribosomal
subunit
Pre-mRNA
mRNA
Template
strand of
DNA
Nucleus (site
of transcription)
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Newly made (and edited)
mRNA leaves nucleus and
travels to a ribosome for
decoding.
Cytosol (site
of translation)
Slide 2
Slide 3
Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a
particular polypeptide.
1
Getting Ready
• Making mRNA (transcription)
• Attaching amino acid to tRNA
• tRNAs diffuse to ribosome
2 Elongation:
Amino acids are added one at a time to
the growing peptide chain via a process
that has three repeating steps: 2a, 2b, 2c.
Initiation:
Initiation occurs when four components combine at the P site:
• A small ribosomal subunit
• An initiator tRNA carrying the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next phase, elongation, begins.
Methionine
(amino acid)
Amino
acid that
corresponds
to anticodon
Initiator tRNA
bearing
anticodon
tRNA
A
site
The correct amino acid
is attached to each species of
tRNA by a synthetase enzyme
(aminoacyl-tRNA synthetase).
p
site
Large
ribosomal
subunit
E
site
Start
codon
Small
ribosomal
subunit
Pre-mRNA
mRNA
Template
strand of
DNA
Nucleus (site
of transcription)
© 2016 Pearson Education, Inc.
Newly made (and edited)
mRNA leaves nucleus and
travels to a ribosome for
decoding.
Cytosol (site
of translation)
Leu
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.
E
P
GGC
tRNA
anticodon
A
A U A C C G CU A
Complementary
mRNA codon
Amino acid
corresponding
to anticodon
Slide 4
Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a
particular polypeptide.
1
Getting Ready
• Making mRNA (transcription)
• Attaching amino acid to tRNA
• tRNAs diffuse to ribosome
2 Elongation:
Amino acids are added one at a time to
the growing peptide chain via a process
that has three repeating steps: 2a, 2b, 2c.
Initiation:
Initiation occurs when four components combine at the P site:
• A small ribosomal subunit
• An initiator tRNA carrying the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next phase, elongation, begins.
Methionine
(amino acid)
Amino
acid that
corresponds
to anticodon
Leu
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.
E
P
GGC
tRNA
anticodon
A
A U A C C G CU A
Complementary
mRNA codon
2b Peptide bond formation:
Initiator tRNA
bearing
anticodon
Growing
polypeptide chain
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. A new peptide bond is formed.
tRNA
E
P
A
GG C G A U
A U A C CG C U A
A
site
The correct amino acid
is attached to each species of
tRNA by a synthetase enzyme
(aminoacyl-tRNA synthetase).
p
site
Large
ribosomal
subunit
E
site
Start
codon
Small
ribosomal
subunit
Pre-mRNA
mRNA
Template
strand of
DNA
Nucleus (site
of transcription)
© 2016 Pearson Education, Inc.
Newly made (and edited)
mRNA leaves nucleus and
travels to a ribosome for
decoding.
Amino acid
corresponding
to anticodon
Cytosol (site
of translation)
New peptide
bond
Slide 5
Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a
particular polypeptide.
1
Getting Ready
• Making mRNA (transcription)
• Attaching amino acid to tRNA
• tRNAs diffuse to ribosome
2 Elongation:
Amino acids are added one at a time to
the growing peptide chain via a process
that has three repeating steps: 2a, 2b, 2c.
Initiation:
Initiation occurs when four components combine at the P site:
• A small ribosomal subunit
• An initiator tRNA carrying the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next phase, elongation, begins.
Methionine
(amino acid)
Amino
acid that
corresponds
to anticodon
Leu
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.
E
P
GGC
tRNA
anticodon
Amino acid
corresponding
to anticodon
A
A U A C C G CU A
Complementary
mRNA codon
2b Peptide bond formation:
Initiator tRNA
bearing
anticodon
Growing
polypeptide chain
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. A new peptide bond is formed.
tRNA
E
P
New peptide
bond
A
GG C G A U
A U A C CG C U A
A
site
The correct amino acid
is attached to each species of
tRNA by a synthetase enzyme
(aminoacyl-tRNA synthetase).
p
site
Large
ribosomal
subunit
E
site
Start
codon
2c Translocation:
The entire ribosome translocates,
shifting its position one codon
along the mRNA.
Released
tRNA
The unloaded tRNA from the
P site is now in the E site.
It is released.
E
Small
ribosomal
subunit
Pre-mRNA
Template
strand of
DNA
Nucleus (site
of transcription)
© 2016 Pearson Education, Inc.
Newly made (and edited)
mRNA leaves nucleus and
travels to a ribosome for
decoding.
G AU
The tRNA that was in the
A site is now in the P site.
A
CCG C U A C U C
Direction of ribosome
movement
mRNA
P
Cytosol (site
of translation)
The next codon to be
translated is now in the
empty A site, ready for
step 2a again.
Slide 6
Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a
particular polypeptide.
1
Getting Ready
• Making mRNA (transcription)
• Attaching amino acid to tRNA
• tRNAs diffuse to ribosome
2 Elongation:
Amino acids are added one at a time to
the growing peptide chain via a process
that has three repeating steps: 2a, 2b, 2c.
Initiation:
Initiation occurs when four components combine at the P site:
• A small ribosomal subunit
• An initiator tRNA carrying the amino acid methionine
• The mRNA
• A large ribosomal subunit
Once this is accomplished, the next phase, elongation, begins.
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.
Methionine
(amino acid)
Amino
acid that
corresponds
to anticodon
Leu
P
E
GGC
tRNA
anticodon
Amino acid
corresponding
to anticodon
A
A U A C C G CU A
Complementary
mRNA codon
2b Peptide bond formation:
Initiator tRNA
bearing
anticodon
Growing
polypeptide chain
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. A new peptide bond is formed.
tRNA
E
New peptide
bond
A
P
GG C G A U
A U A C CG C U A
A
site
The correct amino acid
is attached to each species of
tRNA by a synthetase enzyme
(aminoacyl-tRNA synthetase).
p
site
Large
ribosomal
subunit
E
site
2c Translocation:
The entire ribosome translocates,
shifting its position one codon
along the mRNA.
Start
codon
Released
tRNA
The unloaded tRNA from the
P site is now in the E site.
It is released.
E
P
G AU
Small
ribosomal
subunit
Pre-mRNA
3
When a stop codon
(UGA, UAA, or UAG)
arrives at the A site,
elongation ends.
P
Release
factor
CCU
C U G G G A UG A
Nucleus (site
of transcription)
© 2016 Pearson Education, Inc.
Newly made (and edited)
mRNA leaves nucleus and
travels to a ribosome for
decoding.
Stop codon
Cytosol (site
of translation)
The next codon to be
translated is now in the
empty A site, ready for
step 2a again.
Polypeptide
chain
Termination:
E
Template
strand of
DNA
A
CCG C U A C U C
Direction of ribosome
movement
mRNA
The tRNA that was in the
A site is now in the P site.
Release factor triggers the ribosomal subunits to
separate, releasing the mRNA and new polypeptide.
Translation (cont.)
• Role of rough ER in protein synthesis
– A short amino acid segment, called the ER
signal sequence, present on a growing
polypeptide chain, signals associated ribosome
to dock on rough ER surface
– Signal-recognition particle (SRP) on ER directs
mRNA–ribosome complex where to dock
– Once docked, forming polypeptide enters ER
• Sugar groups may be added to protein, and its shape
may be altered
• Protein is then enclosed in vesicle for transport to
Golgi apparatus
© 2016 Pearson Education, Inc.
Slide 1
Figure 3.33 Rough ER processing of proteins.
1 The SRP directs the
2 Once attached to the ER, the SRP is
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
ER signal
sequence
3 An enzyme clips off the signal
Ribosome
sequence. As protein synthesis continues,
sugar groups may be added to the protein.
mRNA
4 In this example, the completed protein
Signal
Signal
recognition
sequence
particle
Growing
(SRP)
polypeptide removed
Receptor site
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 3.17).
Rough ER cistern
Cytosol
© 2016 Pearson Education, Inc.
Transport vesicle
pinching off
Protein-coated
transport vesicle
Figure 3.33 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
© 2016 Pearson Education, Inc.
Slide 2
Slide 3
Figure 3.33 Rough ER processing of proteins.
1 The SRP directs the
2 Once attached to the ER, the SRP is
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
ER signal
sequence
Ribosome
mRNA
Signal
recognition
particle
Growing
(SRP)
polypeptide
Receptor site
Rough ER cistern
Cytosol
© 2016 Pearson Education, Inc.
Slide 4
Figure 3.33 Rough ER processing of proteins.
1 The SRP directs the
2 Once attached to the ER, the SRP is
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
ER signal
sequence
3 An enzyme clips off the signal
Ribosome
sequence. As protein synthesis continues,
sugar groups may be added to the protein.
mRNA
Signal
Signal
recognition
sequence
particle
Growing
(SRP)
polypeptide removed
Receptor site
Rough ER cistern
Cytosol
© 2016 Pearson Education, Inc.
Sugar
group
Slide 5
Figure 3.33 Rough ER processing of proteins.
1 The SRP directs the
2 Once attached to the ER, the SRP is
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
ER signal
sequence
3 An enzyme clips off the signal
Ribosome
sequence. As protein synthesis continues,
sugar groups may be added to the protein.
mRNA
4 In this example, the completed protein
Signal
Signal
recognition
sequence
particle
Growing
(SRP)
polypeptide removed
Receptor site
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
© 2016 Pearson Education, Inc.
Slide 6
Figure 3.33 Rough ER processing of proteins.
1 The SRP directs the
2 Once attached to the ER, the SRP is
mRNA-ribosome complex to the
rough ER. There the SRP binds to
a receptor site.
released and the growing polypeptide
snakes through the ER membrane pore
into the cistern.
ER signal
sequence
3 An enzyme clips off the signal
Ribosome
sequence. As protein synthesis continues,
sugar groups may be added to the protein.
mRNA
4 In this example, the completed protein
Signal
Signal
recognition
sequence
particle
Growing
(SRP)
polypeptide removed
Receptor site
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 3.17).
Rough ER cistern
Cytosol
© 2016 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 are coded to mRNA codons
– mRNA codons are base-paired with tRNA
anticodons to ensure correct amino acid
sequence
• Anticodon sequence of tRNA is identical to DNA
sequence, except uracil is substituted for thymine
© 2016 Pearson Education, Inc.
Figure 3.34 Information transfer from DNA to RNA to polypeptide.
DNA
molecule
Gene 2
Gene 1
Gene 4
Triplets
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
1
T
A C G G
Codons
1
A U G
3
T
A G
2
4
C G
A
5
T
T
T
6
C
3
4
C G C
U
A
A
A
U
U U C
C C
A
U
G G
U
A G
C C
5
A
T
G C
6
G
G
A A
G G
A
C
U G C G
G C
A
8
7
C
9
8
7
A
C
T
9
U U
U
A
A
U G A
Anticodon
U
A C
C G
C
A
tRNA
Met
Start
translation
© 2016 Pearson Education, Inc.
2
Pro
Ser
Leu
Lys
Gly
Arg
Phe
Stop;
detach
Other Roles of DNA
• DNA codes for other types of RNA:
– MicroRNA (miRNA)
• Small RNAs that can bind to and silence mRNAs
made by certain exons
– Riboswitches
• Folded RNAs that act as switches that can turn protein
synthesis on or off in response to certain
environmental conditions
– Small interfering RNAs (siRNA)
• Similar to miRNA, but can also be made to silence
mRNA from pathogenic sources such as viruses
© 2016 Pearson Education, Inc.
3.12 Apoptosis, Autophagy, and
Proteasomes
• Cells that have become obsolete or damaged
need to be taken out of system
• Autophagy (self-eating) is the process of
disposing of nonfunctional organelles and
cytoplasmic bits by forming autophagosomes,
which can then be degraded by lysosomes
• Unneeded proteins can be marked for
destruction by ubiquitins
– Proteasomes disassemble ubiquitin-tagged
proteins, recycling the amino acids and ubiquitin
© 2016 Pearson Education, Inc.
3.12 Apoptosis, Autophagy, and
Proteasomes
• Apoptosis, also known as programmed cell
death causes certain cells (examples: cancer
cells, infected cells, old cells) to neatly selfdestruct
– Process begins with mitochondrial membranes
leaking chemicals that activate enzymes called
caspases
– Caspases cause degradation of DNA and
cytoskeleton, which leads to cell death
– Dead cell shrinks and is phagocytized by
macrophages
© 2016 Pearson Education, Inc.
Developmental Aspects of Cells
• All cells of body contain same DNA, but not all
cells are identical or carry out same function
• 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 is called cell differentiation
© 2016 Pearson Education, Inc.
Cell Destruction and Modified Rates
of Cell Division
• Organs are well formed and functional before
birth, but we need cell division for growth
• Cell division in adults is needed to replace shortlived cells and repair wounds
• Hyperplasia is accelerated growth that
increases cell numbers when needed
• Atrophy is a decrease in size that results from
loss of stimulation or use
© 2016 Pearson Education, Inc.
Cell Aging
• The mechanism of aging is a mystery, but there
are several theories:
– Wear and tear theory: a lifetime of chemical
insults and free radicals have cumulative effects
– Mitochondrial theory of aging: free radicals in
mitochondria diminish energy production
– Immune system disorders: autoimmune
responses, as well as progressive weakening of
immune response
© 2016 Pearson Education, Inc.
Cell Aging (cont.)
– Genetic theory: cessation of mitosis and cell
aging are programmed into genes
• Telomeres are strings of nucleotides that protect ends
of chromosomes (like caps on shoestrings)
• Everytime a cell divides, the telomere shortens, so
telomeres may act like an hour-glass on how many
times a cell can divide
• Telomerase is an enzyme that lengthens telomeres
– Found in germ cells of embryos but absent in adult
cells, except for cancer cells
© 2016 Pearson Education, Inc.