DNA structure replication transcription translation

The Molecular Basis of Inheritance
Figure 16.7a, c
C
T
A
A
T
CG
GC
A
C G
AT
AT
A T
TA
C
TA
0.34 nm
3.4 nm
(a) Key features of DNA structure
G
1 nm
G
(c) Space-filling model
T

1962: Nobel Prize in Physiology and Medicine
James D.
Watson
Francis H.
Crick
Maurice H. F.
Wilkins
What about?
Rosalind Franklin
Watson, J.D. and F.H. Crick, “Molecular Structure of
Nucleic Acids: A Structure for Deoxynucleic Acids”.
Nature 171 (1953), p. 738.

The Structure of DNA
• DNA is composed of four nucleotides,
each containing: adenine, cytosine,
thymine, or guanine.
• The amounts of A = T, G = C, and
purines = pyrimidines [Chargaff’s
Rule].
• DNA is a double-stranded helix with
antiparallel strands [Watson and
Crick].
• Nucleotides in each strand are linked
by 5’-3’ phosphodiester bonds
• Bases on opposite strands are linked
by hydrogen bonding: A with T, and G
with C.

The Basic Principle: Base Pairing to a
Template Strand
• The relationship between structure and
function is manifest in the double helix
• Since the two strands of DNA are
complementary each strand acts as a
template for building a new strand in
replication

DNA replication
• The parent molecule unwinds, and two new
daughter strands are built based on base-
pairing rules
(a) The parent molecule has two
complementary strands of DNA.
Each base is paired by hydrogen
bonding with its specific partner,
A with T and G with C.
(b) The first step in replication is
separation of the two DNA
strands.
(c) Each parental strand now
serves as a template that
determines the order of
nucleotides along a new,
complementary strand.
(d) The nucleotides are connected
to form the sugar-phosphate
backbones of the new strands.
Each “daughter” DNA
molecule consists of one parental
strand and one new strand.
A
C
T
A
G
A
C
T
A
G
A
C
T
A
G
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
A
C
T
A
G
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
T
G
A
T
C
T
G
A
T
C

DNA Replication is “Semi-conservative”
• Each 2-stranded
daughter molecule is
only half new
• One original strand was
used as a template to
make the new strand

DNA Replication
• The copying of DNA is remarkable in its speed and
accuracy
• Involves unwinding the double helix and synthesizing
two new strands.
• More than a dozen enzymes and other proteins
participate in DNA replication
• The replication of a DNA molecule begins at special
sites called origins of replication, where the two strands
are separated

Origins of Replication
• A eukaryotic chromosome may have hundreds or
even thousands of replication origins
Replication begins at specific sites
where the two parental strands
separate and form replication
bubbles.
The bubbles expand laterally, as
DNA replication proceeds in both
directions.
Eventually, the replication
bubbles fuse, and synthesis of
the daughter strands is
complete.
1
2
3
Origin of replication
Bubble
Parental (template) strand
Daughter (new) strand
Replication fork
Two daughter DNA molecules
In eukaryotes, DNA replication begins at many sites along the giant
DNA molecule of each chromosome.
In this micrograph, three replication
bubbles are visible along the DNA of
a cultured Chinese hamster cell (TEM).
(b)(a)
0.25 µm

Mechanism of DNA Replication
• DNA replication is catalyzed by DNA polymerase which needs an
RNA primer
• RNA primase synthesizes primer on DNA strand
• DNA polymerase adds nucleotides to the 3’ end of the growing
strand

Mechanism of DNA Replication
• Nucleotides are added by complementary base pairing with the
template strand
• The substrates, deoxyribonucleoside triphosphates, are
hydrolyzed as added, releasing energy for DNA synthesis.

The Mechanism of DNA Replication
• DNA synthesis on the leading strand is continuous
• The lagging strand grows the same general direction as the leading
strand (in the same direction as the Replication Fork). However,
DNA is made in the 5’-to-3’ direction
• Therefore, DNA synthesis on the lagging strand is discontinuous
• DNA is added as short fragments (Okasaki fragments) that are
subsequently ligated together

DNA polymerase I degrades the
RNA primer and replaces it with
DNA

The Mechanism of DNA Replication
• Many proteins assist in DNA replication
• DNA helicases unwind the double helix, the
template strands are stabilized by other
proteins
• Single-stranded DNA binding proteins make
the template available
• RNA primase catalyzes the synthesis of short
RNA primers, to which nucleotides are added.
• DNA polymerase III extends the strand in the
5’-to-3’ direction
• DNA polymerase I degrades the RNA primer
and replaces it with DNA
• DNA ligase joins the DNA fragments into a
continuous daughter strand

Enzymes in DNA replication
Helicase unwinds
parental double helix
Binding proteins
stabilize separate
strands
DNA polymerase III
binds nucleotides
to form new strands
Ligase joins Okazaki
fragments and seals
other nicks in sugar-
phosphate backbone
Primase adds
short primer
to template strand
DNA polymerase I
(Exonuclease) removes
RNA primer and inserts
the correct bases

Binding proteins prevent single strands from rewinding.
Helicase protein binds to DNA sequences called
origins and unwinds DNA strands.
5’
3’
5’
3’
Primase protein makes a short segment of RNA
complementary to the DNA, a primer.
3’5’
5’3’
Replication

Overall direction
of replication
5’3’
5’
3’
5’
3’
3’5’
DNA polymerase enzyme adds DNA nucleotides
to the RNA primer.
Replication

DNA polymerase enzyme adds DNA nucleotides
to the RNA primer.
5’
5’
Overall direction
of replication
5’
3’
5’
3’
3’
3’
DNA polymerase proofreads bases added and
replaces incorrect nucleotides.
Replication

5’
5’3’
5’
3’
3’
5’
3’
Overall direction
of replication
Leading strand synthesis continues in a
5’ to 3’ direction.
Replication

3’5’ 5’
5’3’
5’
3’
3’
5’
3’
Overall direction
of replication
Okazaki fragment
Discontinuous synthesis produces 5’ to 3’ DNA
segments called Okazaki fragments.
Replication

5’ 5’
5’3’
5’
3’
3’
5’
3’
Overall direction
of replication
3’
Okazaki fragment
Replication

5’
5’ 3’
5’
3’
3’
5’
3’
3’
5’ 5’3’
Replication

3’
5’
3’
5’
5’ 3’
5’
3’
3’
5’ 5’3’
Replication

5’
5’
3’ 3’
5’
3’
5’ 3’
5’
3’
3’
5’
Exonuclease activity of DNA polymerase I
removes RNA primers.
Replication

Polymerase activity of DNA polymerase I fills the gaps.
Ligase forms bonds between sugar-phosphate backbone.
3’
5’
3’
5’ 3’
5’
3’
3’
5’
Replication

Proofreading
• DNA must be faithfully replicated…but
mistakes occur
– DNA polymerase (DNA pol) inserts the wrong
nucleotide base in 1/10,000 bases
• DNA pol has a proofreading capability and can correct
errors
– Mismatch repair: ‘wrong’ inserted base can be
removed
– Excision repair: DNA may be damaged by
chemicals, radiation, etc. Mechanism to cut out
and replace with correct bases

Mutations
• A mismatching of base pairs, can occur at a
rate of 1 per 10,000 bases.
• DNA polymerase proofreads and repairs
accidental mismatched pairs.
• Chances of a mutation occurring at any one
gene is over 1 in 100,000
• Because the human genome is so large,
even at this rate, mutations add up. Each of
us probably inherited 3-4 mutations!

Proofreading and Repairing DNA
• DNA polymerases
proofread newly made
DNA, replacing any
incorrect nucleotides
• In mismatch repair of DNA,
repair enzymes correct
errors in base pairing
• In nucleotide excision DNA
repair nucleases cut out
and replace damaged
stretches of DNA
Nuclease
DNA
polymerase
DNA
ligase
A thymine dimer
distorts the DNA molecule.
1
A nuclease enzyme cuts
the damaged DNA strand
at two points and the
damaged section is
removed.
2
Repair synthesis by
a DNA polymerase
fills in the missing
nucleotides.
3
DNA ligase seals the
Free end of the new DNA
To the old DNA, making the
strand complete.
4

Accuracy of DNA Replication
• The chromosome of E. coli bacteria contains
about 5 million bases pairs
– Capable of copying this DNA in less than an hour
• The 46 chromosomes of a human cell contain
about 6 BILLION base pairs of DNA!!
– Printed one letter (A,C,T,G) at a time…would fill
up over 900 volumes of Campbell.
– Takes a cell a few hours to copy this DNA
– With amazing accuracy – an average of 1 per
billion nucleotides

The Central Dogma of Life.
replication

Protein Synthesis
• The information content of DNA is in the form
of specific sequences of nucleotides along
the DNA strands
• The DNA inherited by an organism leads to
specific traits by dictating the synthesis of
proteins
• The process by which DNA directs protein
synthesis, gene expression includes two
stages, called transcription and translation

Transcription and Translation
• Cells are governed by a cellular chain of
command
– DNA → RNA → protein
• Transcription
– Is the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide, which
occurs under the direction of mRNA
– Occurs on ribosomes

• In prokaryotes transcription and translation
occur together
Figure 17.3a
Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
(a)
TRANSLATION
TRANSCRIPTION
DNA
mRNA
Ribosome
Polypeptide

• In a eukaryotic cell the nuclear envelope separates
transcription from translation
• Extensive RNA processing occurs in the nucleus
Eukaryotic cell. The nucleus provides a separate
compartment for transcription. The original RNA
transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
(b)
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclear
envelope

Transcription
• Transcription is the DNA-
directed synthesis of RNA
• RNA synthesis
– Is catalyzed by RNA
polymerase, which pries the
DNA strands apart and hooks
together the RNA nucleotides
– Follows the same base-pairing
rules as DNA, except that in
RNA, uracil substitutes for
thymine

RNA
Table 17.1
• RNA is single stranded, not double stranded like DNA
• RNA is short, only 1 gene long, where DNA is very long and
contains many genes
• RNA uses the sugar ribose instead of deoxyribose in DNA
• RNA uses the base uracil (U) instead of thymine (T) in DNA.

Synthesis of an RNA Transcript
• The stages of
transcription
are
– Initiation
– Elongation
– Termination
Promoter
Transcription unit
RNA polymerase
Start point
5′
3′
3′
5′
3′
5′
5′
3′
5′
3′
3′
5′
5′
3′
3′
5′
5′
5′
Rewound
RNA
RNA
transcript
3′
3′
Completed RNA
transcript
Unwound
DNA
RNA
transcript
Template strand of
DNA
DNA
1 Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5′ → 3 ′. In the wake of
transcription, the DNA strands re-form a double helix.
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.

• Promoters signal the
initiation of RNA
synthesis
• Transcription factors help
eukaryotic RNA
polymerase recognize
promoter sequences
• A crucial promoter DNA
sequence is called a
TATA box.
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
T A T AAA A
ATAT T T T
TATA box Start point Template
DNA strand
5′
3′
3′
5′
Transcription
factors
5′
3′
3′
5′
Promoter
5′
3′
3′
5′5′
RNA polymerase II
Transcription factors
RNA transcript
Transcription initiation complex
Eukaryotic promoters1
Several transcription
factors
2
Additional transcription
factors
3
Synthesis of an RNA Transcript - Initiation

Synthesis of an RNA Transcript - Elongation
Elongation
RNA
polymerase
Non-template
strand of DNA
RNA nucleotides
3′ end
C A E G C A
A
U
T A G G T T
A
A
C
G
U
A
T
C
A
T C C A A T
T
G
G
3′
5′
5′
Newly made
RNA
Direction of transcription
(“downstream”) Template
strand of DNA
• RNA polymerase synthesizes a single strand of RNA against the DNA
template strand (anti-sense strand), adding nucleotides to the 3’ end of
the RNA chain
• As RNA polymerase moves along the DNA it continues to untwist the
double helix, exposing about 10 to 20 DNA bases at a time for pairing
with RNA nucleotides

• Specific sequences in the DNA signal
termination of transcription
• When one of these is encountered by the
polymerase, the RNA transcript is
released from the DNA and the double
helix can zip up again.
Synthesis of an RNA Transcript - Termination

• Most eukaryotic mRNAs aren’t ready to be translated into protein directly after being
transcribed from DNA. mRNA requires processing.
• Transcription of RNA processing occur in the nucleus. After this, the messenger RNA
moves to the cytoplasm for translation.
• The cell adds a protective cap to one end, and a tail of A’s to the other end. These
both function to protect the RNA from enzymes that would degrade
• Most of the genome consists of non-coding regions called introns
– Non-coding regions may have specific chromosomal functions or have regulatory purposes
– Introns also allow for alternative RNA splicing
• Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things:
– Add protective bases to the ends
– Cut out the introns
Post Termination RNA Processing

Alteration of mRNA Ends
• Each end of a pre-mRNA molecule is modified
in a particular way
– The 5′ end receives a modified nucleotide cap
– The 3′ end gets a poly-A tail
A modified guanine nucleotide
added to the 5′ end
50 to 250 adenine nucleotides
added to the 3′ end
Protein-coding segment Polyadenylation signal
Poly-A tail3′ UTR
Stop codonStart codon
5′ Cap 5′ UTR
AAUAAA AAA…AAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
G P P P
5′
3′

RNA Processing - Splicing
• The original transcript
from the DNA is called
pre-mRNA.
• It contains transcripts of
both introns and exons.
• The introns are removed
by a process called
splicing to produce
messenger RNA
(mRNA)

RNA Processing - Splicing
• Ribozymes are catalytic RNA molecules that
function as enzymes and can splice RNA
• RNA splicing removes introns and joins exons
Figure 17.10
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
5′ Cap
Exon Intron
1
5′
30 31
Exon Intron
104 105 146
Exon 3′
Poly-A tail
Poly-A tail
Introns cut out and
exons spliced together
Coding
segment
5′ Cap
1 146
3′ UTR3′ UTR
Pre-mRNA
mRNA

RNA Processing
• RNA Splicing can also be carried out by spliceosomes
RNA transcript (pre-mRNA)
Exon 1 Intron Exon 2
Other proteins
Protein
snRNA
snRNPs
Spliceosome
Spliceosome
components
Cut-out
intron
mRNA
Exon 1 Exon 2
5′
5′
5′
1
2
3

Alternative Splicing (of Exons)
• How is it possible that there are millions of
human antibodies when there are only about
30,000 genes?
• Alternative splicing refers to the different
ways the exons of a gene may be combined,
producing different forms of proteins within
the same gene-coding region
• Alternative pre-mRNA splicing is an important
mechanism for regulating gene expression in
higher eukaryotes

RNA Processing
• Proteins often have a modular architecture
consisting of discrete structural and functional
regions called domains
• In many cases different exons code for the
different domains in a protein
Figure 17.12
Gene
DNA
Exon 1 Intron Exon 2 Intron Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 1
Domain 2
Polypeptide

Translation
• Translation is the RNA-
directed synthesis of a
polypeptide
• Translation involves
– mRNA
– Ribosomes - Ribosomal RNA
– Transfer RNA
– Genetic coding - codons
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Polypeptide
Amino
acids
tRNA with
amino acid
attachedRibosome
tRNA
Anticodon
mRNA
Trp
Phe Gly
A
G C
A A A
C
C
G
U G G U U U G G C
Codons5′ 3′

The Genetic Code
• Genetic information is encoded as a sequence of nonoverlapping
base triplets, or codons
• The gene determines the sequence of bases along the length of an
mRNA molecule
DNA
molecule
Gene 1
Gene 2
Gene 3
DNA strand
(template)
TRANSCRIPTION
mRNA
Protein
TRANSLATION
Amino acid
A C C A A A C C G A G T
U G G U U U G G C U C A
Trp Phe Gly Ser
Codon
3′ 5′
3′5′

The Genetic Code
• Codons: 3 base code for the production of a specific amino acid,
sequence of three of the four different nucleotides
• Since there are 4 bases and 3 positions in each codon, there
are 4 x 4 x 4 = 64 possible codons
• 64 codons but only 20 amino acids, therefore most have more
than 1 codon
• 3 of the 64 codons are used as STOP signals; they are found at
the end of every gene and mark the end of the protein
• One codon is used as a START signal: it is at the start of every
protein
• Universal: in all living organisms

The Genetic Code
• A codon in messenger RNA is either translated into an
amino acid or serves as a translational start/stop signal
Second mRNA base
U C A G
U
C
A
G
UUU
UUC
UUA
UUG
CUU
CUC
CUA
CUG
AUU
AUC
AUA
AUG
GUU
GUC
GUA
GUG
Met or
start
Phe
Leu
Leu
lle
Val
UCU
UCC
UCA
UCG
CCU
CCC
CCA
CCG
ACU
ACC
ACA
ACG
GCU
GCC
GCA
GCG
Ser
Pro
Thr
Ala
UAU
UAC
UGU
UGC
Tyr Cys
CAU
CAC
CAA
CAG
CGU
CGC
CGA
CGG
AAU
AAC
AAA
AAG
AGU
AGC
AGA
AGG
GAU
GAC
GAA
GAG
GGU
GGC
GGA
GGG
UGG
UAA
UAG Stop
Stop UGA Stop
Trp
His
Gln
Asn
Lys
Asp
Arg
Ser
Arg
Gly
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
FirstmRNAbase(5′end)
ThirdmRNAbase(3′end)
Glu

Transfer RNA
• Consists of a single RNA strand that is only about 80
nucleotides long
• Each carries a specific amino acid on one end and has an
anticodon on the other end
• A special group of enzymes pairs up the proper tRNA molecules
with their corresponding amino acids.
• tRNA brings the amino acids to the ribosomes,
Two-dimensional structure. The four base-paired regions and
three loops are characteristic of all tRNAs, as is the base sequence
of the amino acid attachment site at the 3′ end. The anticodon triplet
is unique to each tRNA type. (The asterisks mark bases that have
been chemically modified, a characteristic of tRNA.)
(a)
3′
C
C
A
C
G
C
U
U
A
A
GACACCU
*
G
C
* *
G U G U
*CU
* G AG
G
U
*
*A
*
A
A G
U
C
A
G
A
C
C
*
C G A G
A G G
G
*
*
GA
CUC*AU
U
U
A
G
G
C
G
5′
Amino acid
attachment site
Hydrogen
bonds
Anticodon
A
The “anticodon” is the 3 RNA bases that
matches the 3 bases of the codon on the
mRNA molecule

Transfer RNA
• 3 dimensional tRNA molecule is roughly “L” shaped
(b) Three-dimensional structure
Symbol used
in the book
Amino acid
attachment site
Hydrogen
bonds
Anticodon
Anticodon
A A G
5′
3′
3′ 5′
(c)

Ribosomes
• Ribosomes facilitate the specific coupling of tRNA anticodons
with mRNA codons during protein synthesis
• The 2 ribosomal subunits are constructed of proteins and RNA
molecules named ribosomal RNA or rRNA
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Exit tunnel
Growing
polypeptide
tRNA
molecules
E
P A
Large
subunit
Small
subunit
mRNA
Computer model of functioning ribosome. This is a model of a bacterial
ribosome, showing its overall shape. The eukaryotic ribosome is roughly
similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules
and proteins.
(a)
5′
3′

Ribosome
• The ribosome has three binding sites for tRNA
– The P site
– The A site
– The E site
E P A
P site (Peptidyl-tRNA
binding site)
E site
(Exit site)
mRNA
binding site
A site (Aminoacyl-
tRNA binding site)
Large
subunit
Small
subunit
Schematic model showing binding sites. A ribosome has an
mRNA binding site and three tRNA binding sites, known as the A,
P, and E sites. This schematic ribosome will appear in later
diagrams.
(b)

Amino end Growing polypeptide
Next amino acid
to be added to
polypeptide chain
tRNA
mRNA
Codons
3′
5′
Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-
pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A
site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged
tRNA leaves via the E site.
(c)
Building a Polypeptide

Building a Molecule of tRNA
• A specific enzyme called an aminoacyl-tRNA
synthetase joins each amino acid to the correct tRNA
Figure 17.15
Amino acid
ATP
Adenosine
Pyrophosphate
Adenosine
Adenosine
Phosphates
tRNA
P P P
P
P Pi
Pi
Pi
P
AMP
Aminoacyl tRNA
(an “activated
amino acid”)
Aminoacyl-tRNA
synthetase (enzyme)
Active site binds the
amino acid and ATP.
1
ATP loses two P groups
and joins amino acid as AMP.
2
3 Appropriate
tRNA covalently
Bonds to amino
Acid, displacing
AMP.
Activated amino acid
is released by the enzyme.
4

Building a Polypeptide
• We can divide translation into three stages
– Initiation
– Elongation
– Termination
• The AUG start codon is recognized by methionyl-tRNA
or Met
• Once the start codon has been identified, the ribosome
incorporates amino acids into a polypeptide chain
• RNA is decoded by tRNA (transfer RNA) molecules,
which each transport specific amino acids to the
growing chain
• Translation ends when a stop codon (UAA, UAG, UGA)
is reached

Initiation of Translation
• The initiation stage of translation brings together
mRNA, tRNA bearing the first amino acid of the
polypeptide, and two subunits of a ribosome
Large
ribosomal
subunit
The arrival of a large ribosomal subunit completes
the initiation complex. Proteins called initiation
factors (not shown) are required to bring all the
translation components together. GTP provides
the energy for the assembly. The initiator tRNA is
in the P site; the A site is available to the tRNA
bearing the next amino acid.
2
Initiator tRNA
mRNA
mRNA binding site Small
ribosomal
subunit
Translation initiation complex
P site
GDPGTP
Start codon
A small ribosomal subunit binds to a molecule of
mRNA. In a prokaryotic cell, the mRNA binding site
on this subunit recognizes a specific nucleotide
sequence on the mRNA just upstream of the start
codon. An initiator tRNA, with the anticodon UAC,
base-pairs with the start codon, AUG. This tRNA
carries the amino acid methionine (Met).
1
Met
Met
U A C
A U G
E A
3′
5′
5′
3′
3′5′ 3′5′

Elongation of the Polypeptide Chain
• In the elongation stage, amino acids are added one
by one to the preceding amino acid
Amino end
of polypeptide
mRNA
Ribosome ready for
next aminoacyl tRNA
E
P A
E
P A
E
P A
E
P A
GDP
GTP
GTP
GDP
2
2
site site5′
3′
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Codon recognition. The anticodon
of an incoming aminoacyl tRNA
base-pairs with the complementary
mRNA codon in the A site. Hydrolysis
of GTP increases the accuracy and
efficiency of this step.
1
Peptide bond formation. An
rRNA molecule of the large
subunit catalyzes the formation
of a peptide bond between the
new amino acid in the A site and
the carboxyl end of the growing
polypeptide in the P site. This step
attaches the polypeptide to the
tRNA in the A site.
2
Translocation. The ribosome
translocates the tRNA in the A
site to the P site. The empty tRNA
in the P site is moved to the E site,
where it is released. The mRNA
moves along with its bound tRNAs,
bringing the next codon to be
translated into the A site.
3

Termination of Translation
• The final step in translation is termination. When the ribosome reaches
a STOP codon, there is no corresponding transfer RNA.
• Instead, a small protein called a “release factor” attaches to the stop
codon.
• The release factor causes the whole complex to fall apart: messenger
RNA, the two ribosome subunits, the new polypeptide.
• The messenger RNA can be translated many times, to produce many
protein copies.
Release
factor
Free
polypeptide
Stop codon
(UAG, UAA, or UGA)
5′
3′ 3′
5′
3′
5′
When a ribosome reaches a stop
codon on mRNA, the A site of the
ribosome accepts a protein called
a release factor instead of tRNA.
1 The release factor hydrolyzes
the bond between the tRNA in
the P site and the last amino
acid of the polypeptide chain.
The polypeptide is thus freed
from the ribosome.
2 3 The two ribosomal subunits
and the other components of
the assembly dissociate.

Translation: Initiation
• mRNA binds to a ribosome, and the transfer RNA
corresponding to the START codon binds to this complex.
Ribosomes are composed of 2 subunits (large and small), which
come together when the messenger RNA attaches during the
initiation process.

Translation: Elongation
• Elongation: the ribosome moves down the messenger RNA,
adding new amino acids to the growing polypeptide chain.
• The ribosome has 2 sites for binding transfer RNA. The first
RNA with its attached amino acid binds to the first site, and then
the transfer RNA corresponding to the second codon bind to the
second site.

Translation: Elongation
• The ribosome then removes the amino acid from the
first transfer RNA and attaches it to the second amino
acid.
• At this point, the first transfer RNA is empty: no
attached amino acid, and the second transfer RNA
has a chain of 2 amino acids attached to it.

• The elongation cycle repeats as the ribosome moves
down the messenger RNA, translating it one codon
and one amino acid at a time.
• The process repeats until a STOP codon is reached.
Translation: Termination

Polyribosomes
• A number of ribosomes can translate a single mRNA
molecule simultaneously forming a polyribosome
• Polyribosomes enable a cell to make many copies of
a polypeptide very quickly
Growing
polypeptides
Completed
polypeptide
Incoming
ribosomal
subunits
Start of
mRNA
(5′ end)
End of
mRNA
(3′ end)
Polyribosome
An mRNA molecule is generally translated simultaneously
by several ribosomes in clusters called polyribosomes.
(a)
Ribosomes
mRNA
This micrograph shows a large polyribosome in a prokaryotic
cell (TEM).
0.1 µm

Comparing Gene Expression In Prokaryotes And Eukaryotes
• In a eukaryotic cell:
– The nuclear envelope separates transcription from translation
– Extensive RNA processing occurs in the nucleus
• Prokaryotic cells lack a nuclear envelope, allowing translation to
begin while transcription progresses
RNA polymerase
DNA
Polyribosome
RNA
polymerase
Direction of
transcription
mRNA
0.25 µm
DNA
Polyribosome
Polypeptide
(amino end)
Ribosome
mRNA (5′ end)

A summary of transcription and translation in a eukaryotic cell
Figure 17.26
TRANSCRIPTION
RNA is transcribed
from a DNA template.
DNA
RNA
polymerase
RNA
transcript
RNA PROCESSING
In eukaryotes, the
RNA transcript (pre-
mRNA) is spliced and
modified to produce
mRNA, which moves
from the nucleus to the
cytoplasm.
Exon
Poly-A
RNA transcript
(pre-mRNA)
Intron
NUCLEUS
Cap
FORMATION OF
INITIATION COMPLEX
After leaving the
nucleus, mRNA attaches
to the ribosome.
CYTOPLASM
mRNA
Poly-A
Growing
polypeptide
Ribosomal
subunits
Cap
Aminoacyl-tRNA
synthetase
Amino
acid
tRNA
AMINO ACID ACTIVATION
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
Activated
amino acid
TRANSLATION
A succession of tRNAs
add their amino acids to
the polypeptide chain
as the mRNA is moved
through the ribosome
one codon at a time.
(When completed, the
polypeptide is released
from the ribosome.)
Anticodon
A C C
A A A
U G G U U U A U G
U
A CE A
Ribosome
1
Poly-A
5′
5′
3′
Codon
2
3 4
5

Post-translation
• The new polypeptide is now floating loose in the
cytoplasm if translated by a free ribosome.
• Polypeptides fold spontaneously into their active
configuration, and they spontaneously join with other
polypeptides to form the final proteins.
• Often translation is not sufficient to make a functional
protein, polypeptide chains are modified after
translation
• Sometimes other molecules are also attached to the
polypeptides: sugars, lipids, phosphates, etc. All of
these have special purposes for protein function.

Targeting Polypeptides to Specific Locations
• Completed proteins are targeted to specific sites
in the cell
• Two populations of ribosomes are evident in cells:
free ribsomes (in the cytosol) and bound
ribosomes (attached to the ER)
– Free ribosomes mostly synthesize proteins that
function in the cytosol
– Bound ribosomes make proteins of the endomembrane
system and proteins that are secreted from the cell
• Ribosomes are identical and can switch from free
to bound

• Polypeptide synthesis always begins in the cytosol
• Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to
attach to the ER
• Polypeptides destined for the ER or for secretion are marked by a signal
peptide
• A signal-recognition particle (SRP) binds to the signal peptide
• The SRP brings the signal peptide and its ribosome to the ER
Targeting Polypeptides to Specific Locations
Ribosomes
mRNA
Signal
peptide
Signal-
recognition
particle
(SRP)
SRP
receptor
protein
CYTOSOL
ER LUMEN Translocation
complex
Signal
peptide
removed
ER
membrane
Protein

Mutation Causes and Rate
• The natural replication of DNA produces occasional
errors. DNA polymerase has an editing mechanism
that decreases the rate, but it still exists
• Typically genes incur base substitutions about once
in every 10,000 to 1,000,000 cells
• Since we have about 6 billion bases of DNA in each
cell, virtually every cell in your body contains several
mutations
• Mutations can be harmful, lethal, helpful, silent
• However, most mutations are neutral: have no effect
• Only mutations in cells that become sperm or eggs—
are passed on to future generations
• Mutations in other body cells only cause trouble when
they cause cancer or related diseases

Mutagens
• Mutagens are chemical or physical agents that interact
with DNA to cause mutations.
• Physical agents include high-energy radiation like X-rays
and ultraviolet light
• Chemical mutagens fall into several categories.
– Chemicals that are base analogues that may be substituted into
DNA, but they pair incorrectly during DNA replication.
– Interference with DNA replication by inserting into DNA and
distorting the double helix.
– Chemical changes in bases that change their pairing properties.
• Tests are often used as a preliminary screen of chemicals
to identify those that may cause cancer
• Most carcinogens are mutagenic and most mutagens are
carcinogenic.

Viral Mutagens
• Scientists have recognized a number of tumor
viruses that cause cancer in various animals,
including humans
• About 15% of human cancers are caused by viral
infections that disrupt normal control of cell
division
• All tumor viruses transform cells into cancer cells
through the integration of viral nucleic acid into
host cell DNA.

Point mutations
• Point mutations involve alterations in the
structure or location of a single gene.
Generally, only one or a few base pairs are
involved.
• Point mutations can signficantly affect protein
structure and function
• Point mutations may be caused by physical
damage to the DNA from radiation or
chemicals, or may occur spontaneously
• Point mutations are often caused by mutagens

Point Mutation
• The change of a single nucleotide in the DNA’s template
strand leads to the production of an abnormal protein
In the DNA, the
mutant template
strand has an A where
the wild-type template
has a T.
The mutant mRNA has
a U instead of an A in
one codon.
The mutant (sickle-cell)
hemoglobin has a valine
(Val) instead of a glutamic
acid (Glu).
Mutant hemoglobin DNAWild-type hemoglobin DNA
mRNA mRNA
Normal hemoglobin Sickle-cell hemoglobin
Glu Val
C T T C A T
G A A G U A
3′ 5′ 3′ 5′
5′ 3′5′ 3′

Types of Point Mutations
• Point mutations within a gene can be divided
into two general categories
– Base-pair substitutions - is the replacement of
one nucleotide and its partner with another pair
of nucleotides
– Base-pair insertions or deletions - are additions
or losses of nucleotide pairs in a gene

Base-Pair Substitutions
• Silent - changes a codon but codes for the same amino acid
• Missense - substitutions that change a codon for one amino acid into a
codon for a different amino acid
• Nonsense -substitutions that change a codon for one amino acid into a
stop codon
Wild type
A U G A A G U U U G G C U A A
mRNA
5′
Protein Met Lys Phe Gly
Stop
Carboxyl end
Amino end
3′
A U G A A G U U U G G U U A A
Met Lys Phe Gly
Base-pair substitution
No effect on amino acid sequence
U instead of C
Stop
A U G A A G U U U A G U U A A
Met Lys Phe Ser Stop
A U G U A G U U U G G C U A A
Met Stop
Missense A instead of G
Nonsense
U instead of A

Insertions and Deletions
– Are additions or losses of nucleotide pairs in a gene
– May produce frameshift mutations that will change the
reading frame of the gene, and alter all codons
downstream from the mutation.
mRNA
Protein
Wild type
A U G A A G U U U G G C U A A
5′
Met Lys Phe Gly
Amino end Carboxyl end
Stop
Base-pair insertion or deletion
Frameshift causing immediate nonsense
A U G U A A G U U U G G C U A
A U G A A G U U G G C U A A
A U G U U U G G C U A A
Met Stop
U
Met Lys Leu Ala
Met Phe Gly
Stop
MissingA A G
Missing
Extra U
Frameshift causing
extensive missense
Insertion or deletion of 3 nucleotides:
no frameshift but extra or missing amino acid
3′

DNA structure replication transcription translation

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