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The Molecular Basis of Inheritance
G
C
A
T
T
A
1 nm
C
G
C
A
C
3.4 nm
G
T
G
T
A
T
A
A
T
T
A
G
A
Figure 16.7a, c
C
0.34 nm
T
(a) Key features of DNA structure
(c) Space-filling model
Transcription
• Transcription is the DNAdirected 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
Home Work Questions
TATA BOX
-It is a type of promoter
sequence, which
specifies to other
molecules where
transcription begins.
Home Work Questions
- INTRONS:
intervening non-coding sequence in a eukaryotic gene.
- EXONS:
the coding region of a eukaryotic gene. Each gene is
composed of one or more exons.
*Translation*
• Translation is the RNA-directed
synthesis of a polypeptide
• Translation involves
–
–
–
–
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
mRNA
Ribosomes - Ribosomal RNA
Transfer RNA
Genetic coding – codons
Amino
acids
Polypeptide
tRNA with
amino acid
Ribosome attached
Gly
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
3
*The Genetic Code*
•
Genetic information is encoded as a sequence of nonoverlapping base
triplets, or codons
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand
(template)
3
A C C A A A C C G A G
T
5
TRANSCRIPTION
mRNA
5
U G G U U U G G C U C A
Codon
TRANSLATION
Protein
Trp
Amino acid
Phe
Gly
Ser
3
*The Genetic Code*
Codons:
• Basic unit of genetic code
• set of 3 nucleotides in mRNA that codes for an amino acid placement
on polypeptides.
•
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 which mark the end of
the protein
•
One codon is used as a START signal: start of every protein
*The Genetic Code*
Second mRNA base
U
C
A
UAU
UUU
UCU
Tyr
Phe
UAC
UUC
UCC
U
UUA
UCA Ser UAA Stop
UAG Stop
UUG Leu UCG
CUU
CUC
C
CUA
CUG
CCU
CCC
Leu CCA
CCG
Pro
AUU
AUC
A
AUA
AUG
ACU
ACC
ACA
ACG
Thr
GUU
G GUC
GUA
GUG
lle
Met or
start
GCU
GCC
Val
GCA
GCG
Ala
G
U
UGU
Cys
UGC
C
UGA Stop A
UGG Trp G
U
CAU
CGU
His
CAC
CGC
C
Arg
CAA
CGA
A
Gln
CAG
CGG
G
U
AAU
AGU
Asn
AAC
AGC Ser C
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
C
GAC Asp GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
First mRNA base (5 end)
• A codon in messenger RNA is either translated into an amino acid
or serves as a translational start/stop signal
*Transfer RNA*
•
•
•
•
Consists of a single RNA strand, only 80 nucleotides long
Each carries a specific amino acid on one end and has an anticodon on
the other end
Enzymes pair up the proper tRNA molecules with their corresponding
amino acids.
tRNA brings the amino acids to the ribosomes,
3
A
C
C
A 5
C G
The “anticodon” is the 3 RNA bases that
G C
C G
matches the 3 bases of the codon on the
U G
U A
mRNA molecule
A U
A U
U C
UA
C A C AG
*
G
*
U
G
U
G
C
C
*
* *
U C
*
* G AG C
(a) Two-dimensional structure. The four base-paired regions and three
G C
U A
loops are characteristic of all tRNAs, as is the base sequence of the
* G
amino acid attachment site at the 3 end. The anticodon triplet is
A
A*
C
unique to each tRNA type. (The asterisks mark bases that have been
U
*
chemically modified, a characteristic of tRNA.)
A
G
A
Amino acid
attachment site
Anticodon
C U C
G A G
A G *
*
G
A G G
Hydrogen
bonds
*Transfer RNA*
• 3 dimensional tRNA molecule is roughly “L” shaped
5
3
Amino acid
attachment site
Hydrogen
bonds
A AG
3
Anticodon
(b) Three-dimensional structure
5
Anticodon
(c) Symbol used
in the book
*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
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Growing
polypeptide
Exit tunnel
tRNA
molecules
Large
subunit
E
P A
Small
subunit
5
mRNA
3
(a) 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.
Building a Polypeptide
Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
tRNA
3
mRNA
5
Codons
(c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon basepairs 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.
*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
P site
3 U A C 5
5 A U G 3
Initiator tRNA
GTP
GDP
E
A
mRNA
5
Start codon
mRNA binding site
1
5
3
Small
ribosomal
subunit
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).
3
Translation initiation complex
2
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.
Elongation of the Polypeptide Chain
• In the elongation stage, amino acids are added one by one to
the preceding amino acid
TRANSCRIPTION
1 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.
Amino end
of polypeptide
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
E
mRNA
Ribosome ready for
next aminoacyl tRNA
5
3
P A
site site
2 GTP
2 GDP
E
E
P
P
A
GDP
3 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.
GTP
E
P
A
A
2 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.
Termination of Translation
•
The final stage is termination when the ribosome reaches a stop codon
in the mRNA
Release
factor
Free
polypeptide
5
3
3
5
5
3
Stop codon
(UAG, UAA, or UGA)
1 When a ribosome reaches a stop 2 The release factor hydrolyzes 3 The two ribosomal subunits
codon on mRNA, the A site of the
the bond between the tRNA in and the other components of
ribosome accepts a protein called
the P site and the last amino
the assembly dissociate.
a release factor instead of tRNA.
acid of the polypeptide chain.
The polypeptide is thus freed
from the ribosome.
*Termination of Translation*
• The final step in translation is termination.
• ribosome reaches a STOP codon, there is no
corresponding transfer RNA.
• 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.
A summary of transcription and translation in a eukaryotic cell
DNA
TRANSCRIPTION
1RNA is transcribed
from a DNA template.
3
RNA
transcript
5
RNA
polymerase
Exon
RNA PROCESSING
2
In eukaryotes, the
RNA transcript (premRNA) is spliced and
modified to produce
mRNA, which moves
from the nucleus to the
cytoplasm.
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
NUCLEUS
Amino
acid
FORMATION OF
INITIATION COMPLEX
CYTOPLASM
AMINO ACID ACTIVATION
tRNA
3 After leaving the
4
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
nucleus, mRNA attaches
to the ribosome.
mRNA
Growing
polypeptide
Activated
amino acid
Ribosomal
subunits
5
TRANSLATION
5
E
A
A A A
U G G U U U A U G
Figure 17.26
Codon
Ribosome
Anticodon
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.)
Post-translation
• The new polypeptide is now floating loose in the
cytoplasm if translated by a free ribosme.
• It might also be inserted into a membrane, if translated by
a ribosome bound to the endoplasmic reticulum.
• ***Polypeptides fold spontaneously into their 4 levels of
structure (quaternary) , and they spontaneously join with
other polypeptides to form the final proteins.***
• Sometimes other molecules are also attached to the
polypeptides: sugars, lipids, phosphates, etc. All of these
have special purposes for protein function and GENE
EXPRESSION