Transcript DNA PowerPoint, page 4

Introduction
The Central Dogma
of Molecular Biology
Cell
DNA
Transcription
mRNA
Translation
Ribosome
Polypeptide
(protein)
Protein Synthesis
Flow of Information:
DNA
RNA
Proteins
Transcription
Translation
Transcription is the process by which a
molecule of DNA is copied into a
complementary strand of RNA.
This is called messenger RNA (mRNA)
because it acts as a messenger between DNA
and the ribosomes where protein synthesis is
carried out.
Protein Synthesis Transcription
Transcription process
•RNA polymerase (an enzyme) attaches to
DNA at a special sequence that is a start
signal called a “promoter”.
•The DNA strands are separated and one strand
serves as a template.
•The RNA bases attach to the complementary
DNA template, thus synthesizing mRNA.
Protein Synthesis: Transcription
Transcription process continued
•The RNA polymerase recognizes a
termination site on the DNA molecule and
releases the new mRNA molecule.
(mRNA leaves the nucleus and travels to the
ribosome in the cytoplasm.)
Protein Synthesis: Transcription
Eukaryotic Transcription
Cytoplasm
DNA
Transcription
RNA
RNA
Processing
mRNA G
G
AAAAAA
Nucleus
Export
AAAAAA
Protein Synthesis: Translation
Translation is the process of decoding a
mRNA molecule into a polypeptide chain or
protein.
Each combination of 3 nucleotides on mRNA
is called a codon or three-letter code word.
Each codon specifies a particular amino acid
that is to be placed in the polypeptide chain
(protein).
Protein Synthesis: Translation
A Codon
OH
P
HO
NH2
O
N
O
N
CH2
H
P
O
O
N
O
CH2
P
NH
N
Guanine
NH2
N
O
Arginine
H
O
HO
N
O
O
HO
Adenine
N
NH2
O
N
O
CH2
N
O
OH
H
N
N
Adenine
Protein Synthesis: Translation
•A three-letter code is used because there are
20 different amino acids that are used to
make proteins.
•If a two-letter code were used there would
not be enough codons to select all 20 amino
acids.
•That is, there are 4 bases in RNA, so 42 (4x
4)=16; where as 43 (4x4x4)=64.
Protein Synthesis: Translation
Protein Synthesis: Translation
•Therefore, there is a total of 64 codons with
mRNA, 61specify a particular amino acid.
• This means there are more than one codon for
each of the 20 amino acids.
•The remaining three codons (UAA, UAG, &
UGA) are stop codons, which signify the end
of a polypeptide chain (protein).
•Besides selecting the amino acid methionine,
the codon AUG also serves as the “initiator”
codon, which starts the synthesis of a protein
Protein Synthesis: Translation
Protein Synthesis: Translation
Transfer RNA (tRNA)
•Each tRNA molecule has 2 important sites
of attachment.
•One site, called the anticodon, binds to the
codon on the mRNA molecule.
•The other site attaches to a particular amino
acid.
•During protein synthesis, the anticodon of a
tRNA molecule base pairs with the
appropriate mRNA codon.
Protein Synthesis: Translation
Met-tRNA
Methionine
16 Pu
17
9
A
17:1
13 12 Py 10
1
2
3
4
5
6
U* 7
A
C
C
73
72
71
70
69
68
67
Py 59A*
66
65 64 63 62 C
Pu
49 50 51 52 G T C
y
Py
G*
22 23 Pu 25
G
26
2020:120:2A
27
1
28
29
30
31
Py*
47:16
47:15
43 44
42 45
41 46
47
40
47:1
39
38
Pu*
U
34
U 35
C
A 36
Anticodon
Protein Synthesis: Translation
Ribosome:
•Are made up of 2 subunits, a large one and a
smaller one, each subunit contains ribosomal
RNA (rRNA) & proteins.
•Protein synthesis starts when the two subunits
bind to mRNA.
•The initiator codon AUG binds to the first
anticodon of tRNA, signaling the start of a
protein.
Protein Synthesis: Translation
Ribosome:
•The anticodon of another tRNA binds to the
next mRNA codon, bringing the 2nd amino
acid to be placed in the protein.
•As each anticodon & codon bind together a
peptide bond forms between the two amino
acids.
Protein Synthesis: Translation
Ribosome:
•The protein chain continues to grow until a
stop codon reaches the ribosome, which
results in the release of the new protein and
mRNA, completing the process of
translation.
Protein Synthesis: Translation
Translation - Initiation
fMet
Large
subunit
E
P
A
UAC
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
Small mRNA
subunit
3’
Translation - Elongation
Polypeptide
Arg
Met
Phe
Leu
Ser
Aminoacyl tRNA
Gly
Ribosome
E
P
A
CCA
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
mRNA
3’
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ser
Gly
Arg
Aminoacyl tRNA
Ribosome
E
P
A
CCA UCU
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
mRNA
3’
Protein Synthesis
AMINE H
H
O
N
ACID
C
C
ANYTHING
R
H
Amino Acid
Alanine
OH
H
H
Serine
H
O
N
C
OH
H
C
H
H
H
H2O
H
H
C
H
H
C
H
O
C
C
C
C
HO
N
H
N
H
H
C
O
N
H
O
C
C
H
H C
H
H HO
H
OH
OH
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ser
Gly
Arg
Ribosome
E
P
A
CCA UCU
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
mRNA
3’
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ala
Ser
Gly
Aminoacyl tRNA
Arg
Ribosome
E
P
A
CCA
UCU
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
mRNA
3’
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ser
Gly
Arg
Ribosome
E
Ala
P
A
UCU CGA
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
mRNA
3’
Transcription And Translation In
Prokaryotes
5’
3’
3’
5’
RNA
Pol.
Ribosome
mRNA
5’
Ribosome