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Genetic Coding in Cells
DNA
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•
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Double stranded
Deoxyribose sugar
Bases: C,G A,T
Self replicate
vs.
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•
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RNA
Single stranded
Ribose sugar
Bases: C,G,A,U
Can’t self replicate
mRNA, tRNA, rRNA
Both contain a sugar, phosphate, and base.
DNA Replication
1. DNA divides in half by splitting down the center
of the ladder starting at one end.
2. Each rung separates in the middle, between the 2
bases. C splits from G and A from T.
DNA Replication cont.
3. The cell has spare DNA Units.
4. The correct DNA unit attaches itself to the
appropriate rungs on each of the 2 half-ladders as
the DNA molecule splits. A spare AT, a spare
TA, a spare GC, & a spare CG.
5. After the DNA finishes “unzipping” and the spare
DNA units join up with the rungs on the half
ladders, 2 identical molecules are formed.
Introduction to The Central Dogma
of Molecular Biology
Protein Synthesis
Flow of Information:
DNA
RNA
Transcription
Proteins
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.
Transcription occurs in the Nucleus.
1. RNA polymerase (an
enzyme) attaches to DNA
at a special sequence that
serves as a “start signal”.
2. The DNA strands are
separated and one strand
serves as a template.
3. The RNA bases attach to
the complementary DNA
template, thus
synthesizing mRNA.
Transcription cont.
4.
5.
6.
The RNA polymerase recognizes a termination
site on the DNA molecule and releases the new
pre - mRNA.
Next the pre - mRNA under goes splicing. This
is when the non-coding sequences or introns are
eliminated. The coding mRNA sequence can
be described as an exon.
Now the mature mRNA leaves the nucleus and
travels to the ribosome in the cytoplasm.
Transcription
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 3-letter code
word.
Each codon specifies a particular amino
acid that is to be placed in the
polypeptide chain (protein).
Translation
•A three-letter code is used because there
are 20 amino acids that are used to make
proteins.
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
Translation
•There is a total of 64 codons with mRNA,
only 61 specify a particular amino acid.
•There are more than 1 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.
Translation
Transfer RNA (tRNA)
•Each tRNA molecule has 2 important sites of
attachment.
1.anticodon, binds to the codon on the mRNA
molecule.
2.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.
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
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