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Introduction to
Central Dogma:
DNA Replication, Transcription
and Translation
What is Central Dogma?
• It is the flow of genetic information from
DNA to Proteins
Transcription
DNA
mRNA
Replication
DNA
Translation
Proteins
What is DNA?
• It is the fascinating
molecule that
contains the Code
of Life.
What Is the Code of Life?
• It is the Genetic
Code, which is the
set of “messages”
that are “translated”
by ribosomes into
proteins that define
YOU
Structure of Nucleic Acids
• Recall that DNA and
RNA belong to the
class of biomolecules
called Nucleic Acids,
which are made up of
nucleotide monomers
Structure of Nucleotides
 The nucleotide subunits consist of:
3. A Phosphate
Group
1. A Nitrogen
Base
2. A Sugar (either
Deoxyribose or Ribose)
Nitrogen Bases
Nitrogen Bases Can Be:
Adenine A
Guanine G
Cytosine C
Thymine (only DNA) T
Uracil (only RNA) U
Chargaff’s Rule of Base Pairing
 Erwin Chargaff analyzed
DNA from different
organisms and found that
A=T & G=C
According to Chargaff,
in DNA:
A always bonds with T
G always bonds with C
The Sugar Group
The Sugar Group Can
Be Either:
Deoxyribose (in DNA)
orRibose (in RNA)
Image by: Riedell
Purines & Pyrimidines
 Purines consist of Adenine


and Guanine bases; they are
double-ring structures
Pyrimidines consist of
Thymine and Guanine
bases; they are single-ring
structures
A Purine ALWAYS bonds
with a Pyrimidine….A
with T and G with C
Nitrogen Base Difference DNA &
RNA
DNA has no URACIL
RNA has no THYMINE
Bonding in DNA
• The nitrogen bases
are held together by
weak hydrogen
Covalent
bonds
Phosphodiester
Bonds
• The sugar and
phosphate groups
are held together by
strong covalent
phosphodiester
bonds
DNA Replication (occurs in the nucleus)
Semi-Conservative:
• Strand separation,
followed by copying
of each strand.
• Each separated
strand acts as a
template for the
synthesis of a new
complementary
strand.
DNA Replication (occurs in the nucleus)
Step 1:
• Unwinding of the double helix
 Helicase enzyme unwinds the double helix & creates
a replication fork
DNA Replication (occurs in the nucleus)
Step 2:
• One DNA strand is
used as a template
to create the new
copy
DNA DNA
A
G
C
A
A
–
–
–
–
–
T
C
G
T
T etc
DNA Transcription (occurs in the nucleus)
•
Transcription
produces genetic
messages in the
form of mRNA
mRNA
DNA
Template
DNA Transcription (occurs in the nucleus)
•
STEP 1: As in
replication, the DNA
mRNA
double helix unzips
•
STEP 2: RNA
nucleotides line up
along one strand of
DNA, following the
base-pairing rules
DNA
Template
DNA Transcription (occurs in the nucleus)
•
STEP 3: DNA is
transcribed into
RNA
DNA
G
T
A
G
G
T
mRNA
C
A
U
G
C
A
mRNA
DNA
Template
DNA Transcription (occurs in the nucleus)
•
Single-stranded
mRNA peels away
from DNA and
prepares to move
into the cytoplasm
DNA
Nucleus
mRNA
Cytoplasm
DNA Transcription (occurs in the nucleus)
•
Eukaryotic RNA is
processed before
leaving the nucleus
 The non-coding
segments, called
introns, are
spliced out
 A cap & tail are
added to the ends
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
NUCLEUS
CYTOPLASM
DNA Translation (occurs in the cytoplasm)
•
mRNA leaves the
nucleus and moves
into the cytoplasm
where it will be
“translated” into a
polypeptide (a
fancy word for a
protein)
TRANSCRIPTION
NUCLEUS
TRANSLATION
PROTEIN
CYTOPLASM
DNA Translation (occurs in the cytoplasm)
•
•
The words that will
be translated are
triplets of mRNA
bases called
codons
The codons in a
gene determine the
amino acids in the
polypeptide
sequence
DNA
molecule
Gene 3
Gene 1
Gene 2
DNA strand
TRANSCRIPTION
RNA
Codon
TRANSLATION
Polypeptide
Amino acid
Codon
Codon
Codon
DNA Translation (occurs in the cytoplasm)
•
•
In the cytoplasm, a ribosome attaches to the mRNA
A tRNA pairs with each codon, adding an amino acid
to the growing polypeptide
mRNA Genetic Code Chart
Second Base
C
U
UUU
UUC
UUA
UUG
C
CUU
CUC
CUA
CUG
A
AUU
AUC
ile
AUA
AUG met (start)
ACU
ACC
ACA
ACG
G
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
phe
leu
leu
val
UCU
UCC
UCA
UCG
CCU
CCC
CCA
CCG
A
ser
UAU
UAC
UAA
UAG
pro
CAU
CAC
CAA
CAG
thr
AAU
AAC
AAA
AAG
ala
GAU
GAC
GAA
GAG
G
tyr
stop
stop
his
gln
asn
lys
asp
glu
UGU
UGC
UGA
UGG
CGU
CGC
CGA
CGG
AGU
AGC
AGA
AGG
GGU
GGC
GGA
GGG
cys
stop
trp
arg
ser
arg
gly
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
Third Base
First Base
U
Table 14.2
Types of RNA
Type of RNA
Functions in
Messenger RNA
(mRNA)
Nucleus,
migrates
to ribosomes
in cytoplasm
Transfer RNA
(tRNA)
Cytoplasm
Provides linkage
between mRNA
and amino acids;
transfers amino
acids to ribosomes
Ribosomal RNA
(rRNA)
Cytoplasm
Structural
component
of ribosomes
Function
Carries DNA
sequence
information to
ribosomes
Mutations
•
Mutations are changes in the DNA base sequence
 Are caused by errors in DNA replication
 Are caused by environmental factors, such as
exposure to radiation and chemicals or
temperature changes
•
Mutations most likely cause altered proteins to be
produced
•
2 Kinds of Mutations: Gene Mutations &
Chromosomal Mutations
Gene Mutations
•
Point mutations – changes of a single DNA
nucleotide – can cause sickle-cell anemia and many
other disorders
Normal hemoglobin DNA
mRNA
Mutant hemoglobin DNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Gene Mutations
•
Types of Point mutations

Substitutions

Insertions

Deletions
Gene Mutations
NORMAL GENE
mRNA
Protein
Met
Lys
Phe
Gly
Ala
Lys
Phe
Ser
Ala
BASE SUBSTITUTION
Met
Missing
BASE DELETION
Met
Lys
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Leu
Ala
His
Chromosomal Mutations
Deletion
Homologous
chromosomes
Duplication
Inversion
Reciprocal
translocation
Non-homologous
chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Chromosomal Mutations
•
Human karyotype
showing addition of 1
chromosome on
autosome 21 – Down
Syndrome
Chromosome painting