Download Central dogma I and II

Central dogma I and II
the flow of genetic information
1 Th
1.
The Transforming
T
f
i
Principle
2. Overview of Central
Dogma
3. Nucleic Acid Structure
4. The Organization of
DNA iin C
Cells
ll
5. DNA Replication
6. Gene Structure and the
Genetic Code
7. Transcription
8. Translation
9. Post-Translational
Modification
DNA as Genetic Material
Transformation – Transforming principle
•
Griffith
G
iffith iin 1928 observed
b
d the
th change
h
off non-virulent
i l t
organisms into virulent ones as a result of “transformation”
– MacLeod and McCarty in 1944 showed that the
transforming principle was DNA
Transforming principle
The Flow of Genetic Information
– DNA stores genetic
information
DNA
Replication
– Information is
d li t d b
duplicated
by replication
li ti
and is passed on to
next generation
Genotype
Transcription
RNA
– transcription yields a
ribonucleic acid (RNA)
copy of specific genes
Translation
Polypeptide
yp p
– translation uses
information in
messenger RNA
(mRNA) to synthesize a
polypeptide
l
tid
• Also involves
activities of transfer
RNA (tRNA) and
ribosomal
ib
l RNA
(rRNA)
Posttranslational
modification
Protein
Phenotype
Cell
Nucleic Acid Structure
•
The nucleic
Th
l i acids,
id DNA and
d RNA are polymers
l
off nucleotides
l tid
– linked together by phosphodiester bonds
•
DNA and RNA differ in
– the nitrogenous bases they contain
– the sugars they contain
– whether they are single or double stranded
Deoxyribonucleic
y
Acid ((DNA))
•
•
•
•
•
Polymer of nucleotides
Contains the bases adenine, guanine, cytosine and thymine
S
Sugar
is deoxyribose
Molecule is usually double stranded
Base pairing
– Adenine (purine) and thymine (pyrimidine) pair by 2
hydrogen bonds
– Guanine (purine) and cytosine (pyrimidine) pair by 3
hydrogen bonds
Ribonucleic Acid (RNA)
• Polymer
P l
off nucleotides
l tid
• Contains the bases
adenine, guanine,
cytosine and uracil
• Sugar is ribose
os RNA molecules
o ecu es
• Most
are single stranded
• Th
Three diff
differentt types
t
which differ from each
other in function, site of
synthesis (in eucaryotic
cells) and in structure
– messenger RNA
(mRNA)
– ribosomal RNA
(rRNA)
– transfer RNA
(tRNA)
The Organization of DNA in Cells
• IIn allll Archaea
A h
and
d
most bacteria DNA
is a circular double
helix
• Further twisting
results in
supercoiled DNA
– In bacteria the
DNA is associated
with basic proteins
• Help organize the
DNA into a coiled
chromatin like
structure
DNA Forms
Eucaryotic DNA Organization
• DNA is more
highly
organized in
eucaryotic
chromatin
where it is
associated
with histones,
small basic
proteins
• The
combination of
DNA and
proteins is
called a
nucleosome
DNA Replication
• Complex process
involving numerous
proteins which help
ensure accuracy
• The 2 strands separate,
each serving as a
t
template
l t for
f synthesis
th i
of a complementary
strand
• Synthesis is semiconservative; each
daughter cell obtains
one old
ld and
d one new
strand
In most procaryotes bidirectional from
a single origin of replication
Rolling Circle Replication
• some smallll
circular genomes
(e.g., viruses and
plasmids
– replicated by
rolling-circle
lli
i l
replication
Eucaryotic DNA Replication
• eucaryotic DNA is ~1,400 times longer than
procaryotic
ti DNA and
d iis lilinear
• many
y replication
p
forks are used simultaneously
y with
many replicons present
1.
2
2.
3.
4
4.
5.
Ori
Helicase
DNA Gyrase
SSB
Primase
6. RNA primer
7 DNA polymerase
7.
8. Leading/lagging strand
9 DNA ligase
9.
10. Termination
DNA Polymerase
Proofreading, removal of mismatched base from 3’ end of
growing strand by exonuclease activity of enzyme
• Gene, defined as the nucleic acid sequence that codes for a
•
•
•
•
•
polypeptide, tRNA or rRNA
Template strand directs RNA synthesis (3’
(3 to 5
5’ direction)
Promoter is located at the start of the gene and the binding
site for RNA polymerase
Leader sequence is transcribed into mRNA but is not
translated into amino acids
Shine-Delgarno sequence important for initiation of translation
reading frame, organization of codons such that they can be
read to give rise to a gene product
Genetic Code
• code degeneracy
– up to six different codons can code for a single amino acid
• sense codons
d
– the 61 codons that specify amino acids
• stop (nonsense) codons
– the three codons used as translation termination signals
– do not encode amino acids
Importance of reading
frame
Transcription
• RNA synthesis under the direction of DNA
– RNA produced has complementary sequence to the template DNA
– Three types off RNA are produced
• mRNA carries the message for protein synthesis
• tRNA carries amino acids during protein synthesis
• rRNA molecules are components of ribosomes
• Polygenic mRNA often found in bacteria and archaea
– contains directions for > 1 polypeptide
• Catalyzed by a single RNA polymerase
– Reaction similar to that catalyzed by DNA polymerase
• ATP,GTP,CTP and UTP are used to produce a complementary RNA
copy of the template DNA sequence
Transcription in procaryotes
• Initiation
• Elongation
• Termination
- the sigma factor has
no catalytic activity
but helps the core
enzyme recognize the
start of genes
– holoenzyme = core
enzyme + sigma
factor
• Only the holoenzyme
can begin transcription
Promoter
- site where RNA polymerase binds to initiate
transcription
The Transcription Bubble
(elongation)
• After binding, RNA
polymerase unwinds
the DNA
• Transcription bubble
produced
– Moves with the
polymerase
p
y
as it
transcribes mRNA
from template strand
• Within the bubble a
temporary RNA:DNA
hybrid is formed
Transcription Termination
• Occ
Occurs
rs when
hen
core RNA
polymerase
dissociates from
template DNA
• DNA sequences
q
mark the end of
gene in the
trailer and the
terminator
• Some
terminators
require the aid
of the rho factor
for termination
Transcription in Eucaryotes
•
Several RNA polymerases
– Promotes
P
t differ
diff from
f
those in bacteria by
having combinations of
many elements
– Catalyzes production of
heterogeneous nuclear
RNA (hnRNA) which
undergoes
posttranscriptional
modification to generate
mRNA
•
eucaryotic genes
– have
h
exons ((expressed
d
sequences) and introns
(intervening sequences)
that code for RNA that
is never translated into
protein
Transcription in the Archaea
• RNA p
polymerase
y
has similarities to both
bacteria and eucaryotic enzyme
• similarities with eucaryotes
– archaeal gene promoters and binding of the
RNA polymerase
– introns present in some archaeal genes
• similarities with procaryotes
– mRNA is polygenic
Translation
•
•
•
synthesis of polypeptide directed by sequence of nucleotides
in mRNA
direction of synthesis N terminal  C-terminal
ribosome
– site of translation
– polyribosome – complex of mRNA with several ribosomes
The Ribosome
•
•
•
•
•
•
Procaryotes, 70S ribosomes = 30S + 50S subunits
Eucaryotes, 80S ribosomes = 40S + 60S subunits
mitochondrial and chloroplast ribosomes resemble procaryotic
ribosomes
peptidyl (donor; P) site, binds initiator tRNA or tRNA attached
to growing polypeptide (peptidyl-tRNA)
aminoacyl (acceptor; A) site, binds incoming aminoacyl-tRNA
exit (E) site, briefly binds empty tRNA before it leaves
ribosome
Aminoacyl tRNA
Aminoacyl-tRNA
• attachment
tt h
t off
amino acid to
tRNA
• catalyzed by
aminoacyl-tRNA
synthetases
th t
– at least 20
• each specific
p
for
single amino acid
and for all the
tRNAs to which
each may be
properly attached
(cognate tRNAs)
Coupled Transcription and
Translation in Procaryotes