1. THE CENTRAL
DOGMA
ROQUE, ARYANA ROSE B.
SCHULLER, KRIS JANE MARIE
AAPD2F
2. CENTRAL DOGMA OF MOLECULAR
BIOLOGY
“The central dogma of molecular biology deals with the
detailed residue-by-residue transfer of sequential
information. It states that such information cannot be
transferred back from protein to either protein or
nucleic acid.”
Francis Crick, 1958
3. … IN OTHER WORDS
Protein information
cannot flow back to
nucleic acids
Fundamental
framework to
understanding the
transfer of
sequence
information
between
biopolymers
6. THE BASICS: ADDITIONAL POINTS
DNA => A T C G, RNA => A U C G
Almost always read in 5' and 3' direction
DNA and RNA are dynamic - 2° structure
Not all DNA is found in chromosomes
Mitochondria
Chloroplasts
Plasmids
BACs and YACs
Some extrachromosomal DNA can be useful in
Synthetic Biology
7. … AN EXAMPLE OF A PLASMID
VECTOR
Gene of interest
Selective markers
Origin of
replication
Restriction sites
9. DNA REPLICATION
The process of copying double-stranded DNA molecules
Semi-conservative replication
Origin of replication
Replication Fork
Proofreading mechanisms
11. DNA REPLICATION: ENZYMES
INVOLVED
Initiator proteins (DNApol clamp loader)
Helicases
SSBPs (single-stranded binding proteins)
Topoisomerase I & II
DNApol I – repair
DNApol II – cleans up Okazaki fragments
DNApol III – main polymerase
DNA primase
DNA ligase
13. DNA REPLICATION: PROOFREADING
MECHANISMS
DNA is synthesised from dNTPs. Hydrolysis of (two)
phosphate bonds in dNTP drives this reduction in entropy.
- Nucleotide binding error rate =>c.10−4, due to extremely short-lived imino and enol tautomery.
- Lesion rate in DNA => 10-9.
Due to the fact that DNApol has built-in 3’ →5’ exonuclease activity, can chew back
mismatched pairs to a clean 3’end.
14. TRANSCRIPTION
Process of copying DNA to RNA
Differs from DNA synthesis in that only one strand
of DNA, the template strand, is used to make mRNA
Does not need a primer to start
Can involve multiple RNA polymerases
Divided into 3 stages
Initiation
Elongation
Termination
22. The regulatory response requires the lactose repressor
The lacI gene encoding repressor lies nearby the lac operon
and it is consitutively (i.e. always) expressed
In the absence of lactose, the repressor binds very tightly to a
short DNA sequence just downstream of the promoter near
the beginning of lacZ called the lac operator
Repressor bound to the operator interferes with binding of
RNAP to the promoter, and therefore mRNA encoding LacZ
and LacY is only made at very low levels
In the presence of lactose, a lactose metabolite called
allolactose binds to the repressor, causing a change in its
shape
The repressor is unable to bind to the operator, allowing
RNAP to transcribe the lac genes and thereby leading to high
levels of the encoded proteins.
25. During Translation, a ribosome will attach
itself onto the strand of mRNA molecule waiting
to be translated. It will cover a single triplet code
at a time. The Ribosome has sockets where tRNA
molecules can be inserted. The tRNA molecules
are linked to a specific amino acids at one one
end, and has 3 bases at the other end. The tRNA
molecule whose bases are able to pair with the
triplet code on mRNA can enter the socket, and
release its amino acid before leaving the socket.
The ribosome will move on to the next triplet, and
another tRNA will be able to enter the socket. The
process repeats itself until the end of the mRNA
molecule. The amino acids that are released by
the tRNA will join together to form a linear chain.
The sequence of amino acids is determined by the
sequence of triplets on the mRNA molecule.
28. STEPS IN TRANSLATION
1. Initiation
The small subunit of the ribosome binds to a site
"upstream" (on the 5' side) of the start of the message.
It proceeds downstream (5' -> 3') until it encounters
the start codon AUG. (The region between the mRNA
cap and the AUG is known as the 5'-untranslated
region [5'-UTR].)
Here it is joined by the large subunit and a
special initiator tRNA.
The initiator tRNA binds to the P site (shown in pink)
on the ribosome.
In eukaryotes, initiator tRNA carries methaionine.
29. 2. Elongation
An aminoacyl-tRNA (a tRNA covalently bound to its amino acid) able
to base pair with the next codon on the mRNA arrives at the A
site (green) associated with:
an elongation factor
GTP (the source of the needed energy)
The preceding amino acid is covalently linked to the incoming amino
acid with a peptidebond (shown in red).
The initiator tRNA is released from the P site.
The ribosome moves one codon downstream.
This shifts the more recently-arrived tRNA, with its attached peptide, to
the P site and opens the A site for the arrival of a new aminoacyl-tRNA.
This last step is promoted by another protein elongation factor and
the energy of another molecule of GTP.
Note: the initiator tRNA is the only member of the tRNA family that can
bind directly to the P site. The P site is so-named because, with the
exception of initiator tRNA, it binds only to a peptidyl-tRNA molecule;
that is, a tRNA with the growing peptide attached.
The A site is so-named because it binds only to the incoming aminoacyl-
tRNA; that is the tRNA bringing the next amino acid. So, for example,
the tRNA that brings Met into the interior of the polypeptide can bind
only to the A site.
30. 3. Termination
The end of translation occurs when the ribosome
reaches one or more STOP codons
(UAA, UAG, UGA). (The nucleotides from this
point to the poly(A) tail make up the 3'-
untranslated region [3'-UTR] of the mRNA.)
There are no tRNA molecules with anticodons for
STOP codons.
However, protein release factors recognize these
codons when they arrive at the A site.
Binding of these proteins —along with a molecule
of GTP — releases the polypeptide from the
ribosome.
The ribosome splits into its subunits, which can
later be reassembled for another round of protein
