Download L2 - DNA Replication and Transcription

The Central Dogma:
DNA Replication and Transcription
Importance of DNA
So, why is DNA important?
• The long strands of DNA in the nucleus of a
cell contains the genetic information for every
characteristic of an organism.
• We now know that the characteristics of an
organism are dependent upon the proteins
that the organism synthesizes.
• Therefore, DNA must hold the information
needed to synthesize the proteins of living
things.
Importance of DNA, cont.
The Central Dogma
• DNA does not direct the synthesis of
carbohydrates, lipids or other non-protein
molecules essential for life; however, these
other materials are manufactured by the cell
through reactions made possible by the
specificity of enzymes (proteins) produced
under the direction of DNA.
• The accepted principle stating that genetic
information contained in DNA molecules is
copied into daughter cells as well as
transferred to RNA molecules that direct the
synthesis of protein molecules in living
organisms is called The Central Dogma.
• The process outline follows:
• A gene is defined as a segment of a DNA
molecule that directs the synthesis of one
kind of protein molecule.
1. Replication (copying) of DNA
• Human DNA encodes approximately 35,000
genes.
3. Translation (“change language”) of RNA to
synthesize proteins primary structure
The Flow of Genetic Information According to
The Central Dogma
• Genes are segments of DNA that contain
information necessary for the synthesis of
proteins according to the following sequence:
For the
passing of
genetic
information
to future
generation
cells
For the
production of
proteins
2. Transcription of DNA (rewriting) to RNA
DNA Replication
• Because DNA contains the genetic
information for living things it is important
that an exact copy can be made to pass on
to the next generation of cells.
• Watson and Crick proposed along with
their model of DNA a model for the
replication of DNA molecules in the nucleus
called semiconservative replication.
• Semiconservative replication - new DNA
molecule is formed which has one strand from
the parent nucleic acid along with a new
complementary strand.
1
DNA Replication, cont.
DNA Replication, cont.
• Once the replication fork is
formed (1), base paring of
new nucleotides form new
DNA strands under the
direction of DNA
polymerase (2).
Occurs in three steps:
Step 1: Unwinding of the DNA by helicase
and formation of a replication
fork or replication bubble.
Step 2: Synthesis of DNA strands and
broken segments, called
Okazaki fragments, by DNA
polymerase between replication forks.
• Notice that since the new
DNA strands must form
from the 5’ to 3’ end, one
strand is duplicated
uninterrupted while the
other is formed in small
fragments (Okazaki
Fragments).
Step 3: Joining the Okazaki fragments by
DNA ligase.
DNA Replication, cont.
•The fragments
are then joined
together the DNA
ligase enzyme (3).
DNA Replication, cont.
• DNA replication usually occurs
at multiple sites within a
molecule, and the replication is
bi-directional from these sites.
•The result is two
daughter DNA
molecules with
identical base
sequences; each
also having one of
the two original
DNA strands
Transcription
• DNA replication ensures that DNA
molecules can be duplicated for the passing of
genetic material from cell to cell. However,
within the cell, DNA’s primary role is to code
for protein production.
• Transcription is the process where the RNA
polymerase enzyme catalyzes the synthesizes
mRNA from a gene sequence of DNA.
• Transcription differs for prokaryotic and
eukaryotic cells.
Transcription, cont.
• Prokaryotic transcription occurs in three
steps:
1. Unwinding of DNA helix
2. Synthesis of mRNA starting at an initiation
sequence in the 5’ to 3’ direction, moving
along the DNA strand in the 3’ to 5’
direction, until RNA polymerase reaches a
termination sequence. (U replaces T on the
RNA strand as it is transcripted from DNA)
3. Separation of mRNA and rewinding of
DNA helix
2
Transcription, cont.
Transcription, cont.
• Prokaryotic vs. eukaryotic transcription.
DNA in prokaryotic cells codes directly
for amino acid sequencing
DNA in eukaryotic cells contains regions
of bases that do not code for proteins
called introns. The sections that do code
for amino acids are called exons.
It is still unclear what the purpose for
introns due to the fact they are cut out
during the formation of mRNA.
Transcription, cont.
• Transcription in eukaryotic cells first produces
hnRNA, or heterogeneous nuclear RNA. The
hnRNA then undergoes a series of enzymecatalyzed reactions that cut and splice the
hnRNA to produce mRNA containing only the
series of bases that code for amino acids
The Genetic Code
• The nucleosides of the mRNA molecule must
somehow contain a code to direct the
attachment of the 20 alpha amino acids in
sequence to form proteins.
• Since there are only 4 bases in RNA
(remember, U replaces T during transcription)
there must be more than one base to code for
one amino acid.
• It was discovered that three nucleotide bases
code for one amino acid. Each sequence of 3
nucleotide bases is called a codon.
The Genetic Code, cont.
• On the DNA molecule ATG, transcribed as
AUG on mRNA, codes for the initiation of a
protein sequence. AUG is the codon for
methionine which is usually cleaved after
protein synthesis.
• Most amino acids are represented by more than
one codon, any of which will signal the addition
of the amino acid to the protein chain.
• Termination is coded by UAA, UAG, and UGA.
• Code is considered universal for all organisms.
The Genetic Code, cont.
Summary:
3
The Genetic
Code, cont.
•3 bases / codon
•4 bases
•Therefore:
43 = 64 possible
combinations
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