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Control of Gene Expression
Pieces of Chapter 16
Pieces of Chapter 17
Pieces of Chapter 18
Objectives
• Understand the process of DNA replication
• Understand why DNA is synthesized from the 5’
end to the 3’end
• Recognize the function of telomeres
• Understand how protein structure and function are
affected by genetic mistakes
• Be familiar with the kinds of mutations that may
occur during replication of DNA
• Understand the role of an operon
• Be aware that gene expression can be regulated at
many points from DNA to polypeptide synthesis
DNA Replication
• DNA replication is semiconservative in that each
new molecule incorporates an old strand that serves
as a template
• Requires many enzymes for assistance
• Few mistakes (~1/billion nucleotides)
Semiconservative Replication
• Early works tested
several potential
methods of replication
• Matthew Meselson and
Frank Stahl determined
that DNA replication
was semiconservative
DNA Polymerase
• Enzymes called DNA
polymerases are
responsible for the
assembly of DNA
• These enzymes
convert nucleoside
triphosphates into
linked nucleotides
through their action
Five principle proteins are used in
the synthesis of DNA
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DNA Replication: The Process
•
Origins of Replication: regions on the DNA where
synthesis begins
• Synthesis occurs in both directions of the “bubble”
along the replication fork (site of DNA elongation)
1. Helicase: responsible for unwinding the DNA
–
Topoisomerases prevent torque
2. Single-strand binding proteins: keep original
complimentary strands separated
DNA Must Be Primed
•
DNA Polymerase III is
unable to replicate DNA
directly and requires that
the original DNA be
primed
3. Primase makes the initial
nucleotide (RNA primer)
to which DNA polymerase
attaches
• RNA primer is replaced
with DNA nucleotides later
by DNA Polymerase I
DNA Replication
4. Elongation of DNA is
catalyzed by DNA
Polymerase III and
driven by the hydrolysis
of phosphate groups
from nucleoside
triphosphates added to
the 3’ end hydroxyl
group of the growing
molecule
DNA Strands are Antiparallel
• New DNA “grows” from 5’3’ as DNA Polymerase III only
adds nucleotides to the 3’ end of the DNA strand.
Continuously synthesized piece is called the leading strand.
• Okazaki fragments, short pieces of discontinuously
synthesized DNA, are formed and joined together by (5) DNA
ligase to form the lagging strand of DNA
Leading Strand Synthesis
Lagging Strand Synthesis
Both Together
• http://www.youtube.com/watch?v=49f
mm2WoWBs
Other things to consider
• DNA polymerase cannot
synthesize the extreme ends of
a DNA molecule
• Gradual shortening with each
replication could lead to
deletion of important
information
• Telomerase adds many copies
of TTAGGG nucleotide
sequence (Telomere) to ends of
DNA
• Telomerase is usually only
found in germ cells and sex
cells
• Presence in cancerous cells may
lead to proliferation of tumors
Other things
to consider
• Placement of mismatched
nucleotides during synthesis
is not rare and is repaired
immediately by DNA
Polymerase III. DNA
polymerase I can repair
“uncaught” mistakes through
a mechanism called
mismatch repair
• Excision repair takes place
in DNA to repair damaged
DNA (not related to
replication) that could
eventually lead to problems
Mutations:
Changes in the
genetic material of
a cell
• Point mutations:chemical changes in just a single or a
few base pairs in a gene
– Base-pair substitutions: replacement of one nucleotide with
another
• Silent
• Missense
• Nonsense
– Insertion/Deletion: change in the number of nucleotide pairs
• Frame shift
Sickle Cell Disease
Controlled Expression: Operons
• Genes that are used together are often found associated
(linked) on the same chromosome and may require a
single promoter for transcription
• An Operator may regulate transcription by interaction
with a repressor protein controlled through allosteric
regulation
• An Operon is the entire stretch of DNA required for the
synthesis of enzymes in a specific enzymatic pathway
(Operator, promotor, and genes)
We have only scratched the surface of gene
expression. Regulatory mechanisms may occur
at many different stages from DNA to Protein
-methylation: leads to inactivation of a gene
-histone acetylation: ease of transcription
-Transcription factors: enable transcription to occur
-intron/exon regulation
-modification of mRNA
-degradation of mRNA: elimination is eminent
-inhibition/activation of ribosome binding
-processing of protein/assembly of subunits
-ubiquitin enhances degradation