Chapter 6

Consists of
 Deoxyribose sugar
 Phosphate group
 A, T, C, G

Double stranded molecule (Double
Helix)
 Two strands of DNA run antiparallel to
each other (opposite direction)
 5’ to 3’
 5’ is the end with the phosphate group
 3’ is where deoxyribose sugar is located

Nitrogenous bases
 Held together by hydrogen bonds
 A pairs with T ( forms double bond)
 C pairs with G (forms a triple bond)
DNA
Four Requirements for DNA to be
Genetic Material
Must carry information
 Cracking the genetic code
Must replicate
 DNA replication
Must allow for information to change
 Mutation
Must govern the expression of the
phenotype
 Gene function
DNA Replication
Process of duplication of the
entire genome prior to cell
division
S
Biological significance


extreme accuracy of DNA
replication is necessary in
order to preserve the integrity
of the genome in successive
generations
In eukaryotes , replication
only occurs during the S
phase of the cell cycle.
phase
G1
interphase
Mitosis
-prophase
-metaphase
-anaphase
-telophase
G2
Basic rules of replication
A.
B.
C.
D.
E.
Semi-conservative
Starts at the ‘origin’
Synthesis always in the 5-3’ direction
Semi-discontinuous
RNA primers required
Mechanism of DNA Replication

Step 1: Strand Separation
 Proteins bind to DNA and open up double helix
 Prepare DNA for complementary base pairing

Step 2: Building Complementary Strands
 Proteins connect the correct sequences of
nucleotides into a continuous new strand of DNA

Step 3: Dealing With Errors during DNA
Replication
 Proteins release the replication complex
DNA Replication is
Semi-Conservative
Separating the two parent
strands and building new
complementary strand for
each
 New DNA has one new
strand and one old strand

Strand Separation

Double Helix
 Unwound at replication origins (many origins on DNA)
 Enzyme called helicase binds to origins and unwinds
the two strands creating replication bubbles
 Two strands separating creates a replication fork
Strand Separation

Unwinding DNA creates tension
 Enzymes called topoisomerases relieves tension by
cutting strands near the replication fork (supercoil)

Single strands want to join back together
 Prevented by single-strand binding proteins (SSBs)
by attaching to the DNA strands stabilizing them
Topoisomerase
Enzyme
DNA
Enzyme
Strand Separation
Multiple replication origins
decrease the overall time
of DNA replication to
about 1 hour
Building Complementary Strands

DNA polymerase III
 Adds nucleotides to
the 3’ end of a strand
 New strands are
always assembled 5’
to 3’
 Builds new strand
using nucleoside
triphosphates
Building Complementary Strands

RNA primase begins the replication process
 Builds small complementary RNA segments on strand
at beginning of replication fork
 RNA primers
 DNA polymerase III can start to add nucleotides
Building Complementary Strands

Leading Strand
 DNA that is copied
in the direction
toward the
replication fork

Lagging Strand
 DNA that is copied
in the direction
away from the
replication fork
Leading and Lagging Strands
3
5
5
3
DNA polymerase III
leading strand
5
3
3
5
3
5
5
5
3
lagging strand
3
5
3
5
lagging strand
5
5
leading strand
growing
replication fork 5
3
growing
replication fork
leading strand
3
lagging strand
5 5
5
5
3
Building Complementary Strands
Anti parallel strands replicated simultaneously
 Leading strand synthesis continuously in 5’–
3’
 Lagging strand synthesis in fragments in 5’-3’
Leading Strand
Single primer is used to start strand
 DNA polymerase III moves towards
replication fork 5’ to 3’ direction
 Continuous

Lagging Strand
DNA polymerase III moves away from replication
fork
 Discontinuous
 Okazaki fragments are used to solve problem

 1000 – 2000 base pairs long

Multiple primers are used
Lagging Strand
DNA polymerase I removes RNA primers and
replaces with DNA nucleotide
 Fills the gaps

Building Complementary Strands

DNA ligase
 Links last nucleotide to Okazaki fragment
 Formation of phosphodiester bond
Dealing With Errors

DNA polymerase
 Proofread and correct
errors
 Errors are usually base
pair mismatches

After replication
 Average of 1 error per
million base pairs

DNA polymerase II
 Repairs damage after
strands have been
synthesized
Chromosome Erosion
DNA polymerases can
only add to 3 end of
an existing DNA strand
DNA polymerase I
5
3
3
5
5
growing
3
replication fork
DNA polymerase III
5
Loss of bases at 5 ends
in every replication
DNA polymerase I cannot replace final RNA primer
3
Does it Create a Problem?
Telomeres
Repeating, non-coding sequences at the end
of chromosomes = protective cap

limit to ~50 cell divisions
5
3
3
5
5
growing
3
replication fork
telomerase
5



enzyme extends telomeres
can add DNA bases at 5 end
different level of activity in different cells
 high in stem cells & cancers -- Why?
TTAAGGG TTAAGGG 3
Cells Aging Process

Cell senescence
 Cells loses ability to function
properly as a person ages

Decrease in telomeres
with age
 No longer provide protection
for the chromosome
Known as the Hayflick limit
 Possibly links to agerelated diseases

 Dementia, atherosclerosis,
macular degeneration
Packing of Eukaryotic DNA

Organization
 Negative DNA wraps around positive histones
 Nucleosome – cluster of 8 histones
 Solenoids – coiled strings of nucleosomes (chromatin
fibres)
Prokaryotic DNA Organization

Eubacteria/Archaea DNA
 One chromosome – circular
in shape
 Unbound by a nuclear
membrane
Genetic Variation Among Bacteria

Plasmids
 Smaller circular pieces of DNA that float throughout cell

Conjugation
 Plasmids are able to exit one cell and enter another
(when two bacteria are close)

Useful in genetic engineering