chapter 6 dna consists of deoxyribose sugar phosphate group a, t, c, g double stranded molecule...
TRANSCRIPT
DNA: HEREDITARY MOLECULES OF
LIFE
Chapter 6
DNA Consists of
Deoxyribose sugarPhosphate groupA, 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 group3’ is where deoxyribose sugar is located
Nitrogenous basesHeld together by hydrogen bondsA pairs with T ( forms double bond)C pairs with G (forms a triple bond)
Four Requirements for DNA to be Genetic Material
Must carry informationCracking the genetic code
Must replicateDNA replication
Must allow for information to changeMutation
Must govern the expression of the phenotypeGene function
DNA ReplicationProcess of duplication of
the entire genome prior to cell division
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.
Mitosis-prophase-metaphase-anaphase-telophase
G1 G2
Sphase
interphase
Basic rules of replication
A. Semi-conservativeB. Starts at the ‘origin’C. Synthesis always in the 5-3’ directionD. Semi-discontinuous E. RNA primers required
Mechanism of DNA Replication
Step 1: Strand SeparationProteins bind to DNA and open up double helixPrepare DNA for complementary base pairing
Step 2: Building Complementary StrandsProteins connect the correct sequences of
nucleotides into a continuous new strand of DNA Step 3: Dealing With Errors during DNA
ReplicationProteins 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 HelixUnwound at replication origins (many origins on DNA)Enzyme called helicase binds to origins and unwinds
the two strands creating replication bubblesTwo 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 togetherPrevented 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 strandNew 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 primersDNA polymerase III can start to add nucleotides
Building Complementary Strands Leading Strand
DNA that is copied in the direction toward the replication fork
Lagging StrandDNA that is copied
in the direction away from the replication fork
Leading and Lagging Strands
DNA polymerase III
5
3 5
3
leading strand
lagging strand
leading strand
lagging strandleading strand
5
3
3
5
5
3
5
3
5
3 5
3
growing replication fork
growing replication fork
5
5
5
5
53
3
5
5lagging strand
5 3
Building Complementary StrandsAnti 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 fragmentFormation 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
Loss of bases at 5 ends in every replication
DNA polymerase I cannot replace final RNA primer
DNA polymerase III
DNA polymerases can only add to 3 end of an existing DNA strand
Chromosome Erosion
5
5
5
5
3
3
3
3
growing replication fork
DNA polymerase I
Does it Create a Problem?
Repeating, non-coding sequences at the end of chromosomes = protective cap
limit to ~50 cell divisions
enzyme extends telomeres can add DNA bases at 5 end different level of activity in different cells
high in stem cells & cancers -- Why?
telomerase
Telomeres
5
5
5
5
3
3
3
3
growing replication fork
TTAAGGGTTAAGGGTTAAGGG
Cells Aging Process Cell senescence
Cells loses ability to function properly as a person ages
Decrease in telomeres with ageNo longer provide protection
for the chromosome Known as the Hayflick limit Possibly links to age-
related diseasesDementia, atherosclerosis,
macular degeneration
Packing of Eukaryotic DNA OrganizationNegative DNA wraps around positive histonesNucleosome – cluster of 8 histonesSolenoids – coiled strings of nucleosomes (chromatin
fibres)
Prokaryotic DNA Organization
Eubacteria/Archaea DNAOne chromosome – circular
in shapeUnbound 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