campbell and reece chapter 16 the molecular basis of inheritance
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CAMPBELL AND REECECHAPTER 16
The Molecular Basis of Inheritance
The Search for Genetic Material
once Morgan proved genes are in chromosomes big debate started:
Is the genetic material in chromosomes the DNA or the proteins?
@ first case for proteins seemed stronger very heterogenous great specificty
Evidence that DNA can Transform Bacteria
1928 Griffith studied Streptococcus pneumoniae
Transformation
term coined by Griffithchange in genotype & phenotype due to
the assimilation of external DNA by a cell
Avery spent the next 14 years identifying the “transforming agent”
Avery’s Experiment
Avery
& his colleagues announced DNA was the transforming agent
many were skeptical Was bacterial DNA anything like eukaryotic
DNA? Nothing much known about DNA Most scientists held to belief that proteins
had to be transforming agent
Bacteriophages
are viruses that infect bacteria“phages” for shortVirus made of a protein coat covering
genetic material to produce more viruses it must invade a cell & take over the cell’s metabolic machinery
Hershey & Chase Experiment
Additional Evidence that DNA is Genetic Material
Chargraff already knew DNA made up of:
Deoxyribose Phosphate group Nitrogenous Base (A, G, C, T)
analyzed DNA from # of species 1950: base composition of DNA varies
between species made DNA more credible
Chargraff’s Rule
no matter what the source of DNA tested:
What is the structure of DNA?
early 1950’s: scientists convinced DNA carried genetic info
focus now on DNA’s structure
knew arrangement of DNA’s covalent bonds
Watson & Crick
Cambridge, England2 young, unknown scientistssame lab: Franklin & Wilkins doing x-ray
crystallography on protein structure
X-Ray Crystallography of DNA
Rosalind Franklin had purified some DNA and showed results to Watkins who was familiar with pattern made by a helical structure
Watson & Crick
began building models that satisfied: known chemical properties of DNA
nitrogenous bases relatively hydrophobic phosphate groups carry (-) charge
Chagraff’s rules helical structure how could this structure pass on genetic
information?
DNA Structure
antiparallel: arrangement of sugar-phosphate backbones in a DNA double helix
means 1 strand runs 5’ 3’ going “up” * the other runs 5’ 3’ going “down”
DNA Structure
because of size differences in dbl ringed purines vs. single ringed pyrimidines Watson & Crick knew could not have a purine linked with itself or the other purine
also knew that adenine & thymine could form H bonds (2) with each other & cytosine & guanine could for 3 H bonds
Watson & Crick
their 1 page paper published in Nature in April 1953
Watson, Crick, and Wilkins received Nobel Prize in 1962 (Franklin died in 1958)
DNA Replication
Watson and Crick’s 2nd paper stated their hypothesis on how DNA replicates:
DNA model is pair of complimentary templates
prior to replication H bonds broken & chains separate & unwind
each chain then acts as template for formation onto itself of a new complimentary chain
allows for exact duplication
DNA Replication
Watson & Crick’s Semiconservative Model of DNA Replication
predicts when a dbl helix replicates, each of the 2 daughter molecules will have 1 old strand and 1 new strand
Conservative Model: 1 new daughter molecule with 2 new strands & the original molecule
Dispersive Model: all 4 strands of DNA after replication have mixture of old & new parts
3 Models of DNA Replication
DNA Replication
begins @ particular sites called:Origins of Replication
short stretches of a specific sequence of nucleotides
many bacterial loops of DNA have single origin
proteins that initiate DNA replication recognize the sequence / attach to the DNA / separate the 2 strands by breaking H bonds creating “bubbles”
Prokaryotic Replication of DNA
Eukaryotic DNA Replication
Replication Bubble
Replication Forks
@ each end of the replication bubbleY-shaped region where DNA is unwindingproteins that participate in the unwinding:1. helicases
unwind double helix
2. single-strand binding proteins bind to single strands prevents them from
rewinding
3. topoisomerases untwisting dbl helix puts strain on ahead of
replication fork, these proteins relieve strain by breaking, swiveling, & rejoining DNA strands
Replication Forks
Replication of DNA
initial nucleotide chain made during DNA synthesis is actually a strand of RNA
this RNA chain called a primer which is made by an enzyme called primase (last slide)
primase starts a complementary RNA chain from a single RNA nucleotide then adds 1 @ time
Primers
when primer 5 – 10 nucleotides long...new DNA strand will start from the 3’ end of the RNA primer
DNA Polymerase
enzyme that catalyzes the synthesis of new DNA by adding nucleotides to a pre-existing chain 2 major one in prokaryotes 11 different ones in eukaryotes
most require a primer & DNA template strand
rate: ~500 nucleotides/s in bacteria ~ 50 nucleotides/s in human cells
Source of Nucleotides
are in form of nucleoside triphosphates
dATP
Nucleoside Triphosphates
are chemically reactive (like ATP, except sugar is deoxyribose, not ribose)
as each nucleotide joins the growing end of a DNA strand 2 of the phosphate groups are lost as a molecule of inorganic phosphate in a couple exergonic reaction that drives the polymerization reaction
Polymerization Reaction
Antiparallel Elongation
each strand of DNA has directionality (1-way street)
& each strand oriented in opposite directions to each other
DNA polymerase III can add nucleotides only to the free 3’ end of a primer or growing DNA strand along 1 template DNA polymerase
synthesizes complementary strand continuously (5’ 3’ direction)
called the Leading Strand
Antiparallel Elongation
along opposite strand because of orientation, DNA polymerase III must work in direction away from the replication fork
called Lagging Strand synthesized in short segments called:
Okazaki Fragments ~ 1,000 – 2,000 nucleotides long in E. coli ~ 100 – 200 nucleotides long in eukaryotes
DNA Replication Complex
easy to think of DNA polymerase as a locomotive moving down template track but not really how it works:
1. various proteins that participate in DNA replication form a large complex
2. DNA replication complex doesn’t move, the DNA template moves thru the complex
DNA Replication Complex
Proofreading & Repairing DNA
~ 1/10 billion base pairs in completed DNA will be incorrect
but right after strands replicated errors ~ 100,000 times more common
DNA polymerases “proofread” each nucleotide against its template as soon as it is added
when error found, incorrect nucleotide removed, correct 1 inserted
Proofreading & Repairing
some errors evade DNA polymerase…other enzymes remove & replace incorrectly paired nucleotides
some errors arise after replication: damage to DNA relatively common: usually corrected by b/4 becoming permanent mutations
cells continuously monitor & repair damaged DNA
Repair Enzymes
~100 in E. coli~ 130 in humansmost organisms use same mechanism to
repair errors or damage involves cutting out damaged area using
DNA-cutting enzyme called nuclease gap then filled with correct nucleotides done
by a DNA polymerase & DNA ligase 1 of these systems called nucleotide excision
repair
Nucleotide Excision Repair
Telomeres
repetitive sequences @ ends of eukaryotic chromosomes shorter as we age (with each round of DNA
replication) so preserves the ends of linear DNA &
postpones erosion of genes telomerase catalyzes the lengthening of
telomeres in germ cells
Telomeres
Prokaryotic DNA
usually loop of DNAsome associated proteinsloop of DNA + proteins = nucleoid
Eukaryotic Chromatin
includes:1. DNA2. histones3. other proteins
Nucleosomes
histones bind to each other & to the DNA to form nucleosomes: the most basic unit of DNA packing
histone tails extend outward from each bead-like nucleosome cone
additional coiling & folding highly condensed chromosome as seen in mitosis
Euchromatin: term for the less coiled chromatin seen in interphase cells easily accessible for transcription
Heterochromatin: portions of chromatin that remains highly condensed even in interphase mostly inaccessible to transcription
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