Chapter 11 DNA and Its Role in Heredity

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Chapter 11 DNA and Its Role in
Heredity
Biology 101
Tri-County Technical College
Pendleton, SC
Start with a Review
Generic nucleotide contains 5-carbon sugar,
one of four nitrogenous bases, and a
phosphate group
DNA has deoxyribose and ATGC
RNA has ribose and AUGC
Numbering carbons in ringed sugar…
“At the Grand Canyon” will save your sack
lunch more times than you care to count…
Dr. Ruth is right on…
Remember how to manipulate the
mnemonic device and pairing and size will
not be a problem
Chalk talk time on ATGC
Purines are double-ringed nitrogenous
compounds
Pyrimidines are single-ringed nitrogenous
compounds
World will never be the same…
Griffin’s transformation experiments
Hershey & Chases bacteriophage work
Watson and Crick
**Chargaff’s Rule
Strands always equidistant; 2 H bonds between A
& T, and 3 H bonds between G & C
Known as “complementary base pairing”
Specific pairing is KEY to replicating genetic
material
DNA Visual
Structure of DNA
DNA is double-stranded helix
Right-handed; twists to the right
Stands are antiparallel
Covalent bonds link sugar, base, and
phosphate
Hydrogen bonds holds strands together
Remember, H bonds NOT very strong, but
DNA has so many, it is quite stable
DNA Replication
Strands separate and each strand serves at template
(mold) for making new complementary strand
When replication is over…there will be TWO
molecules of DNA
Each molecule will be composed of one old strand
and one new strand
Meselson and Stahl’s work confirmed
semiconservative replicaiton is correct model
Replication Requirements
DNA must act as template for
complementary base pairing
Four deoxyribonucleoside triphosphates
(dATP, dTTP, dGTP, dCTP), must be
present
DNA polymerase needed to bring substrates
to template and catalyze reaction
Source of chemical energy needed
Process of Replication
Occurs in two stages
DNA locally denatured (unwound)
New nucleotides linked by covalent bonding to each
growing strand
Current model suggests huge protein replication
complex and DNA moves through the complex
All chromosomes have at least one sequence of
nucleotides recognized by replication complex
Process, cont.
Sequence called origin of replication
Forms replication bubble with 2 replication
forks
DNA helicase opens up double helix
Single-stranded binding proteins keep two
strands separated
RNA primase makes primer strand needed
to get replication underway
Process, cont.
DNA polymerase III adds nucleotides to primer,
proofreads the DNA, and repairs it
Leading/lagging strands and Okazaki
fragments
DNA Polymerase I responsible for removing
RNA primer and replacing it with DNA
DNA ligase responsible for linking Okazaki
fragments together and for linking any repaired
strands back together
Replication Visual
Replication Forks Visual
Lagging Strand Visual
Proofreading and Repair
DNA polymerase(s) “proofs” each
nucleotide as it is added to growing strand
Repair enzymes designed for 2 basic
functions
Mismatch repair (should be AT and is AC
for example)
Excision repair works for mismatched
pairs, chemically modified bases, or points
where strand has more bases than the other
Proofreading, cont.
Cuts out errors, DNA polymerase and DNA
ligase synthesize and seal up new piece to
replace the excised one
DNA ligase responsible for linking
segments of strand back together after
repairs are made
Error rate of one base in 106; after repair,
the error rate reduced to one base in 109
Proofreading Visual

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