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CHAPTER 12: DNAThe Blueprint of Life
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DNA Video
Importance of DNA video clip
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DNA = Deoxyribonucleic Acid
Role of DNA Molecule that stores
genetic information in living cells
Genetic information can be copied for new cells
Information is transmitted to new offspring
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12.2 Structure of DNA
Objectives Identify the chemical
components of DNA. Discuss the experiments
leading to the identification of DNA as the molecule that carries the genetic code.
Describe the steps leading to the development of the double-helix model of DNA.
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Components of DNA
DNA is a polymer of nucleotides These nucleotides have three parts:
Phosphate group 5 carbon sugar: deoxyribose Nitrogen base: A, G, C, T
A = adenine G = guanine C = cytosine T = thymine
Covalent bonds join nucleotides
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Check-in
What are the three roles of DNA?
DNA is a polymer of _______________. A nucleotide is made of what three
things?
What type of bond joins nucleotides together?
Storing, copying and transmitting genetic information
Phosphate group, deoxyribose sugar, nitrogenous base
Covalent bond
nucleotides
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Solving the Structure of DNA
Chargaff’s Rule – true for all organisms Amount of A = Amount of T Amount of C = Amount of G
Rosalind Franklin Used X-ray diffraction Took picture of DNA in 1952 Picture gave clues about DNA structure:
Strands twisted into helix (like coiled spring) Two strands in DNA Nitrogenous bases in center
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Solving the Structure of DNA
James Watson and Francis Crick Built 3-D models of DNA Shown Franklin’s Photo 51 Led to discovery of double helix shape in 1953
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Check-in
Describe Chargaff’s rule?
What clues did Rosalind Franklin’s picture give about the structure of DNA?
What method did she use to take the picture?
Looking at DNA in all organisms, amount of adenine equals amount of thymine and amount of guanine equals amount of cytosine.
DNA was helix shaped and contained two strands; bases are in the center of DNA molecule.
X-ray diffraction
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Who is credited with the discovery of the structure of DNA?
What method did they use to discover its structure?
James Watson and Francis Crick
Built three dimensional models.
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Double-Helix Model
DNA is shaped like a double helix Looks like: spiral staircase or twisted ladder
Sides = Phosphate groups and sugars Steps or rungs = Nitrogen bases Bases are held together by weak hydrogen
bonds
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Double-Helix Model
Stands are antiparallel – run in opposite directions
Which bases are complementary to each other? A with T G with C Called base pairing Explains Chargaff’s rule
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Structure of DNA
Chromatin, Chromosomes and DNA subunits video clip
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Check-in
What does a double helix look like?
What forms the backbone of DNA or sides of the ladder?
What forms the rungs of the ladder?
What type of bond holds the bases together?
Twisted ladder or spiral staircase
Sugar (deoxyribose) and phosphate groups.
Nitrogenous bases
Hydrogen bonds
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What base bonds with A (adenine)?
What base bonds with C (cytosine)?
What is the complementary DNA strand for the following: AGGTCT
T (thymine)
G (guanine)
TCCAGA
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12.3 DNA Replication
Objectives Summarize the events of DNA
replication. Compare DNA replication in
prokaryotes with that of eukaryotes.
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DNA Replication
What is it? Process by which chromosomal DNA is copied
before mitosis and meiosis
When does it happen? Interphase – S phase
Why does it happen? So each new cell gets a complete copy of DNA
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Steps of DNA Replication1. Strands of DNA
Separate: Hydrogen bonds between
bases are broken by an enzyme and the strands separate
2. Base Pairing: Free-floating nucleotides
pair up with bases on the exposed strand (matches with the complementary base – AT & CG)
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Steps of DNA Replication3. Bonding Nucleotides Backbone
(sugar/phosphates) of the once-free nucleotides are bonded together by an enzyme (DNA polymerase) to form a new complementary strand
4. Result of Replication Two DNA molecules, each
with a new strand and an old strandAnimation of DNA Replication
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DNA Replication
Role of enzymes One enzyme unzips the strands (breaks
hydrogen bonds) DNA polymerase creates sugar-phosphate
bonds and “proofreads” new DNA strand Telomerase adds short DNA sequences to
telomeres
Telomere
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Prokaryotes vs. Eukaryotes
Prokaryotes Single circular DNA molecule in cytoplasm Replication starts at one point and continues
in both directions
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Prokaryotes vs. Eukaryotes
Eukaryotes Chromosomes much bigger than prokaryotes Replication starts at many places and
continues in both directions
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Check-in
Describe the three steps of DNA replication?
What is the result of DNA replication?
What are the two functions of DNA polymerase?
1. DNA strands separate2. Free-floating nucleotides pair with exposed bases3. Sugar-phosphate backbone bonds together
Two DNA molecules, each with one new and one old strand
1. Bond sugar and phosphate together2. Proofread new DNA strand
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Check-in
What is the function of telomerase?
What is the major difference in DNA replication between prokaryotes and eukaryotes?
Add DNA sequences to tips of chromosomes to prevent genes from being lost or damaged during replication
In prokaryotic cells, replication starts in one spot on the chromosome, while in eukaryotic cells it starts in multiple spots.