dna structure and function. watson and crick’s dna model
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DNA Structure and Function
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Watson and Crick’s DNA Model
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Discovery of DNA Structure
DNA consists of two strands of nucleotides, coiled into a double helix
Each nucleotide has • A five-carbon sugar (deoxyribose)• A phosphate group• A nitrogen-containing base (adenine, thymine,
guanine, or cytosine)
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Base Pairing
Bases of two DNA strands pair in only one way• Adenine with thymine (A-T)• Guanine with cytosine (G-C)
The DNA sequence (order of bases) varies among species and individuals
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The DNA Double Helix
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Key Concepts: THE DNA DOUBLE HELIX
A DNA molecule consists of two chains of nucleotides, hydrogen-bonded together along their length and coiled into a double helix
Four kinds of nucleotides make up the chains: adenine, thymine, guanine, and cytosine
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Watson, Crick, and Franklin
Rosalind Franklin’s research produced x-ray diffraction images of DNA• Helped Watson and Crick
build their DNA model, for which they received the Nobel Prize
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DNA Replication and Repair
A cell replicates its DNA before dividing• Enzymes unwind the double helix • DNA polymerases assemble complementary
DNA strands on templates from free nucleotides• DNA ligase seals gaps in new DNA strands
Two double-stranded DNA molecules result• One strand of each is new
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Fig. 12.8, p.192
new
newold
old
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Fig. 12.9, p.193
Any gaps between basesof the “new” DNA are joinedto form a continuous strand.The base sequence of eachhalf-old, half-new DNAmolecule is identicalto that of the parent.
Part of a parentDNA molecule, with twocomplementary strands ofbase-paired nucleotides.
Replication starts.The strands are unwoundat many sites alongthe molecule’s length.
Each of the two parentstrands guides the assemblyof new DNA strands from freenucleotides, according tobase-pairing rules.
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Key Concepts: HOW CELLS DUPLICATE THEIR DNA
Before a cell divides, enzymes and other proteins copy its DNA
Newly forming DNA strands are monitored for errors, most of which are corrected
Uncorrected errors are mutations
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Cloning
Clones• Genetically identical individuals• Produced by artificial twinning, nuclear transfers
To clone an adult animal• Cell’s DNA must be reprogrammed to function
like an embryonic cell and direct development
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Nuclear Transfer