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Chapter 12 DNA
12.1 The Substance of Genes
(p. 342-343)
A. Role of DNA (hereditary molecule)
1. Storing Information
Genes written into DNA control development of each trait (ex: eye color gene)
2. Copying Info
Every gene must be copied b4 division
3. Transmitting Info
Genes made of DNA are sorted and passed on w/cell division/each generation
Section 12.2 A. Structure of DNA
1. Nucleic Acids (NA) and Nucleotides
Acidic macromolecules
Nucleotides are the building blocks (subunits) of DNA
Deoxyribose (5C sugar)
Phosphate group
Nitrogenous base (molecule w/Nitrogen)
2. Nitrogenous Bases & Covalent Bonds
Contain nitrogen
4 bases: Adenine/Guanine (2 rings) and Cytosine/Thymine (1 ring); A; G; C; T
Form covalent bonds between sugar-phosphate
Bases absorb UV light=tells amt of DNA in cell
B. Structure of DNA 1. Chargaff’s Rule: Amt of (A)=(T) Amt of (G)=(C) 2. Franklin’s X-rays diffraction X-rayed purified, stretched DNA 2 twisted strands w/ N-bases in middle 3. Watson and Crick: used Franklin’s work to build 3D-model of DNA
C. Double Helix Model
1. Backbone: sugar/phosphate
2. Rungs: N-base
3. Antiparallel strands: “mirror”
opposite strand arrangement
4. Hydrogen Bonding: weak
center bond between
N-bases (permits replication)
5. Base pairing=Chargaff’s Rule
A H-bonds w/T; G H-bonds w/C
12.3 DNA Replication
A. Copying the Code
1. Base pairing permits DNA to copy
Bases pair w/only 1 other base
on opposite strand
Complementary strands (mirror
images) contain info to
reconstruct other strand
2. The Replication Process & Enzymes Replication: DNA copied during S phase of Interphase
Old strand “template” for 2 new complementary strands
Enzymes help:
1. Helicase unzips
2. DNA Polymerase adds complementary
N-bases (also “proofreads” so no mistakes)
Old: TACGTT/New: ATGCAA
2 identical DNA molecules (1/2 old:1/2 new)
3. Telomeres
Repeated DNA code at tips of chromosome
Telomerase builds telomere tip and prevents gene damage or loss during replication
Switched off in adulthood
Cancers can activate (w/rapid division)
B. Replication in Living Cells
1. Prokaryotic DNA Replication
Regulatory proteins initiate S phase
Single start point proceeding in two directions
2 attached identical chromosomes (split w/division)
2. Eukaryotic DNA Replication
1000 x’s more DNA than prokaryotes
Chromosomes made of chromatin=DNA coiled around histones
Multiple start points
Proteins check for damage (not foolproof-change may alter gene)
Copied chromosomes remains attached until Anaphase
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