Chapter 12 DNA.pdf

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