CH 14 - DNA DOUBLE HELIX
I. INTRODUCTION
A. CARRIER OF TRAITS
1. DNA confirmed as genetic material: 1940’s – 1950’s (p. 256-258)
5. DNA structure - 1953 Watson & Crick (p. 261)
6. Genetic code cracked - 1960's
B. FLOW OF GENETIC INFORMATION
Replication « DNA (transcription) è RNA (translation) è protein
C. NUCLEIC ACIDS (p. 259)
1. Building blocks = nucleotides
2. Nucleotides = a. 5C sugar
b. phosphate group
c. nitrogenous base
3. DNA vs RNA: (p. 259)
DNA RNA
a. Sugar Deoxyribose OH, H Ribose OH, OH
b. Bases A,C,G,T A,C,G,U
c. Strands Double Single
4. Base pairing: Purines (2 rings) Pyrimidines (1 ring)
(p. 262) Adenine Cytosine
Guanine Thymine
5. Chargaff's rule (p. 260)
a. Know % of 1 base, know % of all.
b. If 20% is A, what is % of other 3?
20% T; 30% C; 30% G
D. MOLECULAR ORGANIZATION OF DNA (p. 262)
1. Complementary - base pairing: A-T; C-G
2. Double helix = "twisted ladder w/cross rungs"
a. ladder = i. sugar w/ 3' OH end
(p. 261) ii. phosphate w/ 5' end
iii. phosphodiester bonds – strong!
b. cross rungs = base pairs linked by H-bonds (p. 262)
i. A=T; 2 H-bonds
ii. C=G; 3 H-bonds
3. Antiparallel - arrangement of strands
II. DNA REPLICATION
A. INTRODUCTION (p. 263)
1. DNA rep. is SEMICONSERVATIVE
a. each strand a template
b. 1/2 of each new DNA is from "parent" DNA
c. 1/2 of each new DNA is newly synthesized
d. not CONSERVATIVE-"parent" DNA intact, completely new DNA
e. not DISPERSIVE-mixed old & new
2. Entire DNA molecule replicated
3. Result is 2 EXACT copies of original
B. STEPS OF REPLICATION - Begins at origin, bidirectional (p. 266)
REPLISOME = 5 enzymes involved:
1. Helicase - cuts H-bonds & unwinds (p. 269)
2. Gyrase - relieves torque or supercoiling (p. 267)
3. RNA primase - begins RNA primer
4. DNA polymerase - a. DNA is "read" 3' è5'; DNA is assembled 5'è 3'
b. adds new complementary nucleotides 1 by 1
c. RNA primer removed, correct DNA nucleotides added
5. DNA ligase - joins fragments together
6. Continuous/leading strand (p. 267 bottom)
7. Discontinuous/lagging strand - Okazaki fragments
8. DNA polymerase proofreads & corrects any errors (p. 269 bottom; p. 274 bottom)
C. PROBLEM: Rate of replication
1. Human chrom #1 has 285,000,000 b.p.
2. DNA polymerase assembles at approx.1,000 nucleotides/second
3. 800 hours to replicate! (actually just 8 hours) HOW?
4. Multiple (100's) of REPLICONS = REPLICATION BUBBLES (p. 271)
D. TELOMERES - @ ends of chromosomes, not replicated, “aging”
– like sand running
Through an hourglass (p. 272 – 273)
E. FACTS
1. DNA from 1 human cell - 46 chrom - 2 meters
2. IMMENSE info carrying capacity
a. 6 billion base pairs!
b. 600,000 printed words @ 500 words/page
c. library of 1,000 books
3. Each b.p. site w/ 4 possible nucleotides: A,C,G,T
a. 1 site = 41 = 4 possible combos
b. 2 sites = 42 = 16 " "
c. 3 sites = 43 = 64 " "
4. Short DNA (100 b.p.) = 4100 possible sequences
5. Human chrom #1 = 4 285,000,000 possible sequences!!!
6. Yet upwards of 90-99% of our DNA is "nonsense", >10% codes for proteins!