1. 12.2 Replication of DNA
DNA replication is the process of copying a DNA molecule.
Semiconservative replication - each strand of the original double helix (parental molecule) serves as a template (mold or model) for a new strand in a daughter molecule.

2. Semiconservative Replication
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5′ 3′ G C G C G A T A
region of parental
DNA double helix T A C G
DNA
polymerase
enzyme A T A T G G T C G C G C G C A A C A T C G C
region of
replication:
new nucleotides
are pairing
with those of
parental strands T G A A T T A T T A G C T C A T A G A T A T A A G A G T G T
region of
completed
replication A C C C A G C 3′ G n e w o l d A s t r a n d s t r a n d 5′
daughter DNA double helix
old
strand
new
strand
daughter DNA double helix

3. Replication of DNA Replication requires the following steps:
Unwinding, or separation of the two strands of the parental DNA molecule
Complementary base pairing between a new nucleotide and a nucleotide on the template strand
Joining of nucleotides to form the new strand
Each daughter DNA molecule contains one old strand and one new strand 3

4. Replication of DNA Prokaryotic Replication
Bacteria have a single circular loop of DNA
Replication moves around the circular DNA molecule in both directions
Produces two identical circles
The process begins at the origin of replication
Replication takes about 40 minutes, but the cell divides every 20 minutes
A new round of replication can begin before the previous round is completed

5. Replication of DNA Eukaryotic Replication
DNA replication begins at numerous points along each linear chromosome
DNA unwinds and unzips into two strands
Each old strand of DNA serves as a template for a new strand
Complementary base-pairing forms a new strand paired with each old strand
Requires enzyme DNA polymerase

6. Replication of DNA Eukaryotic Replication
Replication bubbles spread bidirectionally until they meet
The complementary nucleotides are joined to form new strands. Each daughter DNA molecule contains an old strand and a new strand.
Replication is semiconservative:
One original strand is conserved in each daughter molecule, i.e., each daughter double helix has one parental strand and one new strand.

7. Prokaryotic versus Eukaryotic Replication
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origin
replication is
complete
replication is
occurring in
two directions
a. Replication in prokaryotes
replication fork
replication bubble
parental strand
new DNA
duplexes
daughter strand
b. Replication in eukaryotes

8. Aspects of DNA Replication
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. OH P
Is attached here
base is attached here 5′
CH2 O OH 4′ C C H H 1′ 1 H C H 3′ C 2′ OH H D e o x y r i b o s e m o l e c u l e 2
DNA polymerase
attaches a new
nucleotide to the
3 ′ carbon of the
previous nucleotide. 5′ e n d P T P A P P G C 3′ e n d P P C G 5′ P P T A 3′ P C G P
template
strand P
3 ′end
DNA polymerase
5′ end 4
leading
new strand
template strand
new strand
Direction of replication 3′ 3
helicase at replication fork
RNA primer
template
strand 6
Okazaki fragment 5
lagging
strand 3′ 5′ 5′
parental DNA helix 7
DNA ligase
DNA polymerase 3′
Replication fork introduces complications

9. Replication of DNA Accuracy of Replication
DNA polymerase is very accurate, yet makes a mistake about once per 100,000 base pairs.
Capable of identifying and correcting errors

10. 12.3 The Genetic Code of Life
Genes Specify Enzymes
Beadle and Tatum:
Experiments on the fungus Neurospora crassa
Proposed that each gene specifies the synthesis of one enzyme
One-gene-one-enzyme hypothesis

11. The Genetic Code of Life
The mechanism of gene expression
DNA in genes specify information, but information is not structure and function
Genetic information is expressed into structure and function through protein synthesis
The expression of genetic information into structure and function:
DNA in a gene determines the sequence of nucleotides in an RNA molecule
RNA controls the primary structure of a protein

12. The Central Dogma of Molecular Biology
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3' 5' A G C G A C C C C
DNA T C G C T G G G G 3' 5' 12

13. The Central Dogma of Molecular Biology
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
nontemplate strand 5 3 A G C G A C C C C
DNA T C G C T G G G G 3 5
template strand
transcription
in nucleus 5 3 A G C G A C C C C
mRNA 13

14. The Central Dogma of Molecular Biology
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
nontemplate strand 5 3 A G C G A C C C C
DNA T C G C T G G G G 3 5
template strand
transcription
at ribosome 5 3 A G C G A C C C C
mRNA
codon 1
codon 2
codon 3 14

15. The Central Dogma of Molecular Biology
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
nontemplate strand 5 3 A G C G A C C C C
DNA T C G C T G G G G 3 5
template strand
transcription
in nucleus 5 3 A G C G A C C C C
mRN A
translation
at ribosome
codon 1
codon 2
codon 3 O O O
polypeptide N C C N C C N C C R1 R2 R3 15
Serine
Aspartate
Proline

16. The Genetic Code of Life
RNA is a polymer of RNA nucleotides
RNA nucleotides contain the sugar ribose instead of deoxyribose
RNA nucleotides are of four types: uracil (U), adenine (A), cytosine (C), and guanine(G)
Uracil (U) replaces thymine (T) of DNA
Types of RNA
Messenger (mRNA) - Takes a message from DNA in the nucleus to ribosomes in the cytoplasm
Ribosomal (rRNA) - Makes up ribosomes, which read the message in mRNA
Transfer (tRNA) - Transfers the appropriate amino acid to the ribosomes for protein synthesis

17. RNA polymerases 3 RNA polymerase enzymes RNA polymerase 1
only transcribes rRNA genes
makes ribosomes
RNA polymerase 2
transcribes genes into mRNA
RNA polymerase 3
only transcribes tRNA genes
each has a specific promoter sequence it recognizes

18. Structure of RNA 5′ end G U S A base is uracil instead of thymine C S
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5′ end P G U S A
base is
uracil instead
of thymine P C S P S P S
ribose
3′ end
one nucleotide

19. RNA Structure Compared to DNA structure

20. The Genetic Code of Life
The unit of the genetic code consists of codons, each of which is a unique arrangement of symbols
Each of the 20 amino acids found in proteins is uniquely specified by one or more codons
The symbols used by the genetic code are the mRNA bases
Function as “letters” of the genetic alphabet
Genetic alphabet has only four “letters” (U, A, C, G)
Codons in the genetic code are all three bases (symbols) long
Function as “words” of genetic information
Permutations:
There are 64 possible arrangements of four symbols taken three at a time
Often referred to as triplets
Genetic language only has 64 “words”

21. Messenger RNA Codons First Base Second Base Third Base U C A G UUU
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First
Base
Second Base
Third
Base U C A G
UUU
phenylalanine
UCU
serine
UAU
tyrosine
UGU
cysteine U
UUC
phenylalanine
UCC
serine
UAC
tyrosine
UGU
cysteine C U
UUA
leucine
UCA
serine
UAA
stop
UGA
stop A
UUG
leucine
UCG
serine
UAG
stop
UGG
tryptophan G
CUU
leucine
CCU
proline
CAU
histidine
CGU
arginine U
CUC
leucine
CCC
proline
CAC
histidine
CGC
arginine C C
CUA
leucine
CCA
proline
CAA
glutamine
CGA
arginine A
CUG
leucine
CCG
proline
CAG
glutamine
CGG
arginine G
AUU
isoleucine
ACU
threonine
AAU
asparagine
AGU
serine U
AUC
isoleucine
ACC
threonine
AAC
asparagine
AGC
serine C A
AUA
isoleucine
ACA
threonine
AAA
lysine
AGA
arginine A
AUG (start)
methionine
ACG
threonine
AAG
lysine
AGG
arginine G
GUU
valine
GCU
alanine
GAU
aspartate
GGU
glycine U
GUC
valine
GCC
alanine
GAC
aspartate
GGC
glycine C G
GUA
valine
GCA
alanine
GAA
glutamate
GGA
glycine A
GUG
valine
GCG
alanine
GAG
glutamate
GGG
glycine G

22. The Genetic Code of Life
Properties of the genetic code:
Universal
With few exceptions, all organisms use the code the same way
Encode the same 20 amino acids with the same 64 triplets
Degenerate (redundant)
There are 64 codons available for 20 amino acids
Most amino acids encoded by two or more codons
Unambiguous (codons are exclusive)
None of the codons code for two or more amino acids
Each codon specifies only one of the 20 amino acids
Contains start and stop signals
Punctuation codons
Like the capital letter we use to signify the beginning of a sentence, and the period to signify the end