1. DNA is made up of nucleotides containing a sugar (deoxyribose), phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, or guanine).
2. DNA replication ensures that new cells have a complete set of DNA by separating the DNA double helix and using each original strand as a template to produce two new complementary strands.
3. Transcription and translation are the processes by which the information in DNA is used to synthesize proteins. Transcription involves RNA polymerase making an mRNA copy of a gene, and translation involves ribosomes using the mRNA to produce a polypeptide chain.
2. Make up of DNA
• DNA is a polymer of nucleotides, each
consisting of a nitrogenous base, a sugar, and
a phosphate group
• Sugar: deoxyribose
• Nitrogenous base: Adenine, Thymine,
Cytosine, Guanine
5. 3.4 nm
1 nm
0.34 nm
Hydrogen bond
(a) Key features of
DNA structure
(b) Partial chemical structure
3′ end
5′ end
3′ end
5′ end
T
T
A
A
G
G
C
C
C
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
G
T
T
T
T
T
T
A
A
A
A
A
A
Figure 16.7a
6. Figure 16.UN01
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data
9. DNA replication
• Ensures that new cells will have a complete
set of DNA
• During replication the DNA molecule
separates into 2 strands, then produces 2 new
complementary strands
• Each original strand serves as a template for
the new strands to form from
• Each new DNA is composed of an old strand
and a new complementary strand
10. Figure 16.9-3
(a) Parent molecule (b) Separation of
strands
(c) “Daughter” DNA molecules,
each consisting of one
parental strand and one
new strand
A
A
A
A
A
A
A
A
A
A
A
A
T
T
T
T
T
T
T
T
T
T
T
T
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
11. Semiconservative Model of
Replication
• When DNA replicates, each daughter
molecule will have one old strand (derived or
“conserved” from the parent molecule) and
one newly made strand
• Competing models were the conservative
model (the two parent strands rejoin) and the
dispersive model (each strand is a mix of old
and new)
13. DNA replication
• The copying of DNA is remarkable in its speed
and accuracy
• More than a dozen enzymes and other
proteins participate in DNA replication
14. DNA replication speed
• In prokaryotes DNA replication begins at one
point then spreads in 2 directions until the
entire chromosome is replicated
• In eukaryotes DNA replication occurs at
hundreds of places at once and proceeds in
both directions until the entire chromosome is
copied.
• Replication forks- sites where separation (of
the double helix) and replication are occurring
15. The players
• Helicases are enzymes that untwist the double helix at
the replication forks
• Single-strand binding proteins bind to and stabilize
single-stranded DNA
• Topoisomerase corrects “overwinding” ahead of
replication forks by breaking, swiveling, and rejoining
DNA strands
• DNA polymerases: add nucleotides to the 3′ end of
DNA
• The initial nucleotide strand is a short RNA primer
16. • Primase: enzyme that makes the RNA primer;
(uses the parental DNA as a template)
• The primer is short (5–10 nucleotides long),
and the 3′ end serves as the starting point for
the new DNA strand
17. Antiparallel elongation
• The antiparallel structure of the double helix
affects replication
• DNA polymerases add nucleotides only to the
free 3′ end of a growing strand; therefore, a
new DNA strand can elongate only in the
5′ to 3′ direction
18. • Along one template strand of DNA, the DNA
polymerase synthesizes a leading strand
continuously, moving toward the replication fork
• To elongate the other new strand, called the
lagging strand, DNA polymerase must work in
the direction away from the replication fork
• The lagging strand is synthesized as a series of
segments called Okazaki fragments, which are
joined together by DNA ligase
19. Proofreading and repairing DNA
• DNA polymerases proofread newly made DNA,
replacing any incorrect nucleotides
• In mismatch repair of DNA, repair enzymes
correct errors in base pairing
• DNA can be damaged by exposure to harmful
chemical or physical agents such as cigarette
smoke and X-rays; it can also undergo
spontaneous changes
• In nucleotide excision repair, a nuclease cuts out
and replaces damaged stretches of DNA
20. Figure 16.UN03
DNA pol III synthesizes
leading strand continuously
Parental
DNA DNA pol III starts DNA
synthesis at 3′ end of primer,
continues in 5′ → 3′ direction
Origin of
replication
Helicase
Primase synthesizes
a short RNA primer
DNA pol I replaces the RNA
primer with DNA nucleotides
3′
3′
3′
5′
5′
5′
5′
Lagging strand synthesized
in short Okazaki fragments,
later joined by DNA ligase
21. Basic Principles of Transcription and
Translation
• RNA is the bridge between genes and the
proteins for which they code
• Transcription is the synthesis of RNA under
the direction of DNA
• Transcription produces messenger RNA
(mRNA)
• Translation is the synthesis of a polypeptide,
using information in the mRNA
• Ribosomes are the sites of translation
22. • In prokaryotes, translation of mRNA can begin
before transcription has finished
• In a eukaryotic cell, the nuclear envelope
separates transcription from translation
• Eukaryotic RNA transcripts are modified
through RNA processing to yield finished
mRNA
27. • During transcription, one of the two DNA
strands, called the template strand, provides
a template for ordering the sequence of
complementary nucleotides in an RNA
transcript
• The template strand is always the same strand
for a given gene
• During translation, the mRNA base triplets,
called codons, are read in the 5′ to 3′ direction
28. Cracking the code
• All 64 codons were deciphered by the mid-1960s
• Of the 64 triplets, 61 code for amino acids; 3
triplets are “stop” signals to end translation
• The genetic code is redundant (more than one
codon may specify a particular amino acid) but
not ambiguous; no codon specifies more than
one amino acid
• Codons must be read in the correct reading
frame (correct groupings) in order for the
specified polypeptide to be produced
29. Figure 17.5
Second mRNA base
FirstmRNAbase(5′endofcodon)
ThirdmRNAbase(3′endofcodon)
UUU
UUC
UUA
CUU
CUC
CUA
CUG
Phe
Leu
Leu
Ile
UCU
UCC
UCA
UCG
Ser
CCU
CCC
CCA
CCG
UAU
UAC
Tyr
Pro
Thr
UAA Stop
UAG Stop
UGA Stop
UGU
UGC
Cys
UGG Trp
GC
U
U
C
A
U
U
C
C
C
A
U
A
A
A
G
G
His
Gln
Asn
Lys
Asp
CAU CGU
CAC
CAA
CAG
CGC
CGA
CGG
G
AUU
AUC
AUA
ACU
ACC
ACA
AAU
AAC
AAA
AGU
AGC
AGA
Arg
Ser
Arg
Gly
ACGAUG AAG AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
Val Ala
GGU
GGC
GGA
GGG
Glu
Gly
G
U
C
A
Met or
start
UUG
G
30. Transcription
• Making mRNA starts with RNA polymerase,
which pries the DNA strands apart and hooks
together the RNA nucleotides
• The RNA is complementary to the DNA
template strand
• RNA synthesis follows the same base-pairing
rules as DNA, except that uracil substitutes for
thymine
31. • The DNA sequence where RNA polymerase
attaches is called the promoter; in bacteria,
the sequence signaling the end of
transcription is called the terminator
32. Figure 17.7-4 Promoter
RNA polymerase
Start point
DNA
5′
3′
Transcription unit
3′
5′
Elongation
5′
3′
3′
5′
Nontemplate strand of DNA
Template strand of DNA
RNA
transcript
Unwound
DNA
2
3′
5′3′
5′
3′
Rewound
DNA
RNA
transcript
5′
Termination3
3′
5′
5′
Completed RNA transcript
Direction of transcription (“downstream”)
5′
3′
3′
Initiation1
33. mRNA editing
• Enzymes in the eukaryotic nucleus modify pre-
mRNA (RNA processing) before mRNA leave
the cytoplasm
• During RNA processing, both ends of the
primary transcript are usually edited
• Also, usually some interior parts of the
molecule are cut out, and the other parts
spliced together
34. • Each end of a pre-mRNA molecule is modified
in a particular way
– The 5′ end receives a modified nucleotide 5′ cap
– The 3′ end gets a poly-A tail
• These modifications share several functions
– They seem help mRNA leave the nucleus
– They protect mRNA from hydrolytic enzymes
– They help ribosomes attach to the 5′ end
35. RNA splicing
• Most eukaryotic genes and their RNA
transcripts have long noncoding stretches of
nucleotides that lie between coding regions
• These noncoding regions are called
intervening sequences, or introns
• The other regions are called exons because
they are eventually expressed, usually
translated into amino acid sequences
• RNA splicing removes introns and joins exons,
creating an mRNA molecule with a continuous
coding sequence
37. The Basis of Translation
• A cell translates an mRNA message into
protein with the help of transfer RNA (tRNA)
• tRNA transfer amino acids to the growing
polypeptide in a ribosome
39. tRNA
• Molecules of tRNA are not identical
– Each carries a specific amino acid on one end
– Each has an anticodon on the other end; the
anticodon base-pairs with a complementary
codon on mRNA
40. Ribosomes
• Ribosomes help join tRNA anticodons with
mRNA codons in protein synthesis
• The two ribosomal subunits (large and small)
are made of proteins and ribosomal RNA
(rRNA)
41. Figure 17.17b
Exit tunnel
A site (Aminoacyl-
tRNA binding site)
Small
subunit
Large
subunit
P A
P site (Peptidyl-tRNA
binding site)
mRNA
binding site
(b) Schematic model showing binding sites
E site
(Exit site)
E
42. Ribosome binding sites
• A ribosome has three binding sites for tRNA
– The P site holds the tRNA that carries the
growing polypeptide chain
– The A site holds the tRNA that carries the next
amino acid to be added to the chain
– The E site is the exit site, where discharged
tRNAs leave the ribosome
43. Figure 17.17c
Amino end
mRNA
E
(c) Schematic model with mRNA and tRNA
5′
Codons
3′
tRNA
Growing polypeptide
Next amino
acid to be
added to
polypeptide
chain
44. Termination of polypeptide
• Termination occurs when a stop codon in the
mRNA reaches the A site of the ribosome
• The A site accepts a protein called a release
factor
• The release factor causes the addition of a
water molecule instead of an amino acid
• This reaction releases the polypeptide, and
the translation assembly then comes apart
45. Mutations
• changes in the genetic material of a cell or
virus
• Point mutations are chemical changes in just
one base pair of a gene
• The change of a single nucleotide in a DNA
template strand can lead to the production of
an abnormal protein
46. Types of Substitution Mutations
• Silent mutations have no effect on the amino
acid produced by a codon because of
redundancy in the genetic code
• Missense mutations still code for an amino
acid, but not the correct amino acid
• Nonsense mutations change an amino acid
codon into a stop codon, nearly always
leading to a nonfunctional protein
48. Figure 17.24e
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causing
extensive missense
Wild type
missing
missing
A
U
A A AT T TC C A T TC C G
A AT T TG GA A ATCG G
A G A A GU U U C A AG G U 3′
5′
3′
3′
5′
Met Lys Leu Ala
1 nucleotide-pair deletion
5′
