Chapter 14
From DNA to Protein
Objectives

1. Know how the structure and behavior of DNA
determines the structure and behavior of the
three forms of RNA during transcription.

2. Understand the role the three forms of RNA
play in determining the primary structure of
polypeptide chains during translation.

3. Understand the types of mutations and their
role in genetic variation.
14.0: Ricin and Your Ribosomes
– comes from the castor oil plant,
and can be used as a bioweapon.
 Ricin has deadly effects – it inactivates
ribosomes, the protein-building machinery
of all cells.
 Protein synthesis stops, cells die, the
individual dies, there is no antidote.
 Ricin
Life depends on enzymes
 Enzymes
are proteins. All proteins consist
of polypeptide chains.
 The chains are sequences of amino acids
that correspond to genes – sequences of
nucleotide bases in DNA.
 Enzymes may selectively unwind certain
regions of DNA to encode information to
make proteins.
The path from genes to proteins
 DNA
 The
RNA
two steps include:
 Transcription
 Translation
Protein
Steps from DNA to Proteins
Same two steps produce all proteins:
1) DNA is transcribed to form RNA


Occurs in the nucleus
RNA moves into cytoplasm
2) RNA is translated to form polypeptide
chains, which fold to form proteins
14.1 How is RNA Transcribed
from DNA
 The
first step in protein synthesis, is
transcription, a sequence of nucleotide
bases is exposed in an unwound region of
a DNA strand.
 That sequence is the template upon which
a single strand of RNA is assembled from
adenine, cytosine, guanine, and uracil
subunits.
Three Classes of RNAs
 Messenger

Carries protein-building instruction or code
 Ribosomal

RNA (rRNA)
Combines with proteins to form ribosomes
 Transfer

RNA (mRNA)
RNA (tRNA)
Delivers the correct amino acids to ribosomes
RNA molecule
 One
phosphate group
 A five carbon sugar – ribose
 Four bases:




Adenine
Cytosine
Guanine
Uracil – instead of Thymine
A Nucleotide Subunit of RNA
uracil (base)
phosphate
group
sugar
(ribose)
Fig. 14-2, p. 220
Base Pairing
during
Transcription
DNA
base pairing
during
transcription
RNA
DNA
base pairing
during DNA
replication
DNA
Fig. 14-2c, p.220
Transcription & DNA Replication
 Like

DNA replication
Nucleotides added in 5’ to 3’ direction
 Unlike



DNA replication
Only small stretch of DNA is template
RNA polymerase catalyzes nucleotide
addition
Product is a single strand of RNA
Promoter

The promoter is a base sequence in
the DNA that signals the start of a
gene.
 For
transcription to occur, RNA
polymerase must first bind to a
promoter, and then move along to the
end of the gene.
Promoter
promoter region
RNA polymerase, the enzyme
that catalyzes transcription
a RNA polymerase initiates transcription at a promoter region in DNA. It
recognizes a base sequence located next to the promoter as a template. It
will link the nucleotides adenine, cytosine, guanine, and uracil into a strand
of RNA, in the order specified by DNA.
Fig. 14-3a, p.220
Gene Transcription
newly forming
RNA transcript
DNA template winding up
DNA template
at selected
transcription site
DNA template unwinding
b All through transcription, the DNA double helix becomes unwound in front
of the RNA polymerase. Short lengths of the newly forming RNA strand
briefly wind up with its DNA template strand. New stretches of RNA unwind
from the template (and the two DNA strands wind up again).
Fig. 14-3b, p.220
Adding Nucleotides
3´
direction of transcription
5´
5´
3´
growing RNA transcript
c What happened at the assembly site? RNA polymerase catalyzed the
assembly of ribonucleotides, one after another, into an RNA strand, using
exposed bases on the DNA as a template. Many other proteins assist this
process.
Fig. 14-3c, p.221
Transcript Modification of mRNA
unit of transcription in a DNA strand
exon
intron
exon
intron
exon
transcription into pre-mRNA
3’ end poly-A
tail
5’ end
Cap
snipped out noncoding introns
snipped out noncoding introns
mature mRNA transcript
Fig. 14-4, p.221
Modifications of mRNA
 In




nucleus
5’ end is capped with guanine – “start” signal
for translations. Cap will also help bind the
mRNA to a ribosome
3’ end a “poly-A-tail” is added. 100 – 200
molecules of adenine is added
Introns (non-coding) are snipped out, exons
(coding portions) are spliced together
Alternative splicing results in different proteins
14.2 Genetic
Code
 Set
of 64 base
triplets
 Codons
 61
specify amino
acids
3
stop translation
Fig. 14-6, p.222
The Genetic Code
 Every
three bases in mRNA (triplet) code
of an amino acid.
 Both DNA and its RNA transcript are
linear sequences of nucleotides carrying
the hereditary code.
 The genetic code consists of 61 triplets
that specify amino acids, AUG – “start”
codon Methionine, and three “stops”
Genetic Code
DNA
mRNA
mRNA
codons
amino
acids
threonine
proline
glutamate
glutamate
lysine
Fig. 14-5, p.222
14.3 The other RNA’s - tRNA
codon in mRNA
anticodon
amino-acid
attachment site
amino
acid
OH
Figure 14.7
Page 223
tRNA
 Each
kind of tRNA has an anticodon that
is complementary to an mRNA codon
 Each tRNA also carries one specific amino
acid.
 After the mRNA arrives in the cytoplasm
an anticodon on a tRNA bonds to the
codon on the mRNA, and thus a correct
amino acid is brought into place.
Ribosomes
A ribosome has two subunits
 Each ribosome is composed of rRNA and
structural proteins) that come together
only during translation.
 Polypeptide chains are built on ribosomes.

Ribosomes
funnel
small ribosomal
subunit
+ large ribosomal
subunit
intact ribosome
Fig. 14-8, p.223
14.4 Three Stages of
Translation
Initiation
Elongation
Termination
Initiation

Initiator tRNA binds to small
ribosomal subunit
 Small subunit/tRNA
complex attaches to mRNA
and moves along it to an
AUG “start” codon
 Large ribosomal subunit
joins complex
Binding Sites
binding site for mRNA
P (first
binding site
for tRNA)
A (second
binding site
for tRNA)
Elongation
 mRNA
passes through ribosomal subunits
 tRNAs
deliver amino acids to the
ribosomal binding site in the order
specified by the mRNA
 Peptide
bonds form between the amino
acids and the polypeptide chain grows
Elongation
Termination
 Stop
codon into place
 No tRNA with anticodon
 Release factors bind to
the ribosome
 mRNA and polypeptide
are released into the
cytoplasm.
mRNA
new
polypeptide
chain
What Happens to the
New Polypeptides?
 Some
just enter the cytoplasm
 Many
enter the endoplasmic reticulum and
move through the cytomembrane system
where they are modified
 Polysome-
several ribosomes moving
along mRNA at the same time.
Transcription
Overview
mRNA
Mature mRNA
transcripts
Translation
rRNA
ribosomal
subunits
tRNA
mature
tRNA
binding site for mRNA
P (first
binding
site for
tRNA)
A (second
binding
site for
tRNA)
c Initiation ends when a large and small
ribosomal subunit converge and bind together.
elongation
Amino
Acid
1
d The initiator tRNA
binds to the ribosome.
e One of the rRNA
molecules
b Initiation, the first stage of translating
mRNA, will start when an initiator tRNA
binds to a small ribosomal subunit.
initiation
a A mature mRNA
transcript leaves the
nucleus through a pore
in the nuclear envelope.
Fig. 14-9a-e, p.224
f The first tRNA is
released
g A third tRNA
binds with the next
codon
h Steps f and g are
repeated
termination
i A STOP codon moves
into the area where the
chain is being built.
j The new polypeptide
chain is released from the
ribosome.
k The two ribosomal
subunits now separate,
also.
Fig. 14-9f-k, p.224
14.5 Gene Mutations
A gene mutation is a change in one to several bases in the
nucleotide sequence of DNA, which can result in a
change in the protein synthesized.
Base-Pair Substitutions
Insertions
Deletions
Base-Pair Substitution
a base substitution
within the triplet (red)
original base triplet
in a DNA strand
During replication, proofreading
enzymes make a substitution
possible outcomes:
or
original, unmutated
sequence
a gene mutation
Frameshift Mutations
 Insertion

Extra base added into gene region
 Deletion

Base removed from gene region
 Both
shift the reading frame
 Result
in many wrong amino acids
Frameshift Mutation
part of DNA template
mRNA transcribed from DNA
THREONINE
PROLINE
GLUTAMATE
GLUTAMATE
LYSINE
resulting amino acid sequence
base substitution in DNA
altered mRNA
THREONINE
PROLINE
VALINE
GLUTAMATE
LYSINE
altered amino acid sequence
deletion in DNA
altered mRNA
THREONINE
PROLINE
GLYCINE
ARGININE
altered amino acid sequence
Fig. 14-10, p.226
Transposons
 DNA
segments that move
spontaneously about the genome
 When
they insert into a gene region,
they usually inactivate that gene
 See
the non-uniform coloration of
kernels in strains of Indian Corn
Mutation Rates


Mutations occur spontaneously while DNA is
being replicated, but fortunately special
enzymes correct most of the mistakes.
Each gene has a characteristic mutation rate
 Average rate for eukaryotes is between 10-4 and
10-6 per gene per generation
 Only mutations that arise in germ cells can be
passed on to next generation
Mutagens
 Ionizing
radiation (X rays)
 Nonionizing
 Natural
radiation (UV)
and synthetic chemicals alkylating agents – add methyl or
ethyl groups to DNA making it more
vulnerable to mutations. (smoking)
Mutations
 If
a mutation arises in a somatic cell, it will
affect only that cell and will not be passed
on to offspring.
 If, however, the mutation arises in a
gamete, it may be passed on and thus
enter the evolutionary arena.
 Either kind of mutation may prove to be
harmful, beneficial, or neutral in its effects.