From DNA to Protein
Chapter 14
Ricin and your Ribosomes
DNA and RNA
DNA has one function: It
permanently stores a cell’s
genetic information, which
is passed to offspring.
RNAs have various
functions. Some serve
as disposable copies of
DNA’s genetic message;
others are catalytic.
DNA template
Transcription
mRNA
Translation
protein
stop
Amino acid
Gly
Amino acid
Glu
Amino acid
Converting a Gene to an RNA

Transcription
Enzymes use the nucleotide sequence of a
gene to synthesize a complementary strand
of RNA
 Occurs in the nucleus

Converting mRNA to Protein

Translation

The information carried by mRNA is
decoded into a sequence of amino acids,
resulting in a polypeptide chain
RNA in Protein Synthesis
3 Different Kinds of RNA:

Messenger RNA (mRNA)
Contains information transcribed from DNA

Ribosomal RNA (rRNA)
Main component of ribosomes, where
polypeptide chains are built

Transfer RNA (tRNA)
Delivers amino acids to ribosomes
Gene Expression

Gene expression
A multistep process in which genetic
information encoded by a gene is converted
into a structural or functional part of a cell
or body
Transcription: DNA to RNA


RNA polymerase assembles RNA by
linking RNA nucleotides into a chain
A new RNA strand is complementary in
sequence to the DNA strand from which
it was transcribed
Transcription: DNA to RNA
DNA template
Transcription
mRNA
Base-Pairing in DNA Synthesis
and Transcription
The Process of Transcription


RNA polymerase and regulatory proteins
attach to a promoter (a specific binding
site in DNA close to the start of a
gene)
RNA polymerase moves over the gene in
a 5' to 3' direction, unwinds the DNA
helix, reads the base sequence, and
joins free RNA nucleotides into a
complementary strand of mRNA
(gene)
Fig. 14-5a, p. 218
Post-Transcriptional
Modifications
gene
exon
DNA
cap
PremRNA
intron exon intron
exon
transcription into RNA
poly-A
tail
3’
5’
snipped out
snipped out
mRNA
Fig. 14-7, p. 220
mRNA – The Messenger

Codon-A sequence of three mRNA
nucleotides that codes for a specific
amino acid

The order of codons in mRNA determines
the order of amino acids in a polypeptide
chain
Genetic Information
• From DNA to mRNA to amino acid
sequence
codon
Codons of the Genetic Code
rRNA and tRNA – The
Translators

tRNAs deliver amino acids to ribosomes

tRNA has an anticodon
Ribosomes
2 subunits
Ribosomes
made of
rRNA &
proteins
Translation: RNA to Protein

Translation converts genetic
information carried by an mRNA into a
new polypeptide chain
Translation


Translation occurs in the cytoplasm of
cells
Translation occurs in three stages
1.
2.
3.
Initiation
Elongation
Termination
Elongation
Elongation
C An initiator tRNA
carries the amino acid
methionine, so the first
amino acid of the new
polypeptide chain will be
methionine. A second
tRNA binds the second
codon of the mRNA (here,
that codon is GUG, so the
tRNA that binds carries
the amino acid valine).
A peptide bond
forms between
the first two
amino acids
(here, methionine
and valine).
Fig. 14-12c, p. 223
D The first tRNA is
released and the
ribosome moves to the
next codon in the mRNA.
A third tRNA binds to the
third codon of the mRNA
(here, that codon is UUA,
so the tRNA carries the
amino acid leucine).
A peptide bond
forms between the
second and third
amino acids
(here, valine
and leucine).
Fig. 14-12d, p. 223
E The second tRNA
is released and the
ribosome moves to the
next codon. A fourth
tRNA binds the fourth
mRNA codon (here, that
codon is GGG, so the
tRNA carries the amino
acid glycine).
A peptide bond
forms between the
third and fourth
amino acids (here,
leucine and
glycine).
Fig. 14-12e, p. 223
Termination

When the ribosome encounters a stop
codon, polypeptide synthesis ends
Release factors bind to the ribosome
 Enzymes detach the mRNA and polypeptide
chain from the ribosome

Mutated Genes
& Their Protein Products


If the nucleotide sequence of a gene
changes, it may result in an altered gene
product, with harmful effects
Mutations

Small-scale changes in the nucleotide
sequence of a cell’s DNA that alter the
genetic code
Common Mutations


Base-pair-substitution
Deletion or insertion
THREONINE PROLINE
GLUTAMATE GLUTAMATE
LYSINE
a. Normal
Hemoglobin chain
b. Base pair
substitution
THREONINE
PROLINE
VALINE
GLUTAMATE
LYSINE
c. Frame shift
THREONINE
PROLINE
GLYCINE
ARGININE
Fig. 14-13, p. 224
What Causes Mutations?
1.
Spontaneous mutations

Uncorrected errors in DNA replication
Harmful environmental agents
2. UV radiation
3. chemicals

Mutations Caused by Radiation

Ionizing radiation damages
chromosomes, nonionizing (UV) radiation
forms thymine dimers
Inherited Mutations


Mutations in somatic cells of sexually
reproducing species are not inherited
Mutations in a germ cell or gamete may
be inherited, with evolutionary
consequences
Transcription Assembly of RNA on unwound
regions of DNA molecule
mRNA
processing
mRNA
rRNA
proteins
mature mRNA ribosomal
transcripts
subunits
Translation
At an intact
ribosome,
synthesis of a
polypeptide
chain at the
binding sites
for mRNA and
tRNAs
Convergence
of RNAs
tRNA
mature
tRNA
cytoplasmic
pools of
amino
acids,
ribosomal
subunits, and
tRNAs
Protein
Fig. 14-16, p. 226