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Central Dogma; Big Idea 3 , Essential Knowledge 3.A.1 b-c and 3.B.1
CH. 17 DNA TO PROTEIN
(TRANSCRIPTION AND
TRANSLATION)
OVERVIEW AND BACKGROUND
INFO
DNA
- Double stranded
molecule
- Contains thymine
- Contains deoxyribose
sugar
- Found only in nucleus
RNA
- Made of
nucleotides
- Contain adenine,
guanine, and
cytosine
- Single stranded
molecule
- Contains uracil
- Contains ribose
sugar
- Found in nucleus
and cytoplasm
Does this diagram represent DNA or
RNA? …how can you tell?
Central Dogma- DNA to protein
 DNA serves as a genetic code
for the synthesis (creation) of
proteins
 We eat food, and that food is
reassembled to make US (you
are made of proteins, which
are made of amino acids)
 DNA codes for RNA, which
guides the synthesis of
proteins (basically in order to
read and express genes, it
goes from DNA to RNA to
protein)
Types of RNA
 The 3 main types of RNA are:
 Messenger RNA (mRNA) –
these travel from the
nucleus to the ribosome to
direct the synthesis of a
specific protein
 Ribosomal RNA (rRNA) –
help form ribosomes in the
cytoplasm (remember,
ribosomes help with protein
synthesis); reads and
decodes RNA
 Transfer RNA (tRNA) –
transport amino acids
(building blocks of protein)
to the ribsomes
Central Dogma - Protein
synthesis
 Protein synthesis is the assembly of amino acids
(by RNA) into proteins
 Involves two steps:
 1. Transcription – copying DNA code into mRNA; occurs
in nucleus
 2. Translation – reading the mRNA code and assembling
amino acids into a polypeptide chain (protein); occurs in
cytoplasm
Transcription overview
 Performed in nucleus by mRNA
 mRNA “reads” single DNA strand and forms the
complementary copy (replacing thyamine with
uracil)
Chinese characters transcribed to English alphabet:
Transcription

First step of the central dogma
involves the creating of mRNA
from DNA
 Occurs in nucleus
 1. Initiation – The DNA is unzipped
in the nucleus (by the enzyme
helicase)


RNA polymerase (another enzyme) binds
to a specific section where an mRNA
will be synthesiszed.
Transcription factors mediate the
binding of RNA polymerase and
initiation of transcription

2. Elongation - Uracil is used
instead of thymines when the
bases pair up and mRNA is made
 The mRNA then gets a 5’ cap on
one end, a poly A tail on the other
3’ end (which is just a bunch of
adenines in a row)

We get rid of introns, which are areas
that don’t code for a gene (exons are
areas that do code for a gene) and
then we splice together the mRNA

3. Termination - The mRNA takes
the copied code into the
cytoplasm for protein synthesis
Termination of Transcription
 The mechanisms of termination are different in
prokaryotes and eukaryotes
 In prokaryotes, the polymerase stops transcription
at the end of the terminator
 In eukaryotes, the polymerase continues
transcription after the pre-mRNA is cleaved from
the growing RNA chain; the polymerase
eventually falls off the DNA
Summary of Transcription
 The three stages of transcription:
 Initiation (RNA polymerase attaches to the DNA)
 Elongation (mRNA is created)
 Termination (DNA zips back together because of the
hydrogen bonds holding the base pairs together)
Practice
 Please create a strand of mRNA from this
template strand of DNA (write this in your
notes):
 TTAACGCATGCATAC
Vocab
 The DNA sequence where RNA polymerase
attaches is called the promoter; in
prokaryotes, the sequence signaling the end
of transcription is called the terminator
 The stretch of DNA that is transcribed is
called a transcription unit
Animation: Transcription
Translation
 Translation occurs in ribosomes (in cytoplasm)
 All three types of RNA work together during
translation to produce proteins
Transcribed Chinese words translated to English words:
Translation
 Once the mRNA is made, it
moves out into the cytoplasm
and attaches to a ribosome (can
be located on the E.R.)
 When it connects to the
ribosome, the code is read and
makes a protein through a
process called translation
 tRNA act as interpreters of the
mRNA
 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 basepairs with a complementary
codon on mRNA
Building a Polypeptide –
Overview of translation
 The three stages of translation:
 Initiation
 Elongation
 Termination
 All three stages require protein “factors” that aid
in the translation process
Steps of Translation







1. mRNA leaves the nucleus
2. mRNA attaches to a ribosome
(between the 2 subunits, which are
made of protein and rRNA)
3. tRNA molecules bring amino acids
(building blocks of protein) to the
ribosome
4. Every 3 letters in the mRNA code
for a single amino acid – 3 bases form
a “codon”
 The tRNA has a 3 letter message
that matches the codon on the
mRNA, called the ANTICODON
5. Amino acids get linked together in a
“polypeptide chain”, which form a
protein
6. The chain folds into a 3-D protein
(looks kind of like a 3 leaf clover)
*DNA – mRNA – ribosome - amino
acids are brought by tRNA –
polypeptide chain – protein
Targeting Polypeptides to
Specific Locations
 Two populations of ribosomes are evident in cells:
free ribsomes (in the cytosol) and bound
ribosomes (attached to the ER)
 Free ribosomes mostly synthesize proteins that
function in the cytosol
 Bound ribosomes make proteins of the
endomembrane system and proteins that are
secreted from the cell
 Ribosomes are identical and can switch from free
to bound
 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
Termination of Translation
 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
The Genetic Code
 DNA is a three base code
(eg. ATC).
 Three bases form a
“codon”.
 DNA codons are
converted into mRNA
codons and are then
interpreted by the gentic
code.
 DNA->mRNA->Amino
Acids->Protein
Genetic Code cont.
 There are 20 amino
acids, but only 4
different nucleotide
bases
 they can combine in so
many different ways,
that they can create
over 10,000 different
proteins
Practice
 The genetic code is a




set of rules (see the
chart) used to specify
which amino acid is
used during protein
synthesis
Here is a chart of the
genetic code ->
DNA codon: TAC
mRNA:
Amino Acid
More Practice
 DNA:
TACGGGTCTGGCATT
 mRNA:
 Amino Acid Sequence:
Evolution of the Genetic
Code
 The genetic code is nearly universal, shared by the
simplest bacteria to the most complex animals
 Genes can be transcribed and translated after
being transplanted from one species to another
Concept 17.6: Comparing gene expression in
prokaryotes and eukaryotes reveals key
differences
 Prokaryotic cells lack a nuclear envelope, allowing
translation to begin while transcription progresses
 In a eukaryotic cell:
 The nuclear envelope separates transcription from
translation
 Extensive RNA processing occurs in the nucleus
Mutations – a permanent change
in a cell’s DNA

Point mutations involve a chemical change in
just one base pair and can be enough to
cause a genetic disorder (a point mutation
where one base is exchanged for another is
called a substitution)
 The change of a single nucleotide in a
DNA template strand can lead to
production of an abnormal protein

Base-pair substitution can cause missense or
nonsense mutations
 Missense mutations still code for an
amino acid, but not necessarily the right
amino acid
 Nonsense mutations change an amino
acid codon into a stop codon, nearly
always leading to a nonfunctional protein
 Missense mutations are more common
*Ex – can cause some cancers, attributes to
Tay-Sachs symptoms, color blindness
Mutation cont.
 Frameshift mutations – change
the multiples of three from the
point of insertion or deletion and
change the “frame” of the amino
acid sequence
 Insertions are additions of a
nucleotide to the DNA
sequence
 Deletion – the loss of a
nucleotide
 These mutations have a
disastrous effect on the resulting
protein more often than
substitutions do
 Ex – can be a cause of cystic
fibrosis, colorectal cancer,
Crohn’s disease (inflammatory
bowel disease)
Insertions and Deletions
 Insertions and deletions are additions or losses of
nucleotide pairs in a gene
 These mutations have a disastrous effect on the
resulting protein more often than substitutions do
 Insertion or deletion of nucleotides may alter the
reading frame, producing a frameshift mutation
Mutations
 Can be caused spontaneously or
by mutagens (certain chemicals
or radiation)
 Can be found in somatic cells or
gametes with different results
 Somatic cells will pass the
mutation to all its daughter
cells – can be a cause of
cancer in the body
 Gametes don’t necessarily
affect the function of the
organism, but may drastically
affect their offspring
 Can be good, bad, or neutral
(may not even know you have it
until there is a change in the
environment)
Mutagens
 Spontaneous mutations can occur during DNA
replication, recombination, or repair
 Mutagens are physical or chemical agents that can
cause mutations
 Ex – ultraviolet light, radiation, benzene (an
industrial solvent found in synthetic rubber and
some dyes), virus’s, sodium azide (found in car air
bags)