IB Biology Review
DNA Transcription and Translation
What is Transcription?
The synthesis of mRNA from a DNA template
When the DNA nucleotides are copied into segments
of RNA
Transcription occurs when a protein is needed by the
body
The “blueprint” for the protein is copied from DNA
Transcription Vocab.
• Template Strand/Anti-sense Strand/Non-Coding Strand. The DNA
strand that is a template for mRNA complementary base pairing during
transcription
• Non-template Strand/Sense Strand/Coding Strand: The DNA strand that
is complementary to the template DNA strand during transcription
• Promoter Region: The region of DNA (a specific sequence of nucleotides)
to which RNA polymerase binds to start transcription. (it signals the RNA
polymerase where to begin translation along the DNA molecule)
• Terminator Region: The region of DNA that signals RNA polymerase to
end transcription
• RNA processing: Removing the introns in pre-mRNA to produce mature
mRNA that is ready for translation
• DNA Triplet: Three (3) DNA nucleotides such as ATA or GAC
• mRNA Codon: Three mRNA nucleotides such as UAU or CUG that were
transcribed from the DNA triplet. Codons form the basis of protein
translation because they code for specific amino acids that form proteins
What is Translation?
The synthesis of a polypeptide from the genetic
information on a mRNA molecule when it couples with
a ribosome. It occurs in 4 steps
When the RNA segment tells the ribosome which
amino acids to use to build a protein
Translation turns the DNA code into a protein
RNA is the “blueprint” for the protein
The ribosome follows the RNA’s directions
Translation Vocab.
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tRNA Anticodon: The molecule that transfers/transports specific amino acids to the ribosome where
polypeptides
Polypeptides: These are the precursors of proteins and are the product of Translation. They contain
all the amino acids in the proper sequence, but the amino acid chain has not yet folded up into its final
shape. After folding, carbohydrates are covalently bonded to the folded proteins in the Rough ER .
Then, the molecule become a full-fledged protein and is sent to the Golgi Apparatus.
Golgi Apparatus: This is where the proteins produced in the Rough ER are sorted and shipped
mostly as secretory proteins used outside the cell such as insulin.
Ribosomes: This organelle is the site of protein synthesis
Bound ribosomes: Ribosomes attached to the walls of the rough ER. Most extra-cellular proteins
which will be exported from the cell by exocytosis are synthesized on these ribosomes
Free ribsomes: Ribosomes that are floating free in the cytoplasm. Most proteins used within the cell
(such as the transport proteins in cell membrane) are synthesized on these ribosomes
Polyribosomes: An aggregation of several ribosomes attached to one mRNA molecule. (see Figure
17.22). Polyribosomes speed up the translation of a protein.
Point Mutations: A change in only one base pair of DNA in a gene
Substitutions: The replacement of one nucleotide and its complementary nucleotide with another
pair of nucleotides. Sometimes these types of mutations are silent mutations as when GAA mutates
to GAG (they both code for glutamic acid. Others, such as the Sickle Cell Anemia Case Study
described below, cause a change in one of the amino acids in a polypeptide
Insertions and Deletions. These types of mutations lead to frameshift mutations and always
produce proteins that are non-functional
What is the difference between
DNA and RNA nucleotides?
DNA nucleotides are
Adenine
Thymine
Guanine
Cytosine
RNA nucleotides are
Adeneine
Uracil
Guanine
Cytosine
What are the steps of transcription?
1. RNA polymerase unwinds DNA helix
2. Transcription starts at promoter region
3. RNA polymerase starts
adding free RNA nucleosides
to template strand of DNA
Transcription goes in a
5’ to 3’ direction
What are the steps of transcription? Cont.
4. RNA polymerase continues to add nucleosides
Nucleosides lose two phosphates as they bind to each other
Become nucleotides
5. RNA polymerase reaches terminator region
6. RNA polymerase detaches from template strand
7. RNA strand, now called mRNA, is released from DNA
8. DNA double helix rewinds
The Three Steps of Transcription
1. Initiation
RNA polymerase binds to helix
RNA polymerase starts to add RNA
nucleotides
2. Elongation
RNA polymerase continues to add
nucleotides
3. Termination
Transcription finishes
RNA polymerase reaches
terminator region
mRNA breaks off
RNA polymerase detaches from
helix
Helix rewinds
What are the steps of translation?
1. mRNA binds to a ribosome
tRNA
Small sub-unit first
Then large sub-unit
Anticodon
mRNA translated in what
direction?
5’ to 3’, like always
mRNA
2. The first transfer RNA
(tRNA) binds its anticodon
to the start codon of the mRNA
The start codon is always AUG
Codon
Codons and Anticodons
Codon is complementary to the DNA strand
Anticodon has the same sequence of nucleotides as
DNA (except thymine is uracil)
64 different codons code for 20 different amino acids
That means there are 64 anticodons that are attached
to 20 different amino acids
Codons and Anticodons Sample Problem
The template strand of DNA is
TAC-TTG-CTA-CAG-ATT
What will the mRNA (codon) sequence be?
AUG-AAC-GAU-GUC-UAA
What will the tRNA (anticodon) sequence be?
UAC-UUG-CUA-CAG-AUU
tRNA sequence is the same as the template strand,
only with U in place of T
HL Only: tRNA Activating Enzymes
each amino acid has a
specific tRNA-activating
enzyme
called aminoacyl-tRNA
synthetase
this enzyme binds the
specific/correct amino acid
to the correct tRNA
uses ATP for energy
HL Only: Ribosome Binding Sites
P site holds the tRNA
carrying the growing
polypeptide chain
A site holds the tRNA
carrying the next
amino acid to be
added to the chain
Discharged tRNAs
leave the ribosome
from the E site
HL Only: DNA Sense and Antisense Strands
DNA sense strand / coding strand has the same base
sequence as is as
mRNA
Except uracil replaces thymine
DNA antisense strand / template strand has the same
base sequence as
Strand of DNA that is being transcribed (copied)
What are the steps of translation? cont.
3. Second tRNA binds to
ribosome and mRNA
Each tRNA brings a specific
amino acid with it
4. The large subunit moves
the mRNA strand down after
the second tRNA binds
5. Amino acid on the first
tRNA is bound to the second
tRNA by a peptide bond
Process continues until
ribosome reaches stop codon
6. Then polypeptide (protein)
is released
Genetic Code Table
Be familiar with the genetic code table
Be able to use this to translate a segment of mRNA
into a protein
Using the Genetic Code Table
The letters refer to the mRNA sequence
What is the amino acid sequence if the mRNA
sequence is
AUG-UCU-GCC-UUA-UCC-UAC
Sequence is
Start/Met.
Serine
Alanine
Leucine
Serine
Tyrosine
HL Only: Free and Bound Ribosomes
Free ribosomes
floating in the cytoplasm
produce proteins that stay inside the cell (intracellular
proteins)
Bound ribosomes
on the rough endoplasmic reticulum
produce proteins that go outside the cell (extracellular
proteins)
mRNA Processing
mRNA must be “processed” to remove what?
Introns
What are introns?
Non-coding segments of DNA
Why would introns have to be taken out?
You wouldn’t want a protein made from them because
they don’t do anything!
IB Exam Question
1. A certain gene codes for a polypeptide that is 120 amino acids
long. Approximately how many nucleotides long is the mRNA
that codes for this polypeptide likely to be?
A.
B.
C.
D.
30
40
360
480
Correct answer: C
Each amino acid has three nucleotides
120 x 3 = 360
(1 mark)
IB Exam Question
2. Distinguish between the structure of DNA
and RNA.
(3 marks)
RNA
ribose
(normally) single stranded
uracil
no double helix
DNA
deoxyribose
double stranded
thymine
double helix
IB Exam Question
3. Explain the process of DNA Transcription including the role of the
promoter region, RNA polymerase, and the terminator and 5’ to 3’
direction (7)
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Transcription is the synthesis of mRNA from a DNA template
It occurs in the 5’ to 3’ direction
It occurs in 3 stages: Initiation, Elongation, and Terminations
In Inititiation, RNA polymerase binds to the promoter region
RNA polymerase unwinds the DNA and begins adding
complementary RNA nucleotides to the template strand of DNA
• In Elongation, the mRNA molecule lengthens
• In termination, the RNA polymerase reaches the terminator
region and releases the mRNA molecule
IB Exam Question
4. Explain the process of translation.
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(15 marks)
Translation consists of initiation, elongation, translocation and termination;
the binding of a ribosome to mRNA is called initiation;
the small ribosomal sub-unit attaches to mRNA, followed by the large subunit;
next, the first tRNA binds it’s anticodon to the start codon (AUG) of mRNA
This occurs at the A site of the large ribosomal subunit
each tRNA has an anticodon and the corresponding amino acid attached;
this is accomplished by a tRNA activating enzyme which links the correct amino acid
to each tRNA;
next, the first tRNA moves to the P site on the ribosome and a second tRNA binds to
A site
This is called translocation, the tRNA moves from the A site to the P Site after it
delivers its “payload” amino acid
The amino acid on first tRNA is bonded to amino acid on second tRNA;
this forms a peptide bond between the amino acids
the mRNA is translated in the 5' to 3' direction;
and the polypeptide lengthens (called elongation)
this process continues until the ribosome reaches a stop codon (called termination)
at this point, the polypeptide is released;
IB Exam Question
5. Compare DNA transcription with translation.
(5 marks)
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both in 5' to 3' direction;
both require ATP;
DNA is transcribed and mRNA is translated;
transcription produces RNA and translation produces
polypeptides;
• transcription occurs in the nucleus (of eukaryotes) and
translation occurs in the cytoplasm at the rough ER;
• tRNA is needed for translation but not transcription;
IB Exam Question
6. What is removed to form mature eukaryotic
mRNA?
(1 mark)
A.
B.
C.
D.
RNA primers
Exons
RNA polymerases
Introns
Correct answer: D
IB Exam Question
7. Discuss the relationship between one gene
and one polypeptide.
(1 mark)
• This means that each gene codes for one unique
polypeptide
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8. Describe the consequence of a base substitution
mutation with regards to sickle cell anemia.
(Total 7 marks)
A mutation is a change in the DNA sequence;
This “mistake” often occurs during DNA replication
This changes the mRNA during transcription;
Which, in turn, can change the amino acid sequence;
A substitution mutation leads to a change to one
codon;
In sickle cell anemia, glutamic acid is changed to
valine / GAG to GTG;
This changes the shape of hemoglobin
The resulting hemoglobin cannot carry oxygen as well;
9. Describe the Genetic Code.
(6
marks)
• composed of mRNA base triplets;
• These are called codons;
• each codes for the addition of an amino acid to a
growing polypeptide chain;
• the genetic code is degenerate;
• this means that more than one codon can code for a
particular amino acid;
• In other words, there are 64 different codons that code
for 20 amino acids
• the genetic code is universal;
• this means it is the same in almost all organisms;