7.3 Transcription in prokaryotes
7.3.1. State that transcription is carried out in a 5’→ 3’ direction.
Nucleotides are added in the
form of ribonucleoside
triphosphates
7.3.2. Distinguish between the sense and antisense strands of DNA
Sense strand
Does it get
transcribed?
Is its base sequence
the same as mRNA?
No
The sense strand has
the same base
sequence as the
mRNA molecule,
except that it contains
thymine instead of
uracil.
Anti-sense strand
Yes
The anti-sense strand
has a complementary
base sequence to the
mRNA molecule
.
A section of DNA on the antisense strand has the sequence
ATT TAG GCG CTA TGC TAG
What is the complementary DNA sequence?
What is the sequence on the codons of mRNA?
What is the sequence on the anti-codons of t-RNA
What is the sequence of amino acids?
7.3.3: Explain the process of transcription in prokaryotes, including the role of the
promoter region, RNA polymerase, nucleoside triphosphates and the terminator.
Promoter and Terminator
DNA in the nucleus of a eukaryotic cell or DNA of the nucleoid region of a
prokaryotic cell
5’
3’
3’
5’
Transcription: Going from the DNA code to the code of mRNA
RNA polymerase has two functions
The promoter is a region of the DNA of a specific sequence that signals
the RNA polymerase where to start transcription
RNA polymerase unwinds the DNA and breaks the hydrogen bonds between the
complementary bases
Free floating nucleotides are added in a 5’ to 3’ direction. They are also added as
ribonucleoside triphosphate (not show in the drawing)
RNA nucleotides are added forming a mRNA transcript
RNA polymerase also adds complementary RNA nucleotides to the antisense strand of DNA.
Nucleotides are added in a 5’ to 3’ direction.
The terminator is a region of DNA that signals to the RNA polymerase where to stop
transcription
At the terminator the RNA polymerase falls off and a mRNA transcript has been
formed
IB Question: Explain the process of transcription in eukaryotes.
N08/4/BIOLO/HP2/ENG/TZ0/XX+
[8]
IB Question: Explain the process of transcription in eukaryotes.
N08/4/BIOLO/HP2/ENG/TZ0/XX+
[8]
DNA (sequence) is copied to (m)RNA; DNA separated;
RNA polymerases separate the strands;
only one strand is copied/transcribed / antisense strand is transcribed; RNA polymerase binds to
promoter/initiation region (of template strand);
ATP provides energy for attachment; nucleotides exist as nucleotide triphosphates;
removal of phosphates (2) from nucleotide triphosphates provides energy for linkage;
RNA polymerase catalyses the formation of the polymer / helps the nucleotides join;
(m)RNA lengthens in 5' to 3' direction;
RNA polymerase reaches terminator on DNA to stop transcription process;
mRNA is separated from DNA; [8 max]
IB Question: Explain the process of transcription leading to the formation of mRNA. [8]
N10/4/BIOLO/HP2/ENG/TZ0/XX
IB Question: Explain the process of transcription leading to the formation of mRNA. [8]
N10/4/BIOLO/HP2/ENG/TZ0/XX
RNA polymerase; (polymerase number is not required)
binds to a promoter on the DNA;
unwinding the DNA strands;
binding nucleoside triphosphates;
to the antisense strand of DNA;
as it moves along in a 5'→3' direction;
using complementary pairing/A-U and C-G;
losing two phosphates to gain the required energy;
until a terminator signal is reached (in prokaryotes);
RNA detaches from the template and DNA rewinds;
RNA polymerase detaches from the DNA;
many RNA polymerases can follow each other;
introns have to be removed in eukaryotes to form mature mRNA;
[8 max]
One gene may code for multiple polypeptides due to alternative splicing
7.3.4: State that eukaryotic RNA needs the removal of introns to form
mature mRNA.
How many different mRNA codons can we make with
adenine (A), uracil (U), guanine (G) and cytosine (C)?
AAA
AAU
….
….
mRNA codon table
Three mRNA codons are stop codons
(UAA, UAG and UGA) , so they do not code for
any amino acid.
So how many mRNA codons code for an amino
acid?
7.4 : Translation
tRNA molecule about
to fold
tRNA molecule
Once in the cytoplasm, mRNA is translated from the nucleic acid language to the protein
language by transfer RNA (tRNA) which acts as an interpreter. Transfer RNA does two things: 1) it
recognizes the appropriate codons in mRNA and 2) it picks up the appropriate amino acids.
In the process of translation the mRNA codons bind to the
anti-codons of tRNA molecules.
There are twenty amino acids.
There is a specific tRNA activating enzyme for each
amino acid. So there are 20 different tRNA activating
enzymes.
Each tRNA activating enzyme recognizes all the different
anti- codons on tRNA molecules for a particular amino acid and will
attach the correct amino acid to the tRNA molecule. For example the
tRNA activating enzyme for phenylalanine will recognize both the
anticodons on the tRNA molecules for phenylalanine
7.4.1: Explain that each
tRNA molecule is
recognized by a tRNAactivating enzyme that
binds a specific amino
acid to the tRNA, using
ATP for energy.
t-RNA activating enzyme
20 different t-RNA activating
enzymes; one for each of the
20 amino acids
The type of amino acid which can attach to a tRNA molecule depends on the sequence of
the anticodon. A tRNA activating enzyme recognizes the anticodon and uses ATP to bind
the appropriate amino acid to the 3' end.
Each amino acid has a specific tRNA activating enzyme, and some amino
acids have more than one tRNA.
7.4.2: Outline the structure of ribosomes, including protein and RNA composition,
large and small subunits, three tRNA binding sites and mRNA binding sites.
IB Question: Outline the structure of a ribosome. [4] M10/4/BIOLO/HP2/ENG/TZ2/XX+
IB Question: Outline the structure of a ribosome. [4] M10/4/BIOLO/HP2/ENG/TZ2/XX+
small subunit and large subunit;
mRNA binding site on small subunit;
three tRNA binding sites / A, P and E tRNA binding sites;
protein and RNA composition (in both subunits);
IB Question: Outline the structure of ribosomes. [6]
IB Question: Outline the structure of ribosomes. [6]
made of protein;
made of rRNA;
large subunit and small subunit;
three tRNA binding sites;
Aminacyl/A, Peptidyl/P and Exit/E;
mRNA binding site (on small subunit);
70S in prokaryotes / 80S in eukaryotes;
can be free / bound to RER (in eukaryotes);
7.4.3: STATE: Translation consists of initiation,
elongation, translocation and termination
7.4.4: State that translation occurs in a
5’→ 3’ direction.
7.4.5: Draw and label a diagram showing the structure of a peptide bond
between two amino acids.
7.4.6: Explain the process of translation, including ribosomes, polysomes, start
codons and stop codons.
A mRNA transcript has the codon sequence of bases
UUG GCA AUG CUC UUU
What is the complementary anticodon sequence?
Transfer RNA is a small molecule made up of only about 80 nucleotides.
It is shaped like a cloverleaf. One of the loops on tRNA contains a base
triplet called the anticodon. The anticodon is complementary to a codon
triplet on the mRNA. At the other end of the tRNA molecule is a specific
sequence of three nucleotides (CCA). The last nucleotide is the (A) and
an amino acid can attach to it.
Translation: the process of going from the code of mRNA to making a polypeptide
AUG is always the start codon one
the mRNA
Initiation: tRNA with complementary anticodon
to the mRNA start codon bind together and
small ribosomal subunit attaches to mRNA and
then large ribosomal subunit attaches. Occurs at
the P-site of the ribosome
Initiation is completed
The three sites of a ribosome
Second tRNA molecule carrying the next amino with its anti-codon
complementary to the second codon of the mRNA bind together at the A-site
First amino attaches breaks away from its tRNA molecule and bonds with the second
amino acid forming a dipeptide
Ribosome translocates a distance of one codon. First tRNA molecule is now
in the E-site and second tRNA molecule is in the P-site. A-site is now vacant
tRNA molecule in the E-site exits the ribosome and can now have another amino acid
attached to it.
Next tRNA molecule with an anti-codon complementary to the next codon
on the mRNA bind together at the A-site
Ribosome translocates one codon and a peptide bond is formed between the amino
acids forming a polypeptide consisting of three amino acids. Elongation is the process
of forming the polypeptide. The ribosome reads the mRNA from a 5’ to 3’ direction.
tRNA in E-site exits the ribosome
Elongation continues until one of three
stop codons on the mRNA is reached.
At the stop codon the small and large
robosomal subunits fall of the mRNA and
the polypeptide is completed
IB Question: Translation occurs in living cells. Explain how translation is carried out, from the
initiation stage onwards. 8
M10/4/BIOLO/HP2/ENG/TZ1/XX
IB Question: Translation occurs in living cells. Explain how translation is carried out, from the
initiation stage onwards. 8
M10/4/BIOLO/HP2/ENG/TZ1/XX
translation involves initiation, elongation/translocation and termination;
mRNA binds to the small sub-unit of the ribosome;
ribosome slides along mRNA to the start codon;
anticodon of tRNA pairs with codon on mRNA:
complementary base pairing (between codon and anticodon);
(anticodon of) tRNA with methionine pairs with start codon / AUG is the
start codon;
second tRNA pairs with next codon;
peptide bond forms between amino acids;
ribosome moves along the mRNA by one codon;
movement in to direction; 5 3
tRNA that has lost its amino acid detaches;
another tRNA pairs with the next codon/moves into A site;
tRNA activating enzymes;
link amino acids to specific tRNA;
stop codon (eventually) reached; [9 max]
IB Question: The diagram below shows the structure of a ribosome during protein synthesis.
M07/4/BIOLO/HP2/ENG/TZ1/XX+
(b)State the names of the structures labelled above.
(c) State the name of a structure shown on the diagram that has an anticodon. [1]
(d) Explain why the process used during protein synthesis in cells is called translation. [2]
IB Question: The diagram below shows the structure of a ribosome during protein synthesis.
M07/4/BIOLO/HP2/ENG/TZ1/XX+
(b)State the names of the structures labelled above.
(c) State the name of a structure shown on the diagram that has an anticodon. [1]
(d) Explain why the process used during protein synthesis in cells is called translation. [2]
Award [2] for four correct and [1] for three or two correct.
I: small (sub)unit (of ribosome);
II: large (sub)unit (of ribosome);
III: transfer RNA/tRNA;
IV: messenger RNA/mRNA; [2 max]
(c) transfer RNA/tRNA [1]
(d) codon/triplet of bases to amino acid;
nucleic acid / base sequence / (m)RNA to polypeptide / protein / amino acid sequence;
genetic code has to be translated; [2 max]
IB Question: Explain briefly how termination of translation occurs. [2]
M07/4/BIOLO/HP2/ENG/TZ1/XX+
IB Question: Explain briefly how termination of translation occurs. [2]
M07/4/BIOLO/HP2/ENG/TZ1/XX+
stop/terminator / nonsense codon (is reached);
polypeptide is released;
mRNA detaches from ribosome;
subunits of ribosome separate; [2 max]
Ignore references to specific codons.
7.4.7: STATE: Free ribosomes synthesize proteins for use primarily within the cell, and
that bound ribosomes synthesize protein primarily for secretion or for lysosomes.
mRNA codon table
The sense strand of DNA has the sequence TTA GAT GCGC TGG TGC
what would be the sequence of amino acids in the polypeptide?
The anti-sense strand of DNA has the sequence AGA GAG TAT ATA
what would be the sequence of amino acids in the polypeptide?
sense strand of DNA
AAC CGC GTA TGC TGA TTT
anti-sense sense strand of DNA
mRNA codons
tRNA anti-codons
amino acids
IB Question: Explain the significance of complementary base pairing for replication,
transcription and translation.
IB Question: Explain the significance of complementary base pairing for replication,
transcription and translation.