7.3 Transcription in prokaryotes. 7.3.1. State that transcription is carried out in a 5’→ 3’...
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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
7.3.1. State that transcription is carried out in a 5 3
direction. Nucleotides are added in the form of ribonucleoside
triphosphates
Slide 3
7.3.2. Distinguish between the sense and antisense strands of
DNA. Sense strandAnti-sense strand Does it get transcribed? No Yes
Is its base sequence the same as mRNA? The sense strand has the
same base sequence as the mRNA molecule, except that it contains
thymine instead of uracil. The anti-sense strand has a
complementary base sequence to the mRNA molecule
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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?
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7.3.3: Explain the process of transcription in prokaryotes,
including the role of the promoter region, RNA polymerase,
nucleoside triphosphates and the terminator.
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Promoter and Terminator
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3 5 DNA in the nucleus of a eukaryotic cell or DNA of the
nucleoid region of a prokaryotic cell
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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
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RNA polymerase unwinds the DNA and breaks the hydrogen bonds
between the complementary bases
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Free floating nucleotides are added in a 5 to 3 direction. They
are also added as ribonucleoside triphosphate (not show in the
drawing)
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RNA nucleotides are added forming a mRNA transcript
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RNA polymerase also adds complementary RNA nucleotides to the
antisense strand of DNA. Nucleotides are added in a 5 to 3
direction.
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The terminator is a region of DNA that signals to the RNA
polymerase where to stop transcription
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At the terminator the RNA polymerase falls off and a mRNA
transcript has been formed
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IB Question: Explain the process of transcription in
eukaryotes.[8] N08/4/BIOLO/HP2/ENG/TZ0/XX+
Slide 29
IB Question: Explain the process of transcription in
eukaryotes.[8] N08/4/BIOLO/HP2/ENG/TZ0/XX+ 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]
Slide 30
IB Question: Explain the process of transcription leading to
the formation of mRNA. [8] N10/4/BIOLO/HP2/ENG/TZ0/XX
Slide 31
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]
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One gene may code for multiple polypeptides due to alternative
splicing
Slide 34
7.3.4: State that eukaryotic RNA needs the removal of introns
to form mature mRNA.
Slide 35
How many different mRNA codons can we make with adenine (A),
uracil (U), guanine (G) and cytosine (C)? AAA AAU . mRNA codon
table
Slide 36
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?
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tRNA molecule about to fold 7.4 : Translation
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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.
Slide 40
In the process of translation the mRNA codons bind to the
anti-codons of tRNA molecules.
Slide 41
There are twenty amino acids.
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There is a specific tRNA activating enzyme for each amino acid.
So there are 20 different tRNA activating enzymes.
Slide 43
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
Slide 44
t-RNA activating enzyme 20 different t-RNA activating enzymes;
one for each of the 20 amino acids 7.4.1: Explain that each tRNA
molecule is recognized by a tRNA- activating enzyme that binds a
specific amino acid to the tRNA, using ATP for energy.
Slide 45
Each amino acid has a specific tRNA activating enzyme, and some
amino acids have more than one tRNA. 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.
Slide 46
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.
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IB Question: Outline the structure of a ribosome. [4]
M10/4/BIOLO/HP2/ENG/TZ2/XX+
Slide 49
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);
Slide 50
IB Question: Outline the structure of ribosomes. [6]
Slide 51
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);
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7.4.3: STATE: Translation consists of initiation, elongation,
translocation and termination
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7.4.4: State that translation occurs in a 5 3 direction.
Slide 54
7.4.5: Draw and label a diagram showing the structure of a
peptide bond between two amino acids.
Slide 55
7.4.6: Explain the process of translation, including ribosomes,
polysomes, start codons and stop codons.
Slide 56
A mRNA transcript has the codon sequence of bases UUG GCA AUG
CUC UUU What is the complementary anticodon sequence?
Slide 57
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.
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Translation: the process of going from the code of mRNA to
making a polypeptide 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 AUG is always the start codon one the
mRNA
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Initiation is completed
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The three sites of a ribosome
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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
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First amino attaches breaks away from its tRNA molecule and
bonds with the second amino acid forming a dipeptide
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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
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tRNA molecule in the E-site exits the ribosome and can now have
another amino acid attached to it.
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Next tRNA molecule with an anti-codon complementary to the next
codon on the mRNA bind together at the A-site
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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.
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tRNA in E-site exits the ribosome
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Elongation continues until one of three stop codons on the mRNA
is reached.
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At the stop codon the small and large robosomal subunits fall
of the mRNA and the polypeptide is completed
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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
Slide 87
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]
Slide 88
IB Question: The diagram below shows the structure of a
ribosome during protein synthesis. (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] M07/4/BIOLO/HP2/ENG/TZ1/XX+
Slide 89
IB Question: The diagram below shows the structure of a
ribosome during protein synthesis. (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] M07/4/BIOLO/HP2/ENG/TZ1/XX+ 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]
Slide 90
IB Question: Explain briefly how termination of translation
occurs. [2] M07/4/BIOLO/HP2/ENG/TZ1/XX+
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stop/terminator / nonsense codon (is reached); polypeptide is
released; mRNA detaches from ribosome; subunits of ribosome
separate; [2 max] Ignore references to specific codons.
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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.
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mRNA codon table
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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
Slide 96
IB Question: Explain the significance of complementary base
pairing for replication, transcription and translation.
Slide 97
IB Question: Explain the significance of complementary base
pairing for replication, transcription and translation.