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Transcription and
Translation AHL
IB Biology HL
Outcomes - Transcription
• 7.3.1 State that transcription is carried out in a 5’ to 3’
direction.
• 7.3.2 Distinguish between the sense and antisense strands
of DNA.
• 7.3.3 Explain the process of transcription in prokaryotes,
including the role of the promoter region, RNA
polymerase, nucleoside triphosphates and the terminator.
• 7.3.4 State that eukaryotic RNA needs the removal of
introns to form mature mRNA.
Outcomes - Translation
• 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.
• 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.
• 7.4.3 State that translation consists of initiation, elongation, translocation and termination.
• 7.4.4 State that translation occurs in a 5’ to 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.
• 7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes.
Let’s see an overview
• http://www.youtube.com/watch?feature=player_e
mbedded&v=itsb2SqR-R0
Transcription – the details
• RNA Polymerase can only work in a 5’ to 3’
direction – in other words, it can only add to a 3’
end so… which strand of DNA is used as a
template?
• The template strand is called the antisense strand
Sense and Antisense Strands
Remember this… 5’ to 3’ direction (always!)
Let’s see…
• http://www.stolaf.edu/people/giannini/flashanima
t/molgenetics/transcription.swf
Promoter and Terminator
RNA Polymerase
• RNA Polymerase will begin at the promoter and it
will read the antisense strand and build a
complementary mRNA strand with free nucleoside
triphosphates until it reaches the terminator.
• Once the terminator is reached, the mRNA will be
released and the DNA will re-anneal unchanged
• The mRNA will then undergo post-transcriptional
modifications
Introns removed
Post-Transcriptional
Modification
http://bcs.whfreeman.com/thelifewire/content/ch
p14/1401s.swf
Translation – the details!
• Once Introns are removed, the mRNA will travel to
the cytoplasm to a ribosome
tRNA
• Each tRNA molecule has a 5’
and a 3’ end just like DNA and
mRNA strands.
• Hydrogen bonds cause the
folding of and resulting three
dimensional shape of the tRNA
molecule.
• The anticodon is located on one
of three loops in the tRNA clover
leaf shape. The anticodon pairs
with a codon located on the
mRNA strand.
tRNA Enzymes
• The 3’ end of the tRNA contains the base sequence
CCA and this is the amino acid attachment site.
• There is a group of 20 enzymes collectively known as
tRNA activating enzymes.
• These enzymes bind each of the 20 amino acids to the
amino acid attachment site of a tRNA molecule with
the help of energy supplied by ATP.
• Each enzyme is specific to a certain amino acid and a specific
tRNA molecule.
• Once the amino acid is attached it is called an activated amino
acid.
• The tRNA can now
deliver the amino
acid to a ribosome
where it can be used
in a polypeptide.
Translation Overview
• http://www-
class.unl.edu/bio
chem/gp2/m_bio
logy/animation/
m_bio_ga.html#
Ribosomes
• The structure of ribosomes
consists of a large subunit and a
small subunit.
• The subunits are composed of
rRNA and many distinct, small
proteins.
• Ribosomes are constructed in the
nucleolus and exit the nucleus
through pores in the nuclear
membrane.
Ribosome binding sites
• Located in the space between the two subunits there are
binding sites for mRNA and three binding sites for
tRNA.
• The rRNA is responsible for binding the mRNA and
tRNA.
Site Function
A site Holds the tRNA carrying the next amino acid to be added to the polypeptide chain.
P site Holds the tRNA carrying the elongating polypeptide
E site The site from which tRNA that has lost its amino acid is discharged from the ribosome.
Three sites for tRNA Stages of Translation
• Similar to transcription, the process of translation
involves several stages including:
• Initiation
• Elongation
• Translocation
• Termination
Direction of Translation
• Translation also occurs in a 5’ to 3’ direction similar
to that of DNA replication and transcription.
Peptide bond between amino acids
KNOW THIS!!!
The Process of Translation
• We will discuss 4 steps in Translation:
• Initiation
• Elongation
• Translocation
• Termination
Initiation
• Initiation, the first step in translation, begins when the
activated amino acid methionine combines with a
mRNA molecule and the small sub unit of the ribosome.
• When the ribosome reads AUG (start codon) hydrogen
bonds begin forming between the initiation tRNA and the
start codon. Now the larger subunit of the ribosome
attaches along with proteins called initiation
factors. These are attached using GTP (guanosine
triphosphate ).
Elongation
• The tRNA molecules start bringing in amino acids
to the mRNA ribosomal complex in the order set out
by the mRNA.
• Proteins known as elongation factors bind the
tRNA’s to the A-site. The initiator tRNA moves to
the P-site and a peptide bond is formed between the
two amino acids.
Translocation
• This phase technically occurs during the elongation
phase.
• Translocation is the movement of the tRNA
molecules from one site to the next as the
polypeptides is being built.
Termination
• This phase begins when one of three stop codons appear
in the A-site.
• A protein known as a release factor enters the A-site
and it catalyzes the release of the now complete
polypeptide from the tRNA in the P-site.
• The released polypeptide signals the separation of the
ribosomal subunits and the mRNA strand.
• A polysome is a string of ribosomes all going through
the process of translation on one single mRNA strand.
Free and bound Ribosomes
• Free ribosomes are found in the cytoplasm and they
synthesize proteins that are used within the cell.
• Ribosomes that are bound would be attached to the
endoplasmic reticulum and the proteins synthesized
here would travel in vesicles to the Golgi apparatus
where they would be modified.