IB Biology
Protein Synthesis
Madden/Van Roekel
Transcription
Transcription is carried out in a 5' - 3' direction (of the new RNA strand)
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DNA consists of two polynucleotide strands, only one of which is transcribed into RNA
The antisense strand is transcribed into RNA
Its sequence will be complementary to the RNA sequence and will be the "DNA version" of the
tRNA anticodon sequence
The sense strand is not transcribed into RNA
Its sequence will be the "DNA version" of the RNA sequence (identical except for T instead of U)
A gene is a sequence of DNA which is transcribed into RNA and contain three main parts:
Promoter: Responsible for the initiation of transcription (in prokaryotes, a number of genes
may be regulated by a single promoter - this is an operon)
Coding Sequence: The sequence of DNA that is actually transcribed (may contain introns in
eukaryotes)
Terminator: Sequence that serves to terminate transcription (mechanism of termination
differs between prokaryotes and eukaryotes)
Transcription is the process by which a DNA sequence (gene) is copied into a complementary
RNA sequence and involves a number of steps:
RNA polymerase binds to the promoter and causes the unwinding and separation of the DNA
strands
Nucleoside triphosphates (NTPs) bind to their complementary bases on the antisense strand
(uracil pairs with adenine, cytosine pairs with guanine)
RNA polymerase covalently binds the NTPs together in a reaction that involves the release of
two phosphates to gain the required energy
RNA polymerase synthesizes an RNA strand in a 5' - 3' direction until it reaches the terminator
At the terminator, RNA polymerase and the newly formed RNA strand both detach from the
antisense template, and the DNA rewinds
Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large
number of transcripts
Post-transcriptional modification is necessary in eukaryotes
Eukaryotic genes may contain non-coding sequences called introns that need to be removed
before mature mRNA is formed
The process by which introns are removed is called splicing
The removal of exons (alternative splicing) can generate different mRNA transcripts (and
different polypeptides) from a single gene
IB Biology
Protein Synthesis
Madden/Van Roekel
Overview of Transcription
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Translation
tRNA Molecules
Each different tRNA molecule has a unique shape and
chemical composition that is recognised by a specific
tRNA-activating enzyme
The enzyme (aminoacyl-tRNA synthetase) first binds
the amino acid to a molecule of ATP (to form an amino
acid-AMP complex linked by a high energy bond)
The amino acid is then transferred to the 3'-end of the
appropriate tRNA, attaching to a terminal CCA
sequence on the acceptor stem and releasing the AMP
molecule
The tRNA molecule with an amino acid attached is thus
said to be 'charged' and is now capable of participating
in translation
The energy in the bond linking the tRNA molecule to
the amino acid will be used in translation to form a
peptide bond between adjacent amino acids
Ribosome Structure
Ribosomes are made of protein (for stability) and ribosomal RNA (rRNA - for catalytic activity)
They consist of two subunits:
The small subunit contains an mRNA binding site
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Protein Synthesis
Madden/Van Roekel
The large subunit contains three tRNA binding sites - an aminacyl (A) site, a peptidyl (P) site and
an exit (E) site
Ribosomes can be either found freely in the cytosol or bound to the rough ER (in eukaryotes)
Ribosomes differ in size in eukaryotes and prokaryotes (eukaryotes = 80S ; prokaryotes = 70S)
Ribosomes floating freely in the cytosol produce proteins for use within the cell
Ribosomes attached to the rough ER are primarily involved in producing proteins to be
exported from the cell or used in the lysosome
These proteins contain a signal recognition peptide on their nascent polypeptide chains which
direct the associated ribosome to the rough ER
Translation occurs in four main steps:
The start codon (AUG) is located at the 5' end of the mRNA sequence and the ribosome moves
along it in the 3' direction
Hence translation occurs in a 5' - 3' direction
Initiation: Involves the assembly of an active ribosomal complex
Elongation: New amino acids are brought to the ribosome according to the codon sequence
Translocation: Amino acids are translocated to a growing polypeptide chain
Termination: At certain "stop" codons, translation is ended and the polypeptide is released
Pre-Initiation:
Specific tRNA-activating enzymes catalyse the attachment of amino acids to tRNA molecules,
using ATP for energy
Initiation:
The small ribosomal subunit binds to the 5' end of mRNA and moves along it until it reaches the
start codon (AUG)
Next, the appropriate tRNA molecule binds to the codon via its anticodon (according to
complementary base pairing)
Finally, the large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a
complex with the small ribosomal subunit
Elongation:
A second tRNA molecule pairs with the next codon in the ribosomal A-site
The amino acid in the P-site is covalently attached via a peptide bond to the amino acid in the
A-site
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Protein Synthesis
Madden/Van Roekel
Translocation:
The ribosome moves along one codon position, the deacylated tRNA moves into the E-site and
is released, while the tRNA bearing the dipeptide moves into the P-site
Another tRNA molecules attaches to the next codon in the newly emptied A-site and the
process is repeated
The ribosome moves along the mRNA sequence in a 5' - 3' direction, synthesising a polypeptide
chain
Multiple ribosomes can translate a single mRNA sequence simultaneously (forming polysomes)
Termination:
Elongation and translocation continue until the ribosome reaches a stop codon
These codons do not code for any amino acids and instead signal for translation to stop
The polypeptide is released and the ribosome disassembles back into subunits
The polypeptide may undergo post-translational modification prior to becoming a functional
protein
Overview of the Process of Translation