8.4 DNA
Transcription
8.5 Translation
8.4 – DNA Transcription
KEY CONCEPT
Transcription converts a gene into a single-stranded RNA molecule.
Gene: A specific region of DNA that codes for a particular protein
RNA carries DNA’s instructions.
 The
central dogma
states that
information flows in
one direction,
starting with DNA
 Through
Transcription, DNA
changes into RNA
 Through
Translation, RNA
changes into a
Protein.
•
The central dogma includes three processes.
– Replication
– Transcription
– Translation
•
DNA contains the instructions to
make proteins. RNA is a link
between DNA and proteins.
replication
transcription
translation
•
RNA differs from DNA in three major ways.
1. DNA has a deoxyribose sugar, RNA has a ribose sugar.
2. RNA has uracil instead of thymine (found in DNA)
1. A pairs with U
3. DNA is a double stranded molecule, RNA is single-stranded.
Transcription makes three types of
RNA.
 Transcription
copies a piece of DNA (a gene) to make a
strand of RNA.
•
1. Transcription is catalyzed (run) by RNA polymerase (an enzyme).
RNA polymerase and other proteins form a transcription complex.
The transcription complex recognizes the start of a gene and
unwinds a segment.
start site
transcription complex
5. nucleotides
2. ONE strand of DNA serves as a template
Nucleotides pair with one strand of the DNA.
RNA polymerase bonds the nucleotides together.
The DNA helix winds again as the gene is transcribed.
4. DNA
7. RNA polymerase
moves along the DNA
3. The RNA strand detaches from the DNA once the gene is transcribed.
6. RNA
•
Transcription makes three types of RNA.
– Messenger RNA (mRNA) carries the message that will be translated to form
a protein.
– Ribosomal RNA (rRNA) forms part of ribosomes where proteins are made.
– Transfer RNA (tRNA) brings amino acids (protein building blocks) from the
cytoplasm to a ribosome to build the protein.
The transcription process is similar to
replication.
 Transcription
and replication both involve complex
enzymes and complementary base pairing.

Both processes take place in the nucleus.
 The


two processes have different end results.
Replication copies
all the DNA;
transcription copies
a gene.
Replication makes
one copy;
transcription can
make many copies.
one
gene
growing RNA strands
DNA
8.5 – Translation
KEY CONCEPT
Translation converts an mRNA message into a polypeptide, or
protein.
Amino acids (protein building blocks)
are coded for by mRNA base sequences.
 A codon
is a sequence of three nucleotides that codes
for an amino acid.
codon for
methionine (Met)
codon for
leucine (Leu)
•
The genetic code matches each codon to its amino acid or function.
– one start codon
(AUG), codes for
methionine and to
start translation
– three stop codons
(UAA, UAG, UGA)
signal the end of a
chain of amino
acids.
The genetic code matches each RNA codon with its amino acid or function.
•
•
Reading frame: multiple codons that code for a chain of amino acids
A change in the order in which codons are read changes the resulting
protein – this is why having a clear “start” and “stop” is important
•
Common language: Regardless of the organism, codons code for the same
amino acid.
Amino acids are linked to become a
protein.
 An
anticodon is a set of three nucleotides that is
complementary to an mRNA codon.
 An anticodon is carried by a tRNA. tRNA carries amino
acids from cytoplasm to the ribosome to become part of
the growing protein.
EXAMPLE:
mRNA codon=GUU
tRNA anticodon=CAA
Amino acid=Valine
Where does translation happen?
•
•
•
Ribosomes are the site of protein synthesis. They are located in the cytoplasm
& on the rough ER.
Ribosomes are made of rRNA & proteins. They have a large & small subunit.
Helps form peptide bonds between amino acids.
How does translation happen?
1. For translation to begin, tRNA binds to a start codon (Met in picture) and
signals the ribosome to assemble.
– A complementary tRNA molecule binds to the exposed codon
(Leu in picture), bringing its amino acid close to the first amino acid.
2. The ribosome helps form a polypeptide bond between the amino acids.
The ribosome pulls the mRNA strand the length of one codon.
3. The now empty tRNA molecule exits the ribosome.
– A complementary tRNA molecule binds to the next exposed codon.
– Once the stop codon is reached, the ribosome releases the protein
and disassembles.