Molecular
Biology
2.7 DNA Replication,
transcription and translation
DNA to Protein Synthesis
 The
function of most genes is to specify
the sequence of amino acids in a
particular polypeptide.
 Two processes are needed to produce a
polypeptide using the base sequence of
a gene.


Transcription
Translation
Transcription

Transcription is the synthesis
of mRNA copied from the
DNA base sequences by RNA
polymerase.

The enzyme RNA polymerase
binds to a site on the DNA at
the start of a gene
The RNA polymerase
separates the double strand
of DNA and pairs up RNA
nucleotides with
complementary bases.

Transcription
 The
RNA polymerase forms
covalent bonds between
the nucleotides
 Transcription stops at the
end of the gene. The
completed mRNA is
released.
Transcription
 The
completed mRNA has
a base sequence that is
complementary to the
template DNA strand.

Note: Uracil replaces
Thymine
Complete the transcription portion of
the practice sheet
Translation
 Translation
is synthesis of
polypeptides on
ribosomes
 Ribosomes
are complex
structures that consist of
a small and large subunit with binding sites for
each of the molecules
used in translation.
Translation
 The
amino acid
sequence of
polypeptides is
determined by mRNA
according to the
genetic code
Translation
 Codons
of three bases
on mRNA correspond to
one amino acid in a
polypeptide
 “Genetic
Code” = The
translation “dictionary”
that enables the cellular
machinery to convert
the base sequence on
mRNA into the amino
acid sequence
Translation
 There
are 20 amino
acids and only 4
nucleotide bases.
 Living organisms use a
triplet code with 3 bases
coding for an animo
acid.
 The sequence of 3
bases on the mRNA =
CODON
Examine the list of codons
Translation
 There
are 3 “stop”
codons that code for
the end of translation.
 Different codons can
code for the same
amino acid.

The code is
“degenerate”
Translation
 Translation
depends on
complementary base
pairing between codons
on mRNA and
anticodons on tRNA
 The
amino acids are
carried on tRNA
 The tRNA has a 3 base
ANTICODON
complementary to the
mRNA codon
Complete the
translation portion of
the worksheet