Transcript Chapter 3 - Cell Protein Production

Cell Protein Production
Chapter 3
© 2010 McGraw-Hill Australia
To learn about the stages of protein synthesis click on the stage names in blue.
Transcription
Protein Formation
Summary
Translation
© 2010 McGraw-Hill Australia
Transcription
• Transcription is the process
by which the mRNA is formed
using the code in a DNA
molecule
• The genetic instructions are
copied (or transcribed) from
the DNA to the mRNA
molecule
• The enzyme RNA polymerase
begins the process of making
mRNA
© 2010 McGraw-Hill Australia
Transcription (cont.)
• RNA polymerase makes the
double-stranded DNA
molecule come apart, usually
about 17 base pairs at a time
• RNA polymerase then
transcribes (copies) the bases
on one strand of the DNA to
make a complementary
molecule of mRNA
© 2010 McGraw-Hill Australia
Transcription (cont.)
• RNA molecules have the
base uracil (U) instead of
thymine
• At the end of the gene
there is a sequence of
bases that tells the RNA
poly-merase to stop
copying and as a
consequence the mRNA
molecule is released
© 2010 McGraw-Hill Australia
Transcription (cont.)
• The strand that is copied is
called the template strand
because it is the template
from which the mRNA is
made. The other strand is
known as the coding strand
• Not all of the bases in the
DNA molecule are code for
an amino acid. The noncoding sequences are called
introns
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Transcription (cont.)
• The base sequences that
code for amino acids are
called exons
• The functional mRNA will
leave the nucleus and direct
protein synthesis in the
cytoplasm
Return
© 2010 McGraw-Hill Australia
Translation
• Translation is the production of a protein
using the information that is coded in the
mRNA molecule
• In the cytoplasm a
ribosome attaches to
one end of the
mRNA molecule.
using a sequence that
is the binding site for
the ribosome
© 2010 McGraw-Hill Australia
Translation (cont.)
• The ribosome then moves along the mRNA
three bases at a time, reading the bases as it goes
• When the ribosome
reaches the start
codon (AUG) it starts
making the protein
Return
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Protein Formation
• Transfer RNA (tRNA) bring the
individual amino acids to the
ribosome to be joined together as
proteins
• Each tRNA molecule has a
section that binds to its
corresponding amino acid
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Protein Formation (cont.)
• The loop of the tRNA has three
nitrogen bases that form an
anticodon. These three bases
bind with the complementary
bases of a codon on the mRNA
molecule
• The anticodon determines the
type of amino acid carried by the
tRNA
© 2010 McGraw-Hill Australia
Protein Formation (cont.)
• As the ribosome reads the
codons on the mRNA, tRNA
molecules with the matching
anticodon are brought in
• The amino acids carried by the
tRNA are joined together so the
protein is assembled with the
amino acids in the correct
sequence
© 2010 McGraw-Hill Australia
Protein Formation (cont.)
• Once the tRNA has delivered its
amino acid it detaches from the
ribosome and can pick up another
amino acid from the cytoplasm
• One mRNA often has 10 or 20
ribosomes reading its code at the
same time
• This means that a cell could produce
over 150 000 protein molecules per
second
Return
© 2010 McGraw-Hill Australia
Summary
•
Each gene in the DNA in the cell nucleus carries the code for
making a protein from amino acids
Transcription
•
The two strands of the DNA in a gene separate
•
mRNA forms with bases that are complementary to those on the
template strand of the DNA
•
The mRNA molecule that is formed undergoes modification –
the introns are removed leaving just the exons
•
mRNA travels from the nucleus to the cytoplasm
Translation
•
mRNA attaches to a ribosome
•
The ribosome moves along the mRNA reading the code. At the
start codon it begins making the protein
•
For each codon on the mRNA, a tRNA with a matching
anticodon brings the correct amino acid
•
The amino acids are bonded together to form the protein
© 2010 McGraw-Hill Australia