Blueprint of Life
Topic 18: Protein Synthesis
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT(s)
 explain the relationship between proteins and
polypeptides
 outline, using a simple model, the process by which
DNA controls the production of polypeptides
Introduction
Proteins are large, complex
macromolecules made up of one or
more long chains called polypeptides.
www.answers.com
Introduction
Each polypeptide chain consists of a
linear sequence of many amino acids
joined by peptide bonds. (There are
about 20 different amino acids and these
can be linked together in chains of up to
300.) One or more polypeptides can
be twisted together into a particular
shape, resulting in the overall
structure of a protein.
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Introduction
The sequence and arrangement
of amino acids determines the
configuration of the protein.
Any change in the amino acid
sequence that results in a
change in the shape of the
protein molecule could affect
the ability of the protein to
carry out its function in the
cell.
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Protein Synthesis
Multicellular organisms are made up of a variety of different
cells. For example, humans have over 200 different types of cells,
including skin cells, muscle cells, blood cells and many others.
Despite differing in structure, every cell that has a nucleus has a
full copy of the same coded genetic information in its DNA.
globalmedicaldiscovery.com
Protein Synthesis
This encoded information
directs the production of cell
products such as polypeptides
which form proteins, the key to
cell specialisation and
differentiation. In specialised
cells, coded instructions for the
production of a particular
protein (or group of proteins) are
‘switched on’.
www.dailytelegraph.com.au
Protein Synthesis
This ensures that the cell develops a particular structure, in
keeping with the type of tissue to which it belongs. For example
in skin tissue, genes for the pigment protein melanin and for the
protein keratin will be switched on in each cell, ensuring that the
cells become skin cells.
www.sciencedaily.com
The Process
DNA never leaves the nucleus—it holds the original copy of all
instructions. In order for a cell to make proteins, only the
relevant instructions for those proteins are accessed in the DNA
nucleotide sequence.
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The Process
Since the DNA instructions must remain in the nucleus, an
intermediate molecule—messenger RNA (mRNA)—is created;
this carries a transcribed copy of the relevant instructions from
the nucleus to the ribosomes in the cytoplasm. The ribosomes can
be considered as the ‘machinery’ that translates the message
carried by the mRNA into a cell product such as protein.
www.biologycorner.com
The Process
The sequence of information transfer necessary for DNA to
direct the production of proteins is summarised below, in a
framework known in genetics as the central dogma.
DNA → RNA → protein
www.nvo.com
The Chemicals Involved in Protein
Synthesis
DNA
DNA consists of long chains of
nucleotides wound into a
double helix. The sequence of
nucleotide bases determines the
meaning of the message—
because it codes for the
sequence of RNA nucleotides
and ultimately the sequence of
amino acids that form the
polypeptide chain.
academic.pgcc.edu
The Chemicals Involved in Protein
Synthesis
RNA
Like DNA, RNA is a nucleic acid
made from a chain of
nucleotides, but it differs from
DNA in the following ways:
■ Most RNA is single-stranded.
■ The sugar in RNA is ribose
sugar (not deoxyribose sugar as
in DNA).
■ RNA has the nitrogenous base
uracil (U ) instead of thymine
(T).
www.makingthemodernworld.org.uk
The Chemicals Involved in Protein
Synthesis
There are three types of RNA:
messenger RNA (mRNA), transfer RNA
(tRNA) and ribosomal RNA (rRNA):
■ mRNA is single-stranded and is not
twisted into a helix. mRNA molecules
are a few thousand bases long, much
shorter than DNA. They are found in
both the nucleus and cytoplasm.They
function as an intermediate molecule,
carrying information from DNA in the
nucleus to the ribosomes in the
cytoplasm.
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The Chemicals Involved in Protein
Synthesis
■ tRNA molecules occur in the cytoplasm
that carry amino acids which build the new
proteins. Each one is 75 nucleotides long
and twisted into the shape of a clover leaf.
On one end of the tRNA there are three
unpaired bases called an anticodon, which
attach the tRNA to its complementary
bases on the mRNA strand. The other end
of the tRNA is able to bind with an amino
acid temporarily. Each tRNA molecule
will only attach to one particular amino
acid. The specific sequence of three bases
at the anticodon end determines which
amino acid will be carried by that tRNA.
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The Chemicals Involved in Protein
Synthesis
■ rRNA forms a structural part of the ribosomes.
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Protein Synthesis
Protein synthesis can be thought of using a ‘computer’ analogy.
The DNA could be considered as equivalent to the software, the
mRNA is an intermediate made using a software program (e.g. a
graph orWord document or diagram) and the protein would be the
hard copy, printed out.
www.astropix.com
The Steps Involved in Protein Synthesis
1. An enzyme, RNA polymerase, binds
to a part of the DNA called the
promoter and the DNA ‘unzips’— that
is, the DNA unspirals, hydrogen bonds
between the two strands break, and the
strands separate over a short length,
just in that part of the DNA that holds
the gene to be used. Only one strand
of DNA contains the genetic
information to make a protein; rather
confusingly, it is called the non-coding
strand or sense strand; the other strand
is called the coding strand or anti-sense
strand.
The Steps Involved in Protein Synthesis
2. Transcription of the gene
occurs, controlled by the enzyme
RNA polymerase: the sense
strand of the DNA acts as a
template and RNA nucleotides
are assembled, forming a
complementary single stranded
mRNA molecule (that is, DNA is
transcribed into mRNA). The
sequence of nucleotide bases on
the mRNA molecule is the same
as the DNA coding strand, except
that it has U instead of T.
The Steps Involved in Protein Synthesis
3. The mRNA moves out of the nucleus and into the cytoplasm,
where it encounters some of the millions of ribosomes in the
cell.
The Steps Involved in Protein Synthesis
4. Translation: the ribosomes move along the mRNA molecule
and, as they do so, they attach tRNA molecules by temporarily
pairing the bases of the tRNA anticodons with their
complementary triplets of bases (codons) on the mRNA.
The Steps Involved in Protein Synthesis
5. The amino acids are linked together by another enzyme to
form a polypeptide chain. The amino acids are then spliced off
their tRNA carriers.
The Steps Involved in Protein Synthesis
6. The tRNAs move away from the mRNA, leaving the growing
chain of amino acids and move back into the cytoplasm where
they can pick up another amino acid and be reused.
The Steps Involved in Protein Synthesis
7. The polypeptide chain may be joined by one or more other
polypeptides; they are further processed and folded into their
correct shape, forming a protein.
The Steps Involved in Protein Synthesis
8. The mRNA is broken down into its individual nucleotides
which can be reused.
Advances in Understanding
A gene was at first defined as a
sequence of nucleotides that
codes for one protein. Advances
in understanding of the
biochemical functioning of cells
led to the definition changing to
a sequence of nucleotides that
codes for one polypeptide chain.
en.wikipedia.org
Advances in Understanding
More recent research has shown
that some genes code mRNA and
tRNA, which are not proteins at all,
and that in other instances one gene
may code for more than one
polypeptide sequence (due to the
splicing and rearrangement of blocks of
mRNA before translation). Therefore
the definition of a gene may need to
change to a more functional
concept: a sequence of nucleotides
that codes for any molecular cell
product.
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Activity
-Students to watch video:
http://youtu.be/983lhh20rGY
-Students watch video:
http://highered.mcgrawhill.com/olc/dl/120077/micro06.swf
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Homework
-Students to complete The Process of Protein Synthesis
worksheet
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