Text: Chapter 3
Human Biology Stage 3
Keywords
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DNA
Protein synthesis
mRNA
Transcription
 RNA polymerase
 Template strand
 Coding strand
 Codon
 Intron
 Exon
 Ribosomes
 tRNA
 Anticodon
 Translation
 Start codon methionine
 Protein structure
 Endoplasmic reticulum
(ER)
 Golgi bodies
Protein synthesis
 Synthesis is the
combining of small
molecules to make larger
molecules
 Proteins are long chains
of amino acids. They
are very large and consist
of several hundred
amino acids
 Proteins have a coiled
shape
Functions of proteins
Proteins have many different
functions:
 Structure: the protein
keratin gives strength to
hair, nails etc.
 Transport across
membranes: some proteins
form channels in cell
membranes to aid the
transport of molecules
 Communication: many
hormones are proteins.
 Cell metabolism: enzymes
are proteins . Enzymes
help speed up the chemical
reactions in a cell.
 Recognition: proteins in
the cell membrane are
unique to a particular
person
 Movement: protein
molecules are able to
change shape and this is
the basis for the movement
of structures within the
cell.
The genetic code
 The blue print for the body’s
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proteins are contained within DNA
The genes which form DNA
determine the types of proteins a
cell is able to make
Genes are made up of a
combination of 4 chemical ‘letters’
These chemical ‘letters’ are:
 Adenine & thymine
 Guanine & cytosine
They always occur in pairs
Each amino acid is coded for in a
sequence of 3 bases. This is called a
codon
Transcription
 The DNA molecule is too
large to leave the nucleus,
so the code for each amino
acid is taken from the DNA
to the ribosomes by a
special molecule called
RNA (ribonucleic acid)
 This process is called
transcription
 RNA is slightly different to
DNA. Instead of the
chemical ‘letter’ thymine,
it has the chemical ‘letter’
uracil
Transcription
 There are 2 types of RNA
 mRNA
 tRNA
 The RNA that takes the
code from the DNA to
the ribosomes is called
messenger RNA
(mRNA)
 mRNA is small enough
to pass through the pores
in the membrane around
the nucleus
Transcription
 Transcription is the process
by which the mRNA molecule
is formed using the code in
the DNA molecule
 The genetic instructions are
copied from the DNA to the
mRNA molecule
 Transcription is triggered by
chemical messengers that
enter the nucleus from the
cytoplasm and bind to the
DNA at the relevant gene
Transcription
 This causes an enzyme
called RNA polymerase
to begin the process of
making mRNA
 RNA polymerase makes
the double stranded
DNA molecule come
apart, about 17 base pairs
at a time
Transcription
 RNA polymerase then
transcribes (copies) the
bases from one strand of
the DNA to make a
complementary molecule
of mRNA
 At the end of the gene
there is a sequence of bases
that tells the RNA
polymerase to stop copying
and the mRNA molecule is
released
Transcription
 Only one of the strands of DNA is copied during
transcription
 This strand is called the template strand
 The other strand is called the coding strand
 Because the bases always form complementary pairs,
the order of bases in the mRNA molecule will be the
same as in the coding strand, but opposite to the
template strand
Transcription
 Not all the bases in the DNA
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molecule are code for an
amino acid
The non-coding sequences
are called introns
The base sequences that code
for amino acids are called
exons
The mRNA that has been
copied, contains both introns
and exons
Introns must be removed
before the mRNA can be used
to assemble amino acids into
a protein
Translation
 Translation is the process
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whereby a protein is
synthesised from the
information contained within
a molecule of mRNA
Translation occurs at the
ribosome
The starting sequence of
bases (AUG) code for the
amino acid methionine
This is the start codon that
every protein begins with (it
may be removed later)
This ensures that the
ribosome attaches to the
correct end of the mRNA
Translation
 The ribosome then
moves along the mRNA
three bases at a time
 The ribosome pulls the
mRNA through like a
ribbon, reading the
bases as it goes
Translation
 Small molecules of RNA
called transfer RNA
(tRNA) bring the
individual amino acids to
the ribosome to be joined
together as proteins
 There is at least one kind
of tRNA for each of the 20
amino acids
 Each tRNA molecule has a
section that binds to its
corresponding amino acid
Translation
 Half way along the molecule,
the tRNA forms a tight loop.
The loop has 3 nitrogen bases
that form an anticodon
 These 3 bases of the
anticodon can bind with the
complementary bases of a
codon in the mRNA molecule
 The anticodon determines
the type of amino acids
carried by the tRNA
Example: the tRNA with the
anticodon CGG always carries
the amino acid alanine
Translation
 As the ribosome reads the
codon on the mRNA, the 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 right order
 Each bond formed between the
amino acid requires 1 ATP
 Once the tRNA has delivered its
amino acid it detaches from the
ribosome, it can then pick up
another amino acid from the
cytoplasm
http://www.cjhs.org/teacherssites/taylor/ac
cbio/DNA-PROTEINSYNTHESIS/PROTEIN-SYNTHESISMOVIES/3%20translation.swf
 After a protein has been synthesized, it then must be
organised in a particular shape
 Protein shape is very important for the correct
functioning of the protein
 Proteins have 3 or 4 levels of complexity in their
structure
Protein Structure
1.
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4.
Primary structure: is the
sequence of amino acids in
the chain that makes up the
protein
Secondary structure: this is
either a coiled or folded shape
that is brought by bonds
between different parts of the
amino acid chain
Tertiary structure: this is
formed by further bending
and folding of the protein into
globular or fibrous shapes
Quaternary structure: when 2
or more chains of amino acids
interact
Packaging of proteins
 If proteins are to be used
outside the cell, they
must be packaged for
secretion
 After proteins are made
at the ribosome, they
travel through the ER
into the golgi bodies
where they are modified
and packaged via
exocytosis
Lipid and carbohydrate synthesis
 DNA only contains code for the manufacture of
proteins
 However, it contains code for the production of
enzymes (which are proteins) and therefore DNA
indirectly controls the synthesis of lipids and
carbohydrates
PROTEIN SYNTHESIS – TRANSLATION & TRANSCRIPTION
Class resources
https://www.youtube.com/watch?v=h3b9ArupXZg&index=46&list=PL7A750281106
CD067
Bozeman Science
Transcription and Translation: 12 mins
Student resources
• http://www.egs.ie/files/translation.swf
• http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sit
es/dl/free/0072437316/120077/micro06.swf::Protein+Synthesis
• http://www.wiley.com/college/test/0471787159/biology_basics/animations/fromG
eneToProtein.swf
• http://highered.mheducation.com/sites/dl/free/0073525634/291136/mRNA_synt
hesis.swf
• http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf