(4) DNA coding and full details of protein synthesis in ribosomes
A brief summary before all the
details
The sequence of bases in a gene is the genetic code for putting
together amino acids in the correct order to make a specific protein -
triplet codes indicate the order in which amino acids are assembled
together to synthesise the specific protein..
DNA controls cell function by controlling the production of proteins
which can enzymes, antibodies, receptors for neurotransmitters etc.
etc. are all made!
Summarising the synthesis of a protein involves:
The gene coding for the protein remains in the nucleus.
The mRNA molecules carry a copy of the gene to the cytoplasm.
The mRNA molecule passes through a ribosome.
The ribosome assembles amino acids in the correct order to
produce a specific protein molecule.
The specific order of amino acids is determined by the sequence
of bases in the mRNA
(Knowledge of the details of transcription or translation is may
or may not be required)
As already mentioned, DNA polymer molecules contain
the genetic codes that determine which proteins are synthesised.
These synthesised proteins control how all the
cells in an organism function, in other words the DNA controls the
production of all proteins - protein synthesis in the ribosomes, one of
the sub-cellular structures in the cytoplasm of cells.
A short section of DNA that codes for a
particular protein is known as a gene.
This means
each gene codes for a particular
set of amino acids that form a protein.
It is the order of the bases in the gene
that determines the order of the amino acids in the protein.
Every gene has a different sequence of
bases to code for all the proteins an organism requires.
Only 20 different amino acids are used to
synthesise all the thousands of different proteins.
It is the genes of the DNA that tells the ribosomes in cells the correct order to assemble the amino acids
to make a specific protein.
A ribosome, a tiny structure in the cytoplasm, is essentially a
protein factory
that makes everything from enzymes, keratin, muscle fibre cells, red
blood cells etc. and all based on the DNA codes.
Every protein, a polymer chain of amino acids, has
a unique structure based on a specific number of amino acids
AND a specific sequence of amino acids.
Each protein also has a specific
3D shape, essential for it to carry out its particular function e.g. an
enzyme or type of tissue.
The order of bases in a gene of the DNA determines
the order of amino acids which will combine to form a specific the protein,
which in turn, will perform a specific function in the living organism.
Every amino acid is coded for by a sequence of three
bases in the gene, known as a
triplet code (illustrated by
diagram below for three 'fictitious' amino acids).
Every gene contains a different sequence of
bases so it can code for a particular protein.
The order of bases on an organism's DNA is called
the genetic code
of the genome.
The genome is the whole of an organism's genetic material.
See
The GENOME and gene expression
- considering chromosomes, alleles, genotype, phenotype, variations
Examples of triplet base codes for amino acids
Example of three triplet codes based on the four
bases: adenine A, thymine T, cytosine C, guanine G along the DNA molecule.
The triplet base codes are called
codons.
Triplet code and amino acid: CCA is for proline, TCG
is for serine and AGA for arginine
The amino acids are joined together to make the
various proteins dependent on the order of the bases in the gene.
The diagram above shows how the triplet codes on DNA
work.
A sequence of three bases (e.g. CCA) on a single strand of DNA codes
for a particular amino acid. A sequence of three triplet codes will code for
three amino acids in that particular sequence on that part of the gene.
Using letters to represent the sequence of bases
on a strand of DNA is an example of a scientific model.
Reminder: The double helix
structure of DNA is another spatial scientific model and this model
must be tried and tested in the laboratory and all observations must
back up any hypothesis to become a workable scientific model.
The cell chemistry allows the reading of the
genetic triplet codes (sequence of bases) on the DNA code to eventually join these three amino acids together
in the precise
order dictated by the DNA code. In fact for any protein you are
actually dealing with sequences of dozens-hundreds of triplet codes for
a particular protein.
The
formation of mRNA and the
actual synthesis of proteins in cytoplasmic ribosomes
DNA is found in a cell's nucleus and cannot
move from it through the nucleus membrane because of the large size of its
molecules.
Therefore there must be a means of getting the genetic
information from the nucleus to the tiny structures, called ribosomes in
the cytoplasm, in which the
proteins are synthesised.
This is achieved using a molecule called messenger
ribonucleic acid (mRNA, a type of RNA) i.e. how the cell gets the
code from the nucleus to the ribosomes - the mRNA is a sort of
'messenger'.
mRNA is shorter than DNA and a single strand
molecule, but still another polymer of nucleotides, but small enough to
exit through the membrane of the nucleus.
The mRNA is the code used in the ribosomes to
connect the amino acids together in the right order to assemble the
protein molecule.
Note that there is an important difference between DNA and
RNA.
In RNA the base thymine (T) is replaced by the base
uracil (U),
so the base pairings in RNA are C-G (as in DNA) but A-U in
RNA
(not A-T as in DNA).
As illustrated above, the DNA contains the gene's
triple coding system for the amino acids to needed to be combined to form a
specific protein - with specific molecular properties to perform a
particular chemical function in an organism.
The process of
TRANSCRIPTION - transferring the genetic code
The
mRNA is made by copying the DNA base sequence of a gene - the process of
transcription.
In the nucleus, using enzymes, the two strands of the
DNA double helix unzip and become a template for the production of
mRNA (messenger ribonucleic acid).
The enzyme RNA polymerase binds to the
non-coding DNA in front of a gene sequence of bases.
The two DNA strands of the double helix unzip and the RNA
polymerase moves along one of the strands of the DNA (see diagram on right).
Therefore the RNA polymerase uses the DNA coding of a gene as a
template to make the mRNA.
Note: In the mRNA molecule, the base uracil
(U) replaces the base thymine (T) in pairing up with adenine (A).
By pairing up the complementary bases on the DNA and
RNA, the correct sequential nucleotides in the nucleus are brought together to form a
complementary strand of mRNA, a step in the overall process called
transcription taking place in the nucleus.
This means the mRNA is complimentary to the gene.
The smaller mRNA molecule can now
migrate out of
the cell nucleus into the cytoplasm and attach themselves to a ribosome
(the actual protein 'factory'!).
The process of TRANSLATION - building the amino acid chain of the protein
In the cytoplasmic ribosomes, the mRNA now
itself acts as a template of triplet codes for amino acids to be joined
together in the correct sequence for a specific protein.
In order for this to happen, the amino acids in the cytoplasm are drawn into the
ribosome complex and assembled in order to match the complementary
triplet codes.
The correct amino acids are brought to the ribosomes
by a carrier molecule called transfer ribonucleic acid (tRNA).
The amino acids are then joined together, by enzymes,
in the correct order to make a particular protein in the ribosome.
The order of the amino acids connected together in
the ribosome will match the order of the base triplets (called codons) on the
mRNA molecule.
The complimentary triplet base sequence on the
tRNA structure is called the anticodon.
This production of the protein, dictated by the complementary triplet codes on the mRNA, is called the
translation
stage, and this takes place in the cytoplasm.
So, the RNA and appropriate enzymes in the
ribosome, join the amino acids together to form the protein - a
polypeptide - meaning a polymer formed from the amino acid monomer
units.
Immediately after its synthesis, the protein adopts its own unique 3D
structure - its specific shape.
Translation
The above diagram shows translation in more detail,
including the role of another type of RNA - transfer ribonucleic acid (tRNA)
which brings the amino acids together onto the mRNA.
-
Points to consider when studying the
translation diagram above
-
The joining together of the amino acids on the mRNA is done using
transfer ribonucleic acid (tRNA).
-
These relatively short molecules of tRNA
actually bring the amino acids together to match the mRNA triplet codes.
-
In
other words the triplet codes of tRNA and mRNA are also complementary.
-
Note that In RNA (mRNA or tRNA) the base thymine (T) has been
replaced by the base uracil (U), so complimentary base pairing is now U-A (not
A-T), but C-G retained and its still all about matching complimentary
base pairs.
-
The sequence of events is as follows:
-
The attachment of the mRNA to the ribosome
-
The coding
by triplets of bases (codons) in the mRNA for specific amino acids
-
The
transfer of amino acids to the ribosome by tRNA (transfer ribonucleic acid)
-
After the mRNA joins onto a ribosome,
molecules of transfer RNA (tRNA) bring the amino acid that matches the
code on the mRNA, the complimentary base codes of the mRNA and tRNA
ensure that all proteins are synthesised with their specific protein
sequence, so all proteins are completely reproducible.
-
The tRNA is then 'empty' and free to collect
another set of amino acids for the ribosome to join up.
-
The linking of amino acids to
form polypeptides
-
The ribosome then acts as the catalytic
site for linking the amino acids together to synthesise a specific
protein.
-
This second process is called translation
because the triplet base code sequence is read and
translated into the amino acid sequence of a protein.
-
A sequence of amino acids joined
together in a chain is called a polypeptide, a natural polymer or
macromolecule.
-
All of these reaction are catalysed by
enzymes.
|
(6)
DNA-proteins: Diagrammatic summary of the synthesis of proteins in the
ribosomes
Doc Brown's GCSE level Biology exam study revision notes
INDEX
of notes: DNA, RNA, synthesis of proteins and functions of
proteins
(6) A SUMMARY diagram of
protein synthesis
So, to summarise, you start with DNA in the nucleus,
then to complementary mRNA in the nucleus (transcription stage), mRNA moves
into the cytoplasm and then the amino acids are joined together in the
ribosomes via the complementary triplet codes (translation stage).
The
diagram 'sketch' below also 'attempts' to summarise what is actually a very
complicated process!
The diagram below puts protein synthesis in
perspective of starting with the genome of a cell's nucleus.
|
Key points
about
protein synthesis
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
including DNA
triplet coding and protein synthesis, broken
down into clear sections to help you master this essential topic.
DNA Triplet Coding
DNA (deoxyribonucleic
acid) carries the genetic
instructions for building proteins.
It’s made of two strands forming a
double helix, with four nitrogenous bases: Adenine (A), Thymine (T),
Cytosine (C), and Guanine (G).
-
Triplet Code:
A sequence of three DNA bases codes for one amino acid. This is
called a base triplet or codon.
-
The sequence of triplets
determines the exact order of amino acids, which in
turn determines the protein’s structure and function.
The Role of RNA in Protein
Synthesis
Because DNA is too large to
leave the nucleus, the instructions are carried out by RNA
(ribonucleic acid).
Types of RNA:
-
mRNA (messenger
RNA) – carries the genetic
code from DNA to the ribosome.
-
tRNA (transfer
RNA) – brings specific
amino acids to the ribosome during translation.
-
rRNA (ribosomal
RNA) – part of the
ribosome’s structure, helps catalyse the reaction.
RNA uses Uracil (U)
instead of Thymine (T).
So where DNA has A–T, RNA has A–U.
Stages of Protein Synthesis
Protein synthesis happens in
two main stages: Transcription and Translation.
1.
Transcription (in the
nucleus)
-
Goal:
Copy the DNA code into a strand of mRNA.
-
The enzyme RNA
polymerase binds to the DNA at the beginning of a gene.
-
It separates the two
strands and uses one as a template.
-
RNA bases (A, U, C, G)
pair with the complementary DNA bases (T → A, A → U, C ↔ G).
-
This forms a
single-stranded mRNA copy, which then exits the nucleus and
travels to a ribosome in the cytoplasm.
2.
Translation (at the
ribosome)
-
Goal:
Convert the mRNA code into a chain of amino acids (a polypeptide).
-
The mRNA attaches to a
ribosome.
-
tRNA
molecules bring amino acids to the ribosome. Each tRNA has an
anticodon that matches a codon on the mRNA.
-
The ribosome matches each
mRNA codon with the correct tRNA anticodon and joins the amino acids
using peptide bonds.
-
This process continues
until a stop codon is reached. The polypeptide chain is
released and folds into a functional protein.
Importance of Protein
Synthesis
Understanding this process is
key to grasping how:
-
Genes control
traits – because they
determine protein structure.
-
Mutations affect
health – a single base
change can alter an amino acid, potentially damaging the protein’s
function.
-
Enzymes, hormones,
antibodies, and structural proteins
are all built from the same basic process.
Summary of learning objectives and key words or phrases
Be able to understand and describe how the DNA triplet codes
for protein polypeptide synthesis in the ribosomes.
Know the functions of the RNA, mRNA, tRNA molecules involved
with transcription, translation and decoding processes.
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