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A* A Level Biology essay: Importance of complementary shapes and molecules

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This essay explains the vital role of complementary shapes and molecular structures within biological systems. It covers key examples including complementary base pairing in DNA and RNA, antibody-antigen interactions, haemoglobin function, neurotransmitter-receptor binding at neuromuscular junctions, and hormone-receptor specificity in the endocrine system. The essay connects these molecular interactions to essential physiological processes such as transcription, immune response, gas exchange, muscle contraction, and blood glucose regulation.

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July 26, 2025
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3
Written in
2024/2025
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Essay
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The importance of complementary shapes and molecules within
organisms

In DNA, the nitrogenous bases are complementary to one another.
Adenine and Thymine are complementary so bond forming 2 hydrogen
bonds and cytosine and guanine form 3 hydrogen bonds. The
complementary base pairs are essential in creating the stable rigid double
helix structure of DNA, consisting of 2 strands of DNA joined by hydrogen
bonds between bases. The hydrogen bonds are strong enough to keep the
structure together in normal conditions. However, they are weak enough
that DNA can unwind readily like in transcription DNA helicase causes the
hydrogen bonds between complementary bases to break, exposing the
strands to act as templates in semi-conservative replication. And again,
complementary base pairs are importance when the free nucleotides line
up against the exposed bases as they bond using RNA polymerase. This
pre-mRNA is spliced and used for translation to produce polypeptides. So,
without the breaking and reforming of hydrogen bonds between
complementary bases, transcription and translation cannot occur, so no
polypeptides produced so no essential proteins are made – such as carrier
proteins, enzymes, hormones, receptors, and antibodies. These are all
essential to bodily function and without them we would struggle to
survive. For example, without production of carrier proteins we would not
be able to transport anything by facilitated diffusion, so no ions, no resting
potentials established, and action potentials will be faulty due to the lack
of Na+ and K+ channels. Thus, the nervous system will fail so no impulses
are relayed between the brain, CNS, and parts of the body – which is
extremely dangerous in fight or flight situations.



Antibodies and antigens are important complementary molecules in the
immune system. Antibodies are produced by lymphocytes; they are
globular proteins (so again not present without complementary DNA bases
causing transcription and translation). They have two heavy and two light
chains, forming the constant and variable regions in a Y shape. The
variable region is the specific binding site, complementary only to one
antigen due to its specific tertiary protein structure (due to hydrogen
bonds, ionic bonds, and disulfide bridges). This means when an antigen is
detected the specific antibody can bind forming an antibody-antigen
complex and many of these can agglutinate, increasing efficiency of
phagocytosis. Thus, the antigens are destroyed and removed quickly for a
quick recovery. So, it’s evident that antibodies having a complementary
binding site to a specific antigen is important for the immune response,
fighting off pathogens to reduce disease for a healthy and long life. A
R123,59
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