OCR 2024 A Level Biology B (Advancing Biology) H422/02 Scientific
literacy in biology Question paper and mark scheme merged
Oxford Cambridge and RSA
By: Click2distinction
Friday 14 June 2024 – Morning
A Level Biology B (Advancing Biology)
H422/02 Scientific literacy in biology
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The Eye Melanin
[1]
Melanin is a brown or black pigment that is synthesised from the amino acid tyrosine. Melanin is present in
melanocytes in the skin and the choroid layer of the eye (uveal melanocytes). Uveal melanocytes have
different biological characteristics to melanocytes in the skin. Uveal melanocytes have roles in light
absorption, regulation of oxidative stress, immune regulation and the formation of new blood vessels.
Melanin absorbs infrared light, visible light and ultraviolet radiation. In the front of the eye, melanocytes
block 99.9% of these wavelengths. In the back of the eye, pigment granules reduce light-induced
oxidative stress and act as a shield against scattered light.
Melanin and eye colour [2]
Eye colour is the result of pigmentation of the iris, which helps control how much light enters the eye. The
phenotype of iris pigmentation varies on a continuum: iris colour ranges from very light blue to dark brown. A
person’s genotype determines iris colour, and this involves several genes. Most of
the genes associated with iris colour are involved in the production, transport or storage of melanin. People
with brown eyes have high levels of melanin in the iris, whereas people with blue eyes have much less
melanin.
In humans, a region of chromosome 15 plays a major role in eye colour. There are two genes located very
close together: OCA2 and HERC2. The OCA2 gene produces a protein known as the P protein, which is
involved in the maturation of pigment granules. Several alleles of the OCA2 gene produce lower levels of
functional P protein. This reduces the amount of melanin present in the iris and leads to blue eyes rather than
brown eyes.
A region of the HERC2 gene known as intron 86 contains a DNA sequence that controls the activity of the OCA2
gene. At least one allele in this region has been shown to reduce the expression of OCA2, which can also lead to
the production of less melanin in the iris and therefore lighter-coloured eyes.
Corneal tissue-resident T memory cells [3]
Barrier tissues of the body, such as the skin and mucous membranes, are frequently exposed to pathogens.
These tissues contain networks of immune cells that help protect against infection. Some tissues, such as the
cornea of the eye, have more restricted networks of immune cells and reduced inflammatory responses.
Populations of antigen-presenting cells reside in the cornea. Other immune cells, including T lymphocytes,
are recruited during infection. During infection of the eye by herpes simplex virus (HSV), T helper cells are
involved in the response to infection.
T memory cells can circulate in the blood, spleen, lymph and tissue fluid. In many tissues, these
T memory cells differentiate into tissue-resident T memory cells. The tissue-resident T memory cells consist
of T helper memory cells and T killer memory cells. These cells remain in specific tissues for long periods and
can provide greater protection compared with the circulating pool of T memory cells.
A recent study examined T lymphocyte responses in the cornea of mice after HSV infection. The results
showed that virus-specific T lymphocytes enter the cornea in significant numbers following HSV infection.
Moreover, tissue-resident T memory cells form in the cornea after the virus has been removed and continue
to provide protection against future infection.
1. Photoprotection role of melanin in the human retinal pigment epithelium. Imaging techniques for retinal
melanin. Istrate, et al. (2020) Romanian Journal of Ophthalmology, 64, 100–104.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7339703/
2. Is eye colour determined by genetics? In ‘Genetics and human traits’ MedlinePlus Genetics.
https://medlineplus.gov/genetics/understanding/traits/eyecolor/
3. Corneal tissue-resident memory T cells form a unique immune compartment at the ocular surface.
Loi et al. (2022) Cell Reports, 39, 110852. https://doi.org/10.1016/j.celrep.2022.110852
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