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34(^1) DNA
Long molecules of DNA (deoxyribonucleic acid) are
found in every body cell. When magnified, DNA
resembles a twisted ladder with “rungs” made
from four types of chemicals called bases (shown in
colour). The sequence of bases along a section of
DNA forms an instruction – a gene – for making
one of the proteins that build and run a cell.
(^2) CHROMOSOMES
A cell’s DNA molecules are packaged into 23 pairs
of chromosomes, which are found in the nucleus,
the cell’s control centre. Normally they are long and
thin, but when a cell prepares to divide into two new
cells, each chromosome shortens – as its DNA coils up
- and duplicates, taking on the X-shape shown here.
(^3) INHERITANCE
We inherit half of our chromosomes from each of
our parents. A set of 23 chromosomes contains about
25,000 genes. Maternal and paternal chromosomes
carry matching pairs of each gene – such as the gene
that controls eye colour – but not necessarily the
same version. If two versions are present, only
one – the dominant gene – has an effect.
(^4) GENETIC SIMILARITY
As brothers and sisters inherit a selection of genes
from the same two people, their parents, they
are more likely to resemble each other than
they would a non-relative. Identical twins
share almost identical genes, so they look
the same and are of the same sex.
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