Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
416
At some later time, probably around 1892, faulty
cell division in S. townsendii somehow produced cells
with double the number of chromosomes. A tetraploid
plant was produced, probably from the fusion of two
abnormal diploid gametes from S. townsendii. So this
tetraploid has two sets of chromosomes that originally
came from S. maritima, and two sets from S. alterniflora.
It is an allotetraploid. These chromosomes can pair up
with each other, two and two, during meiosis, so this
tetraploid plant is fertile. It has been named S. anglica. It is
more vigorous than any of the other three species, and has
spread so widely and so successfully that it has practically
replaced them in England.
Molecular comparisons
between species
Molecular evidence from comparisons of the amino acid
sequences of proteins and of the nucleotide sequences of
mitochondrial DNA can be used to reveal similarities
between related species.
Comparing amino acid sequences of
proteins
As you saw in Chapter 16, changing a single amino acid in
the primary structure of a protein may cause a dramatic
change in its structure and function. However, for many
proteins, small changes in the amino acid sequence leave
the overall structure and the function of the protein
unaltered. Typically, the part of the molecule essential for
its function (such as the active site of an enzyme) remains
the same, but other parts of the molecule may show
changes. When the amino acid sequence of a particular
protein is compared in different species, the number of
differences gives a measure of how closely related the
species are.
Let us take cytochrome c as an example. Cytochrome c
is a component of the electron transfer chain in oxidative
phosphorylation in mitochondria (pages 273–275). A
protein with such an important function is expected to
have a similar sequence of amino acids in different species
since a poorly adapted cytochrome c molecule would
result in the death of the organism.
When the sequences of cytochrome c from humans,
mice and rats were compared, it was found that:
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all three molecules consist of 104 amino acids
the sequences of mouse and rat cytochromes c are
identical
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nine amino acids in human cytochrome c are different
from the mouse or rat sequence
most of these substitutions in human cytochrome c
are of amino acids with the same type of R group
(Appendix 1).
This comparison suggests that mice and rats are closely
related species, sharing a recent common ancestor, and
that humans are more distantly related, sharing a common
ancestor with mice and rats less recently.
When the sequences of cytochrome c from other species,
such as a fruit fly or a nematode worm, are also examined,
the number of differences from the human sequence
increases. These organisms are less closely related to humans.
Comparing nucleotide sequences of
mitochondrial DNA
Differences in the nucleotide sequences of mitochondrial
DNA (mtDNA) can be used to study the origin and spread
of our own species, Homo sapiens. Human mitochondrial
DNA is inherited through the female line. A zygote contains
the mitochondria of the ovum, but not of the sperm.
Since the mitochondrial DNA is circular (page 276) and
cannot undergo any form of crossing over, changes in the
nucleotide sequence can only arise by mutation.
Mitochondrial DNA mutates faster than nuclear DNA,
acquiring one mutation every 25 000 years. Unlike nuclear
DNA, mitochondrial DNA is not protected by histone
proteins (page 95) and oxidative phosphorylation in the
mitochondria (page 273) can produce forms of oxygen that
act as mutagens.
Different human populations show differences in
mitochondrial DNA sequences. These provide evidence for
the origin of H. sapiens in Africa and for the subsequent
migrations of the species around the world. These studies
have led to the suggestion that all modern humans, of
whatever race, are descendants from one woman, called
Mitochondrial Eve, who lived in Africa between 150 000
and 200 000 years ago. This date is derived from the
ʻmolecular clockʼ hypothesis, which assumes a constant
rate of mutation over time and that the greater the
number of differences in the sequence of nucleotides, the
longer ago those individuals shared a common ancestor.
The ‘clock’ can be calibrated by comparing nucleotide
sequences of species whose date of speciation can be
estimated from fossil evidence.