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NEL Molecular Genetics 691

Section20.4

Interspersed Elements

Other DNA analyses focus on intervening sequences inserted into DNA. For example,

SINEs(short interspersed elements) and LINEs(long interspersed elements) are often

associated with the genes of retroviruses within the genome and are thought to have

been inserted by those viruses. SINEs and LINEs are often located in areas of the DNA

that appear to be noncoding regions. That is, the DNA in these areas does not code for

one of the known gene products of that species. Although the function of the DNA in

these regions is not known, it is inherited; therefore, changes to these DNA sequences,

such as insertions, are passed to succeeding generations.

If two species have the same SINE or LINE located at precisely the same position in

their DNA, it can be assumed that the insertion occurred only once in a common ancestor.

SINEs and LINEs make ideal markers for tracing evolutionary pathways. They are easy

to find and identify, even if they undergo small mutational changes, because they are

relatively large and recognizable segments of DNA often hundreds of base pairs in length.

The possibility of a mutation reverting to an older form is extremely remote, as the

chances of a SINE or LINE being inserted in exactly the same location in two different

species is highly unlikely.

SINEsrepeated DNA sequences

300 base pairs long that alternate

with lengths of DNA sequences

found in the genomes of higher

organisms

LINEsrepeated DNA sequences

5000 to 7000 base pairs long that

alternate with lengths of DNA

sequences found in the genomes of

higher organisms

Purpose Design Analysis

Problem Materials Evaluation

Hypothesis Procedure Synthesis

Prediction Evidence

Looking for SINEs of Evolution

In this activity, you will use DNA sequences to predict and

chart phylogenetic relationships among species.

Suppose you find a pattern in the noncoding SINE DNA of

two different species, and do not find that pattern in other

species. Evolution can explain the situation by saying that the

two species recently had a common ancestor, and that both

species inherited this pattern from their ancestor. The pre-

dicted family tree is shown in Figure 4.

sections of DNA that appear to be homologous.

These homologous sequences have been aligned

vertically so that similarities and differences can be

easily seen and colours are used to highlight those

nucleotides that are not matches (Figure 6).

LAB EXERCISE 20.B Report Checklist

Time

species A

lacks the SINE

species B

inherits the SINE

common
ancestor
species of
A, B, and C
X
species C
inherits the SINE

common

ancestor

species

of B and C

Figure 4
X indicates the time when the SINE became inserted into the
genome. Since the SINE insertion occurs only once, at time X,
the size and precise location of the SINE will be identical in
species B and C.

Z AAATGGGATAGCGCGTA

Y AAATAGTTAAAGTTACGCATAAATAC

X

W

Species

Species

Species

Species

AAATAGCGCGTAAATAG

AGATAGCGCGTAAAAAG

Figure 5

Sequenced DNA fragments from four distantly related

species

Figure 6

DNA sequences from Figure 5aligned for comparison.

Note that spaces appear in the sequences only to facilitate

comparisons.

Z- AG ATAG

Y- AAATAGTTAAAGTTACGCA TAAATAC

X-

W-

AAATAG

AG ATAG

CGCGTAAATG G

CGCGTAAATAG

Species CGCGTAAAA AG

Species

Species

Species

Part I: Looking for a SINE

Procedure I

1. Examine the hypothetical DNA code from four
   different species (Figure 5). These species have large