DNA Similarities
Suppose you could compare the total DNA sequences of various organisms (some billions of base
pairs). How much similarity would you expect between a whale and a fish? A whale and a dog? A dog
and a shrimp? A shrimp and a bacterium? As always, there are two types of similarity to be
considered: analogy and homology. If the sequences show similarity because they code for proteins
with similar functions, analogy would be the explanation. If the sequences show similarity because of
common ancestry, then it is homology.
We now have techniques for determining DNA sequences rapidly, and we know the sequences of the
entire genome for many organisms. The entire human genome was finished in 2003. Surprisingly,
only a small portion of the DNA of higher plants and animals is actually genes. Some of the rest is
involved in controlling when genes are transcribed; but a lot of the DNA seems to be just "filler," or
"junk DNA." Experiments in which changes are made in these regions show no effect on gene
function at all. For most higher plants and animals, the amount of DNA which actually codes for
proteins is 5% or less. Even allowing for control sequences, at least 90% of the DNA is functionless,
and rather variable even within species.
Suppose there is a species of mice, and a small population becomes isolated. Reproductive isolation
occurs, and there are now two species of mice.
I know you are not a molecular biologist. Just speculate.
1: How could the sequences of their filler DNA change?
2: Does every mutation that changes the DNA sequence within a gene cause a change in protein
function? Refer to the attached genetic code table. Notice that different codons often specify the
same amino acid. Of the three nucleotides of a codon, first, second, and third, which one may be
changed most often without changing the amino acid coded for?
3: Suppose you have two species which have originated from a common ancestor. You compare their
DNA sequences. What factors would influence the amount of difference between the sequences?
FIRST
LETTER
U
SECOND LETTER
U
C
A
G
THIRD
LETTER
UUU UCU UAU UGU
phe ser tyr cys
U
UUC UCC UAC UGC
phe Ser tyr cys
C
UUA UCA UAA UGA
leu ser (stop) (stop)
A
UUG UCG UAG UGG
leu ser (stop) trp
G
CUU CCU CAU CGU
leu pro his arg
U
CUC CCC CAC CGC
C
C
A
G
leu
pro
his
arg
CUA CCA CAA CGA
leu pro glu arg
A
CUG CCG CAG CGG
leu pro glu arg
G
AUU ACU AAU AGU
ile
thr asn ser
U
AUC ACC AAC AGC
ile
thr asn ser
C
AUA ACA AAA AGA
ile
thr lys arg
A
AUG ACG AAG AGG
(start)
met thr lys arg
G
GUU GCU GAU GGU
val ala asp gly
U
GUC GCC GAC GGC
val ala asp gly
C
GUA GCA GAA GGA
val ala glu gly
A
GUG GCG GAG GGG
val ala glu gly
G