Name:________________________________________________
Date:_____________________
Period:_____
Biochemical Evidence
Question 1:
Question 2:
Question 3:
Question 4:
Scientists have long believed that organisms with similar DNA sequences are
related due to a common ancestor. The more similar the DNA of two living
organisms, the more closely related they may be to each other. One method
scientists use to help determine these evolutionary relationships between organisms
is to analyze and compare the molecular structure of proteins. Recall that proteins
are made up of chains of amino acids. There are 20 amino acids that are used to
make the proteins found in all species. Proteins vary due to the number and that
order of the amino acids used. The particular type and order of amino acids is
determined by the DNA code. If two organisms have similar DNA molecules, they
must have similar amino acids and thus similar proteins. Therefore, DNA
comparisons can be made by examining the sequences of amino acids within a
protein.
To make a given protein, this information is first transcribed into a
messenger RNA (mRNA) code, which is “read”, or translated into a protein, three
nucleotides at a time. With a few exceptions, each triplet, called a codon, codes for
a certain amino acid. For example, the mRNA codon UCA (that is Uracil, Cytosine,
and Adenine) code for the amino acid called Serine. Some amino acids can even be
coded for by multiple codons. For example, the amino acid Glycine will be brought
into a protein by any of the following codons: GGU, GGC, GGA, or GGG.
To better understand ancestral relationships between organisms, scientists
compare the amino acids sequences of the same proteins in different organisms.
Scientists have concluded that the more a sequence from one organism matches the
sequence from another organism, the more closely related the two types of
organisms are. For example, scientists determined that light-sensing proteins in the
brain of an ancient marine worm are very similar to those found in the vertebrate
organism’s eye. This evidence supports the idea that worms and vertebrates share a
common ancestor.
Scientists have examined the amino acid sequences of proteins such as
hemoglobin and myoglobin in many organisms and compared them to the
corresponding sequences in humans. Hemoglobin is the protein in red blood cells
that binds to oxygen and carbon dioxide. Myoglobin is a protein found in muscle
tissue that helps muscles contract. Scientists have used the information from these
analyses to strengthen conclusions drawn from other evidence of the ancestral
relationships between certain organisms.
Question:
How do scientists use DNA sequences to prove common ancestry?
In the following lab, you will compare the amino acid sequences of hemoglobin
from several vertebrate organisms with the amino acid sequence for human hemoglobin.
Name:________________________________________________
Date:_____________________
Period:_____
“Biochemical Evidence” Lab
Hypothesis: Which organism is most closely related to human? Which organism is least closely
related to humans. Explain your reasoning based upon what you know about the organisms’
characteristics versus those of a human.
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Procedure:
1) Table 1 shows part of the amino acid sequence for hemoglobin in several different vertebrates, including
humans. The abbreviations of the amino acids have been used in the sequences to save space. Table 2
shows which amino acid each abbreviation stands for.
2) Look at the amino acid sequence of hemoglobin for each organism listed in Table 1 and compare it to the
sequence of the amino acids in human hemoglobin. Count the number of amino acids that match the
human amino acids in the same position. Record this number in Table 3.
3) Count the number of amino acids that do not match the human amino acids in the same position. Record
this number in Table 3.
4) Use the information from Table 3 to help you to figure out which are Rank the organisms.
Table 1. Hemoglobin Amino Acid Sequence for Various Vertebrates
Chimp
Fish
Human
Mouse
Fly
Val
Arg
Val
Val
Ala
Leu
His
Leu
Phe
Leu
Ser
Ser
Ser
Ser
Met
Pro
Thr
Pro
Pro
Ser
Abbreviation
Ala
Arg
Asp
AsA
Cys
GlA
Glu
Gly
His
Iso
Organism
Chimpanzee
Fish
Mouse
Fly
Ala
Ala
Ala
Ala
GlA
Asp
Glu
Asp
Asp
Asp
Val
Val
Val
Val
Asp
Lys
Iso
Lys
Lys
Val
Thr
Thr
Thr
Ala
His
Asp
Asp
Asp
Asp
Cys
Val
Val
Val
Val
Iso
Lys
Met
Lys
Lys
Trn
Ala
Ala
Ala
Try
AsA
Ala
Ala
Ala
Ala
Phe
Table 2. Abbreviations for Amino Acids
Amino Acid
Abbreviation
Alanine
Leu
Arginine
Lys
Asparagine
Met
Aspartic Acid
Phe
Cysteine
Pro
Glutamic Acid
Ser
Gluttamine
Thr
Glycine
Trn
Histidine
Tyr
Isoleucine
Val
Lys
Lys
Lys
Lys
Tyr
Ala
Ser
Ala
Ala
Val
Thr
His
His
His
Iso
Ala
Ala
Ala
Ala
Asp
Gly
Gly
Gly
Gly
Cys
Glu
Cys
Glu
Glu
Leu
Amino Acid
Leucine
Lysine
Methionine
Phenyalanine
Proline
Serine
Threonine
Tryptophan
Tyrosine
Valine
Table 3. Matching and Nonmatching Amino Acids
# of Matching Amino Acids
# Nonmatching Amino Acids
Name:________________________________________________
Date:_____________________
Period:_____
Rank the organisms by ordering them from most similar to least similar to humans.
2.
3.
4.
1.
Calculate the percent similarity between the amino acid sequence for humans and each of the other organisms using the
formula below. Record your results in Table 4.
Percent Similarity = # of matching AA x 100
Total # AA
Percent Difference = 100% - Percent Similarity
Table 4. Percent Similarity and Difference in Amino Acid Sequences Compared to Humans
Organism
Percent Similarity
Percent Difference
Chimpanzee
Fish
Mouse
Fly
Graphing:
Use the data above to construct a bar graph showing how similar each organisms’ AA sequence is to a human.
100%
80%
60%
40%
20%
Chimp
Fish
Mouse
Fly
Analysis:
Based on the amino acid sequences for hemoglobin, which organism is most closely related to humans? Which is the least
closely related? Support your claims with evidence from the reading passage and the data we collected.
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Conclusion:
Review your original hypotheses. Analyze whether each was accurate or not, and explain why.
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