Download Analyzing Amino-Acid Sequences to Determine Evolutionary

Name
Date
Class
Analyzing Amino-Acid Sequences to
Determine Evolutionary Relationships
Skills
. comparing and contrasting sequencesof amino acids
. interpreting data
Objectives
. Identifythe differencesin the amino-acid sequencesof the cytochromec and
hemoglobin molecules of severalspecies.
. Inferthe evolutionary relationshipsamong severalspeciesby comparing
amino-acid sequencesof the same protein in different organisms.
Materials
Purpose
. paper
. pencil
~
1
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1
Youare a zoologistwho specializesin the classificationof vertebratesaccording
to theirevolutionaryrelationships.In yourresearch,youexaminethe amino-acid
sequencesof particular protein moleculesfound in vertebratesto determine the
degreeof biochemicalsimilaritybetweenvertebrate species.Todayyou willcompare portions of human cytochromec and hemoglobin moleculeswith the same
portionsofthosemoleculesin othervertebratespecies.Yourgoalsareto deter'mine the differencesin the amino-acid sequencesof the moleculesand to deduce
/
Background
The biochemicalcomparison of proteins is a technique used to determine evolutionary relationshipsamong organisms.Proteins consist of chains of amino
acids.The sequence,or order,of the amino acids in a protein determinesthe
type and nature of the protein. In turn, the sequenceof amino acidsin a protein
is determined by the sequenceof nucleotidesin a gene.A changein the DNA
nucleotidesequence (mutation) of a gene that codes for a protein may resultin a
changein the amino-acid sequenceof the protein.
Biochemicalevidenceof evolutioncomparesfavorablywith structural evidence
of evolution.Evenorganismsthat appear to have fewphysicalsimilaritiesmay
havesimilar sequencesof amino acidsin their proteins and be closelyrelated
through evolution.Researchersbelievethat the greater the similarityin the
amino-acid sequencesof two organisms,the more closelyrelatedthey are in an
evolutionarysense.Conversely,the greaterthe time that organismshavebeen
divergingfrom a common ancestor,the greaterthe differencesthat can be
expectedin the amino-acid sequencesof their proteins.
Twoproteins are commonly studied in attempting to deduce evolutionaryrelationships from differencesin amino-acid sequences.One is cytochromec, and
the other ishemoglobin. Cytochrome c is a protein used in cellularrespiration
and found in the mitochondria 'of many organisms.HeIrloglobin is the oxygencarryingmolecule found in red blood cells.
HOLTBIOSOURCES
Lab Program: Laboratory Techniques C19 87
I
LABORATORY TECHNIQUES c191
1
continued
Part 1-Cytochrome
c
1. A cytochrome c molecule consists of a chain of 104 amino acids. The chart
below shows the amino-acid sequence in corresponding parts of the
cytochrome c molecules of nine vertebrates.The numbers along the side of
the chart referto the position of these sequences in the chain.The letters
identify the specific amino acids in the chain.
Cytochrome
AA#
Horse Chicken Tuna
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Sequences
Frog Human Shark Turtle Monkey Rabbit
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LABORATORYTECHNIQUES c191 continued
~ 2. On a pieceof scratchpaper,write the name of eachvertebratein the chart on
the previouspage.Comparethe amino-acid sequenceof human cytochrome c
j . with that of each of the other eightvertebrates.For eachvertebrate'ssequence,
j
count the.number of amino acidsthat differfrom those in the human
j
sequence.Writethe number of differencesin the amino-acidsequencesunder
j
the vertebrate'sname.When you have completedyour comparisons,transfer
your data to the data tablebelow.As you do, listthe eightvertebratesin order
~
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Cytochrome c Amino-Acid Sequence Differences
Between Humans.and Other Vertebrate Species
Species
Number of differences
from human cytochrome c
...........................................................................................
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...........................................................................................
.........................................................................'...........".....
...........................................................................................
............................................................................................
""""""""""""""""""""""""""""""""""".....................
"""""""'..............................,..............................................
...........................................................................................
....................................................................""""""""""""
. According to this line of evidence, which organism is most closely related
to humans? Which is least closely related to humans?
. Frog and turtle cytochromec moleculeshavethe same number of differencesfrom human cytochrome c.Which vertebrate,frog or turtle, would
you put higher on the list?Explain.
HOLTBIOSOURCES
Lab Program: Laboratory TechniquesC19 89
I
LABORATORY TECHNIQUES c191
continued
3. Look at the amino-acid sequences shown below. These sequences areportions of the hemoglobin molecules of five organisms. The portion of the
chains shown are from amino acid number 87 to amino acid number 116 in
a sequence of 146 amino acids.
Hemoglobin
Amino-Acid
Sequences
Gorilla
THR
Monkey
LEU
LEU
SER
SER
GLU
GLU
LEU
LEU
92
HIS
HIS
93
CYS
CYS
CYS
94
ASP
ASP
ASP
95
LYS
LYS
LYS
AA #
87
Human
THR
Chimpanzee
THR
88
LEU
LEU
89
SER
SER
90
GLU
91
GLN
Horse
ALA
..............................
LEU
..............................
,~
SER
..............................
.,
GLU
GLU
..............................
~
LEU
LEU.
LEU
..............................
HIS
HIS
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"""""""""""""""
"""""""""""""""
CYS
"""""""""""""""
ASP
LYS
"""""""""""""""
96
LEU
LEU
LEU
LEU
97
HIS
HIS
HIS
HIS
98
VAL
VAL
VAL
VAL
99
ASP
ASP
ASP
ASP
100
PRO
PRO
PRO
PRO
101
GLU
GLU
GLU
GLU'
102
ASN
ASN
ASN
ASN
103
PHE
PHE
PHE
PHE
HIS
,~
..............................
CYS
..............................
ASP
..............................
LYS
..............................
LEU
..............................
HIS
..............................
VAL
..............................
ASP
..............................
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GLU
..............................
ASN
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.
104
ARG
ARG
LYS
LYS
105
LEU
LEU
LEU
LEU
106
LEU
LEU
LEU
LEU
107
GLY
GLY
GLY
GLY
108
ASN
ASN
ASN
109
VAL
VAL
VAL
110
LEU
LEU
LEU
111
VAL
VAL
VAL
VAL
112
CYS.
CYS
CYS
CYS
113
VAL
VAL
VAL
114
LEU.
LEU
LEU
LEU
115
ALA
ALA
ALA
ALA
116
HIS
HIS
HIS
HIS
ASN
ARG
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HOLT
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LABORATORY
TECHNIQUES
~.
.
~
C19
I
continued
4. Compare the amino-acid sequence of human hemoglobin moleculeswith
that of eachof the other four vertebrates.For eachvertebrate'ssequence,
j
j
count the number of amino acids that differ from the human sequence and
list them in the table below. Be sure to list the animal species ill descending
I
order accordingto their degreeof evolutionaryclosenessto humans.
j
Hemoglobin Amino-Acid Sequence Similarities
Number of differences
from human hemoglobin
Species
..........................................................................................
............................................................................................
..........................................................................................
............................................................................................
... ........ ........ .................
.........................
................
........ ..... ............................................................................................
. In the study of hemoglobin, which vertebrate is most closely related to
humans? Least closely related?
Analysis
5. What are somemethods biologists use to determine evolutionary
relationships?
6. Why can it be said that proteins behavelikemolecularclocks?
HOLTBIO$OURCES
Lab Program: Laboratory Techniques C19 91
:
I
LABORATORY TECHNIQUES C19
I
continued
7. When the portions of the gorilla and human hemoglobin molecules were
compared,there was only one differencein the amino-acid sequence.What
could havebeen responsiblefor this change?
(
.>-"
,
,
8. If the amino-acid sequencesare similar in gorillasand humans, will the
nucleotide sequenceof their DNA alsobe similar?Why or why not?
Conclusions
9. Examinethe data table you completed in step 2 of Part 1.The valueslistedfor
the chickenand the horse differby only one. Can you deduce from this that
the chickenand the horse are closelyrelatedto each other?Why or why not?
j 10. How isbiochemicalcomparison differentfrom other methods of determin-
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Extensions
!11. Do libraryor on-line research to discoverwhat other types of moleculescan
j
be used to determine the evolutionaryrelationshipsamong organismsbased
!
on biochemicalcomparisons.
~
i
92
.
12. Do researchto findout howbiologistsdeterminethe amino-acidsequenceof
a protein molecule.
HOLT BIOSOURCES
Lab Program: Laboratory Techniques
C19
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