chemistry 3
TRANSCRIPT
- The Structure and Function of Macromolecules
Overview: The Molecules of Life Another level in the hierarchy of biological
organization is reached when small organic molecules are joined together
Macromolecules Are large molecules composed of smaller
molecules Are complex in their structures Include proteins, carboydrates, lipids, and nucleic
acids like DNA
Most macromolecules are polymers, built from monomers
Three of the classes of life’s organic molecules are polymers
Carbohydrates Proteins Nucleic acids
A polymer Is a long molecule consisting of many
similar building blocks called monomers
The Synthesis and Breakdown of Polymers
Monomers form larger molecules by condensation reactions also called dehydration reactions
(a) Dehydration reaction in the synthesis of a polymer
HO H1 2 3 HO
HO H1 2 3 4
OH
H2O
Short polymerUnlinked monomer
Longer polymer
Dehydration removes a watermolecule, forming a new bond
Polymers can disassemble by Hydrolysis (also called digestion)
(b) Hydrolysis of a polymer
HO 1 2 3 OH
HO H1 2 3 4
H2O
HHO
Hydrolysis adds a watermolecule, breaking a bond
The Diversity of Polymers Each class of polymer
Is formed from a specific set of monomers All living organisms are composed of the
same types of polymers made up of the same monomer types – proteins, carbohydrates and nucleic acids.
However, each organism is composed of many unique polymers (unique proteins, carbohydrates and nucleic acids) based on the arrangement of monomers
An immense variety of polymers can be built from a small set of monomers
Carbohydrates serve as fuel and building material
Carbohydrates Include both simple sugars and their polymers
Monosaccharides (simple sugars) Are the simplest sugars Can be used for fuel - glucose Can be converted into other organic molecules
Nucleotides include a 5 carbon sugar, ribose or deoxyribose
Can be combined into polymers
Examples of monosaccharides
Triose sugars(C3H6O3)
Pentose sugars(C5H10O5)
Hexose sugars(C6H12O6)
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
HO C H
H C OH
H C OH
H C OH
H C OH
HO C H
HO C H
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
H C OH
C OC O
H C OH
H C OH
H C OH
HO C H
H C OH
C O
H
H
H
H H H
H
H H H H
H
H H
C C C COOOO
Ald
os
es
Glyceraldehyde
RiboseGlucose Galactose
Dihydroxyacetone
Ribulose
Ke
tos
es
Fructose
Monosaccharides May be linear Can form rings
H
H C OH
HO C H
H C OH
H C OH
H C
OC
H
1
2
3
4
5
6
H
OH
4C
6CH2OH 6CH2OH
5C
HOH
C
H OH
H
2 C
1C
H
O
H
OH
4C
5C
3 C
H
HOH
OH
H
2C
1 C
OH
H
CH2OH
H
H
OHHO
H
OH
OH
H5
3 2
4
(a) Linear and ring forms. Chemical equilibrium between the linear and ring structures greatly favors the formation of rings. To form the glucose ring, carbon 1 bonds to the oxygen attached to carbon 5.
OH3
O H OO
6
1
Notice the carbons are numbered and this numbering system remains when they form a ring in water.
DisaccharidesConsist of two monosaccharides
Are joined by a glycosidic linkageDehydration reaction in the synthesis of maltose. The bonding of two glucose units forms maltose. The glycosidic link joins the number 1 carbon of one glucose to the number 4 carbon of the second glucose. Joining the glucose monomers in a different way would result in a different disaccharide.
Dehydration reaction in the synthesis of sucrose. Sucrose is a disaccharide formed from glucose and fructose.Notice that fructose,though a hexose like glucose, forms a five-sided ring.
(a)
(b)
H
HO
H
HOH H
OH
O H
OH
CH2OH
H
HO
H
HOH H
OH
O H
OH
CH2OH
H
O
H
HOH H
OH
O H
OH
CH2OH
H
H2O
H2O
H
H
O
H
HOH
OH
O H
CH2OH
CH2OH HO
OHH
CH2OH
HOH H
H
HO
OHH
CH2OH
HOH H
O
O H
OHH
CH2OH
HOH H
O
HOH
CH2OH
H HO
O
CH2OH
H
H
OH
O
O
1 2
1 41– 4
glycosidiclinkage
1–2glycosidic
linkage
Glucose
Glucose Glucose
Fructose
Maltose
Sucrose
OH
H
H
In living systems, these reactions are always done by enzymes. Cellular enzymes are controlled
Notice that the chemical reactions take place at the functional groups
Polysaccharides
Polysaccharides Are polymers of sugars Serve many roles in organisms
Storage Starch is a polymer of glucose only Glycogen is also a polymer of glucose
Cell wall - structure Cellulose is a polymer of glucose Chitin
StarchIs a polymer consisting entirely of
glucose monomers
Is the major storage form of glucose in plants
Chloroplast Starch
Amylose Amylopectin
1 m
(a) Starch: a plant polysaccharide
Glycogen Consists of glucose monomers Is the major storage form of glucose in animals
Mitochondria Glycogen granules
0.5 m
(b) Glycogen: an animal polysaccharide
Glycogen
Variety from monomers and the covalent bond type
(c) Cellulose: 1– 4 linkage of glucose monomers
H O
O
CH2OH
HOH H
H
OH
OHH
H
HO
4
C
C
C
C
C
C
H
H
H
HO
OH
H
OH
OH
OH
H
O
CH2OH
HH
H
OH
OHH
H
HO
4 OH
CH2OH
O
OH
OH
HO41
O
CH2OH
O
OH
OH
O
CH2OH
O
OH
OH
CH2OH
O
OH
OH
O O
CH2OH
O
OH
OH
HO4
O1
OH
O
OH OHO
CH2OH
O
OH
O OH
O
OH
OH
(a) and glucose ring structures
(b) Starch: 1– 4 linkage of glucose monomers
1
glucose glucose
CH2OH
CH2OH
1 4 41 1
Which type of bond
depends on the enzyme
which is controlled by the cell.
Cellulose Is a major component of the tough walls that
enclose plant cells
Plant cells
0.5 m
Cell walls
Cellulose microfibrils in a plant cell wall
Microfibril
CH2OH
CH2OH
OH
OH
OO
OHO
CH2OHO
OOH
OCH2OH OH
OH OHO
O
CH2OH
OO
OH
CH2OH
OO
OH
O
O
CH2OHOH
CH2OHOHOOH OH OH OH
O
OH OH
CH2OH
CH2OH
OHO
OH CH2OH
OO
OH CH2OH
OH
b Glucos
e monomer
O
O
O
O
O
O
Parallel cellulose molecules areheld together by hydrogenbonds between hydroxyl
groups attached to carbonatoms 3 and 6.
About 80 cellulosemolecules associate
to form a microfibril, themain architectural unitof the plant cell wall.
A cellulose moleculeis an unbranched glucose polymer.
OH
OH
O
OOH
Cellulosemolecules
Figure 5.8
Cellulose is difficult to digest Cows have microbes in their stomachs to
facilitate this process
What do these microbes have that will allow them to break down cellulose?
Chitin, another important structural polysaccharide Is found in the exoskeleton of arthropods Can be used as surgical thread
(a) The structure of the chitin monomer.
O
CH2OH
OHH
H OH
H
NH
CCH3
O
H
H
(b) Chitin forms the exoskeleton of arthropods. This cicada is molting, shedding its old exoskeleton and emergingin adult form.
(c) Chitin is used to make a strong and flexible surgical
thread that decomposes after the wound or incision heals.
OH
Lipids are a diverse group of hydrophobic molecules
Lipids Are the one class of large biological molecules
that do not consist of polymers Share the common trait of being hydrophobic Include
Fats Phospholipids steroids
(b) Fat molecule (triacylglycerol)
H
H O HC
C
C
H
H OH
OH
H
HH H
HH
HH
H
HHH
H
HH
H
HH
HH
HH
H
HH
HH
H
HH
H
HC
CCC
CC
CC
CC
CC
CC
CC
Glycerol
Fatty acid(palmitic acid)
H
H
H
H
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HHHH
HHH
HH
HH
H
H
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH H
HH
HH
HH
HH
HH
HH
H
HH
HH
HH
HH
HH
HHH
HH
HO
O
O
O
O
OC
C
C C CC
CC
CC
CC
CC
CC
CC
C
C
CC
CCCC
CC
CC
CC
CC
CC
C CC
CC
CC
CC
CC
CC
CC
O
O
(a) Dehydration reaction in the synthesis of a fat
Ester linkage
The synthesis and structure of a fat, or triglycerol
Again, notice where the chemical reaction takes
place.
Fatty acidsVary in the length and number and locations of double bonds they contain
Saturated fatty acids Have the maximum number of hydrogen atoms possible (saturated with hydrogen) Have no double bonds
(a) Saturated fat and fatty acid
Stearic acid
(b) Unsaturated fat and fatty acidcis double bondcauses bending
Oleic acid
• Unsaturated fatty acids --Have one or more
double bonds
A single bond allows rotation, is longer and not a strong as a double bond
A double bond is stronger, shorter, and more rigid.
Bonds help to determine the 3-D shape of a molecule.
Phospholipids have only two fatty acids plus a phosphate group instead of a third fatty acid
Consists of a hydrophilic “head” and hydrophobic “tails”
CH2
O
PO O
O
CH2CHCH2
OO
C O C O
Phosphate
Glycerol
(a) Structural formula (b) Space-filling model
Fatty acids
(c) Phospholipid symbol
Hy
dro
ph
ob
ic t
ail
s
Hydrophilichead
Hydrophobictails
–
Hy
dro
ph
ilic
he
ad CH2 Choline
+N(CH3)3
The structure of phospholipids Results in a bilayer arrangement found in cell
membranes
Hydrophilichead
WATER
WATER
Hydrophobictail
Steroids - Are lipids characterized by a carbon skeleton consisting of four fused rings
One steroid, cholesterol Is found in cell membranes Is a precursor for some hormones
HO
CH3
CH3
H3C CH3
CH3
Is this molecule polar or nonpolar?
When written as a ring, all points are carbon unless written in otherwise.
Cholesterol fills in the spaces left by the kinks; stiffens the bilayer and makes it less fluid and less permeable.
How do you think bacteria, which do not use cholesterol, adjust the fluidity of their cell membrane?
Do concept check 5.3
Animal fats found in meat, butter, and cream are usually saturated, and solid at room temperature.
Plant oils like corn oil contain more unsaturated fatty acids.
Peanut and olive oil contain monounsaturated fatty acids.
Both saturated and trans fats correlate with heart problems and high levels or blood cholesterol. Atherosclerosis
Proteins have many structures, resulting in a wide range of functions
Enzymes Are often a type of protein that acts as a
catalyst, speeding up chemical reactions
Substrate(sucrose)
Enzyme (sucrase)
Glucose
OH
H O
H2O
Fructose
3 Substrate is convertedto products.
Substrate binds toenzyme.
1 Active site is available for a molecule of substrate, the
reactant on which the enzyme acts.
22
4 Products are released.
Is this part of the protein polar or nonpolar?
Enzyme remains unchanged, ready to work again.
Polypeptides Polypeptides
Are polymers of amino acids A protein
Can consist of only one large polypeptide Can consists of more than one polypeptides
(subunits) bound together by non-covalent interactions Hemoglobin
Some very small polypeptides are referred to as peptides
Amino acids Are organic molecules possessing both carboxyl and
amino groups Differ in their properties due to differing side chains,
called R groups
Proteins are composed of amino acid building blocks and are diverse in structure (shape) and function.
Amino acids have an amino group and an acid group bound to a central carbon.
This central carbon forms 4 single bonds. One with the amino group, one with the carboxylic acid, one with hydrogen, and the last with a variety of different chemical groups (R group).
Amino group
Acid group
20 different amino acids make up proteins
O
O–
H
H3N+ C C
O
O–
H
CH3
H3N+ C
H
C
O
O–
CH3 CH3
CH3
C C
O
O–
H
H3N+
CH
CH3
CH2
C
H
H3N+
CH3
CH3
CH2
CH
C
H
H3N+ C
CH3
CH2
CH2
CH3N+
H
C
O
O–
CH2
CH3N+
H
C
O
O–
CH2
NH
H
C
O
O–
H3N+ C
CH2
H2C
H2N C
CH2
H
C
Nonpolar
Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)
Methionine (Met) Phenylalanine (Phe)
C
O
O–
Tryptophan (Trp) Proline (Pro)
H3C
S
O
O–
Know the structure of an amino acid, not all the R groups.
O–
OH
CH2
C C
H
H3N+
O
O–
H3N+
OH CH3
CH
C C
HO–
O
SH
CH2
C
H
H3N+ C
O
O–
H3N+ C C
CH2
OH
H H H
H3N+
NH2
CH2
OC
C C
O
O–
NH2 O
C
CH2
CH2
C CH3N+
O
O–
O
Polar
Electricallycharged
–O O
C
CH2
C CH3N+
H
O
O–
O– O
C
CH2
C CH3N+
H
O
O–
CH2
CH2
CH2
CH2
NH3+
CH2
C CH3N+
H
O
O–
NH2
C NH2+
CH2
CH2
CH2
C CH3N+
H
O
O–
CH2
NH+
NHCH2
C CH3N+
H
O
O–
Serine (Ser) Threonine (Thr)Cysteine
(Cys)Tyrosine
(Tyr)Asparagine
(Asn)Glutamine
(Gln)
Acidic Basic
Aspartic acid (Asp)
Glutamic acid (Glu)
Lysine (Lys) Arginine (Arg) Histidine (His)
Know both the name and abbreviation of all amino acids along with their chemical nature – polar, nonpolar, charged, acidic, . . .
A peptide bond forms between the amino group on one amino acid and the carboxyl group on another
amino acid.
The formation of a peptide bonds in a tetrapeptide
Amino acids Are linked by peptide bonds between the amino
group of one amino acid and the acid group of the other amino acid
OH
DESMOSOMES
DESMOSOMESDESMOSOMES
OH
CH2
C
N
H
C
H O
H OH OH
Peptidebond
OH
OH
OH
H H
HH
H
H
H
H
H
H H
H
N
N N
N N
SHSide
chains
SH
OO
O O O
H2O
CH2 CH2
CH2 CH2CH2
C C C C C C
C CC C
Peptidebond
Amino end(N-terminus)
Backbone
(a)
(b) Carboxyl end(C-terminus)
The chemical reaction again takes place at the functional groups!
Each peptide bond is in a plane. This contributes to the shape of the protein.
Protein Conformation and Function
Two models of protein conformation
(a) A ribbon model
(b) A space-filling model
Groove
Groove
A protein’s specific conformation (shape and chemical nature)
determines how it functions.
Four Levels of Protein Structure
Primary structure Is the unique sequence of amino acids in a
polypeptide
–
Amino acid subunits
+H3NAmino
end
oCarboxyl end
oc
GlyProThrGlyThr
Gly
GluSeuLysCysProLeu
MetVal
Lys
ValLeu
AspAlaVal ArgGly
SerPro
Ala
Gly
lle
SerProPheHisGluHis
Ala
GluVal
ValPheThrAlaAsn
AspSer
GlyProArg
ArgTyrThr
lleAla
Ala
Leu
LeuSer
ProTyrSerTyrSerThr
Thr
Ala
ValVal
ThrAsnProLysGlu
ThrLys
SerTyrTrpLysAlaLeu
GluLle Asp
Covalent bonds
Peptide backbone imposes some restrictions on the folding of a protein. Why?
O C
helix
pleated sheetAmino acid
subunits NCH
C
O
C N
H
CO H
R
C NH
C
O H
C
R
N
HH
R C
O
R
C
H
NH
C
O H
NCO
R
C
H
NH
H
C
R
C
O
C
O
C
NH
H
R
C
C
O
N
HH
C
R
C
O
NH
R
C
H C
ON
HH
C
R
C
O
NH
R
C
H C
ON
HH
C
R
C
O
N H
H C R
N HO
O C N
C
RC
H O
CHR
N H
O C
RC
H
N H
O CH C R
N H
CC
N
R
H
O C
H C R
N H
O C
RC
H
H
C
RN
H
CO
C
NH
R
C
H C
O
N
H
C
Secondary structure Is the folding or coiling of the polypeptide into a
repeating configuration Includes the helix and the pleated sheet
H H
All based on hydrogen bonds between the peptide bonds of different amino acids
Tertiary structure Is the overall three-dimensional shape
of a polypeptide after it “folds” into a stable form.
Results from interactions between amino acids and R groups
CH2CH
OH
O
CHO
CH2
CH2 NH3+ C-O CH2
O
CH2SSCH2
CH
CH3
CH3
H3C
H3C
Hydrophobic interactions and van der Waalsinteractions
Polypeptidebackbone
Hydrogenbond
Ionic bond
CH2
Disulfide bridge
What are these?
The distribution of polar and nonpolar amino acids is important in how a protein folds. The nonpolar side chains tend to cluster in the interior of a molecule, avoiding contact with water, while the polar side chains arrange themselves near the outside.
Hydrophobic areas also tend to be found spanning the lipid bilayer of membranes like the plasma membrane.
Transmembrane proteins often cross the membrane in an alpha helix because the peptide bond itself is hydrophic unless all partial charges are equalized in an alpha helix or beta sheet.
Quaternary structure
Is the overall protein structure that results from the aggregation of two or more polypeptide subunits
Hemoglobin contains
two alpha globin subunits and
two beta globin subunits.
Heme is the site where oxygen is carried
There are many large multi-subunit proteins in cells.
Larger protein molecules may contain more than one polypeptide chain or subunit. The region that interacts with another molecule through noncovalent bonds is the binding site.
Sickle-cell disease results from a single amino acid substitution in the protein hemoglobin
Fibers of abnormalhemoglobin deform cell into sickle shape.
Primary structure
Secondaryand tertiarystructures
Quaternary structure
Function
Red bloodcell shape
Hemoglobin A
Molecules donot associatewith oneanother, eachcarries oxygen.
Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen
10 m 10 m
Primary structure
Secondaryand tertiarystructures
Quaternary structure
Function
Red bloodcell shape
Hemoglobin S
Molecules interact with one another tocrystallize into a fiber, capacity to carry oxygen is greatly reduced.
subunit subunit
1 2 3 4 5 6 7 3 4 5 6 721
Normal β hemoglobin
Sickle-cell β hemoglobin . . .. . .
Figure 5.21
Exposed hydrophobic
region
Val ThrHis Leu Pro Glul Glu Val His Leu Thr Pro Val Glu
The sickle-cell hemoglobin does not fold into the proper shape because the amino acid sequence (Primary structure) is incorrect.
20 different amino acids make up proteins
O
O–
H
H3N+ C C
O
O–
H
CH3
H3N+ C
H
C
O
O–
CH3 CH3
CH3
C C
O
O–
H
H3N+
CH
CH3
CH2
C
H
H3N+
CH3
CH3
CH2
CH
C
H
H3N+ C
CH3
CH2
CH2
CH3N+
H
C
O
O–
CH2
CH3N+
H
C
O
O–
CH2
NH
H
C
O
O–
H3N+ C
CH2
H2C
H2N C
CH2
H
C
Nonpolar
Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)
Methionine (Met) Phenylalanine (Phe)
C
O
O–
Tryptophan (Trp) Proline (Pro)
H3C
S
O
O–
Know the structure of an amino acid, not all the R groups.
O–
OH
CH2
C C
H
H3N+
O
O–
H3N+
OH CH3
CH
C C
HO–
O
SH
CH2
C
H
H3N+ C
O
O–
H3N+ C C
CH2
OH
H H H
H3N+
NH2
CH2
OC
C C
O
O–
NH2 O
C
CH2
CH2
C CH3N+
O
O–
O
Polar
Electricallycharged
–O O
C
CH2
C CH3N+
H
O
O–
O– O
C
CH2
C CH3N+
H
O
O–
CH2
CH2
CH2
CH2
NH3+
CH2
C CH3N+
H
O
O–
NH2
C NH2+
CH2
CH2
CH2
C CH3N+
H
O
O–
CH2
NH+
NHCH2
C CH3N+
H
O
O–
Serine (Ser) Threonine (Thr)Cysteine
(Cys)Tyrosine
(Tyr)Asparagine
(Asn)Glutamine
(Gln)
Acidic Basic
Aspartic acid (Asp)
Glutamic acid (Glu)
Lysine (Lys) Arginine (Arg) Histidine (His)
Know both the name and abbreviation of all amino acids along with their chemical nature – polar, nonpolar, charged, acidic, . . .
Protein conformation Depends on
the sequence of amino acid side chains (with R groups) and the physical and chemical conditions of the protein’s environment
Denaturation is when a protein unravels and loses its native conformation
Denaturation
Renaturation
Denatured proteinNormal protein
What kinds of bonds are broken here?
Increased temperature
Change in pH
Organic solvent (hydrophobic)
What kinds of bonds are not broken here?
The Protein-Folding Problem
Most proteins Probably go through several intermediate states
on their way to a stable conformation. Many proteins are being made in the cell all of
the time. How do the fold correctly, how do they interact with their subunits correctly?
Chaperonins Are protein molecules that assist in the proper
folding of other proteins
Hollowcylinder
Cap
Chaperonin(fully assembled)
Steps of ChaperoninAction: An unfolded poly- peptide enters the cylinder from one end.
The cap attaches, causing the cylinder to change shape insuch a way that it creates a hydrophilic environment for the folding of the polypeptide.
The cap comesoff, and the properlyfolded protein is released.
Correctlyfoldedprotein
Polypeptide
2
1
3
X-ray crystallography Is used to determine a protein’s three-
dimensional structureX-raydiffraction pattern
Photographic film
Diffracted X-rays
X-raysource
X-ray beam
Crystal Nucleic acid Protein
(a) X-ray diffraction pattern (b) 3D computer modelFigure 5.24
Do concept check 5.4
Nucleic acids store and transmit hereditary information
Genes Are the units of inheritance
Code for the amino acid sequence of polypeptides Are made of nucleic acids
There are two types of nucleic acids Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA)
Deoxyribonucleic acid (DNA)
Stores information for the synthesis of specific proteins –DNA is the “ genetic material” inherited from parents
Directs RNA synthesis Directs protein synthesis
indirectly through messenger RNA
1
2
3
Synthesis of mRNA in the nucleus
Movement of mRNA into cytoplasm
via nuclear pore
Synthesisof protein
NUCLEUS
CYTOPLASM
DNA
mRNA
Ribosome
AminoacidsPolypeptide
mRNA
The Structure of Nucleic Acids Nucleic acids
Exist as polymers called polynucleotides Each polynucleotide Consists of monomers called
nucleotides
(a) Polynucleotide, or nucleic acid
3’C
5’ end
5’C
3’C
5’C
3’ endOH
O
O
O
O Nitrogenousbase
Nucleoside
O
O
O
O P CH2
5’C
3’CPhosphate
group Pentosesugar
(b) Nucleotide
O
Nucleotide Monomers Are made up of nucleosides and phosphate
groups
(c) Nucleoside componentsFigure 5.26
CHCH
Uracil (in RNA)U
Ribose (in RNA)
Nitrogenous bases Pyrimidines
CN
NC
OH
NH2
CHCH
OC
NH
CH
HNC
O
CCH3
N
HNC
C
HO
O
CytosineC
Thymine (in DNA)T
NHC
N C
CN
C
CH
N
NH2 O
NHC
NHH
CC
N
NH
C NH2
AdenineA
GuanineG
Purines
OHOCH2
H
H H
OH
H
OHOCH2
HH H
OH
H
Pentose sugars
Deoxyribose (in DNA) Ribose (in RNA)OHOH
CH
CH
Uracil (in RNA)U
4’
5”
3’OH H
2’
1’
5”
4’
3’ 2’
1’
Nitrogenousbase
Nucleoside
O
O
O
O P CH2
5’C
3’CPhosphate
group Pentosesugar
(b) Nucleotide
O
pyrimidines
Nucleotide polymers are made up of nucleotides linked by the–OH group on the 3´ carbon of one nucleotide and the phosphate on the 5´ carbon on the next So they “grow” at the 3’ end.
The sequence of bases along a nucleotide polymer Is unique for
each gene
The DNA Double Helix
Anti-parallel
complementary
Complementary base pairs
Do concept check 5.5
DNA and Proteins as Tape Measures of Evolution
Molecular comparisons Help biologists sort out the evolutionary
connections among species Ribosomal RNA gene sequence is conserved. Look for differences.