How to grade essays
 Use a highlighter to mark all the statements that are in
your essay that are on the rubric (give yourself points
according to the guidelines of the rubric)
 Use a red pen to write in the concepts you missed in
the section of the essay where that concept should
have been covered.
 At the top of the page, put the score you received and
circle it in pen.
 Voila! Done!
Essay Prompt #1
AP BIO QUESTION 1996
The unique properties (characteristics) of
water make life possible on Earth. Select
three properties of water and
a) for each property, identify and define the
property and explain it in terms of the
physical/chemical nature of water.
b) for each property, describe one example of
how the property affects the functioning of
living organisms.
Key for Essay #1 -Prop. of water
Note: you will need to
print this slide out to see
the words better.
Essay Prompt #1
AP BIO QUESTION 1991:
Carbon is a very important element in living systems.
a. Describe the various characteristics of the carbon atom
that makes possible the building of a variety of biological
molecules.
b. Explain how reactions involving carbon-containing
compounds can contribute to the greenhouse effect.
c. The following structures are examples of two different
categories of biological compounds. Describe how each
category of compounds is important to the structure and
function of living systems.
KEY TO ESSAY #1
STANDARDS:
A. CHARACTERISTICS OF CARBON ATOMS:
(Max of 4 points)
__Ready availability, abundance
__Atom small in size, outer (valence) electrons close to nucleus, so forms
stable (strong) bonds
__4 electrons in a valence-capacity of 8, forms 4 bonds to 4 other atoms
__Forms covalent bonds
__Can bond to other carbon atoms, no upper limit to size of carbon
compounds
__Bond angles form tetrahedron, resulting in 3-D structures, chains, rings, not
just planar
__Can form multiple C-C, C=C, C=C bonds
__Can form isomers, different structures - same number and kind of atoms
__Functional groups/combine with a variety of other elements
__BONUS POINT: if get 3 above - Uniqueness, only Carbon has all of these
characteristics
B. REACTIONS CONTRIBUTING TO GREENHOUSE EFFEC (Max of 4 points)
__Overview: Increase in gas concentration (CO2, CO, CFC) causes greenhouse effect
__CO & CO2 from respiration and combustion, or volatilization of limestone
__CH4 from correct source - livestock, microbes, landfills, swamps, oil wells, etc.
__CFCs from industrial activities, refrigerants, plastic foam, etc.
PHYSICAL MECHANISMS OF HEATING:
__Ozone destruction / more energy (UV) gets in
__Trapping of Energy - "Blanket" traps heat or Reflection of Sun's Energy (technically
incorrect but common use in texts)
__Good technical description of absorption, reradiation and wavelength shift leading to
production or long wave infrared absorbed by greenhouse gases
__Concept of sinks:
CO2 removal by photosynthesis, CaCO3 formation, soluble in oceans, etc.
CO2 addition by forest destruction, industry, etc.
C. BIOLOGICAL MOLECULES:
(Max of 5 points)
(Cannot get 10 points for this question without mentioning both category I and II)
CATEGORY I:
__Identification of molecule / category:
__Characteristics of molecule:
__Structural uses of molecule:
__Functions of molecule (for example):
(2 Max) Phospholipids:
Fats:
Steroids/Sterols:
Cholesterol:
Fat soluble Vitamins:
Prostaglandins:
Waxes:
Phospholipid, phosphoglyceride, fat, lipid,
phosphatidylethanolamine / polypeptide.
Hydrophobic/hydrophilic, polar/nonpolar,
amphipathic, or non-water soluble, high E
bonds, sat/unsat C-C bonds.
membrane, lipid bilayer, fluid mosaic.
Regulation of membrane permeability, fluidity
Structural, insulation, energy storage, water-proofing
Hormonal, membrane fluidity
Animal membranes
Coenzymes
Neural modulators
Water-proofing
CATEGORY II:
__Identification of molecule / category:
__Recognizing cysteine's role in
disulfide bond formation
__Characteristics of molecule:
__Structural roles:
__Functional roles:
(2 Max)
Amino acid / protein / cysteine
Side chains variable
Peptide bonds may be formed
Subunit (monomer, building block) of
protein
Levels of protein structure / zwitterion / as
buffers
Keratin, collagen, cytoskeletal (tubulin,
actin), etc.
Enzymatic - speed reactions
(pepsin, glucose oxidase, etc.)
Transport (Hb, Myb, permeases, HDL/LDL)
Regulatory (oligopeptides, ex.
hypothalamic releasing
factors, insulin, glucagon, etc.)
Contractile - actin, myosin
Protection - antibodies
ESSAY Prompt #2
CELL QUESTION 1992:
A laboratory assistant prepared solutions of 0.8 M, 0.6 M, 0.4 M, and 0.2
M sucrose, but forgot to label them. After realizing the error, the assistant
randomly labeled the flasks containing these four unknown solutions as
flask A, flask B, flask C, and flask D.
Design an experiment, based on the principles of diffusion and osmosis,
that the assistant could use to determine which of the flasks contains
each of the four unknown solutions. Include in your answer
(a) a description of how you would set up and perform the experiment;
(b) the results you would expect from your experiment; and
(c) an explanation of those results based on the principles involved.
(Be sure to clearly state the principles addressed in your discussion.)
Key to Essay #2
A. EXPERIMENTAL SET-UP (1 point each)
___ 1. Experiment based on concentration gradient
___ 2. Experiment based on semipermeable membrane (dialysis tubing, thistle
tubes, plant or animal cells)
___ *3. Experimental set-up (design) adequate to produce measurable results
___ *4. (2 max) Experimental variables are eliminated (mass, volume, time,
temperature, tissue type, etc.)
___ *5. Experimental set-up is exemplary (must include semipermeable
membrane)
B. RESULTS ( 1 point each)
___ *1. Describes a measurable change
___ *2. Correctly correlates the observed changes with molarities of unknowns
C. APPLICATION OF PRINCIPLES TO RESULTS (1 point each)
___ *1. Correctly applies principles of diffusion and osmosis in the
interpretation
of results (a correct analysis)
___ 2. Demonstrates an understanding of the concept of water potential
(hydrostatic/turgor pressure) in analysis of results
Key to Essay #2 continued
D. PRINCIPLES (1 point each)
___ 1. Demonstrates an understanding, or gives a correct definition of diffusion
___ 2. Demonstrates an understanding, or gives a correct definition of osmosis **
(must include both water and semipermeable membrane)
___ 3. Demonstrates an understanding, or gives a correct definition of selective
permeability
___ 4. Describes how solute size and/or molar concentration (hypertonic/hypotonic)
affects the process of diffusion through a membrane
_______________________ Max possible = 14 * No points if the lab will not work.
**Osmosis: the diffusion of water through a selectively (semi)permeable membrane in
the following directions: -from higher water potential toward lower water potential from hypotonic (hypoosmotic) solution toward hypertonic (hyperosmotic) solution from higher water concentration toward lower water concentration -from lower solute
concentration toward higher solute concentration -from region of lower osmotic
pressure toward regions of higher osmotic pressure -from region of higher osmotic
potential toward region of lower osmotic potential
ESSAY Prompt #3
Enzymes are important biological molecules.
They are one of the primary means of
regulating chemical processes within cells.
A.
B.
C.
Describe how enzymes affect chemical
reactions.
Describe environmental factors that affect
enzyme action.
Describe how enzymes are important for the
process of DNA replication. (see chapter 16)
**NOTE: Be sure to include the structure and what
enzymes are composed in your answer.
Grading Rubric for Essay #3

1.
2.
3.
4.
5.
6.
7.
8.
9.
Part a: (1 point for each of the following)
Enzymes increase the rate of chemical reactions
Enzymes bind to the substrate at the active site
The enzymes and substrate bind through hydrogen
bonding
The substrate that binds to the enzyme is one of the
reactants in the reaction
The substrate is changed to products and consumed by
the reaction
The enzyme is not consumed by the reaction and can be
used again
The enzyme reduces the activation energy of the reaction
causing the increase in reaction rate
The enzyme is a protein with a complicated three
dimensional shape
The shape of the enzyme’s active site had to be
complementary to the substrate molecule

1.
2.
3.
4.
5.
6.
7.
8.
9.
Part b (4 pt max-1 pt for each of the following
An increase in temp. causes substrate and enzyme
molecules to move more rapidly
The increased movement causes more collisions that
increase the reaction rate
Extreme temp. denature enzymes and slow reaction
Competitive inhibitors are shaped similarly to the
substrate
Competitive inhibitors compete for the active site with the
substrate, slowing the reaction rate
Noncompetitive inhibitors bind to the site outside of the
active site
The non competitive inhibitors change the shape of the
enzyme and its active site and slow the reaction
Each enzyme operates most efficiently at an optimum pH
pHs outside of that optimum can disrupt or denature the
enzyme
Part C (3pt max; 1 pt for each of the following)
1.
2.
3.
4.
5.
Helicase causes the DNA double helix to
unwind
DNA polymerase copies the exposed single
strands of DNA
DNA polymerase can only add nucleotides
to a free three prime side
DNA ligase seals Okazaki fragments
together
DNA polymerase proof-reads the new
strands
Other Information about Enzymes
Enzyme question
 QUESTION I-2000
 1. The effects of pH and temperature were studied for an enzymecatalyzed reaction. The following results were obtained.

a) How do (1) temperature and (2) pH affect the activity of this
enzyme? In your answer, include a discussion of the relationship
between the structure and the function of this enzyme, as well as a
discussion of how structure and function of enzymes are affected by
temperature and pH.
b) Describe a controlled experiment that could have produced the data
shown for either temperature or pH. Be sure to state the hypothesis
that was tested here.


Question I
Part a. (maximum 6 points)
• Optimum temperature and pH concept [must include both temp and pH]
• Enzyme/Substrate Fit concept (function dependent on conformation complernentarity between enzyme and
substrate)
• Tertiary (and sometimes quarternary) structure determines function
• Description of enzyme structure or function, e.g.











Structure
Elegant description of primary to tertiary or primary to quarternary levels of structure
Protein folding/coiIing
Co-enzymes/co-factors
Zymogens
Allosteric effectors
Function
Increases rate of reaction
Increases proximity of reactants
Decreases activation energy of the catalyzed reaction
Decreases time to reach equilibrium
Induced fit and/or orbital steering ("bond stress")
• Denaturation concept [temp and/or pH] linked to decreased enzyme activity (e.g. "denaturation" in context or
unfolding or change in 3D shape. not "enzyme breaks down")
• How temperature affects conformation (increased temperature breaks specific bonds e.g. hydrogen, Van dcr
Wuals, eljsrtllide bridges)
• How pH affects conformation (change in H concentration causes a change in specific bond interactions, e.g.
hydrogen: ionic, R-group interactions)
• Kinetics (increased or decreased molecular movement ) linked to effect on enzyme activity due to increase or
decrease in temperature up to the optimum.

Part b. (maximum 6 points)
Experimental design must be relevant to the data shown
• What is measured (e.g. product formed or substrate used)
• How is it measured (titration or spectrophotometry or color change
or bubbles counted. etc.)
• The independent variable (temperature/pH) is manipulated to
produce the results at least 3 data points are identified]
• The described experiment could produce these data
Experimental design included sufficient range, varied the temp/pH of
the reaction mix not the enzyme, what was measured, and how it was
measured)
• Held experimental factors constant (specified at least one)
• Specified a control group for comparison (no enzyme or boiled
enzyme or no Substrate)
• Verified results (e.g. repeated trials: results represent an average)
• Hypothesis clearly related to experiment of choice, and clearly
identified as a hypothesis can use the if/then... form.
Essay Prompt #4


AP BIO CELL RESPIRATION 1977
Explain how the molecular reactions of
cellular respiration transform the chemical
bond energy of Krebs Cycle substrates into
the more readily available bond energy of
ATP. Include in your discussion the structure
of the mitochondrion and show how it is
important to the reactions of the Krebs Cycle
and the Electron Transport Chain.
Answer Key to Essay #4:
STANDARDS: 1/2 point for each of the following
 ___Krebs and ETS occur within mitochondria
 ___Krebs - enzymes freely present in matrix fluid
 ___ETS - respiratory chain (respiratory assembly) arranged in order
inner membrane of mitochondria (Diagram OK)
 ___more active cells - more respiratory assemblies & more cristae
 ___Aerobic - O2 necessary as final H acceptor (-> H2O) (most
eukaryotic cells all of the time)
 ___Glycolysis is 1st required (outside mitochondria)
 ___Glucose (6C) is broken down into 2 Pyruvic Acid (3C) molecules
 ___Phosphorylation must 1st occur
 ___Net production: 2 ATP & 2 NADH MITOCHONDRIA
 ___Pyruvic Acid & 2NADH enter mitochondria
 ___2 NADH will transfer H (electrons) into ETS
 ___yields 2 x 2 ATP = 4 ATP (some loss due to point of entry into ETS)










KREBS CYCLE SUBSTRATES
___2 Pyruvic Acid loses CO2 & H -> 2 NADH &
combines w/CoA -> Acetyl CoA
___(2C) Acetyl CoA + (4C) Oxaloacetic Acid -> (6C)
Citric Acid
___Citric Acid -> Isocitric Acid
___(6C) Isocitric Acid - DEHYDROGENATION &
loss of CO2 -> (5C) Ketogluatric Acid NAD -> NADH
___(5 C) Ketoglutaric Acid - DEHYDROGENATION & loss of
CO2 -> (4C) Succinic Acid NAD -> NADH
___(4C) Succinic Acid - DEHYDROGENATION -> (4C) Malic
Acid FAD -> FADH2
___(4C) Malic Acid - DEHYDROGENATION -> Oxaloacetic Acid
NAD -> NADH
___specific mention of 2 x 3 NADH & 2 x 1 FADH2 produced
during Krebs
___ATP (1) produced in Krebs

ETS RECEIVES THE FOLLOWING: NADH or FADH2 WHICH
RESULTS IN ATP PRODUCTION

___Glycolysis -> 2 NADH x 2 ATP = 4
___Pyruvic Acid -> Acetyl CoA + 2 NADH x 3 ATP = 6
___Krebs -> 8 NADH (FADH2) x 3 ATP = 24 Total = 34
___34 ATP gained through ETS
___Respiratory Assembly: CoQ, cytochromes b, c, a, a3
___Ring Compounds w/Fe (porphyrin ring)
___Changing Oxidation states as "go down" assembly
___Fe III -> Fe II change ionic state as accept electrons
___Release energy in "packets" - small amounts sufficient to
produce ATP (about 7 kcal/mole)
___Occurs at 3 places in the chain for each NADH, FADH2
___mention of various hypotheses: Chemiosmotic,
Conformational, Chemical Coupling
___O2 final acceptor ( -> H2O)
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Essay Prompt #5

1.
2.
3.
4.
5.
Ap Bio question 1995:
Energy transfer occurs in all cellular activities.
For 3 of the following 5 processes involving
energy transfer, explain how each functions in
the cell and give an example. Explain how ATP is
involved in each example you choose.
Cellular movement
Active transport
Synthesis of molecules
Chemiosmosis
Fermentation
KEY TO ESSAY #5
1 pt function of process
 1 pt example
 1 pt additional for detail
 1 pt ATP involvement 4 pts = MAX/process
 ATP <====> ADP + Pi + Energy 1 pt

CELLULAR MOVEMENT
Function: Locomotion, Mitosis, Cytoplasmic
Streaming, Muscle Contraction (sliding
filaments)
 Examples: Cilia, Flagella, Pseudopodia,
Exocytosis, Endocytosis, Microfilaments, Spindle
Fibers
 Details: 9 + 2 Microtubules, Dynein, ActinMyosin (Myosin heads), Kinesins
 ATP Involvement: ATP used Motor molecules
powered by ATP hydrolysis (Conformational
change with ATP use)

ACTIVE TRANSPORT




Function: Concentration gradient low to high /
Protein carrier w/use of ATP
Examples: Sodium/Potassium Pump, Other
Transports Specified
Details: Protein carrier related to example
ATP Involvement: ATP used Conformational
change with ATP use
SYNTHESIS OF MOLECULES




Function: Small to Large / Monomer to Polymer
Examples: Intermediates to Monosaccharides to
Polysaccharides Intermediates to Fatty Acids
(Glycerol) to Lipids Intermediates to Amino Acids
to Proteins Intermediates to Nucleotides to
Nucleic Acids Intermediates to Vitamins
Details: Charging tRNA Reactions catalyzed by
enzymes
ATP Involvment: ATP used ATP's energy stored
in chemical bonds
CHEMIOSMOSIS




Function: Form electrochemical proton (H+)
gradient
Examples: Mitochondria, Chloroplasts, Bacterial
Cell Membrane
Details: Cristae, Thylakoids, Direction/ H+ vs eATP Involvement: ATP formed Proton
pump/gradient with ATP synthase yields ATP
FERMENTATION
Start with glucose
 Function: breakdown of organic compounds
 Examples: Ethanol / Lactic Acid
 Details Anaerobic, Less energy produced
Cytosolic process, Yeast, Muscle
 ATP Involvement: ATP used and formed
Substrate-level
Start with pyruvate
 Function: recycling NADH
 Examples: Ethanol / Lactic Acid
 Details: Anaerobic, Regenerate oxidized NAD,
Cytosolic process Yeast, Muscle
 ATP Involvement: (No direct involvement) Makes
ATP production Phosphorylation possible in
glycolysis
OVERALL COMMENTARY ON QUESTION:

This question was a cell biology energetics question that
required both breadth and depth of knowledge to answer
the question successfully. Full credit could only be obtained
by responding to three of the process examples listed in the
question. Also, if more than three processes were discussed,
readers were directed to grade the first three unless specifi
indications directed one to be omitted. Each process had a
four point maximum score. One point was available for
description of the function of the process, which might also
be a definition or description of a mechanism. One point
came from an example in context with the process. One
point was available for showing how ATP was involved in
the process. For each of these points, it was possible to get
an additional point for detail (elaboration). In addition, it
was possible to obtain a point frordescription of the ATP
<==> ADP cycle.
EssayPrompt #6
Discuss the process of cell division in animals.
Include a description of mitosis and cytokinesis,
and of the other phases of the cell cycle. Do not
include meiosis. Be sure to be detailed in your
response
Answer Key/Grading Rubric for Essay #6
PART I. DESCRIPTION OF MITOSIS IN ANIMAL CELLS: Max. = 7 points General

__ division of nucleus

__ daughter cells acquire the same number and kinds of chromosomes as in the mother cell

__ process for growth or repair or asexual reproduction

__ list phases in correct order (P,M,A.T)
Prophase (one point each / max. 2) __

centrioles move apart

__ chromosomes condense

__ nucleolus is no longer visible

__ nuclear envelope disappears

__ asters and spindle form Metaphase

__ sister chromatids (chromosomes) are in a line at the midpoint of the spindle
Anaphase (one point each / max. 2)

__ centromeres uncouple (split)

__ chromosomes move to opposite poles

__ microtubules involved in the push/pull movement
Telophase (one point each / max. 2)

__ reverse of prophase

__ nuclear envelope reforms

__ nucleolus reappears

__ chromosomes become diffuse

__ spindle and aster disappear

__ centrioles are replicated
Points less frequently mentioned:

__ function of centrioles

__ definition of kinetochores

__ description of polar microtubules and kinetochore microtubules

__ definition of chromatids
*In order to obtain a score of 10, there must be points in all three sections. If only two sections are written the maximum is 9.




PART II. CYTOKINESIS:
__ division of cytoplasm
__ formation of a cleavage furrow
__ occurrence of cytokinesis in the cell cycle
Points less frequently mentioned:
__ function of cytokinesis
__ dense belt of actin and myosin microfilaments
__ purse-string mechanism
__ furrow occurs at location of equatorial plane
__ cytochalasin blocks activity of microfilaments (stops
cytokinesis)
PART III. OTHER PHASES OF THE CELL CYCLE (INTERPHASE):
General
 __ list G1, S, and G2 in correct order
 __ G1, S, and G2 are part of interphase
 __ chromosomes appear as a mass of chromatin material G1
 __ synthesis of cell organelles or cell doubles in size
 __ restriction (decision) point or point of no return S
 __ synthesis or replication of DNA or DNA replication occurs during
interphase G2
 __ synthesis of microtubular assembly, or prepare for mitosis
Points less frequently mentioned:
 __ description of nucleosomes
 __ times in each phase
 __ growth factors
 __ some cells do not go beyond G1
 __ after cell passes "S", mitosis will usually continue
 __ colchicine prevents the formation of microtubules
Essay Prompt #7

a.
b.
c.
The Cell Cycle functions in regulating information flow through
organisms.
Discuss how alleles are distributed by the process of meiosis
to the gametes.
Explain how the cell cycle is controlled.
Explain how these controls prevent errors in inheritance.
Note: It is important t answer all three parts (a-c) and to include all
of the key proteins and concepts discussed in class. ie. Cyclin,
MPF, three checkpoints, p53 genes, tumor suprerssor genes, etc.
Essay Prompt #7 Grading Rubric
Essay Prompt #8





Read the following question and on a separate sheet op paper give your answer in an ESSAY FORM. Outline alone is not
acceptable. Labeled diagrams may be used to supplement discussion, but in no case will a diagram alone suffice. It is
important that your read each question completely before you begin to write.
In Fruit flies, the phenotype for eye color is determined by a certain locus. E indicates the dominant allele and e indicates
the recessive allele. The cross between a male wild-type fruit fly and female white-eyed fly produced the following
offspring:
Wild-type
Wild-type
White-eyed
White eyed
Brown eyed
Male
Female
Male
Female
Female
F1
0
45
55
0
1

The wild-type and white-eyed individuals from F1 generation were then crossed to produce the following offspring:
Wild-type
Wild-type
White-eyed
White eyed
Brown eyed
Male
Female
Male
Female
Female
F2
23
31
22
24
1

1.








Determine the genotype of the original parents (P generation) and explain your reasoning.
You may use Punnett squares to enhance your description, but the results from the Punnett
squares must be discussed in your answer.
2. Use a Chi-squared test on the F2 generation data to analyze your prediction of the
parental genotypes.
Show all your work and explain the importance of your final answer. BE sure to write the
Chi-Square
formula next to your work.
The brown-eyed female in the F1 generation resulted from a mutational change. Explain
what a mutation is, and discuss two types of mutations that might have produced the
brown-eyed female in the F1 generation.
**Note: Your response to these three questions will be a major part ofyour grade for this
lab. Therefore, be thorough and complete in your response
ESSAY Prompt #8: Rubric (Key)

A) maximum 4 pts

1 pt Genotypes of the parents (words or symbols) XEY (or X+Y) and XeXe
1 pt Discuss/show how these resulted in this F1 (may be annotated Punnett)
1 pt Explain that it is a sex-linked (X-linked) gene (not just the word)
1 pt How you know which type is dominant

1 pt F2 results (may be annotated Punnett square)



B) Maximum 4 pts
1 pt Correct F2 hypothesis (1:1:1:1 or 25/genotype)
1 pt Show work (components): o e o-e (o-e)2 (o-e)2/e (or correct numbers (4/25 of 36/25 + 1/25
+ 9/25 = 50/25 = 2; or at least the last term)
1 pt Sum; correct chi-square result ~ 2.0 or 1.85
1 pt degrees of freedom = 3 (critical value is 7.82)
1 pt correct interpretation of chi-square in terms of p
p = probability that the difference between the observed and the expected value is due to
chance alone. This p value shows we accept our hypothesis. The null hypothesis is
supported in this case. (alternative: 2 X2 tests of white vs. red males and white vs. red
females.
C) Maximum 4 pts
1 pt Explain what a mutation is: (heritable) change in the DNA (code)
1-2 pts Discuss 2 types of mutations
May be: point mutation, frameshift (deletion/duplication), insertion, transposition, break,
inversion within gene, base substitution, nonsense/stop, missense)
May NOT be: chromosomal aberration, nondisjunction, silent/neutral, transcription or
translation or processing error
1 pt Molecular or biochemical elaboration beyond the explanation required
Essay Prompt #9

Describe the chemical nature of genes. (a) Discuss
the replication process of DNA naming all of the
steps and key enzymes involved. (b) Name TWO
types of gene mutations that occur during
replication.
Essay Prompt #9-Rubric (Key)
A gene is a hereditary unit located at a specific locus along a
chromosome. Genes are made up of DNA, and DNA is made up of
repeating subunits of nucleotides. A nucleotide consists of three parts:
a five-carbon sugar, a phosphate group, and a nitrogenous base.
When a chromosome replicates, the two DNA strands unwind and the
hydrogen bonds between them are broken. Each strand serves as a
template for the synthesis of a complementary strand. Once the
process is initiated by an RNA primer), DNA polymerase adds
nucleotides to each growing strand. One strand serves as the leading
strand (it is made continuously) and the other serves as the lagging
strand (it is made discontinuously). Each base matches the appropriate
bases in the template strand; they are complementary. Once the
complementary strands are formed, hydrogen bonds form between the
new base pairs, leaving two identical copies of the original DNA
molecule.