DNA STRUCTURE AND REPLICATION NOTES
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DNA stands for ___________________________________________
It is the “blueprint for life”; everyone’s DNA is different!
Each ________________________________ codes for a protein that will represent a
________.
DNA is found in the ____________________ of __________________ cells.
History of DNA Discovery
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Rosalind Franklin (1950) - Used X-ray diffraction to show DNA
fibers were “twisted”.
This was one
smart lady!
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Erwin Chargaff (1952) - DNA is composed of “nucleotides” bonded together in a
paired fashion:
Chargaff’s base pair rule:
_____________ (A) with _______________ (T)
______________ (C) with ______________ (G)
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Using all the accumulated knowledge from previous scientists’ in 1953 James Watson &
Francis Crick published their findings on the structure of DNA. They called this structure
a _______________ ________________. **They won the Nobel Prize in 1962.
Watson & Crick’s findings:
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DNA is composed of two nucleotide chains arranged in a
________________________(twisted ladder)
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Nucleotides combine together, across the ladder, in complementary base pairing:___
___ and ___ ___.
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Nucleotides are held together by weak ____________ ____________.
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DNA is found only in the _________________
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It is composed of subunits called________________, which are the building blocks of all
nucleic acids.
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The DNA nucleotide is made up of three different
components:_______________________________, a _________________________, and one of
four _____________ ___________: adenine, guanine, thymine, and cytosine (A-T, G-C)
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Adenine and guanine are double-ringed molecules known as____________________,
while cytosine and Thymine are single-ringed____________________.
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The general structure of a nucleotide is shown in Figure 1 below. Notice where the
phosphate and the base bond to the sugar.
Figure 1: Structure of a nucleotide is circled and labeled.
The DNA Molecule
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It is “___________________________.” (See Figure 1 or 2 on next page) It is made up
of two strands of nucleotides paired with one another, giving it a “ladder-like”
structure.
The “______________” of the “______________” are composed of alternating units of
_______________________________________& _____________________________
connected together by ____________________________ (strong bonds) to form the
“_____________________” of the molecule.
The phosphates connect to the 5’ of one sugar to the 3’ of the next sugar. The two
sides of the molecule are identical except that they run in opposite directions. The
two strands are said to be ____________________________to one another.
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The “_______________” of the DNA “_____________________” are composed of the
_____________________________which are bonded to the deoxyribose sugar &
project out into the center of the molecule where they pair with one another.
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The chemical structure of the nitrogen bases limits their pairing to very specific
combinations. This pairing of the bases is referred to as
__________________________________base pairing. (refer to figure 2 below)
o ________________________on one strand will always pair only with
___________________on the opposite strand.
o ________________________on one strand will always pair with
____________________ on the opposite strand.
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The two strands are held together by weak ___________________________that form
between the bases. Adenine & Thymine form _______ hydrogen bonds, while
guanine & cytosine form ________ hydrogen bonds. (refer to figure 2 below)
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Watson & Crick described DNA as a _______________________________ because the
two strands of nucleotides are twisted around one another in a spiral.
Figure 2: Structure of DNA
5’
3’
One Nucleotide
3’
5’
Exercise 1: Structure of DNA
1. Refer to Exercise 1 on the data sheet. On the diagram provided, a DNA molecule
will be drawn, colored, & labeled.
2. Note that the sugar/phosphate “backbones” of the two strands are already
drawn. First, they must be colored & labeled. Color the deoxyribose sugars purple
& color the phosphates pink.
3. Now label each sugar with a “D” and each phosphate with a “P”. Label the 3’ &
5’ ends of the strands using Figure 2 as a guide.
4. Now, complete the DNA molecule by adding the nitrogen bases to the strand.
The first pair is already drawn. Using the templates shown below, finish drawing in
the remaining 8 bases on the left side of the DNA diagram. The bases can be
placed on the strand in any order, but make sure you use at least one of each
type. Note that the bases are attached to the sugars, NOT the phosphates. Also,
allow enough room between the bases to indicate the hydrogen bonds (Step 7).
A
Adenine (Red)
G
Guanine (green)
T
Thymine (Yellow)
C
Cytosine (Blue)
5. Next, draw in the complementary bases on the right side of the DNA diagram.
Remember, the bases drawn must be complementary to the ones already drawn
on the left side.
6. Color the bases as indicated above & label each one with the first letter of the
name of the base (A, T, G, C).
7. Now indicate the hydrogen bonds between all the base pairs using dotted lines.
Place 2 dotted lines between A and T, and 3 dotted lines between G & C.
8. Finally, somewhere near the middle of the molecule, draw a circle around a single
nucleotide on each strand and label them “nucleotide”.
PART 2: REPLICATION OF DNA
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DNA is the “____________________________” for an organism. The specific arrangement
o sequence of the paired nitrogen bases in the DNA molecule makes up the
“_______________________” in the cell.
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The DNA molecules must be able to produce ____________________________of itself prior
to ___________________________________in order to ensure that: (1) the
___________________information is carried from generation to generation, and (2) each
new cell will have the _______________same genetic information.
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The process by which the DNA molecule is copied is called __________________________.
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DNA replication has been shown experimentally to be a
“_________________________________” process which means that at the completion of
replication each new DNA molecule consists of one strand from the
______________________DNA molecule and one _____________synthesized strand.
THE STEPS OF DNA REPLICATION ARE AS FOLLOWS:
1. The two original strands composing the DNA double helix____________________.
2. The weak hydrogen bonds are broken by the enzyme __________________
exposing the nitrogen bases.
3. As this occurs, each of the two strands acts as a “______________________” or
mold for the replication of a new strand.
4. New nucleotide bases are attracted to their
___________________________partners on each of the separated strands and pair
with them forming ______________________bonds.
5. Two _______________ molecules of DNA are formed, each made of a
__________and _____________strand (semi-conservative).
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In step 3, as the new nucleotides come in and pair, an enzyme,
__________________________, moves along each newly forming strand, catalyzing the
formation of ______________________bonds between the _____________and phosphates
of the new nucleotides.
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DNA polymerase can only move in a _________________direction. (See Figure 4).
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One of the new strands is synthesized____________________, in one long unbroken piece
in the 5’ to 3’ direction. This is called the “__________________________” and has
______________________________replication.
The DNA polymerase enzyme does not act on the second strand until a stretch of
about __________________nucleotides has been exposed. Then it starts at the
replicating fork and moves along, in the __________________________direction, until the
100-300 nucleotides are paired. This is the “_______________________________” and it has
________________________________replication.
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When another such section of nucleotides of the DNA has opened, the process is
repeated. This results in a series of short replicated pieces called
_______________________fragments, which are later connected together by ligase
enzyme.
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SUMMARY: during replication, the leading strand has continuous replication (into
fork) and the lagging strand has discontinuous replication (out of fork) in the opposite
direction.
FYI- Origins of replication- There are many different sites along a chromosome which are
active in replication. There may be 100’s or 1000’s of initiation points along one DNA
molecule- about 50,000 to 300,000 nucleotide pairs long (15 to 100 um). A single
chromosome is huge. The “X” chromosome is 29,850,000 base pairs in length. (DNA
polymerase can hook about 50 nucleotides together in a second in mammals.)
3’
5’
Original DNA
3’
Figure 4: Continuous & Discontinuous Strands
5’
Continuous
(leading) strand
3’
5’
Discontinuous
(lagging) strand
Exercise 2: Replication
1. Using the DNA sequence that was constructed in Exercise 1, the replication of the
molecule will be illustrated.
2. In Exercise 2 on the data sheet, the strands of DNA are shown to be splitting apart
as the base pairs are separated in preparation for replication. The shorter strands
in the center represent the new strands that are being synthesized off the original
template. The arrows on the diagram indicate the direction that DNA polymerase
is moving down the strands as it joins the new nucleotides together.
3. On the original DNA strands (that are splitting apart; the outside strands) draw in
the same bases that were used in Exercise 1. On the shorter strands (newly
synthesized) draw in bases that are complementary to those in the original DNA
template.
4. Color and label all the components in the diagram, using the same colors as in
Exercise 1.
5. Indicate hydrogen bonds between all the paired bases as in Exercise 1.
6. Label the continuous strand & the discontinuous strand. Also label the 3’ and 5’
ends of the original strands and of each new strand being synthesized.
PAP 2014: The Cell Cycle & Cancer
As we have already learned, the _____________ theory states that all cells come from preexisting _____________. In this unit you will learn how cells reproduce to make
________________ daughter cells.
What role does cellular division play in the lives of organism?
1. _______________________________: when a unicellular organism such as an amoeba
divides, it reproduces an entire organism. The two organisms are identical.
2. _________________ and __________________________: in multicellular organisms, cell
division allows for growth (from a single, fertilized egg) and repair of the organism.
It is important to note that cells do not simply “pinch” in half. A dividing cell duplicates its
DNA and sends each identical copy to two ends of the cell and then splits into two
________________________ cells.
All of a cell’s hereditary information is called its _________________. A typical human cell has
about ______ feet of DNA!! Before the cell can divide, all of this genome must be copied so
that each daughter cell can have its own copy. The replication and distribution of this DNA
is possible because the DNA is packaged into __________________. Chromosomes are made
of the chromatin material that we have been studying. During cell division, they condense
and appear as chromosomes.
Every type of organism has a characteristic number of chromosomes. Human have _____
chromosomes in the nucleus of their _______or body cells (every cell but the ______ and
(______________). Reproductive cells or __________________have half of that number,
_______________. The diagram above illustrates that each chromosome is made up of two
_________________________. Each contains the identical copies of the cell’s DNA. These
copies are held together at the “waist” by the ________________.
A cell with that is about to divide has double the amount of DNA and is referred to as
____________ or 2N. At the end of cell division the cell only has one copy of DNA and it is
referred to as ____________ or N.
The Cell Cycle:
The diagram above represents the life of a cell. The G1, S period, and G2 in the above
diagram is called ___________________________ and accounts for ________% of the cell’s life.
During interphase the cell grows and copies its chromosomes in preparation for cell division.
Mitosis is the division of the nucleus of the cell. Cytokenisis is the division of the
_______________.
Interphase: (________________________) Cells are carrying out their normal functions in the
body and preparing for cell division. This phase may be divided into three parts:
1. G1 phase: period of cellular __________________ and development.
2. S phase: ____________________________________
3. G2 phase: manufacture of _____________ and ___________ needed for cell division.
Mitosis begins!! P.M.A.T.
1. __________________ (prepare)
a. Chromatin appears as _________________.
b. Nuclear membrane ______________.
c. Nucleolus ________________.
d. Spindle fibers __________________.
e. Centrioles move to the ____________.
2. ____________________ (middle)
a. Chromosomes _____________________
____________________________
____________________________
b. Spindle fibers ______________________
____________________________
____________________________
3. ____________________ (away)
a. Chromatids ________________________
_____________________________
_____________________________
Each chromatid is now considered
an individual chromosome.
4. ________________________ (two new cells)
a. Nuclear membrane _________________.
b. Nucleolus _____________________.
c. Chromosomes become __________________.
d. Spindle fibers ________________.
How are plant and animal cells different in cell division?
Plant cells
Animal cells
Cytokinesis is the ___________________________________________. In animal cells, a
_________________ ________________ forms that pinches the cell in two.
In plant cells, there in no cleavage furrow. Instead, packets or ______________ from the Golgi
body move to the center of the cell where they form a _____________ ___________. Cell wall
materials collect here as the plate grows until a new cell wall has developed between the
two daughter cells.
Cell Cycle Regulators & Cancer
Regulating Cell Growth- Cyclins – Proteins that control the “____________” of the cell cycle
(Mitosis!)
____________________occur at end of each cycle before it moves onto next phase. They
ensure that the cell is healthy before allowing it to move onto next phase.
What stops cells from dividing endlessly?
Internal regulators – Class of proteins that respond to events ___________ the cell called
_____________.
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Checkpoint example: Some cyclins do not let cell continue the cell cycle (Mitosis) until
all chromosomes are copies.
External regulators – Class of proteins that respond to events ___________ of the cell.
Especially important in embryonic development and wound healing.
What is Cancer? ____________________________________. Cell won’t stop dividing and
eventually they block nutrients, oxygen and blood flow from tissues.
*on a microscope slide you would see an abnormally high amount of cells going
through mitosis.
Environmental or genetic mutations disrupt healthy cell division.
Environmental: over exposure to ______________, ________________, or sun!
Genetic Mutations: chance mutations that occur during cell division. Can’t prevent 
How do tumors form? Cancer does not respond to the ____________ or ____________
regulators of a normal cell cycle. Cancer is essentially uncontrolled cell division and results in
tumors over time that can disrupt.