2. In 1953, James Watson and Francis Crick worked out that
DNA is double helix like a twisted staircase. The two
sugar-phosphate backbones make up the sides and the
base pairs make up the rungs or steps of the twisted
staircase.
Deoxyribonucleic acid is copied during interphase prior to
mitosis and meiosis. It is important that new copies are
exactly like the original molecule.
3.
4. The structure of the DNA provides a mechanism for
making accurate copies of the molecule. The process of
making copies of DNA is called replication.
When DNA replicates, two identical copies of DNA
molecules are produced, which are exactly the same as
the original.
5. Objectives
1. Explain how DNA replicates;
2. Make a model of a DNA template to determine the
sequence of bases in the new DNA strand; and,
3. Cite the importance od DNA replication.
6. Activity 2: DNA Makes DNA
Objective:
1. Make a model of a DNA template to determine the
sequence of bases in the new DNA strand.
Materials:
• crayons
• scissors
• paste/tape
• 1/4 size illustration board or long size folder
7. Guide Questions:
Q4. Compare the two new strands of DNA. Are they the
same or different? Why?
Q5. How do the nucleotides in DNA pair?
Q6. How do you compare a DNA molecule to a zipper?
Q7. How is information from the DNA passed on from one
cell to another?
Q8. How does the structure of a DNA molecule help
account for the great variety of life that exists on earth?
8. Let us generalize...
Do you understand the process by which DNA copies itself?
The following are the events while DNA copies itself:
• Step 1. An enzyme called helicase breaks the bond
between nitrogenous bases. The two strands of DNA split.
• Step 2. The bases attached to each strand then pair up
with the free nucleotides found in the cytoplasm.
9. Let us generalize...
Step 3. The complementary nucleotides are added to each
strand by DNA polymerase to form new strands. Two new
DNA molecules, each with a parent strand and each with a
new strand are formed. The DNA replication is known as
semi-conservative replication, because one of the old
strands is conserved in each new molecule.
11. KEY CONCEPTS:
• DNA is made up of sugars, phosphate groups, and
nitrogenous bases and its shape is a double helix. The
complementary structure the two strands of DNA allow each
strand to serve as a template during replication.
• The specificity of base pairing in DNA, adenine with
thymine, and cytosine with guanine, allows DNA to replicate
itself with accuracy.
