Download DNA Replication Reading

DNA - The Double Helix
Recall that the nucleus is a small spherical, dense body in a eukaryotic cell. It is often
called the "control center" because it controls all the activities of the cell including cell
reproduction, and heredity. DNA is the molecule containing this code. Chromosomes
are microscopic, threadlike strands composed of the chemical DNA (short
for deoxyribonucleic acid and histone proteins. In simple terms, DNA controls the
production of proteins within the cell. These proteins in turn, form the structural units of
cells and control all chemical processes within the cell.
Chromosomes are composed of genes. A gene is a segment of DNA that codes for a
particular protein, which in turn codes for a trait. Hence you hear it commonly referred
to as the gene for baldness or the gene for blue eyes. Meanwhile, DNA is the chemical
that genes and chromosomes are made of. It stands for deoxyribonucleic acid. DNA is called a nucleic acid
because it was first found in the nucleus. We now know that DNA is also found in organelles, the mitochondria
and chloroplasts, though it is the DNA in the nucleus that actually controls the cell's workings.
In the early 1950s, the British scientist Rosalind Franklin began to study DNA. Using a technique called X-ray
diffraction Franklin provided many clues about the structure of DNA. Building on Franklins work, in 1953,
James Watson and Francis Crick established the structure of DNA. The structure is a double helix, which is
like a twisted ladder. The sides of the ladder are made of alternating sugar and phosphate molecules. The
sugar is deoxyribose.
The rungs of the ladder are pairs of 4 types of nitrogen bases including adenine, guanine, thymine and
cytosine. The bases are known by their coded letters A, G, T, C. These bases always bond in a certain
way. Adenine will only bond to thymine. Guanine will only bond with cytosine. This is known as the Base-Pair
Rule. The bases can occur in any order along a strand of DNA. The order of these bases is the code
containing the instructions. For instance ATGCACATA would code for a different gene than AATTACGGA.
A strand of DNA contains millions of bases. (For simplicity, the image only contains a few.) Note that the bases
attach to the sides of the ladder at the sugars and not the phosphate.
The combination of a single nitrogenous base, a deoxyribose sugar, and a phosphate make up a nucleotide.
DNA is actually a molecule or repeating nucleotides.
The two sides of the DNA ladder are held together loosely by hydrogen bonds. Hydrogen bonds are
relatively weak bonds, which allow the enzyme helicase to open the molecule exposing the nitrogenous bases
to copy the code during interphase. After the DNA molecule is opened DNA polymerase adds nucleotide
bases onto the exposed bases and a new sugar-phosphate backbone is formed.
Questions – Use the information above and page 344 to 348 in Miller and Levine to answer the
following questions
1. Where is DNA located in eukaryotic cells?
2. What is a nucleotide? RSQ
3. How is a nucleotide arranged to form the molecule DNA? (use the ladder analogy) RSQ
4. Base pair rules states that adenine always binds with
5. List the individuals involved in the discovery of the structure of DNA.
and guanine always binds with
6. Match the nitrogenous base pairs. Color the adenine red, thymine green, guanine blue, cytosine yellow,
the phosphates brown, and the deoxyribose black
5. Why is hydrogen bonding essential to the structure of
DNA? RSQ
Read pages 350 and 351 and answer the following
questions. RSQ
6. Why it is necessary for the DNA to make a copy before the
cell divides?
7. What is the role of helicase in DNA replication?
8. What is the role of DNA polymerase in DNA replication?
Now! Copy the code – use your DNA model and the information on page 350 to 351 to model DNA
Replication, must include the opening of the double stranded molecule, something to represent helicase and
DNA polymerase, the addition of new base pairs onto the original strand to show the final product and a new
sugar-phosphate backbone. Label each strand as new and old.
9. Now show the steps involved in DNA replication of the DNA molecule in the image above. There should be
at least four steps.