DNA Structure, Function and Replication[1]
DNA carries the genetic information in all living organisms, including humans, other animals, plants, and bacteria.
Each cell in your body has a nucleus with multiple chromosomes. Each chromosome contains a DNA molecule with multiple genes. Each gene is a segment of DNA that provides the instructions for making a protein. A cell needs many different types of proteins to function, including:
· protein enzymes to carry out all the chemical reactions needed for life
· transport proteins to move ions and molecules into and out of the cell and to move substances around inside the cell
· structural proteins.
1. Which cells in your body contain DNA? Why do these cells need DNA?
The genes in our DNA influence our characteristics by giving the instructions for making the proteins in our bodies. For example, as shown in the chart below, two different versions of a gene give the instructions for producing either normal or defective versions of a protein enzyme which result in either normal skin and hair color or albinism.
DNA / ® / Protein / ® / Characteristic/ ® / / ® /
Version of the gene that provides instructions to make
normal protein enzyme / ® / Normal enzyme that makes the pigment molecule in skin and hair / ® / Normal skin and hair color
Version of the gene that provides instructions to make
defective enzyme / ® / Defective enzyme that does not make this pigment molecule / ® / Albinism (very pale skin and hair)
Skin and hair color are also influenced by:
· other genes (e.g. genes that influence how much pigment is made)
· environmental factors (e.g. sun exposure which influences how much pigment is made in the skin)
2. For most people, when they are injured, the clotting proteins in their blood can produce a blood clot to prevent excessive bleeding. However, some people have a defective version of one type of clotting protein, so clots do not form normally. As a result, they tend to bleed excessively after even minor injuries. These people have hemophilia. Explain how differences in DNA molecules can determine whether or not a person has hemophilia.
3. For each of the following, indicate whether you think it contains DNA (Y = yes; N = no).
bananas ___ fish ___ glass___ meat ___ metal ___ mushrooms ___ spinach ___
Explain your reasoning.
This diagram shows a short section of a DNA double helix with a diagram of four nucleotides in each strand of the double helix. Each nucleotide has:· a phosphate group (P) and a sugar molecule (represented by a pentagon) in the backbone of the DNA strand
· one of the four bases (A = adenine, C = cytosine, G = guanine, or T = thymine) /
(http://bio3400.nicerweb.com/Locked/media/ch01/01_08-DNA_double_helix.jpg)
Since all the nucleotides in DNA are the same except for the base they contain, each nucleotide is given the same symbol as the base it contains (A, C, G, or T).
These diagrams show two short sections of DNA.The base-pairing rules describe which bases pair together in the DNA double helix. Complete the following sentences to give the base-pairing rules.
4. A in one strand always pairs with _____ in the other strand.
C in one strand always pairs with _____ in the other strand. /
(http://www.bio-rad.com/cmc_upload/Literature/12525/4006096G.pdf)
5. / For the two sections of DNA shown above: / Yes or No
Is the arrangement of the sugar and phosphate groups the same in both sections of DNA?
Do both sections of DNA contain the bases, A, C, G, and T?
Are the base-pairing rules the same in both sections of DNA?
Is the sequence of nucleotides the same in both sections of DNA?
What is the only difference between these sections of DNA?
This flowchart shows how a gene influences an organism’s characteristics. This flowchart shows how the sequence of nucleotides in a gene encodes the genetic information.
sequence of nucleotides in the DNA of a gene/
determines the sequence of amino acids in a protein
determines the structure and function of the protein
(e.g. normal vs. defective enzyme
to make skin pigment) /
influences the characteristics or traits of the organism
(e.g. normal skin pigmentation vs. albino) /
Our bodies need to make new cells to grow or to replace damaged cells. New cells are formed by cell division, which occurs when a cell divides into two daughter cells. For example, cell division in the lining of your mouth makes the new cells that replace the cells that are rubbed off whenever you chew food. Before a cell can divide, the cell must make a second copy of all the DNA in each chromosome; this process is called DNA replication.
6. Explain why DNA replication is needed before a cell divides into two daughter cells.
During DNA replication, the two strands of the DNA helix are separated and each old strand provides the instructions for making a new matching strand.The nucleotides in each new strand are added one at a time. Each new nucleotide is matched to a nucleotide in the old strand using the base-pairing rules.
DNA replication results in two new DNA molecules that have the exact same sequence of nucleotides as the original DNA molecule. Thus, each of the new DNA molecules carries the same genetic information as the original DNA molecule.
DNA polymerase is an enzyme that helps to make the new matching DNA strand by adding nucleotides one at a time and joining each new nucleotide to the previous nucleotide in the growing DNA strand. /
(From Biology -- A Human Emphasis, by Starr)
7. DNA replication produces two identical copies of the original DNA molecule. Explain how the double helix structure of DNA and the base-pairing rules work together to result in two identical copies of the original DNA molecule.
8. Based on the function of DNA polymerase, explain why each part of the name DNA polymerase (DNA, polymer, -ase) makes sense.
9. The drawing below shows the short segment of DNA from the first figure on page 3, after the two strands of the DNA molecule have been separated in preparation for replication. Your job is to play the role of DNA polymerase and create the new matching strands of DNA to make two pieces of double-stranded DNA in the drawing below. Use the base-pairing rules and the nucleotides and tape provided by your teacher.
Old Strand New Strand New Strand Old Strand
Look at both of the double-stranded pieces of DNA you have created. Are there any differences between these two pieces of DNA?
Are these new double-stranded pieces of DNA the same as or different from the original piece of DNA shown in the first figure on page 3?
Why is it important that both copies of the DNA molecule have the exact same sequence of nucleotides as the original DNA molecule?
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10. What is wrong with the figure on the right? Why would the type of DNA replication shown in the bottom part of the figure be biologically impossible? (Hint: Think about what determines the sequence of nucleotides as a new strand of DNA is formed.)
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