Experimenting with DNA
|Program||Brown Science Prep|
|Developed by||Joseph Botros|
|Developer Type||Undergraduate students|
Overview / Purpose / Essential Questions
What are the basic components of DNA?
Performance / Lesson Objective(s)
Understand the structure of DNA
Understand how DNA forms genes
Understand the basic rules of genetics
DNA necklace kit
DNA base pair floor puzzle
Cards with pea traits
Blood typing kit
Cells are the basic unit of life and DNA is the blueprint of our genetic material; hence, it is important to understand its structure and function.
Blood typing activity
DNA necklace kit (extraction)
Pea trait activity (Mendel's Laws)
DNA structure floor puzzle
Nucleus of the cell is the “control center” because it contains the genetic material, the DNA, that is, the instructions for making the proteins that control the bodily functions and is responsible for inherited differences between individuals
DNA is referred to as the “blueprints” (similar to the architectural plans for a building)
DNA = deoxy (missing one oxygen relative to ribose) ribo (sugar of DNA is ribose) nucleic acid
DNA is located in the nucleus of nearly every cell of the body. In order to extract DNA, we must first collect a sample of intact cells.
Distribute “DNA Necklace kit instructions”, a small cup into which they can pipet 2ml of sports drink, and a 15ml tube and access to a Sharpie to label it with their name. Direct students to perform steps 1-4. For best results, time the students to ensure they swish and scrape at least a full minute.
Because sports drinks contain salt, at a concentration compatible with the osmotic environment of the cells, the cheek cells maintain their volume. (Using water, which is hypotonic to cells, would have caused the cells to burst.)
To collect the DNA, we must break the membrane barriers of the cell itself and the nucleus. This process is called cell lysis.
Just as dishwashing solution dissolves fats (lipids) to clean a flying pan, the detergent-based cell lysis solution dissolves the phospholipid bilayer that make up the membranes by forming water-soluble complexes with them. Once the membranes are broken, the cell contents flow out and create a soup of dissolved membranes, cellular proteins, DNA, and other contents. This soup is called the cell lysate.
The DNA in the cell lysate is in solution. This means that it is incorporated in the liquid lysate and is not visible (much like a teaspoon of sugar in a cup of water).
However, DNA is insoluble in ethanol (an alcohol), meaning it cannot be incorporated into the liquid (much like a teaspoon of sand in a cup of water).
Therefore, DNA from the cell lysate can be visualized by applying a layer of ethanol on top of the cell lysate.
Direct students to perform steps 6-10. Again, time the students to ensure they wait the full 2 minutes.
A human cell (with the exception of sex cells, namely egg and sperm) possesses DNA in the form of 23 pairs of chromosomes
1 of each pair from Mom
1 of each pair from Dad
The 23 pairs consist of 22 pairs are autosomes and one pair of sex chromosomes
If your sex chromosomes consist of 2 X chromosomes, XX, you are female
If your sex chromosomes consist of an X chromosome and a Y chromosome, XY, you are male
Your mother has only X chromosomes (she always donates an X to the offspring)
Only your father has the Y chromosome that can result in a male offspring
Therefore, the father determines the sex of the child (makes you rethink Henry VIII)
Distribute to each student one red and one blue chip
The red chip which has X on both sides --- represents contribution of the mother
The blue chip has X on one side, Y on the other and represents the contribution of the father
Have the students toss the two chips and allow them to fall on their desk to determine the sex of their first imaginary child; repeat several times
There is a 50/50 chance of having a boy every time, regardless of the sex of the children already born (because the number of Y-bearing sperm are numerous and not significantly diminished)
Structure of DNA
Watson and Crick constructed a model in 1953
Double-helix = a ladder, twisted, 10 rungs per turn
Sides of the ladder = alternating sugar and phosphate (sugar = deoxyribose)
Rungs of the ladder = nitrogen base pairs
Four possible bases:
Adenine (A), cytosine (C), thymine (T), guanine (G)
Bases pair up in a specific pattern
Adenine (A) with thymine (T)
Cytosine (C) with guanine (G)
Adenine and guanine are larger, double-ring structures = Purines
Cytosine and thymine are smaller, single-ring structures = Pyrimidines
Specific binding (small with big) allow for equal “rung” length
Allow students to build DNA using the floor puzzle --- it can be built from both ends
Base-pair mutation = mistakes in base pairing
Base pair mutation can cause diseases such as Tay-Sach’s Disease,
cystic fibrosis, and sickle-cell anemia
many base pair mutations are of no consequence because they either occur in a region of the DNA that does not code for protein OR the base pair change does not result in a change in the amino acid (protein building block) placed in the protein
Mutations can also be caused by chromosomal error
For example, Down’s syndrome is the result of trisomy 21, that is, 3 copies of chromosome #21 rather than just 2 copies
Down’s syndrome occurs more frequently in older mothers, whose eggs are older (all the eggs a woman will ever have are produced at once while she herself is still a fetus)
Gregor Mendel = “Father of Genetics”
Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance. The profound significance of Mendel's work was not recognized until the turn of the 20th century, when the independent rediscovery of these laws initiated the modern science of genetics.
Mendel’s work led to Law of Dominance which states:
1. An organism receives two genes for each trait, one from each parent
2. One of the genes may be stronger; the trait of the stronger gene is displayed and is called the dominant gene. The trait of the weaker gene is “hidden” (is not displayed) and is called the recessive gene
Genotype = genetic makeup ex. Pp
Phenotype = physical trait resulting from that genotype
If the trait always showed in the offspring, Mendel called that gene the dominant gene for that trait
The other gene, weaker and usually hidden by the stronger gene, Mendel called the recessive gene
Ex. (on cards) the gene for flower color
P = dominant gene (purple) ---- upper case letter
p = recessive gene (white) ---- lower case letter
Four possible combinations
PP = homozygous dominant >>> purple flower
Pp or pP = heterozygous >>> purple flower (by appearance, indistinguishable from PP)
pp = homozygous recessive >>> white flower
homo = same hetero = different
Put out cups with pea plant traits. Have students line up and take TWO cards from each cup and using their chart, determine the appearance of their pea plant --- drawing it is optional
Trait dominant characteristic recessive trait
Flower color purple (P) white (p)
Pod color green (G) yellow (g)
Seed color yellow (Y) green (y)
Seed shape round (R) wrinkled (r)
Plant height tall (T) short (t)
Each parent carries two genes for a trait, one of which is passed on to any one offspring
To predict heredity we use a special chart called a Punnett Square
It shows the possible gene combinations for a trait
The parental genes are placed outside the square
Ex. If both parents are heterozygous for pod color G (green)
25% GG homozygous dominant
50% Gg heterozygous (like the parents in this example)
25% gg homozygous recessive
75% display the dominant trait G >>> green pod color
25% display the recessive trait g >>> yellow pod color
Optional: additional examples such as crossing 2 purple flowers, PP X Pp and so forth
This is the simpliest form of inheritance in which a trait is determined by a single gene in a pure dominance pattern
Not all traits are inherited this simply
For example, flower color in carnations
RR >>> red flower
rr >>> white flower
Rr >>> pink flower = pattern called co-dominance
Another example, blood types in humans
Blood type genotype blood type phenotype
AA or AO type A
BB or BO type B
AB type AB
OO type O
A and B are dominant over O
If A and B are both expressed >>> type AB
A denotes the presence of the A antigen (protein) on the surface of the red blood cells.
B denotes the presence of the B antigen (protein) on the surface of the red blood cells.
AB denotes the presence of both the A and B antigens on the surface of the red blood cells.
O denotes the absence of A and B (and an increase amount of H antigen which is present on all the types)
Distribute one chart and a small pile of toothpicks of each color (blue, white, and yellow) to each student or pair of students.
Use taped example to instruct students to place toothpicks on the top row of their sheet (not taped --- simply placed)
In spaces with an A, place a blue toothpick
In spaces with an O, place a white toothpick
In spaces with a B, place a yellow toothpick
This displays the genotypes for all four blood types. Using this as a guide, perform a cross in which the parents are AO (blue and white toothpicks in first parent spot) and BO (yellow and white toothpicks in second parent spot)
***Please collect toothpicks – do not toss in trash
Results prove that these parents could have children of any blood type meaning, that is, the child could have a blood type that isn’t the same as either parent.
The presence of a protein (an antigen) can be detected by using antibodies to that protein. Antibodies are very specific
To determine which proteins are present on the surface of red blood cells, and thus type the blood, antigen-antibody reaction can be used
A reaction occurs when an antigen combines with a corresponding antibody to produce an immune complex. This reaction, in the case of blood, results in agglutination which is seen as clumping
All the blood and anti-sera used in this experiment are simulated products. No actual blood products are used in this experiment and thus there is no risk of acquiring a blood-born disease.
Distribute each pair of students a hema-tag, a mystery blood, and access to anti-A serum and anti-B serum.
*****Students must use only small, single drops of all the materials!!!!
Have students place one small drop of the mystery patient’s blood in each circle at the top of the hema-tag.
Have the student add one small drop of anti-A serum to the left circle.
Have the student add one small drop of anti-B serum to the right circle.
Using two different toothpicks, being careful to avoid cross-contamination, mix the blood and anti-sera and look for evidence of agglutination (clumping) to determine blood type. Share results between other groups in the class and compare lab results to published answers.
Sample 1 type A
Sample 2 type B
Sample 3 type A
Sample 4 type O
Now it’s time to collect our DNA and make the necklaces.
Direct students to perform steps 11-13.
Wrap up / Conclusion
Compare results of DNA necklaces.
Answer questions appear following the necklace lab in the teacher instructions for the lab.
Pre Assessment Plan
Assess students' knowledge of DNA and divide into groups accordingly using the following survey:
1. What does DNA stand for?
2. What is a dominant trait?
3. What are the four different blood types?
4. What does codominance mean?
Post Assessment Plan
Repeat the survey as students wrap up.
|Audience(s)||High school students
|Grade Level(s)||High School
|Created||05/29/2013 01:03 PM|
|Updated||12/20/2018 11:43 AM|