SOURCE Lesson Plans Detail

Experimenting with DNA

Topic Biology
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

Lesson Materials

Sharpies
DNA necklace kit
Test tubes
DNA base pair floor puzzle
Cards with pea traits
Blood typing kit

Lesson Motivation

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.

Lesson Activities

Blood typing activity
DNA necklace kit (extraction)
Pea trait activity (Mendel's Laws)
DNA structure floor puzzle

Procedure

Cell is the basic unit of life

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”

Silesian scientist and Augustinian friar who gained posthumous recognition

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

Pea Traits

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)

Genotypes:

           25% GG   homozygous dominant

           50% Gg    heterozygous (like the parents in this example)

           25% gg    homozygous recessive

Phenotypes:

           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.

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.

Follow up

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.

Alignment Info

Audience(s) High school students
STEM Area(s) Biology
Standard(s)
Life Sciences (RI GSE) LS1.9-11.2a
Students demonstrate an understanding of the molecular basis for heredity by … describing the DNA structure and relating the DNA sequence to the genetic code.
Activity Type(s) Hands-on
Grade Level(s) High School
Version 1
Created 05/29/2013 01:03 PM
Updated 12/20/2018 11:43 AM