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Content Benchmark L.12.A.1
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Students know genetic information passed from parents to offspring is coded in the DNA molecule. E/S

During the first half of the 20th century a debate among biologists centered around whether proteins or deoxyribonucleic acid (DNA) was the molecule of inheritance. In 1928, Fredrick Griffith first proposed that the transfer of DNA between bacteria caused transformation, but could not provide convincing proof. In the 1940s Oswald Avery and others were able to devise experiments that provided the evidence that Griffith lacked. However it took an experiment by Alfred Hershey and Martha Chase in 1952 to finally demonstrate to the scientific community that DNA and not proteins was the molecule of inheritance.

Today we know that the coded message for our traits is based in the four nucleotides of DNA. These four bases adenine (A), thymine (T), guanine (G), and cytosine (C) are divided into two groups called purines (A & G) and pyrimidines (T & C). The Human Genome Project has confirmed that the DNA in a typical human cell contains over 3 billion base pairs (bp). In these 3 billion bp are 20,000 to 25,000 genes that code for proteins, which in turn code for our traits. Each gene is a specific sequence of nucleotides located on one of the DNA strands.

The DNA in the human body is spread over 24 distinct chromosomes which range in size from 50 million to 250 million bp.

To learn more about the findings of the Human Genome Project go to

DNA is a nucleic acid that contains nucleotides running in two strands and twisted into a double helix. Each nucleotide contains three molecules (a base, a deoxyribose sugar and a phosphate group). The nucleotides pairings are shown in the illustration below. These pairings also referred as complementary base pairs.

Figure 1: The DNA molecule showing base pairing.

In the illustration above you will see that the base adenine is paired with the base thymine and the base guanine is paired with the base cytosine. The idea of these pairing was first discovered by Edwin Chargaff in the late 1940’s. In working with cells from different organisms Chargaff discovered that the percentages of adenine in a cell were equal to the percentages of thymine, and the percentages of guanine were equal to the percentages of cytosine in the nucleus. These pairing have become known as “Chargaff’s Rule”. However the meaning of this discovery was not clear until Watson and Crick developed their mode of DNA in 1953.

The genetic code in DNA is passed along via mitosis, meiosis or binary fission. Prior to these processes the molecules of DNA in the parent cell must be copied via DNA replication. In their 1953 paper Watson and Crick proposed that each strand of the DNA molecule makes a complementary copy of itself through DNA replication prior to cell division. In 1957, Meselson and Stahl devised an experiment that demonstrated this semi-conservation nature of DNA replication as first proposed by Watson and Crick. In this process each strand from the original DNA molecule gets a new complementary strand. Thus each new DNA molecule has 1 (one) strand from the “old” molecule and one “new” strand that is an exact copy of the original.

Figure 2: The semi-conservative nature of DNA replication.

To learn more about the history of DNA’s discovery go to

Before the genetic code is passed from parents to offspring via meiosis or from one cell to another new cell via mitosis the DNA must be replicated. As illustrated in Figure 2 each strand of the DNA makes a complementary copy of itself.

In a simplified view the copying can be seen in Figure 3. The DNA “unzips” and complementary bases are brought to each strand and the new stands “rezip” forming two identical copies of the original DNA molecule.

Figure 3: A simplified view of DNA replication.

Biologists have discovered that the actual process is far more complicated. First a molecule called helicase unwinds the DNA double helix. Before DNA polymerase travels along each strand matching complementary bases, a short sequence of RNA nucleotides is matched with the separated strands by RNA primase. The copying of each strand, however, is different. One strand called the leading strand is copied in a continuous fashion, while the other called the lagging strand is copied in fragments called Okazaski fragments as seen in the illustration below. Later DNA ligase will join these fragments.

Figure 4: The replication of DNA along the leading and lagging strands.

The rate at which new nucleotides are added is about 50 per second and would take 53 days to replicate the largest human chromosome if replication began at one end and proceeded to the other end. As such the replication of any chromosome begins at many origins along the chromosome. In fruit fly chromosomes there are some 3500 origin sites where DNA replication begins simultaneously.

To learn more and to view an animation of DNA replication go to

The later is a part of the online textbook “Online Biology Book”. Numerous diagrams are included along with a description of DNA structure and replication. Links are also provided to other websites.

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Content Benchmark L.12.A.1

Students know genetic information passed from parents to offspring is coded in the DNA molecule. E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly assume that DNA is vastly different amongst members of the same species.

Despite the fact that humans contain over 3 billion bp in their DNA, researchers have found that most of DNA is quite similar. Based on sequencing to date it appears that on average two unrelated people have one different nucleotide per 1000 bases. Thus with 3 billion bp total bases this means there are 3 million differences between individuals or less than 0.01% difference between individuals. These differences are called single nucleotide polymorphisms or SNPs (pronounced “snips”)

Aside from the fact each human has a unique combination of genes; these genes are shared by all members of the human species. The goal of the Human Genome Project is to identify these genes and than determine what each gene codes for in humans.

To learn more about SNPs go to

2. Students mistakenly assume that DNA coding for molecules is different between different species.

While a species has a unique gene pool that defines that species, many genes are shared by humans and other organisms. For example 45% of the genes found in fruit flies are also found in humans and we share approximately 96% of our genes with chimpanzees. This should not be surprising considering the number of biochemical pathways that are commonly found in organisms. For example most organisms obtain energy or ATP by cellular respiration and the enzymes (coded for by DNA) involved in this biochemical pathway are found in most organisms.

To learn more about the similarity of human and chimpanzee DNA go to

3. Students incorrectly assume that mutations in DNA are always harmful.

Single base errors in DNA copying are called point mutations, however, these are rare. During DNA replication the error rate is 1 in 10,000 bases being copied. Most of these errors are corrected by DNA proof readers. Secondly if an error is not corrected, because of the redundancy of the genetic code the same amino acid may be coded for by the codon. This is called a silent mutation. Or an amino acid with similar properties can be coded for by the “mutant” codon which is sometimes called a neutral mutation. Note the table below.

At the same time some point mutations can be harmful. In the table below the DNA triplet and mRNA codon are shown for the 6th amino acid for normal hemoglobin. In sickle-cell anemia a mutation from T to A leads to the replacing of glutamic acid with valine. The result is an abnormally shaped hemoglobin molecule during low oxygen concentration in the blood. And if the first C is replace by A, the result is the termination of protein synthesis and no hemoglobin molecule is produced.

DNA Triplet mRNA codon Amino Acid Properties Mutation Type
CTC GAG Glutamic acid Hydrophobic Normal codon
CTT GAA Glutamic acid Hydrophobic Neutral
CTA GAU Aspartic acid Hydrophobic Silent
CAC GUG Valine Hydrophilic Missense
ATC UAG Stop Termination Nonsense

All humans with blood type O are also carrying a mutation. The genes for blood type code for proteins found on the red blood cell. Many inherit the genes for the A and B proteins. However due to a point mutation in our ancestral past the coding for these proteins was lost, thus those who inherit the alleles for O lack coding for either protein.

To learn more about DNA mutations go to

4. Students incorrectly assume that DNA and chromosomes are not the same.

Both of these are names for the same molecule. Humans have 46 chromosomes and each chromosome contains a specific sequence of DNA nucleotides which are codes for our genes. However, each chromosome contains a unique combinations of genes. Genes found in Chromosome #1 are not the same as those found in chromosome #2..

To learn more about what genes have been discovered on each human chromosome and to order a free poster showing human genome landmarks go to

5. Students inaccurately assume that the amount of genetic material is equal in males and females.

While each offspring, regardless of sex, receives 23 chromosomes from each parent, the amount of DNA and the genes they receive is not equal. Both males and female receive the same 22 pairs of autosomes (non-sex chromosomes) and therefore each receives equal number of genes for non-sexual characteristics found on these chromosomes. However the sex chromosomes (X and Y) do not contain equal number of genes or types of genes.

The X chromosome represents approximately 5% of the total DNA in cells and contains approximately 1300 genes. These 1300 genes not only include genes for femaleness, but genes for such traits as blood clotting and color vision. On the other hand the Y chromosome contains approximately 2% of the DNA in a cell or approximately 300 genes which will be inherited by males only, since females do not inherit a Y chromosome. Thus males inherit both the X and Y chromosomes, they will inherit all genes for the same traits as females, but females will lack any genes found on the Y chromosome.

To learn more about the X and Y chromosomes go to

6. Students incorrectly believe that the amount of DNA varies in organisms based upon their “complexity”

While is true that humans have approximately 1000 times more DNA than a typical bacterium, the Human Genome Project has reveled some interesting surprises. Humans have approximately 3.0 X 109 base pairs (bp) and 20,000 to 25,000 genes. But notice the figures for other organisms.

Organism DNA base pairs Approximate # of Genes Chromosome # Mutation Type
Rice 3.9 X 109 37,000 12 Normal codon
Maize 2.5 X 109 Over 50,000 20 Neutral
Mouse 2.5 X 109 22,500 40 Silent
Whisk fern 2.5 X 1011 ? ? Missense
Sea urchin 8.14 X108 23,000 44 Nonsense

To view more examples, go to

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Content Benchmark L.12.A.1

Students known genetic information passed from parents to offspring is coded in the DNA molecule. E/S

Sample Test Questions

1st Item Specification: Identify the location of the DNA molecule and the shape of its double helix structure.

Depth of Knowledge Level 1

  1. DNA is found in all the following cell structures EXCEPT the
    1. nucleus.
    2. mitochondria.
    3. chloroplasts
    4. ribosomes.
  1. Which of the following describes the normal shape of a DNA molecule?
    1. It forms a double helix.
    2. It consists of a single helix.
    3. It contains two straight strands.
    4. It is formed from a single stand.
  1. Which statement below correctly describes the relationship between genome size and organism complexity?
    1. As organisms get more complex their genomes get larger.
    2. Organisms of similar complexity have genomes of similar size.
    3. There appears to be little relationship between complexity and genome size.
    4. Individuals of the same species have vastly different genomes.

Depth of Knowledge Level 2

  1. A molecule contains the original directions for the formation of proteins. In which structure would it most likely be found in a cell?
    1. Ribosome
    2. Nucleus
    3. Endoplasmic reticulum
    4. Golgi apparatus
  1. A molecule that consists of two strands running in the opposite direction generally forms a(n)
    1. antiparrallel double helix.
    2. parallel double helix.
    3. twisted ladder shape.
    4. coiled spring-like shape.
  1. Use the diagram below to answer the next question.

What macromolecule results when these molecules and others like them are
joined together?

  1. Carbohydrate
  2. Protein
  3. Lipid
  4. Nucleic acid
2nd Item Specification: Explain that DNA copies itself.

Depth of Knowledge Level 1

  1. Chargaff’s rule states that in DNA the amount of
    1. adenine equals the amount of thymine.
    2. guanine equals the amount of thymine.
    3. cytosine equals the amount of thymine.
    4. Iadenine equals the amount of guanine.
  1. Which one of the following pairs normally bond together in a DNA molecule?
    1. cytosine–guanine
    2. adenine–cytosine
    3. cytosine–uracil
    4. adenine–cytosine
  1. Which of the following are complimentary base pairs in DNA?
    1. adenine–thymine; uracil–cytosine.
    2. adenine–thymine; guanine–cytosine.
    3. adenine–guanine; thymine–cytosine.
    4. uracil–thymine; guanine–cytosine.
  1. If the nucleotide sequence on one side of a DNA molecule is CCTAGCT, then the sequence on the complimentary strand of DNA will be
    1. TTGCATG.
    2. CCTAGCT.
    3. AAGTATC.
    4. GGATCGA.

Depth of Knowledge Level 2

  1. If the amount of adenine in a DNA molecule is 20%, the amount of cytosine would be
    1. 20%
    2. 30%
    3. 50%
    4. 80%
  1. If the amount of guanine in a DNA molecule is 32%‚ the amount of thymine would be
    1. 16%
    2. 18%
    3. 32%
    4. 36%

3rd Item Specification: Explain that DNA contains hereditary information.

Depth of Knowledge Level 1

  1. Which molecule carries the code for traits of most organisms?
    1. Ribonucleic acids
    2. Carbohydrates
    3. Adenosine triphosphate
    4. Deoxyribonucleic acids
  1. Which statement below best describes the role of DNA?
    1. It is the main supplier of energy for cells.
    2. It is used to store energy in the body.
    3. It carries information for building proteins.
    4. It serves as the memory molecule in brain cells.

Depth of Knowledge Level 2

  1. Use the diagram below to answer the next question. The diagram illustrates an experiment performed in the early part of the 1900’s in work done with two strains of bacteria called the R and S strains. In this experiment, the researcher is trying to determine if he could transform one strain of bacteria into the other strain.


Which statement below correctly describes a possible conclusion for this experiment?

  1. The R and S strains both cause the mice to die.
  2. The R and S strains can be changed into the other.
  3. The S strain is always lethal.
  4. The R strain can be converted into a lethal strain.
  1. Use the table below to answer the next question.
Table 1: Distribution of Bases Found in Different Types of Organisms
Table adapted from Modern Biology test bank.

Which conclusion BEST describes the data in this table in reference to life on Earth?

  1. The genetic code is generally universal among living organisms.
  2. Organisms have different proportions of bases in their DNA.
  3. Animals have similar amounts of bases in their cells.
  4. Plants are least related genetically to other organisms on this table.

4th Item Specification: Describe the process of DNA replication in the formation of sex cells.

Depth of Knowledge Level 1

  1. How many times does DNA replicate during meiosis?
    1. Once
    2. Twice
    3. Three times
    4. Four times
  1. During the process of meiosis DNA replicates
    1. twice and divides once.
    2. twice and divides twice.
    3. once and divides once.
    4. once and divides twice.
  1. An offspring of two parents had an extra chromosome in its karyotype. The most likely cause of this condition would have been
    1. a point mutation.
    2. translocation.
    3. nondisjunction.
    4. a nonsense mutation.

Depth of Knowledge Level 2

  1. Which statement below correctly describes what happens to DNA during the formation of sex cells?
    1. When DNA replicates, each of the two new DNA molecules has one old strand and one new strand.
    2. As a result of DNA replication, one DNA molecule is completely new and one remains as the original.
    3. After the formation of sex cells the DNA molecules are a mix of old and new strands.
    4. Each new sex cell contains a random, but equal amount of the original DNA molecules.
  1. An organism has 3 pairs of chromosomes. How many different combinations of chromosomes are possible as a result of meiosis if crossing over or chromosomal mutations do not occur?
    1. 2
    2. 4
    3. 8
    4. 16

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Content Benchmark L.12.A.1

Students known genetic information passed from parents to offspring is coded in the DNA molecule. E/S

Answers to Sample Test Questions

  1. D, DOK Level 1
  2. A, DOK Level 1
  3. C, DOK Level 1
  4. B, DOK Level 2
  5. A, DOK Level 2
  6. D, DOK Level 2
  7. A, DOK Level 1
  8. A, DOK Level 1
  9. B, DOK Level 1
  10. D, DOK Level 1
  11. B, DOK Level 2
  12. B, DOK Level 2
  13. D, DOK Level 1
  14. C, DOK Level 1
  15. D, DOK Level 2
  16. A, DOK Level 2
  17. A, DOK Level 1
  18. D, DOK Level 1
  19. C, DOK Level 1
  20. A, DOK Level 2
  21. C, DOK Level 2

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Content Benchmark L.12.A.1

Students know genetic information passed from parents to offspring is coded in the DNA molecule. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Movie on the discovery of DNA and its molecular structure
To understand and appreciate the discovery of DNA as the molecule of inheritance teachers can begin by showing the video Biologix: Development of Molecular Genetics which last 29 minutes. It is divide into two parts. The first part talks about the works of Griffith, Avery, and Hershey/Chase that lead to the establishment of DNA as the molecule of inheritance. In the second part the work Levene, Chargaff, Franklin and Watson/Crick is and their efforts to establish the structure of DNA. This video is useful for all levels of biology.

To find this video, go to the KLVX Video Streaming website at

2. The building of the DNA model
One of the important aspects of science is to build model molecular structures that scientists are trying to understand. In their efforts to discover the structure of DNA Watson and Crick resorted to a variety of models. To help students appreciate this aspect of science students can also build their own models of DNA.

To learn more about building DNA models and for lesson plans go to

3. DNA Extraction experiments
Along with model building students can very easily perform DNA extraction experiments at little cost using household items. The Genetic Science Learning Center website provides a simple and straight forward protocol for extracting DNA from wheat germ. In addition it provides information as to why various items are used in the protocol. For example it explains the role of detergent in extracting the DNA from the wheat germ. This site also has a virtual DNA extraction lab.

To review the virtual DNA extraction lab at the Genetics Science Learning Center, go to

4. DNA fingerprinting activity
To understand that our genes are passed from parent to offspring the teacher could have the students look at the role of DNA fingerprinting in helping to identify individuals. At this Nova Online site students can play the role of detective and use genetic fingerprinting to solve a crime.

To review the DNA fingerprinting lab at Nova Online, go to

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