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| TIPS: Targeted Interventions for Proficiency in Science | |||
Life Science
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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
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.
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.
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.
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. Performance 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.
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.
To view more examples, go to Performance Benchmark L.12.A.1 Students know genetic information passed from parents to offspring is coded in the DNA molecule. E/S Sample Test Questions 1. In Chargaff’s rule, the base-pairing
rule, state that in DNA 2. Which one of the following nucleotide pair
bonds would be found in a DNA molecule? 3. The base-pairing rules state that the following
are base pairs in DNA: 4. During DNA replication, a complementary strand
of DNA is made for each original DNA strand. Thus, if a portion of
the original strand is CCTAGCT, then the new strand will be 5. Which statement below correctly describe the
relationship between genome size and organism “complexity”
Students know genetic information passed from parents to offspring is coded in the DNA molecule. E/S Answers to Sample Test Questions 1. (a) Performance 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 find this video, go to the KLVX Video Streaming website at
2. The building of the DNA model To learn more about building DNA models and for lesson plans go to
3. DNA Extraction experiments To review the virtual DNA extraction lab at the Genetics Science
Learning Center, go to
4. DNA fingerprinting activity To review the DNA fingerprinting lab at Nova Online, go to |
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