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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Interlude C.  These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this essay.

  1. What scientific milestone was achieved in February of 2001?
  2. What are SNPs and how do scientists use them to predict an individual’s susceptibility to a genetic disease?
  3. Why did the National Institutes of Health decide to use a mixture of DNA from several individuals for the Human Genome Project?
  4. How is it that a complex organism such as a human can be produced using only ten times the number of genes required to produce a simple bacterium (like E. coli)?
  5. What have scientists learned about human sleep patterns by studying fruit flies?
  6. What is “behavioral genomics” and why are ethicists alarmed about this emerging field?

A Guide to the Reading

When exploring the content in Interlude C for the first time, the following concepts typically give students the most difficulty.  For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas. 

Organismal Complexity

The number of genes present in an organism’s genome does not always reflect how complex the organism may be.  For instance, humans have a genome size of 3.3 billion base pairs with 20-30,000 functional genes (See Table C.1).  Compare this to some of the single-celled organisms listed in the table which have 5-6,000 genes in their genome.  To further illustrate this point, consider that the puffer fish Takifugu rubripes has an estimated 31,000 genes!  So how is it that organisms with such varied complexity can be produced using roughly the same number of genes?  Part of this can be explained by understanding that it is the combination of genes that are turned on or off which helps to generate such diversity.  As described in the essay, an organism with 100 total genes which can be either on or off can have a total of 2100 possible genetic combinations.  To further enhance complexity, recall that the expression of genes depends on several interacting factors.  Also, eukaryotic genes may be processed (removal of introns) in such a manner that makes it possible to generate a number of different mRNA molecules from a single gene.  All of these factors taken together make it easy to see how a relatively small number of genes may work together to produce a highly complex organism.

For more information on this concept, be sure to focus on:

  • In Interlude C, Complexity in organisms is determined by more than just the number of genes
  • Table C.1, A Sample of the Genomes Sequenced to Date
  • Figure C.3, How Genes Can Produce Multiple RNAs

Functional Protein Domains

A protein domain is a portion of a protein, such as an enzyme, that has a particular function.  Life processes that are common between groups of organisms, such as DNA replication and glycolysis, make use of enzymes that are likewise, very similar.  Since any alteration to the amino acid sequence of these important enzymes would likely have a negative impact on the function of the protein, mutation within these domains is not tolerated very well.  This explains the observation that 90 percent of all human protein domains are also found in less complex organisms such as nematode worms and fruit flies.  The pressures to maintain consistent sequences of genes will be discussed in more detail in Chapter 18. 

For more information on this concept, be sure to focus on:

  • In Interlude C, Comparing the genomes of different organisms yields valuable insights
  • Figure C.4, Gene Expression in a Fruit Fly Embryo

Single Nucleotide Polymorphisms

Recall that the DNA from two unrelated individuals is approximately 99.9 percent identical in nucleotide sequence.  Single nucleotide polymorphisms (SNPs) represent the single base-pair differences that make each of us unique.  The Human Genome Project has identified more than 10,000,000 SNPs.  SNPs can be useful for genetic screening when a particular SNP can be associated with a particular genetic disease.  It is important to note that the SNP itself may not necessarily be the cause of the genetic disease, but it does provide a “marker” that can be screened for to determine an individual’s predisposition to developing the associated disease.  Several examples are provided in the essay, including the identification of different SNPs within the apoE gene that are associated with the development of Alzheimer disease.  When combined with the power of the DNA chip, SNPs provide a rapid, accurate method for doctors to obtain a person’s genetic profile.

For more information on this concept, be sure to focus on:

  • In Interlude C, SNPs are a powerful means of characterizing individual genomes
  • In Interlude C, SNP profiling will help identify the risk of getting certain diseases
  • Figure C.6, From Tissue to Test Results

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:


  • Interlude B, Cancer: Cell Division Out of Control

Human Genetic Disorders

  • Chapter 11 – Section 11.5, Human Genetic Disorders

Preimplantation Genetic Diagnosis

  • Chapter 11 – Science Toolkit, Prenatal Genetic Screening

Noncoding DNA Sequences

  • Chapter 13 – Section 13.3, Transcription : Information Flow from DNA to RNA

Gene Expression

  • Chapter 14 – Section 14.4, How Cells Control Gene Expression

DNA Technologies

  • Chapter 15 – Section 15.3, Applications of DNA Technology


  • Chapter 17 – Section 17.7, Natural Selection: The Effects of Advantageous Alleles

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