Some Learning Goals

1. Identify components of a DNA molecule and know how they 11. are arranged in the molecule.

2. Describe the functions of DNA. 12.

3. Describe how a DNA molecule replicates.

4. Know the function of transcription and outline its steps. 13.

5. Know the function of translation and outline its steps.

6. Distinguish between somatic and germ-line mutations. 14.

7. Describe the significance of translocations and inversions. 15.

8. Distinguish between aneuploids and polyploids. 16.

9. Understand the significance of Mendel's experiments with peas. 10. Give the ratios of the offspring in the first two generations 17.

from a monohybrid and a dihybrid cross. Describe the genotypes involved. 18.

Distinguish between genotype and phenotype; heterozygous and homozygous.

Be able to solve simple genetics problems involving dominance and incomplete dominance.

Show how genes may interact with each other to influence phenotype.

Explain how genotype influences phenotype. Describe features of a quantitative trait. Describe where extranuclear DNA is located and how it differs from nuclear DNA.

Explain how linkage can produce ratios that deviate from

Mendelian ratios.

Describe the Hardy-Weinberg law.

very so often, someone comes along who is able

Eto push beyond our limits of understanding in science and change the way we view the natural world. Barbara McClintock was one of the most significant scientists in 20th century biology because she caused a major shift in the way we view gene organization. She was a geneticist who carried out most of her work at the Cold Spring Harbor laboratory in New York in the 1940s and 1950s. While looking at corn seedlings for one of her genetic studies, she noticed that streaks of color often appeared where they did not seem to belong. There were patches of yellow in green leaves or patches of green in white leaves. She also noticed spots of color in corn kernels that should have been colorless, based on her genetic studies (Fig. 13.1). Instead of ignoring these plants as genetic oddities, McClintock undertook years of research to identify the cause of these patterns.

In the early 1950s, McClintock published work introducing the concept of transposition, or the movement of a piece of a chromosome to another chromosomal location. A transposable genetic element (or jumping gene in popular literature) is a gene or a small DNA fragment that can move to a new location on the same chromosome or even to another chromosome. If it moves into an existing gene, then the function of that gene is disrupted. A transposable element can also move out of a gene and restore its original function. So, the patches of yellow in green leaves were regions where transposable elements were inserted into a gene involved in chlorophyll synthesis. Green areas on white leaves resulted when a transposable element moved out of a chlorophyll synthesis gene and allowed it to function again.

Figure 13.1 An ear of corn showing the effects of transposable elements. The speckles in the center kernel are clusters of cells in which a transposable element has removed itself from a purple pigment gene. Yellow areas in that kernel do not express purple pigment because the transposable element has interrupted that pigment gene.

Figure 13.1 An ear of corn showing the effects of transposable elements. The speckles in the center kernel are clusters of cells in which a transposable element has removed itself from a purple pigment gene. Yellow areas in that kernel do not express purple pigment because the transposable element has interrupted that pigment gene.

Despite volumes of data to support her hypothesis, McClintock's efforts to explain her work were initially met with silence. Most of her colleagues could not understand what she was proposing, and so they ignored her work. Others ridiculed her or implied she was crazy. Gene theory at that time suggested a gene to be an indivisible unit, like a bead on a string. It was unthinkable to believe that mutations could happen within genes. And it certainly seemed absurd to propose that pieces of chromosomes could actually move

Chapter 13

to new locations. However, McClintock knew she was right, and she continued her attempts to convince colleagues to accept her ideas.

The discovery of the structure of DNA by James Watson and Francis Crick in 1953 ushered in the new field of molecular genetics. At that time, geneticists thought of McClintock's work as a remnant of a previous era and not relevant to modern genetics. However, studies of DNA and gene structure transformed the concept of a gene. Genes were no longer abstract entities but were actual sequences of nucleic acids that could be isolated and characterized. The concept of mutation within a gene became believable. In addition, numerous studies found transposable elements in other organisms.

McClintock's work was formally recognized in 1983 when she was awarded a Nobel Prize. We now know that transposable elements are widespread and have probably been in organisms for a long time. We do not know, however, if they represent genetic parasites causing mutations as they move about an organism's genetic material or if they perform valuable functions. One theory is that they allow nature to tinker with chromosomes much as human genetic engineers do. It may be evolutionarily beneficial to copy, move, and rearrange pieces of chromosomes, creating new and occasionally better combinations of genes within an organism.

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Responses

  • mike
    Why does transposable elements alter chlorophyll synthesis?
    8 years ago

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