B. McClintock (1953) Induction Of Instability at Selected Loci In Maize. Genetics 32:579-599.

[Presented by: Megan White, Lorielle Pevie (web page), 04 February 2010]

Background and Introduction

Barbara McClintock (1902-1992) received the Nobel Prize in Physiology or Medicine in 1983.
McClintock was the first American Woman to win an unshared Nobel Prize.

McClintock identified the mechanism by which transposition occurred by studying Maize.
            ~This mechanism is known as the AC-DS system (Activator Dissociator).

                    Ds- Dissocator element
                    Ac- Activator element
Ac Ds system can cause changes other than transposition such as;
            ~dicentric chromatid formation, deficiency, duplication of segments, inversions, ring
            chromosomes and reciprocal translocations between chromosomes.

For all of the previously stated changes the chromosome breaks are caused by some type of
initial change that involves the Ds unit.

In the 1950's when McClintock published her paper on transposition it was known that;
            ~ Chromosomes were the units of heredity. (Boveri and Sutton's Chromosome Theory of Inheritance, 1902)
            ~It was also known that DNA not protein is the genetic material that is inherited between generations.
            (Hersey and Chase experiment 1952)

McClintock held a research position at Cold Spring Harbour Laboratories and studied genetic mutations. She did
this by observing the leaves of growing plants and documented the colour changes and the texture of the pigment
in the kernels.

After numerous discoveries in molecular biology the view that genes are universal was accepted and Barbara
McClintock was finally acknowledged for her work with transposition in corn.

Materials & Methods:
Corn Kernels are normally purple which indicates the expression of genes that allows for the synthesis of anthocyanin.
If the expression of the previously stated genes is disrupted a variegated appearance will occur.

Timing of inactivation is significant to the phenotypic outcome. If some genes become inactivated late in kernel
development the spots will be small, but if the inactivation occurs early the spots will be large.

The purpose of this experiment was to show that Ac-controlled mutability can be expressed at different loci where genic action is known.
    Previous reports have shown Ac-controlled mutability at loci C, Bz and Wx found on chromosome 9.
    In this experiment loci A₁ on chromosome 5 is used.

To detect presence and number of Ac elements in a "plant X", cross with an "Ac tester stock"
    constituents of tester stock: known genotype of Ds/Ds A₁/A₁
                                                    contains no Ac element

All resulting progeny of test cross therefore have a chromosome containing Ds and A₁on one arm

Consider possible results of the text cross:
If no Ac elements are present in plant X, there will be no chromosome breakage
    all kernels will express the dominant, colourless, phenotype

If one Ac element is present in plant X, this will be passed on to 1/2 the kernels in the test cross
    these kernels will experience a break at Ds, resulting in the loss of the A₁ allele allowing for expression of the recessive, variegated phenotype.

If two Ac elements are present in plant X, all resulting kernels of the test cross will receive an Ac
    all kernels will express the recessive, variegated phenotype.

Experimental Procedure:

A population of plants homozygous for A₁(colourless) was grown having a Ds element located in chromosome 5.
    These plants also contained an Ac element on separate chromosome.

These plants were pollenated by a plant homozygous for a₁ (variegated) and containing no Ds or Ac elements.
    All resulting kernels should appear colourless if Ds and Ac have no effect on the A₁locus.

Vareigated kernels were observed and a plant was grown from one such kernel.
    This plant was then self-pollinated, producing culture 6424.

To preform a test cross:
19 plants chosen from Culture 6424
1 tester stock (Ds/Ds A₁/A₁ No Ac element)

Cross each plant with tester stock

Count number of kernels on each resulting ear displaying each phenotype
    from these counts determine phenotypic ratio resulting from each cross

Results:

Group 1
    all resulting kernels were colourless, dominant phenotype being expressed
    .·. no Ac factor contributed by plant being tested

Group 2
    1/2 of resulting kernels were variegated
    recessive phentype being expressed in these kernels, Ac present in 1/2 the kernels
    .·. 1 Ac factor present in plant being tested

Group 3
    almost all resulting kernels were variegated, all must contain an Ac factor
    .·. 2 Ac factors present in plant being tested
    small number colourless kernels appear because Ac action delayed so that breakage at Ds occured after the kernel developed

    Transposition Videos

More Information about Transposons and Transposable elements

McClintock's work with transposable elements was subject to intense criticism and opposition at the time of its publication. At this time many scientists
believed that the genome was a linear and unchanging structure and that experiments conducted and the data gathered on genes in maize were not
universal and could not be applied to other biological genomes.

Today it is known that DNA undergoes natural and induced mutations that cause variation in the genome. The genome is also
altered by mobile elements which may rearrange genes within the genome and create new genotypes and phenotypes. (Also seen in
McClintock's experiments with maize)

It is known that eukaryotic genomes (Humans, Corn, Drosophila) transposition can occur within a chromosome or between two
different chromosomes. Conversely in prokaryotic genomes transposition can only occur on the same chromosome because the
chromosome is singular in nature.

In the human genome there are two types of transposable elements.

The human genome is estimated to contain between 20,0000-25,000 genes and the maize genome is estimated to have between 36,000-63,000.
Many genes in the genome were thought to have no significant role or funtction and were labelled "junk genes".

Some scientists now believe that transposable elements (Jumping Genes) many play a large role in the evolution and diversity of
the human genome.

Barbara McClintock quotes

"If you know you are on the right track, if you have this inner knowledge, then nobody can turn you off.... no matter what they say."

"It may seem unfair to reward a person for having so much pleasure over the years, asking the maize plant to solve specific problems and then watching its responses."

"I never thought of stopping, and I just hated sleeping. I can't imagine having a better life."

"When you know you're right, you don't care what others think. You know sooner of later it will come out in the wash."

Quotes about Barbara McClintock

"Barbara McClintock's burning curiosity, enthusiasm and uncompromising honesty serve as a constant reminder of what
drew us all to science in the first place." _Gerald Ralph Fink

Conclusion

Both Ac and Ds elements are involved in transposition and are structurally similar

The Ac element of the 2 unit Ac-Ds system controls when changes occur at Ds

It is important to note that both Ac and Ds elements must be present for transposition to occur.

When test plants are crossed the presence and number of Ac units can be determined by analyzing phenotypic ratios.

Transposable elements are important for the study and potential cure of disease and illness.

There is much still unknown about the genome and the so called "Junk Genes"

Now to test your knowledge of Transposition.