Concept


For centuries people accepted the "spontaneous generation" of life from inanimate matter. When this long-standing myth was finally dispelled in the mid-1800s, it became clear that all life must arise from pre-existing life — via a process of reproduction. If cells are the fundamental units of life, they too must have a reproductive mechanism that maintains the proper chromosome number in each cell. About a decade after the publication of Mendel's paper, scientists carefully documented the behavior of dye-stained chromosomes during cell division (mitosis). First, each chromosome copies itself, and the duplicates line up at the "equator" of the cell. Then, duplicate copies of each chromosome are pulled toward opposite poles. Finally, the cell splits at the equator, producing two new cells with identical sets of chromosomes.

Animation


Hello, I am Walther Flemming. In 1882, I published a detailed description of the process of mitosis — cell division. Mitosis was seen as early as 1844 by Herr Carl Nageli. However, he and many of my colleagues weren't sure if what they were seeing in stained, fixed (dead) cells really happened in living organisms. I showed that mitosis is a natural event. I looked at the cells of developing salamander embryos. These cells had big, thick chromosomes made visible by aniline dyes I used for staining. A good thing about these cells is they divide simultaneously in a fixed amount of time. I took cell samples at increasing time intervals and figured out the sequence of events for mitosis. Mitosis is a continous process, but I described it as a series of phases, or stages. My terminology is still in use today. When a cell is not undergoing mitosis, I call it the resting stage or interphase. During interphase, the nucleus stains as a dense granular body. Of course, the cell is not really resting. Nuclear material is duplicating. Prophase is the first stage of mitosis. The duplicated nuclear material condenses out of the nucleus as fibers. In my day, I called these fibers chromatin threads. After 1888, they were known as chromosomes. Each duplicated chromosome can be seen as a pair of sister chromatids. Although I didn't note this myself, sister chromatids are joined at a structure called the centromere. Let's zoom in and take a look. The next stage is metaphase. The nuclear membrane disappears and the sister chromatids move in line with the cell equator. Then the centromere of each chromatid pair divides and the daughter chromosomes begin to move apart. In my stained slides, I saw that spindle fibers were connected with these chromosome movements. During anaphase, the daughter chromosomes are pulled to opposite poles of the cell. The last stage is telophase. The chromosomes become less distinct and membranes form around the new nuclei. The cell pinches at the equator and splits into two daughter cells. So, mitosis starts with one mother cell and produces two daughter cells. The cells all have the same amount of genetic material. Once I published my paper, there was no doubt that mitosis is a natural process. As Rudolph Virchow said previously in 1858, omnis cellula e cellula, which is Latin for "all cells arise from other cells."

Gallery


(1 of 4) Photomicrograph of a cell dividing: nucleus is visible as dark staining organelle.
(2 of 4) Cell dividing: chromosomes are visible and lined up at the plane of division.
(3 of 4) Cell dividing: chromosomes are being pulled toward the cellular poles.

Audio/Video


Audio Glossary

Cell, Chromosome, Diploid, Haploid, Metaphase

Video Interviews

Garland Allen

Garland Allen is a Professor in the Evolutionary and Population Biology Program at Washington University in St. Louis. He authored Thomas Hunt Morgan: The Man & His Science, and several texts, including Matter, Energy and Life and The Study of Biology.

Clip 1 (0:30)
Human chromosome number and the "notion" of chromosomes.

Biography


 

In 1882, Walther Flemming published the definitive study of the cellular process of mitosis.

WALTHER FLEMMING (1843-1905)

Walther Flemming was born in Sachsenberg, Mecklenburg, now in Germany. He was a military physician during the Franco-Prussian War. Flemming held positions at the University of Prague (1873-76), and at the University of Kiel (1876-1901).

Flemming was one of the first to devote his time to cytology, the study of chromosomes. Cell division had been described as early as 1842 by Carl Nägeli, who thought it was an anomalous event. Flemming was the first to detail the chromosomal movements in the process of mitosis. In 1879, Flemming used aniline dyes, a by-product of coal tar, to stain cells of salamander embryos. He was able to visualize the threadlike material as the cells divide. He described the whole process in his book Zell-substanz, Kern und Zelltheilung (Cell-Substance, Nucleus, and Cell-Division), which was published in 1882. Much of what we know today about mitosis originated with Flemming's observations. He saw that chromosomes were "doubled" when they appeared in prophase, and "solved" the problem of chromosomal partitioning between mother and daughter cells. This was significant for later work in meiosis and the chromosomal theory of inheritance.

Factoid

Links


 

Links

The Biology Project

This site is a good resource in all areas of biology. It has a tutorial on mitosis .

Chromosomes

Dr. John Sedat's group in the University of California at San Franscisco studies chromosomes, their organization, structure and role within the cell. They have a 3D movie of mitosis.

Bibliography

  • Moore, John A., 1985, Science as a Way of Knowing, American Society of Zoologists, Thousand Oaks.

  • Portugal, Franklin H., and Cohen, Jack S., 1977, A Century of DNA: A History of the Structure and Function of the Genetic Substance, The Massachusetts Institute of Technology, Cambridge, Massachusetts.

  • Stubbe, Hans, 1972 (English Translation), History of Genetics, The Massachusetts Institute of Technology, Cambridge, Massachusetts.

  • Sturtevant, A. H., 1965, A History of Genetics, Harper & Row, Publishers, New York.

Glossary


Cell - The basic unit of any living organism. It is a small, watery, compartment filled with chemicals and a complete copy of the organism's genome.
Chromosome - One of the threadlike "packages" of genes and other DNA in the nucleus of a cell. Different kinds of organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes, 46 in all: 44 autosomes and two sex chromosomes. Each parent contributes one chromosome to each pair, so children get half of their chromosomes from their mothers and half from their fathers.
Diploid - Refers to a cell that has two sets of chromosomes. In humans, almost all the cells, except the gametes are diploid. Because there are 23 chromosomes in a single human set, most human cells contain 46 chromosomes.
Haploid - The number of chromosomes in a sperm or egg cell, half the diploid number.
Metaphase - The phase of mitosis, or cell division, when the chromosomes align along the center of the cell. Because metaphase chromosomes are highly condensed, scientists use these chromosomes for gene mapping and identifying chromosomal aberrations.

Children resemble their parents.
Genes come in pairs.
Genes don't blend.
Some genes are dominant.
Genetic inheritance follows rules.
Genes are real things.
Sex cells have one set of chromosomes; body cells have two.
Specialized chromosomes determine gender.
Chromosomes carry genes.
Genes get shuffled when chromosomes exchange pieces.
Evolution begins with the inheritance of gene variation.
Mendelian laws apply to human beings.
Mendelian genetics cannot fully explain human health and behavior.
DNA and proteins are the molecules of the cell nucleus.
One gene makes one protein.
A gene is made of DNA.
Bacteria and viruses have DNA too.
The DNA molecule is shaped like a twisted ladder.
A half DNA ladder is a template for copying the whole.
RNA is an intermediary between DNA and protein.
DNA words are three letters long.
A gene is a discrete sequence of DNA nucleotides.
The RNA message is sometimes edited.
Some viruses store genetic information in RNA.
RNA was the first genetic molecule.
Mutations are changes in genetic information.
Some types of mutations are automatically repaired.
A chromosome is a package for DNA.
Higher cells incorporate an ancient chromosome.
Some DNA does not encode protein.
Some DNA can jump.
Genes can be turned on and off.
Genes can be moved between species.
DNA responds to signals from outside the cell.
Different genes are active in different kinds of cells.
Master genes control basic body plans.
Development balances cell growth and death.
A genome is an entire set of genes.
Living things share common genes.
DNA is only the starting point for understanding human biology.
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