Mendel published his findings in 1865 in a paper titled Experiments on Plant
Hybridization, but contemporary biologists remained unaware of its true importance (Mendel 1996). It was not until 1900—a full 35 years after its publication by
Mendel—that Mendel’s rules were rediscovered independently by three different
people: De Vries in the Netherlands, Correns in Germany, and Tshermark in
Austria.
Perhaps it was the extreme originality of Mendel’s ideas—their characterization
of complex traits of organisms as being passed to offspring from parents by way of
genes, and the use of abstract symbols to represent them—that prevented his
contemporaries from readily understanding them. Fulling understanding the gene,
however, required proof that it was more than simply an abstract symbol, but a
comprehensible entity in and of itself. At least three questions thus remained to be
answered: where genes existed in the cell, how they manifested their traits, and
what chemicals they consisted of.
2.2 Where Do Cells Exist in the Body?
Mendel was a small boy when he learned that all organisms were made up of
microscopic, sack-shaped components known as cells. It was later determined that
cells proliferated through fission, but research in that area had only just begun when
Mendel conducted his hybridization experiments with pea plants. Understanding
the question of where genes exist in the cell requires an understanding of the cells
themselves and their fission process.
For most organisms, cells are surrounded by a membrane and consist of a
gelatinous cytoplasm containing a gene-bearing nucleus and organelles such as the
mitochondrion, endoplasmic reticulum, and chloroplasts (in plant cells). These are
surrounded by a very thin cell membrane. Plant cells contain a cell wall made of
cellulose outside the cell membrane (see Fig. 2.3). When the cell divides, the
nucleus first splits in two, after which the cell separates into two parts.
Prior to division, the resting nucleus appears as a kind of mesh or particle. Once
division begins, however, the nucleus itself transforms into a long, slender,
threadlike chromosome. The chromosome is made up of a combination of DNA and
protein known as a chromonema. The proteins are necessary to maintain the
chromonema’s shape and regulate gene activity.
The number of chromosomes is fixed for different organism types: 46 in the case
of human beings and 14 for pea plants. In higher organisms, each somatic cell
receives one chromosome each from its mother and father, resulting in a homologous chromosome pairing one chromosome each from both parents. These pairs of
chromosomes are called “diploid” (2n), while sex cells and gametes are called
“haploid” because they carry only half the number of somatic cell chromosomes.
When the cells that form the body (somatic cells) undergo division, each chromosome first divides in two vertically for distribution to two new cells; each
resulting cell has the same number and same form of chromosomes (see Fig. 2.4a).
26
2 Introduction to Molecular Biology
Hybridization, but contemporary biologists remained unaware of its true importance (Mendel 1996). It was not until 1900—a full 35 years after its publication by
Mendel—that Mendel’s rules were rediscovered independently by three different
people: De Vries in the Netherlands, Correns in Germany, and Tshermark in
Austria.
Perhaps it was the extreme originality of Mendel’s ideas—their characterization
of complex traits of organisms as being passed to offspring from parents by way of
genes, and the use of abstract symbols to represent them—that prevented his
contemporaries from readily understanding them. Fulling understanding the gene,
however, required proof that it was more than simply an abstract symbol, but a
comprehensible entity in and of itself. At least three questions thus remained to be
answered: where genes existed in the cell, how they manifested their traits, and
what chemicals they consisted of.
2.2 Where Do Cells Exist in the Body?
Mendel was a small boy when he learned that all organisms were made up of
microscopic, sack-shaped components known as cells. It was later determined that
cells proliferated through fission, but research in that area had only just begun when
Mendel conducted his hybridization experiments with pea plants. Understanding
the question of where genes exist in the cell requires an understanding of the cells
themselves and their fission process.
For most organisms, cells are surrounded by a membrane and consist of a
gelatinous cytoplasm containing a gene-bearing nucleus and organelles such as the
mitochondrion, endoplasmic reticulum, and chloroplasts (in plant cells). These are
surrounded by a very thin cell membrane. Plant cells contain a cell wall made of
cellulose outside the cell membrane (see Fig. 2.3). When the cell divides, the
nucleus first splits in two, after which the cell separates into two parts.
Prior to division, the resting nucleus appears as a kind of mesh or particle. Once
division begins, however, the nucleus itself transforms into a long, slender,
threadlike chromosome. The chromosome is made up of a combination of DNA and
protein known as a chromonema. The proteins are necessary to maintain the
chromonema’s shape and regulate gene activity.
The number of chromosomes is fixed for different organism types: 46 in the case
of human beings and 14 for pea plants. In higher organisms, each somatic cell
receives one chromosome each from its mother and father, resulting in a homologous chromosome pairing one chromosome each from both parents. These pairs of
chromosomes are called “diploid” (2n), while sex cells and gametes are called
“haploid” because they carry only half the number of somatic cell chromosomes.
When the cells that form the body (somatic cells) undergo division, each chromosome first divides in two vertically for distribution to two new cells; each
resulting cell has the same number and same form of chromosomes (see Fig. 2.4a).
26
2 Introduction to Molecular Biology