Download Gen.1303 The Scientific Basis of Human Genetics In the 19th

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The Scientific Basis of Human Genetics
In the 19th century, microscopic studies of cells led scientists to suspect that the nucleus
of the cell contains the important mechanisms of inheritance. They found that
chromatin, the substance that gives the nucleus a granular appearance, is observable in
the nuclei of non-dividing cells. Just before a cell undergoes division, the chromatin
condenses to form discrete, dark staining bodies called chromosomes. With the
rediscovery of Mendel's breeding experiments at the beginning of the 20th century, it
soon became apparent that chromosomes contain genes. Genes are transmitted from
parent to offspring and are considered to be the basic unit of inheritance. It is through the
transmission of genes that physical traits such as eye color are inherited in families.
Diseases can also be transmitted through genetic inheritance.
Physically, genes are composed of deoxyribonucleic acid (DNA). DNA provides the
genetic “blueprint” for all proteins in the body. Thus, genes ultimately influence all
aspects of body structure and function.
Chromosomes and its Relation to Heredity
Chromosomes are microscopic structures that contain an individual’s genetic material; a copy of a
person’s chromosomes is located within each cell of their body (except for red blood cells). This
genetic material serves as the “instruction manual” for the body, containing the “directions” the body
needs in order to form and function properly. In general, chromosomes serve as a way to organize
the complex genetic code; if you imagine the genetic code as a library, the chromosomes would be
like the books in the library, and genes would be like the specific chapters in each book, containing
the instructions for each particular task the body needs to perform.
Each human somatic cell (cells other than the gametes, or sperm and egg cells) contains
23 pairs of different chromosomes, for a total of 46. one member of each pair is
derived from the individual's father, and the other member is derived from the mother.
One of the chromosome pairs consists of the sex chromosomes. In normal males, the
sex chromosomes are a Y chromosome inherited from the father and an X chromosome
inherited from the mother. Two X chromosomes are found in normal females, one
inherited from each parent. The other 22 pairs of chromosomes are termed autosomes.
The members of each pair of autosomes are said to be homologous, because their DNA
is very similar. The X and Y chromosomes are not homologous of one another. Somatic
cells, having two of each chromosome, are termed diploid cells. Human gametes have
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the haploid number of chromosomes, 23.
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Cell Reproduction and Divisions
The Cell Cycle
During the course of development, each human progresses from a single-cell zygote (an
egg cell fertilized by a sperm cell) to a complex organism containing approximately 100
trillion individual cells. Because of few cells last for an individual's entire lifetime, new
ones must be generated to replace those that die. Both of these processes – development
and replacement-- require the manufacture of new cells. The cell division processes are
of two types Mitosis and Meiosis.
Mitosis:
Cell division process in which two identical progeny cells are produced from a single
parent cell.
Meiosis:
Cell division process in which haploid gametes are formed from diploid germ cells.
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