Introduction in Genetics

The scientific study of heredity is called genetics. Modern genetics is based on the knowledge that traits are transmitted by means of chromosomes, rod-shaped structures within the nucleus of a cell. Offspring resemble parents because the chromosomes in sperm cells and egg cells contain units of hereditary information. These units are called genes. As an individual formed by a sperm cell and an egg, a cell grows into an adult, the genes influence its development. The genes cause it to resemble the parents who supplied the chromosomes.

The similarities are common biological characteristics or traits of the human species. Each species has its own traits. Heredity is passing of traits from parents to their young - the offspring. Genetics is the study of how traits are passed on and expressed in an organism.

To understand genetics, you must think about cells. Human skin cells, brain cells, and all the body cells have one thing in common. They all have 23 pairs of chromosomes (Fig. 2.3).

Chromosomes [soma, body]: one of the bodies in the cell nucleus containing genes in a linear order; visualized as threads or rods of chromatin, which appear in a contracted form during mitosis and meiosis. They are found within the nuclei of your cells and are made of DNA and many proteins. Chromosomes are visible when a cell is reproducing. All species have a certain number of chromosomes in their body cells. Cattle, for example, have 30 pairs of chromosomes in their body cells. Garden peas have 7 pairs (Table 2.1).

Included in the chromosomes are genes, which are sections of DNA. Genes carry coded instructions. Genes carry information for assembling the molecules that make up an organism’s body.

Long before people understood the basis of heredity, they bred animals and plants for certain desirable traits. Many breeds of dogs were developed during the middle Ages. In the 1800s biologists began to study heredity through scientific experiments. Discoveries made by one of these biologists, Gregor Mendel, became the basis of modern genetics.

Fig. 2.3. Normal human (female) karyotype

Table 2.1

The number of chromosome pairs in body cells of different species

Growth of Living Organisms

Organisms grow larger by increasing the number of their cells. In order for the number of cells to increase, new cells must form from cells that are already present. After a new cell forms, it grows to about twice its original size. As it grows, it doubles all of its parts. Then this double-larger cell splits into two, forming two new cells from one original cell. For this process to work, a cell must double all of its parts, including the chromosomes. The chromosomes are especially important because the DNA in chromosomes carries the instructions for how the new cell should be built. But this criterion alone is inadequate to define growth. Growth is the irreversible increase in the dry mass of an organism and is normally accompanied by an increase in cell number. It involves the uptake of chemicals and the synthesis of new structures. Fresh mass is less reliable indicator of growth due to temporary fluctuations in an organism’s water content.

DNA Makes Exact Copies

Chromosomes are threadlike structures found inside the nucleus of a cell. They contain deoxyribonucleic acid (DNA). DNA is a unique molecule because it is able to reproduce itself exactly. This process is called replication (Fig. 2.4). DNA replicates itself so that every new cell receives a complete copy of the genetic code. Thus DNA replicates before mitosis, when new cells are produced for growth and repair. DNA also replicates before the first division of meiosis so that each gamete receives a copy of the genetic material present in the somatic cells of the organism. The chemical bonds connecting the bases break in several places and the molecule separates down the middle. As the molecule splits into separate strands, special enzymes cause the proper nucleotides to pair with complementary nucleotides on each single strand. Other enzymes

Fig. 2.4. DNA ladder separates to form two identical DNA ladders

then link the new nucleotides into one long strand. Each original strand serves as a template, or pattern, for the creation of the new strand. Every T nucleotide pairs with a nucleotide containing an A. Likewise, every G nucleotide pairs with a nucleotide containing a C. Each completed DNA molecule contains one old and one new strand. The entire process is powered by energy from ATP and the action of enzymes.

DNA replication results in the formation of two new molecules, each of which receives one strand of the original parent molecule. It is therefore said to be semi-conservative.

DNA replication is the means by which new genetic material is produced in the nucleus of a cell during interphase. The two identical daughter DNA molecules formed from each chromosome coil up and become identical chromatids held together by a centromere. The quantity of genetic material has doubled without changing the cell’s chromosome number. It is at this time, following DNA replication but immediately before nuclear division that the cell’s DNA content is at its maximum.

Date: 2014-12-22; view: 1265