MECHANISMS OF HEREDITY

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 The cells of the body are constantly dividing to replace damaged cells. Before a cell divides to create new cells, a process known as mitosis, or to form ovules or spermatozoa, a process called meiosis, the DNA included in each cell needs to copy, or replicate, itself. This process is possible because the DNA strands can open and separate. Each of the two strands of the original DNA serves as a model for a new strand. In this chapter, we will also tell you how human beings vary in height, weight, skin colour, eyes and other physical characteristics despite belonging to the same species. The secret is in the genes.

Chemical Processes

Although it is assumed today that all life-forms are connected to the presence of oxygen, life began on Earth more than three billion years ago in the form of micro-organisms. They determined, and still determine today, the biological processes on Earth. Science seeks to explain the origin of life as a series of chemical reactions that occurred by chance over millions of years and that gave rise to the various organisms of today. Another possibility is that life on Earth originated in the form of microbes that reached the Earth from space, lodged, for instance, within a meteorite that fell to the Earth's surface.


Original Cells

The origin of life on Earth can be inferred from molecular evolution. The first living organisms (prokaryotes) began to develop in groups, giving rise to a process of cooperation called symbiosis. In this way, more complex life-forms called eukaryotes emerged. Eukaryotes have a nucleus that contains genetic information (DNA). In large measure, the development of bacteria was a chemical evolution that resulted in new methods to obtain energy from the Sun and extract oxygen from water (photosynthesis).

Prokaryotes

These were the first life-forms, with no nucleus or enveloping membranes, These single-celled organisms had their genetic code dispersed between the cell walls. Today two groups of prokaryotes survive: bacteria and archaebacteria.

Eukaryotes                                                                                                                                                        Have a central nucleus that contains nucleic acid (DNA). The content of the nucleus is called nucleoplasm. The substance outside the nucleus is called cytoplasm, and it contains various organelles with different functions. Many are involved in generating energy for the organism's development.

The First Reaction

Some four billion years ago, the atmosphere contained very little free oxygen and carbon dioxide. However, it was rich in simple chemical substances, such as water, hydrogen, ammonia and methane. Ultraviolet radiation and discharges of lightning could have unleashed chemical reactions that formed complex organic compounds (carbohydrates, amino acids, nucleotides), creating the building blocks of life. In 1953, Americans Harold Urey and Stanley Miller tested this theory in the laboratory. The scientific evidence supports the theory that life on Earth has evolved and that all species share common ancestors. However, there are no conclusive facts about the origin of life. It is known that the first life-forms must have been prokaryotes, or unicellular beings, whose genetic information is found anywhere inside their cell walls. From this point of view, the archaea are prokaryotes, as are bacteria.             Cladistics                                                                                                                                                             This classification technique is based on the evolutionary relationship of species coming from similar derived characteristics and supposes a common ancestor for all living species. The results are used to form a diagram in which these characteristics are shown as branching points that have evolved; at the same time, the diagram places the species into clades, or groups. Although the diagram is based on evolution, its expression is in present-day characteristics and the possible order in which they developed. Cladistics is an important analytical system, and it is the basis for present- day biological study. It arises from a complex variety of facts: DNA sequences, morphology and biochemical knowledge. The cladogram, commonly called the tree of life, was introduced in the 1950s by the German entomologist Willi Hennig.

Humans                                                                                                                                                             Humans belong to the class Mammalia and specifically share the subclass of the placentals, or eutherians, which means that the embryo develops completely inside the mother and gets its nutrients from the placenta. After birth, it depends on the mother, who provides the maternal milk in the first phase of development. Humans form part of the order Primates one of the 29 orders in which mammals are divided. Within this order, characteristics are shared with monkeys and apes. The closest relatives to human beings are the great apes.

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