II. Listen to fragment b again and fill in the gaps in the text below

The metabolic definition is still popular with many biologists. It describes (1)_______________ as an object with a (2)_______________, continually exchanging (3)_______________ with its surroundings, but without altering its (4)_______________, at least (5)_______________. But again there are exceptions. There are (6)_______________ that remain, so far as is known, perfectly dormant and totally without (7)_______________ at low temperatures for hundreds, perhaps thousands, of years but that (8)_______________ upon being subjected to more clement conditions.

SPEAKING

Speak about Life covering the following issues:

1. The variety of approaches to the problem of life: the physiological, metabolic, biochemical, and genetic definitions of life.

2. Metabolism, growth, reproduction, responsiveness, and adaptation as the main functional activities performed by living organisms.

Lesson 3

THE ORIGIN OF LIFE

PRE-READING TASKS

I. Answer the following questions

• In all cultures there are legends explaining the origin of the Earth and life on it. Which legends do you know? Tell them to your group-mates.

• The scientific and religious views on the problem of origin of life are quite contradictory. Which of them do you support? Do you believe that a compromise between religion and science in relation to this problem will ever be possible?

II. Listen to the following words and practice their pronunciation

Hypothesis, spontaneous, polymer, nucleotide phosphate, primer, enzyme, catalyst, template, scarce, proteinoid microsphere, mitotic spindle, chloroplast, DNA, symbiosis, bacteria, algae, oxygen.

READING COMPREHENSION AND VOCABULARY DEVELOPMENT

I. Match each word on the left to its correct definition on the right

relevant, adj site, n subsequent, adj catalyst, n template, n diverse, adj eliminate, v descendant, n ancestor, n abundance, n

a large quantity of something; happening or coming after something else; to completely get rid of something that is unnecessary or unwanted; directly relating to the subject or problem being discussed or considered; something that is used as a model for another thing; a place where something important happens; a living form that lived in the past, that modern living forms have developed from; something that has developed from something else; a substance that makes a chemical reaction happen more quickly without being changed itself; very different from each other.

II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

All the essential building blocks for life and their polymers may have been produced in some fair concentration on the primitive Earth. This possibility is certainly relevant to the origin of life, but it is not the same thing as the origin of life. By the genetic definition of life a self-replicating, mutable molecular system, capable of interacting with the environment, is required. In contemporary cells the nucleic acids are the sites of self-replication and mutation. Laboratory experiments have already shown that polynucleotides can be produced from nucleotide phosphates in the presence of a specific enzyme of biological origin and a pre-existing “primer” nucleic acid molecule. If the primer molecule is absent, polynucleotides are still formed, but they of course contain no genetic information. Once such a polynucleotide spontaneously forms, it then acts as primer for subsequent syntheses.

Imagine a primitive ocean filled with nucleotides and their phosphates and appropriate mineral surfaces serving as catalysts. Even in the absence of the appropriate enzyme it seems likely, although not yet proved, that spontaneous assembly of nucleotide phosphates into polynucleotides occurred. Once the first such polynucleotide was produced, it may have served as a template for its own reproduction, still of course in the absence of enzymes. As time went on there were bound to be errors in replication. These would be inherited. A self-replicating and mutable molecular system of polynucleotides, eventually leading to a diverse population of such molecules, may have arisen in this way.

So far as is known, polynucleotides have no catalytic properties, and proteins have no reproductive properties. It is only the partnership of the two molecules that makes contemporary life on Earth possible. Accordingly, a critical and unsolved problem in the origin of life is the first functional relation between these two molecules, or, equivalently, the origin of the genetic code. If polynucleotides were initially capable of crude, nonenzymatic replication, and if a crude primitive genetic code existed, then any one of a very large number of catalytic properties was available to some self-replicating polynucleotides on the primitive Earth. This situation is all that would be necessary for the origin of life; those polynucleotides that could code for a primitive protein having catalytic properties furthering the replication of the polynucleotide would preferentially replicate. Other polynucleotides coding for less effective proteins would have replicated more slowly. The foregoing is one of several possibilities for the origin of the first living systems. Many separate and rather diverse instances of the origin of life may have occurred on the primitive Earth, but competition eventually eliminated all but one line. Every organism on Earth today would be a descendant of that line.

Even the evolution of enzymatic reaction chains may have occurred in free nucleic acids before the origin of the cell. The cell may have arisen in response to the need for maintaining a high concentration of scarce building blocks or enzymes, or as protection against the gradually increasing abundance of oxygen on the primitive Earth. Oxygen is a well-known poison to many biological processes, and in contemporary higher organisms the mitochondria that handle molecular oxygen are kept in the cytoplasm, far from contact with the nuclear material. Even today processes are known whereby polyamino acids form small spherical objects, microns to tens of microns across, with some of the properties of cells. These objects, called proteinoid microspheres by Fox, are certainly not cells, but they may indicate processes by which the ancestors of cells arose. Prokaryotic cells almost certainly preceded eukaryotic cells, and the evolution of so extremely complex an apparatus as the mitotic spindle (which ensures equal segregation of replicated chromosomes) must have taken very long periods of time to evolve. The development of mitochondria and chloroplasts (each of which contains its own DNA) in the eukaryotic cell may have been the result of a symbiosis, a cooperative arrangement entered into at first tentatively by originally free-living cells.

Among the oldest known fossils are those found in the Fig Tree chert from the Transvaal, dated at 3,100,000,000 years old. These organisms have been identified as bacteria and blue-green algae. It is very reasonable that the oldest fossils should be prokaryotes rather than eukaryotes. Even prokaryotes, however, are exceedingly complicated organisms and very highly evolved. Since the Earth is about 4,500,000,000 years old, this suggests that the origin of life must have occurred within a few hundred million years of that time.