The living substance of which the cell is made is called protoplasm.
protoplasm is the very basis of life. Protoplasm is made up of inorganic and organic substances, water and minerals. But the protoplasm contains many non-living substances as well. The protoplasm consists of the
nucleus (central region) and the cytoplasm. Scattered throughout the cytoplasm are rod-like bodies called mitochondria which are responsible for cellular respiration. The
chromosomes containing DNA are found within the nucleus. The cell is surrounded by a membrane known as the cell membrane. Protoplasm has all the properties of a living organism, it is capable of nutrition, respiration, excretion, metabolism, growth and reproduction.
The nucleus contains one or more nucleoli and is bounded by a nuclear envelope. The nucleolus is an active site for RNA synthesis. It produces most of the cellular RNA. The nuclear envelope acts as an interface between the nucleus and the cytoplasm. It serves to separate the
genetic material of the cell (the chromosomes) from the protein-synthesis machinery. The nuclear envelope is not a static, but a dynamic barrier as it allows nucleo-cytoplasmic exchange to take place.
The chromosome is a filament-like body which is present in the nucleus, and which becomes visible during cell division. The chromosome consists of a DNA
double helix to which proteins are attached. TheDNA is present as a continuous thread in each chromosome. A set of chromosomes of an individual or species is known as a karyotype. Normally all the individuals of a species have the same number of chromosomes. In man, there are twenty three pairs of chromosomes.
Nucleic acids are present in all living organisms, whether plants or animals. There are two types of nucleic acids – DNA (which is found in the nucleus) and RNA (which is found in the cytoplasm). Double stranded DNA is the genetic material found in most organisms. As per Watson and Crick, the DNA molecule consists of two helically twisted strands (the double helix) interwined in a clockwise direction.
One of the most important properties of DNA is that it can make exact copies of itself, by a process called replication. A new strand is formed around each old strand. The end result is the formation of two double helices, each identical to the original. The process is described as semi-conservative, because each of the double heix formed has one old and one new strand.
The hereditary material of the chromosomes is known as the genome. In almost all cases (except some viruses)
genes consist of DNA. It has been suggested that the granules on the chromosomes are the site of genes. However, there is no one-to-one correspondance between the granules and the genes. The number of genes that a n organism possesses is generally related to its complexity. The gene has three main functions – it is a unit of recombination, it is a unit of
mutation and it is a unit for enzyme synthesis. Genes act by controlling the structure and rate of production of specific enzymes or proteins. This is known as the one gene – one enzyme hypothesis.
A mutation refers to the heritable change in the genetic material of an organism. It may be qualitative or quantitative in nature. It results in a change in the enzyme capacities of an individual. This may effect the traits controlled by that enzyme. Mutations usually occur in a random manner. And the rate of mutations is usually very low. Any mutation which does not result in physical expression is called a silent mutation.
The genes of a cell contain coded information for the maintainance and reproduction of the cell. They direct the arrangement of the different amino acids into chains of protein molecules. The genetic code was first given a theoretical basis by the scientist George Gamow. The genetic code is a triplet or three letter code – three letters specify an amino acid. The genetic code is non-overlapping, in that each letter is read only once. A mutation in one letter thus effects only one word of the genetic code. There is a linear correspondance between a gene and the protein synthesized by it.
The genetic code is universal – it is valid for all organisms ranging from bacteria to man. The code has remained constant since the time it was fixed when life evolved. The work of Khorana shows that the genetic code is read in an uninterrupted manner from one end to the other. As any given gene has to specify the same protein over and over again, the genetic code must be read between fixed starting and ending points. The genetic code is degenerate, in that there are many more triplets than amino acids; but the degeneracy of the genetic code is not random.
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