The filamentous structures that are seen in the nucleus during cell division are called as chromosomes.
Chromosome was discovered in 1875 by Strasburger. In 1900 Wilson told that chromosome is the basis of heredity. It is the most wonderful gift of nature. We have already studied about the structure of chromosome in the previous class.
A. Chromosomal Basis of Inheritance
Sutton and Boveri in 1902 independently established the Chromosomal Theory of Inheritance. According to this theory –The factors that were said to be responsible for heredity by Mendel are Genes that are located on Chromosomes. Chromosomes are segregated and they alone are independently assorted. This incident takes place at the time of Meiotic Cell Division.
In 1910 Morgan experimented on a fly named as Drosophila melanogaster and told that –
Crossing over occurs during Meiotic cell division. When chiasma formation takes place during crossing over exchange of genes and recombination takes place. Genes join chromosomes in pairs. But they get separated during meiosis. Thus, chromosomes themselves are responsible for heredity.
B. Functions of Chromosomes
Chromosomes have following functions –
(i) Chromosomes bear genes that have all the information coded within them.
(ii) Chromosomes control protein synthesis.
(iii) Chromosomes are helpful in cell division and cellular development.
(iv) Chromosomes control the metabolism and synthesis of enzymes.
(v)Chromosomes duplicate and form their own copy and transmit information in the form of codes in the next generation.
(vi) Chromosomes function as connecting links between parents and offspring.
(vii) Some chromosomes are called as sex chromosomes and they determine the sex of the organism.
(viii) Some chromosomes are helpful in the synthesis of ribosomes.
(ix)Chromosomes cause variations through crossing over.
(x) Chromosomes affect the structure and Organisation of genes.
II. DNA or Deoxyribose Nucleic Acid
Any hereditary information remains stored in the form of codes in the hereditary material. Hereditary materials are found in the chromosomes.
The substance which is responsible for the transfer of hereditary materials form one generation to other generation and expresses that information in the form of trait is called as hereditary material.
The bodies of organisms are formed by Bio chemicals. So it is natural that hereditary material is also formed of these biochemicals.Biochemicals are basically of two types- Proteins and Deoxyribos Nucleic Acid. The DNA has the capacity of replication. No other protein has the capacity of replication. Thus it has been proved experimentally that DNA is the Genetic Material.
A. DNA is the basis of heredity
DNA is also considered as the basis for heredity. It has following reasons –
(i) It has capacity of duplication
(ii). It is found in every cell
(iii) The replica of DNA remains similar to DNA molecule
(iv) The replication of DNA completes before cell division
(v) If there is a change in the structure of DNA, it will cause symptoms of mutation or permanent changes.
B. Role of DNA
DNA has following roles in heredity –
(i) DNA is a hereditary material. All the hereditary information remains coded in the arrangement of nitrogenous bases of it.
(ii) DNA has the capacity of replication due to which it can form its replica which remains very similar to it. This action is necessary for passing the information in other cells and in the next generation also.
(iii) Crossing over results in the process of gene recombination.
(iv)The change in the sequence of DNA nucleotide or change in their number cause mutation which leads to variations.
(v) It produces RNA (Ribose Nucleic Acid) through transcription and RNA has important role in protein synthesis.
(vi) It controls the cell metabolism. For this, it takes help from RNA Besides this it also controls the processes conducted by RNA and synthesis of bio-chemicals.
Those physical units found on chromosomes which pass on parental traits from one generation to the other generation are called as genes.
A. Nature of Genes:
Genes are arranged on chromosomes Johanssen named gene in 1909) Morgan and Bridges in 1916 experimentally proved that genes are found on chromosomes and number of genes on each chromosome remain fixed. The chromosome of Lily the number of gene has been estimated to be 2000 to 1500 whereas on the chromosome of Drosophila melanogaster the number of genes is 10,000.
On the specific segment of chromosome genes remain specifically arranged. Such a segment of chromosome is known as locus. It is through gene mutation that various types of variations emerge out. Genes have coded DNA segments that contain information regarding various functions. In some viruses there are RNA segments on the places of DNA segments.
B. Gene Pool:
The total number of genes of members of population having the capacity of sexual reproduction is called as Gene pool.
C. Gene flow:
Transfer of genes from one generation to other generation is called as gene flow. Different types of events are combined in the process of gene flow for example distant movement of pollen grains, movement of human beings of one country to the other country, living of red coloured insects in the colony of black coloured insects etc. In these cases new variations are produced through sexual reproduction. Gene flow brings changes in the gene pool of a particular area. This happens also when some living beings of one population migrate and start living permanently in another population. It is also called as gene migration.
D. General merits of genes
(i) These are units of genetic material having capacity of duplication.
(ii)These are units of recombination and can take part in the process of crossing over.
(iii) Gene mutation causes variations and it is through gene mutation that information encoded in gene is changed.
(iv)These are related to physical characters and activities and are capable of expressing those characters and activities.
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