CONTINUITY OF LIFE
Human beings, animals and plants also inherit characters generation after generation. Parents transmit characters to their offspring. Offspring either resemble or differ from their parents.
The process in which the parental characters are transmitted to offspring is called inheritance or heredity.
The differences from the parents shown by the offspring are called variations. Genetics:
The branch of biology which is concerned with heredity and variations is called genetics.
Gamete—a vehicle of inheritance:
To understand the ‘method of inheritance, a knowledge of the structure and contents of a, cell and sexual reproduction is essential. The cells of the body can be divided into two groups on the basis of contents of the nuclei–the somatic cells or body cells and sex cells or gametes. Gametes are responsible for the inheritance of characters. Male and female gametes fuse to form a zygote. The zygote is a single cell and the nucleus of the zygote has all the information for developing individuals so the nucleus is the part of the cell. that is connected with reproduction and hence with the transmission of characteristics from parents to offspring.
The nucleus is the most important component of a living cell. The nucleus is enclosed in a membrane called the nuclear membrane and contains semi-fluid material called nucleoplasm. The nuclear membrane has minute –pores, which allow the flow of materials between the nucleus and cytoplasm. The nucleolus is a densely stain round body present in the nucleus. The most important structures present in the nucleus are short rodlike bodies or thread-like structures known as chromosomes.
CHROMOSOMES AND GENES
There is a fixed number of chromosomes in every organism of a specie; the number is the same in all body cells of organisms except sex cells (gametes), where it is half. Chromosomes are always in the form of pair.
Each chromosome is seen to be made up of two similar strands called chromatids and these chromatids are attached at the same centromere. Chromosomes are made up of deoxyribonucleic acid (DNA) and proteins. The chromosomes are clearly visible in dividing cells.
Each chromosome carries a number of genes all along its length. These genes are arranged in a linear order at specific positions
on chromosome. Genes are units of inheritance which determine particular characteristics of an individual, for example, what color of eyes or hair is? Genes provide the “instructions” to transmit characters from the parents to the offspring. Genes are short lengths of DNA in the chromosomes.
DEOXYRIBONUCLEIC ACID (DNA)
. A molecule of DNA consists of two
strands like a ladder. These two strands linked with each other by cross bands/pieces like a ladder and twisted around each other. The DNA molecule is therefore in the form of a spiral helix or double helix. DNA consists of a very large number of units called nucleotides. There are four types of nucleotides in DNA. These units are joined to each other to form DNA molecule.
HOW DNA TRANSMITS HEREDITARY CHARACTERISTICS FROM PARENTS TO OFFSPRING?
DNA is the genetic material. DNA stores genetic information in the sequence of its nucleotides in a lineal’ order and is responsible for the physical structure and functioning of the body.
The information is stored in the form of a code known as genetic code. It is recorded in such a way that it can be duplicated and passed from one generation to another generation. So it is DNA that determines as to what type of organism and what characters would be produced in any living organism.
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