CELL DIVISION: It is our common observation that living things grow and increase in size. It is also observed that offspring resemble their parents. We have learnt that all the living things are made up of cells. Hence, more and more cells are needed for growth and development of living things. Where do new cells come from? The answer to the question is that the new cells arise by the divisions of pre-existing cells. In this chapter, we will discuss the process of cell division. Why do offspring resemble their parents? This is the heredity which produces resemblance in the offspring with their parents. Heredity and basis of heredity will also be discussed in this chapter.
Cell Division Cell division is a process by which a cell divides into two daughter cells. The cell which divides is called parent cell. The cells which are produced as a result of cell division are called daughter cells. Before the start of cell division, the parent cell passes through a phase called interphase. During interphase, chromosomes in the nucleus are duplicated, i.e., copies of all the chromosomes are developed. The process of cell division involves two phases, i.e., nuclear division and cytokinesis. Nuclear division is the division of nucleus which is followed by cytokinesis. Cytokinesis is the division of cytoplasm.
Nucleus is part of the cell which controls the activities of the whole cell. § Chromosomes are found in the nucleus of the cell. They consist of proteins and DNA. § DNA stands for Deoxyribonucleic Acid. § DNA is the material that contains complete set of instructions for developing a new cell or an organism. That is why DNA is called hereditary material. § For one kind (species) of organism the number of chromosomes in the cells remain the same. However, when an individual forms gametes (sperms or eggs in animals) or spores (in plants), the number of chromosomes is reduced to half in the gametes or spores.
Cell division is of two types which are called mitosis and meiosis. Mitosis Mitosis is a process by which the parent cell divides into two daughter cells with same number of chromosomes as in the parent cell. The number of chromosomes is doubled during interphase. Two sets of chromosomes are formed. During mitosis when the nucleus of parent cell divides the two set of chromosomes are distributed equally in the two daughter nuclei (Figure 2.1). After nuclear division a shallow groove arises in the middle of the cytoplasm which deepens further and divides the cell into two daughter cells, each having a nucleus. Meiosis Meiosis is a process by which the nucleus of a cell divides twice to form four daughter cells in such a way that the number of chromosomes in each daughter cell is reduced to half, compared to the parent cell The process of meiosis consists of two divisions, meiotic-I division and meioticII division. During meiotic-I division, the number of chromosomes is reduced to half as compared to the parent cell. Meiotic-II division is similar to mitosis because the half number of chromosomes is retained in the four daughter cells.
Differences between Mitosis and Meiosis
2.2 Heredity During reproduction, living things pass on their characteristics to their offspring. This is the reason that babies look like their parents. Plants grown from seeds resemble their parent plants. The transmission of characteristics from parents to offspring is called heredity. The characteristics such as the colour of eyes, skin colour, hair colour, free or attached earlobes, height, intelligence, etc., are the examples of the characteristics that are transmitted from parents to the offspring and are called hereditary characteristics.
Sr. No. Mitosis Meiosis 1. During mitosis, two daughter cells are formed from the parent cell. During meiosis, four daughter cells are formed from the parent cell.
The number of chromosomes in the daughter cells remain the same as in the parent cell.
The number of chromosomes in the daughter cells is reduced to half as compared to that in the parent cell.
3. Mitosis occurs in general body cells. Meiosis occurs to produce gametes (sperms and eggs) in animals or spores in plants.
Closely observe your own body features as well as those of your parents, grandparents (both maternal and paternal), brothers and sisters. Record your observations in the table given below:
Characters Yourself Father Mother Sister Brother Grandparents (Paternal)
Hair (straight or curly) Eye colour (blue, brown, black)
Complexion (fair, less dark, dark)
Height (tall, medium
What feature or features are common to you and your brothers and sisters? § Which of these features are also present in your parents or grandparents? § What conclusions do you draw from the above observations? § What are the characters, which are transferred from parents to offspring, called? § Which of your body features are different from your brothers and sisters? § What are the characteristics, which are different in members of a family or in members of a species, called?
We know that children possess many features similar to those of their parents but they also differ from their parents in certain respects. Similarly, brothers and sisters also show differences in many characteristics. Differences among members of a family or a species are called variations. Beneﬁcial variations help organisms to adapt (live successfully) their environment, have greater chances of survival and continue their race. 2.3 Basis of Heredity The basic physical and functional unit of heredity is called gene. Genes act as instructions to make molecules called proteins. Genes occur in pairs. Every hereditary character in an organism (e.g., tallness, dwarfness, eye colour, free earlobe, attached earlobe, etc.) is controlled by a pair of genes. One member of a gene pair comes from male parent (father) while the other comes from female parent (mother). Where are genes found physically? Genes are the sections of DNA (Deoxyribonucleic Acid) molecule and are located on chromosomes. As different sections of DNA (genes) are a set of information for the development of different characters in an organism, DNA is called hereditary material. DNA and proteins are the components of chromosomes. Chromosomes are thread-like structures found in the nucleus of a cell. They appear as distinct structures only during cell division. A typical chromosome consists of two arms called chromatids which are attached to the same part called centromere.
The number of chromosomes is speciﬁc and constant for every kind (species) of organism. In general body cells (somatic cells), the chromosomes occur in pairs but the gametes (sperms or eggs) or spores which are formed by meiosis contain one member of each chromosome pair. For example; in man, every somatic cell has 46 chromosomes in the form of 23 pairs but every sperm or egg cell has 23 chromosomes.
Watson and Crick Model of DNA Each DNA molecule is made of thousands of small units called nucleotides. There are four types of nucleotides in DNA. These are Adenine (A) nucleotide, Thymine (T) nucleotide, Cytosine (C) nucleotide and Guanine (G) nucleotide. According to Watson and Crick, the DNA molecule consists of two strands formed of nucleotides. The two stands of DNA are linked to each other by cross bands like a ladder (Figure 2.4). Transmission of Characters When an organism forms gametes (sperms or eggs) by meiosis, the number of chromosomes is reduced to half in the gametes, i.e., haploid (n) sperms or eggs are produced. It means, the hereditary material (DNA) is also reduced to half in the gametes. When male and female organisms mate, the haploid (n) sperm cell from male and haploid (n) egg cell from female fuse with each other to form a diploid (2n) cell called zygote. In this way the complete hereditary material (DNA) is restored in the zygote, i.e., the physical and functional units of all the characters (gene pairs) are transferred in the zygote. The zygote after passing through various changes develops into a full organism with speciﬁc characteristics from both parents. Thus, zygote is the ﬁrst cell from which the life of an organism starts. Inheritable and Non-inheritable Characters The characters such as eye colour, skin colour, hair colour, free or attached earlobes, height, intelligence, etc., are transmitted from parents to the offspring. Such characters that are transmitted from one generation (parents) to the next generation (offspring) are called inheritable characters. Inheritable characters are controlled by genes. Many characters of parents are not transferred to their offspring because these are not developed by genes. Such characters are called non-inheritable characters. For example, if a body organ of a person is lost or weakened due to disease, this character is not transferred to his or her children.
Examples of Inheritable Characters i. Eye colour The colour of eyes in an organism is controlled by a pair of gene. Thus, it is an inheritable character. The genes control the production of brown pigment in the iris of the eyes. If the genes work and produce more pigment, the eyes are black. Production of very less pigment results in light brown eyes (Figure 2.5). Blue, green, and hazel eye colours are developed due to the production of brown pigment in different amounts.
ii. Attached and Detached Earlobes In some people the earlobes are attached with the sides of the face while others have free earlobes (Figure 2.6). This character is also controlled by genes. When the said genes work, the earlobes hang freely (detached earlobe). Some people do not have this gene. Their earlobes remain attached with the sides of the face.
Characters of living things are controlled by the genes on the chromosomes. Mitosis is a great § blessing of nature. If there were no mitosis, the number of chromosomes could not have been maintained constant. Similarly, meiosis has also a signiﬁcant importance in controlling the hereditary characters § generation after generation.
The process by which a parent cell divides into two daughter cells is called cell division. § Mitosis is a process in which a parent cell divides into two daughter cells with same number of chromosomes as in the parent cell. § Meiosis is a process in which a cell divides twice to form four daughter cells in such a way that the number of chromosomes in daughter cells is reduced to half compared to that in the parent cell. § The transmission of characters from parents to offspring is called heredity. § The characters which are transmitted to next generation are called inheritable characters. § Eye colour, skin colour, hair colour, free or attached earlobes, height, intelligence, etc., are inheritable characters. § The basic physical and functional unit of heredity is called gene. § Genes act as instructions to make molecules called proteins. Genes are located on chromosomes. § The number of chromosomes is constant for every kind (species) of organisms. § When a sperm and an egg fuse to form zygote, the characters (genes) are transferred in the zygote. Zygote after passing through various changes develops speciﬁc characters in the new baby.