Haploid Cells And Diploid Cells | Reproduction by meiosis

Key Concepts

• Homologous chromosomes
• Genes
• Haploid cells
• Diploid cells

Introduction

A characteristic feature of a living thing is that it reproduces independently of other living things. Here reproduction occurs at the cellular level. In some parts of the body, such as along the gastrointestinal tract, the cells reproduce repeatedly. In other parts of the body, such as in the nervous system, the cells reproduce less often. With the exception of only a few kinds of cells, such as red blood cells (which does not have nuclei when fully mature), all cells of the human body reproduce. 

Reproduction 

Reproduction is the way by which an organism produces the same kind of offspring. In living organisms, there are two types of reproduction—sexual and asexual. Sexual reproduction normally requires two organisms, whereas, in asexual reproduction, a new organism is produced from one organism. The new organism will carry hereditary material identical to the hereditary material of the parent organism. 

In sexual reproduction, two sex cells, an egg and a sperm, come together. Sperms are formed in the male reproductive organs. Eggs are formed in the female reproductive organs. When the egg and a sperm come together, it is called fertilization, and the formation of cell is called a zygote. The egg and the sperm come from two separate organisms of the same species.  

Chromosomes and Chromosome Number 

Each of us has some characteristics passed on to us from our parents. Each characteristic, such as color of hair, height, or color of the eye, is termed a trait. The instructions for each trait are situated on chromosomes, which are present in the nucleus of cells. The DNA on chromosomes is arranged in segments that control the making of proteins. These DNA segments are known as genes. Each chromosome comprises hundreds of genes. Each gene plays an important role in deciding the characteristics and functions of the cell. 

Homologous chromosomes  

The cells of a human body have 46 chromosomes. Each parent provides 23 chromosomes, which results in 23 pairs of chromosomes. The chromosomes that form a pair, one chromosome from each parent, are called homologous chromosomes.  

Homologous chromosomes in body cells have an equal length and the same centromere position, and they carry genes that control the same inherited traits. For example, the gene for earlobe type will be situated at the same position on both homologous chromosomes. Though these genes each code for earlobe type, they might not code for the exact same type of earlobe. 

Haploid and Diploid cells  

To maintain the same number of chromosomes from generation to generation, an organism produces sex gametes, which are sex cells that have half the number of chromosomes. Though the number of chromosomes differs from one species to another, in human beings, each gamete consists of 23 chromosomes. The symbol ‘n’ is used to denote the number of chromosomes in a gamete. A cell with ‘n’ number of chromosomes is known as a haploid cell. Haploid word comes from the Greek word ‘haploos’, which means single. The method by which one haploid gamete unites with another haploid gamete is known as fertilization. Due to fertilization, the cell contains a total of 2n chromosomes, i.e., ‘n’ chromosomes from the mother and ‘n’ chromosomes from the father or male parent. A cell that comprises of 2n number of chromosomes is known as a diploid cell.  

The number of pairs of chromosomes in an organism is also shown by ‘n’. There are 23 pairs of homologous chromosomes formed when two human gametes combine. 

Summary:

• A characteristic feature of a living thing is that it reproduces.
• Reproduction occurs at the cellular level.
• Reproduction is the way by which an organism produces the same kind of offspring.
• Reproduction is of two types – sexual and asexual reproduction.
• Sexual reproduction requires two organisms, i.e., male and female.
• In asexual reproduction, a new organism is produced from one organism.
• A cell with ‘n’ number of chromosomes is known as a haploid cell.