Microbiology

Microbiology Study Guide

2.4 Genetics Variations of Cells and Viruses In the field of genetics, it is necessary to differentiate between genotype and phenotype. Genotype denotes the information about the organism’s full hierarchy and represents the exact structure, including the set of genes possessed by the organism. The organisms are said to be of different types if their genes differ even at one point of locus. In contrast, phenotype refers to an organism’s actual observed properties, or expressed characteristics, such as morphology, development, or behavior. In a biological cell, the number of set of chromosomes is denoted by ploidy. The sex cells in humans, known as gametes (sperm in male and egg in female), are derived from one complete chromosome set from both parents. Somatic cells are produced by combining these gametes and have twice the number of chromosomes. The number of chromosomes in the gamete is denoted by the haploid number (n). The somatic cell in human beings consists of two sets of haploids, containing 23 homologous pairs of chromosomes, thereby, having a total number of 46 chromosomes. Since human beings have two set of haploids, they are known as diploid. Organisms that have more than two sets of homologous chromosomes are known as polypoid. When the offspring produced inherits genes from a single parent, the mode of reproduction is known as asexual. In this type of reproduction, none of the meiosis, ploidy reduction, or fertilization takes place, and the fusion of gametes does not take place. This is often referred to as agamogenesis. Some examples of organisms that go through asexual reproduction are archaea, bacteria, protists, certain fungi, and plants. In contrast, when the genes of two organisms are combined in order to create a new organism, such form of reproduction is termed as sexual reproduction. Sexual reproduction takes place by meiosis in which the chromosomes from each organism is halved, and fertilization in which the gametes from both organisms are blended. The original number of chromosomes are then restored. Mutations When the sequence of the nitrogenous bases of the DNAmolecule is altered permanently, it is referred to as mutation. The outcome of mutation is the alteration in the end product in terms of genes. The mutation can either be beneficial or detrimental, depending on the change in the metabolic activity. In the casewhere the metabolic activity in themicroorganisms rises, themutation is termed beneficial and if the metabolic activity is reduced, the mutation is said to be detrimental. Types of Mutations There are different kinds of mutations. The most common is the point mutation, which involves single base pair in DNA molecules. In this case, the genetic code is altered by changing the normal base with a different base. When the amino acid is inserted in the protein, it is termed as missense mutation. When the mutation alters the existing genetic code, and destroys all information about it, it is known as nonsense mutation.

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