Introduction to Cell Biology and Genetics
Cell Theory
The cell is the smallest form of life and the basic unit of all living organisms. Every cell in a living being originates from another existing cell. Information necessary for the life of cells is passed from one generation to the next.
Chromosome Theory of Heredity
A gene, located on a DNA fragment, carries the information for a particular character. Genes are aligned one after another on chromosomes. Each gene occupies a specific location (loci) on a chromosome. Each character is located on a pair of homologous chromosomes at the same locus. An organism is homozygous if it contains the same information at a specific locus on homologous chromosomes and heterozygous if the information is different. The genotype refers to the set of genes inherited by an organism, present in all cells. The phenotype is the observable expression of the genotype in a specific environment.
DNA
DNA, a nucleic acid localized in the cell nucleus, contains the genetic information that governs cell functions and guides individual development. In eukaryotic cells, DNA exists as multiple linear molecules associated with chromatin proteins. DNA nucleotides contain deoxyribose and four bases: adenine, thymine, guanine, and cytosine.
Mitosis
The onset of mitosis is marked by the appearance of distinct chromosomes. As they become visible, chromosomes adopt a double-stranded appearance, forming chromatids held together at the centromere. Nucleoli disappear at this stage. The nuclear membrane begins to fragment, and the nucleoplasm and cytoplasm merge. The spindle apparatus appears and captures chromosomes. Each chromatid corresponds to a long DNA chain. By the end of prophase, the nuclear membrane and nucleolus have disappeared.
Metaphase
Chromosomes align at the cell’s equatorial plane, with each centromere attached to spindle fibers. Chromosomes line up in the middle, forming the metaphase plate.
Anaphase
Sister chromatids separate and move towards opposite poles of the cell. The separation begins at the centromere. Sister chromatids are abruptly pulled apart and driven to opposite spindle poles as the spindle elongates. The centromere divides, separating each chromosome into its two chromatids. Centromeres migrate along spindle fibers in opposite directions, each dragging a chromatid. Anaphase is crucial for mitosis as it ensures the equal distribution of the two copies of the original genetic information.
Telophase
Chromosomes unwind, and nucleoli reappear, signifying the regeneration of interphase nuclei. The spindle disperses, and a new membrane divides the elongating cytoplasm. The spindle continues to elongate as chromosomes reach the poles and detach from spindle microtubules. The membrane constricts in the center and eventually breaks. The nuclear envelope reconstructs around each chromosome set, defining the new daughter nuclei. Cytoplasmic division follows.
Mendel’s Laws
1st Law: Law of Uniformity of the First Filial Generation
When crossing two purebred varieties or strains of the same species, the first filial generation (F1) offspring are all phenotypically and genotypically identical.
2nd Law: Law of Segregation of Antagonistic Genes or Purity of Gametes
Mendel deduced the second law by studying the second filial generation (F2), resulting from mating F1 individuals. Antagonistic genes that remained together in the F1 zygote separate in the F2 generation, leading to the reappearance of the pure genotypes of the grandparents. The segregation of these antagonistic genes is a major conclusion drawn from Mendel’s experiments.
3rd Law: Law of Independent Assortment of Genes and Their Random Combination in the Third Filial Generation
Mendel arrived at this conclusion by working with dihybrids, individuals differing in two characters. Specifically, he used smooth, yellow peas and wrinkled, green peas.