Introduction
Genetic information is stored in DNA within the nucleus, in the format of chromosome. This information needs to be replicated precisely and transmitted into next generation (heredity). Genetic transmission from cell to cell is through mitosis, whereby DNA is replicated and chromosome duplicates and then segregates evenly into two daughter cells. Genetic transmission from organism to next generation is through meiosis and gamete formation. This transmission follows Mendel’s Law of segregation and Law of Independent Assortment.
Chromosomes
Majority of DNA exist in the format of chromosomes. One chromosome contains one long linear DNA molecule which harbors many genes. Chromosomes are visible under a microscope during mitosis. The structure of a chromosome contains two telomeres which are related to DNA replication and senescence, one centromere which is critical for proper segregation. Each species contain different number of chromosomes.
Mitosis
Mitosis is the process whereby a somatic cell divides into two daughter cells. Prior to this process, DNA is already replicated, and chromosomes are duplicated, containing two sister chromatids. Mitosis can be divided into prophase, prometaphase, metaphase, anaphase and telophase. Each phase has its distinct characteristics, and the final result is even segregation of one set of chromosome into daughter cells. Mitosis is one phase of cell cycle, before and after mitosis the cell cycle contains G2 phase and G1, S phases.
Meiosis
Meiosis is the process whereby gametes are produced. The cell replicates DNA once and initiates two cell divisions, resulting in 4 1N gametes. During the first division, homologous chromosomes pair and exchange genetic material, sister chromatids remain together, migrating to same pole. During the second division, sister chromatids segregate and generating 1N gametes.
Law of Segregation
Mendel found out that during the gamete formation, the two alleles of one gene segregate independently without mixing with each other. This is Mendel’s first Law, the Law of segregation.
Law of Independent Assortment
Mendel’s second Law is the Law of independent assortment. During the gamete formation, genes from different chromosomes assort independently and combine randomly.
Linkage and Recombination
There are exceptions for Mendel’s Laws. Specifically, genes can be linked to each other if they reside on neighboring area of a chromosome. When these genes segregate, they exhibit linkage and recombination, i.e., in some cases they segregate together while in other case they do not. The chance by which that they segregate together is called recombination rate.
