Why are pairs of chromosomes called homologs

There are two ways that a cell can divide and reproduce and these are mitosis and meiosis. Mitosis exactly copies a cell and meiosis creates unique cells. Both of these methods of cellular reproduction are necessary for sustaining human life. Mitosis allows a zygote to replicate until a human is formed and meiosis produces the gametes that make fertilization, and therefore zygotes, possible in the first place. Cellular division by mitosis replicates cells for repair and growth.

Before mitosis begins, chromosomes are copied so that each cell produced retains the original number of chromosomes after division this number is doubled and then halved. Homologous chromosomes replicate by forming identical copies of chromosomes called sister chromatids.

After replication, single-stranded DNA becomes double-stranded and resembles the familiar "X" shape. As a cell further progresses through mitosis, sister chromatids are eventually separated by spindle fibers and distributed between two daughter cells.

Each separated chromatid is considered a full single-stranded chromosome. The stages of mitosis are listed and explained in more detail below. After the cytoplasm is divided during cytokinesis, the final stage of mitosis, two daughter cells are formed with the same number of chromosomes in each cell.

Mitosis preserves the homologous chromosome number. Meiosis is the mechanism of gamete formation that involves a two-stage division process.

Prior to meiosis, homologous chromosomes replicate to form sister chromatids. In prophase I, the first stage of meiosis, sister chromatids pair up to form a tetrad. While in close proximity, homologous chromosomes exchange sections of DNA randomly in a process called crossing over. Homologous chromosomes separate during the first meiotic division and the resulting sister chromatids separate during the second division.

At the end of meiosis, four distinct daughter cells are produced. Each of these is haploid and contains only half of the chromosomes of the original cell.

Plants are characterized by having alternation of generations in their life cycles. This tutorial is a review of plant mitosis, meiosis, and alternation of generations. This tutorial looks at sex determination via the sex chromosomes, X and Y. Read it to get more info on X and Y chromosomes and the genetic traits inherited via these two This tutorial describes the independent assortment of chromosomes and crossing over as important events in meiosis.

Read this tutorial to know more details in each of these meiotic events and how they promote genetic diversity in sexually-reproducing organisms Humans are diploid creatures. This means that for every chromosome in the body, there is another one to match it.

However, there are organisms that have more than two sets of chromosomes. The condition is called polyploidy. Know more about this topic through this tutorial Genes are expressed through the process of protein synthesis. This elaborate tutorial provides an in-depth review of the different steps of the biological production of protein starting from the gene up to the process of secretion. Also included are topics on DNA replication during interphase of the cell cycle, DNA mutation and repair mechanisms, gene pool, modification, and diseases Skip to content Main Navigation Search.

Dictionary Articles Tutorials Biology Forum. Homologous chromosome — definition. Table of Contents. Homologous chromosomes showing sister and non-sister chromatids. Genetic recombination can occur between non-sister chromatids. The homologous chromosomes of humans based on the gene sequences, loci, and centromere location. A somatic cell 2n contains 46 chromosomes and 22 of them are homologous. If the sex chromosomes are both X chromosomes, then the total number of homologous chromosomes is Chromosome Mutations Mutations can also influence the phenotype of an organism.

Meiosis and Alternation of Generations Plants are characterized by having alternation of generations in their life cycles. Independent Assortment and Crossing Over This tutorial describes the independent assortment of chromosomes and crossing over as important events in meiosis. Polyploidy Humans are diploid creatures. Genetic Information and Protein Synthesis Genes are expressed through the process of protein synthesis.

Related Articles No related articles found See all Related Topics. Homologous chromosomes have alleles on the same genes located in the same loci. Heterologous chromosomes have alleles on different genes. Does not pair up during meiosis. For instance, the underlying causes of Klinefelter's syndrome and Turner's syndrome are errors in sex chromosome number, and Down syndrome is caused by trisomy of chromosome However, the severity of phenotypic abnormalities can vary among different types of aneuploidy.

In addition, aneuploidy is rarely transferred to subsequent generations, because this condition impairs the production of gametes. Overall, the inheritance of odd chromosome number arises from errors in segregation during chromosome replication. Often, it is these very exceptions or modifications of expected patterns in mitosis and meiosis that enrich our understanding of how the transfer of chromosomes is regulated from one generation to the next.

Belling, J. On the attachment of non-homologous chromosomes at the reduction division in certain chromosome daturas. Proceedings of the National Academy of Sciences 12 , 7—11 Farmer, J. On the maiotic phase reduction divisions in animals and plants. Quarterly Journal of Microscopical Science 48 , — Gilbert, S. Hirsch, J. Behavior genetics and individuality understood. Science 42 , — doi Uniqueness, diversity, similarity, repeatability, and heritability.

International Journal of Comparative Psychology 17 , — Paweletz, N. Walther Flemming: Pioneer of mitosis research. Nature Reviews Molecular Cell Biology 2 , 72—75 doi Chromosome Theory and the Castle and Morgan Debate. Discovery and Types of Genetic Linkage. Genetics and Statistical Analysis. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory. Genetic Recombination. Gregor Mendel and the Principles of Inheritance. Mitosis, Meiosis, and Inheritance. Multifactorial Inheritance and Genetic Disease.

Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses. Biological Complexity and Integrative Levels of Organization. Genetics of Dog Breeding. Human Evolutionary Tree.

Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Phenotype Variability: Penetrance and Expressivity. In eukaryotes , the genome comprises several double-stranded, linear DNA molecules Figure 6.

Each species of eukaryote has a characteristic number of chromosomes in the nuclei of its cells. Human body cells somatic cells have 46 chromosomes. A somatic cell contains two matched sets of chromosomes, a configuration known as diploid. The letter n is used to represent a single set of chromosomes; therefore a diploid organism is designated 2 n.

Human cells that contain one set of 23 chromosomes are called gametes , or sex cells; these eggs and sperm are designated n , or haploid. The matched pairs of chromosomes in a diploid organism are called homologous chromosomes. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus.