A Genetics Definition of Homologous Chromosomes

A homologous chromosome pair diagram.

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A pair of homologous chromosomes contains chromosomes of similar length, gene position, and centromere location. Chromosomes are important molecules because they contain DNA and genetic instructions for the direction of all cell activity. They also carry genes that determine individual traits that can be inherited through reproduction.

Human Karyotype

A human karyotype shows the complete set of human chromosomes. Each human cells contain 23 pairs of chromosomes or 46 total. Every chromosome pair represents a set of homologous chromosomes. During sexual reproduction, one chromosome in each homologous pair is donated from the mother and the other from the father. In a karyotype, there are 22 pairs of autosomes or non-sex chromosomes and one pair of sex chromosomes. The sex chromosomes in both males (X and Y) and females (X and X) are homologs.

Cellular Reproduction

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.

Mitosis

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.

  • Interphase: Homologous chromosomes replicate to form sister chromatids.
  • Prophase: Sister chromatids migrate toward the center of a cell.
  • Metaphase: Sister chromatids align with the metaphase plate at a cell's center.
  • Anaphase: Sister chromatids are separated and pulled toward opposite cell poles.
  • Telophase: Chromosomes are separated into distinct nuclei.

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

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. The resulting chromosomes have the correct number of genes but different gene alleles.

Meiosis guarantees genetic variation through genetic recombination via prophase crossover and random gamete fusion into diploid zygotes during fertilization.

Nondisjunction and Mutations

Problems occasionally arise in cell division that lead to improper cell division. These usually significantly affect the outcome of sexual reproduction whether the issues are present in gametes themselves or the cells that produce them.

Nondisjunction

Failure of chromosomes to separate during mitosis or meiosis is called nondisjunction. When nondisjunction occurs in the first meiotic division, homologous chromosomes remain paired. This results in two daughter cells with an extra set of chromosomes and two daughter cells with no chromosomes at all. Nondisjunction may also occur in meiosis II when sister chromatids fail to separate prior to cell division. Fertilization of these gametes produces individuals with either too many or not enough chromosomes.

Nondisjunction is often fatal or otherwise results in birth defects. In trisomy nondisjunction, each cell contains an extra chromosome (for a total of 47 instead of 46). Trisomy is seen in Down syndrome where chromosome 21 has an additional whole or partial chromosome. Monosomy is a type of nondisjunction in which only one chromosome is present

Sex Chromosomes

Sex chromosomes can also suffer from nondisjunction. Turner syndrome is a form of monosomy that causes females to have only one X chromosome. Males with XYY syndrome have an extra Y sex chromosome in another example of trisomy. Nondisjunction in sex chromosomes typically has less severe consequences than nondisjunction in autosomal chromosomes, but still impacts the quality of life for an individual.

Chromosome Mutations

Chromosome mutations can affect both homologous and non-homologous chromosomes. A translocation mutation is a type of mutation in which a piece of one chromosome breaks off and is joined to another chromosome. Translocation mutation between non-homologous chromosomes is not to be confused with crossover between homologous chromosomes or regions of chromosomes. Deletion, the loss of genetic material, and duplication, the over-copying of genetic material, are other common chromosome mutations.