Homologous chromosomes are chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location. The position of the genes on each homologous chromosome is the same. However, the genes may contain different alleles. Chromosomes are important molecules, as they contain DNA and the genetic instructions for the direction of all cell activity. They also carry genes that determine individual traits.
Homologous Chromosomes Example
A human karyotype shows the complete set of human chromosomes. Human cells contain 23 pairs of chromosomes, for a total of 46. Each chromosome pair represents a set of homologous chromosomes. One chromosome in each pair is donated from the mother and the other from the father during sexual reproduction. In a karyotype, there are 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. In males, the X and Y sex chromosomes are homologs. In females, both X chromosomes are homologs.
The purpose of mitosis (nuclear division) and cell division is to replicate cells for repair and growth. Before mitosis begins, chromosomes must be replicated to ensure that each cell retains the correct number of chromosomes after cell division. Homologous chromosomes replicate forming sister chromatids (identical copies of a replicated chromosome that are attached). After replication, the single-stranded DNA becomes double-stranded and has the familiar X shape. As the cell progresses through the stages of mitosis, sister chromatids are eventually separated by spindle fibers and divided between two daughter cells. Each separated chromatid is considered a full single-stranded chromosome.
- Interphase: Homologous chromosomes replicate forming sister chromatids
- Prophase: Sister chromatids move toward the center of the cell
- Metaphase: Sister chromatids align along the metaphase plate at the 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, two new daughter cells are formed with the same number of chromosomes in each cell. Mitosis preserves the homologous chromosome number.
Meiosis is the mechanism for gamete formation and involves a two-stage division process. Prior to meiosis, homologous chromosomes replicate forming sister chromatids. In prophase I, sister chromatids pair up forming what is called a tetrad. While in close proximity, homologous chromosomes sometimes exchange sections of DNA. This is known as genetic recombination.
Homologous chromosomes separate during the first meiotic division and sister chromatids separate during the second division. At the end of meiosis, four daughter cells are produced. Each cell is haploid and contains half the number of chromosomes as the original cell. Each chromosome has the appropriate number of genes, however, the alleles for the genes are different. The swapping of genes during homologous chromosome recombination produces genetic variation in organisms that reproduce sexually. Upon fertilization, haploid gametes become a diploid organism.
Nondisjunction and Mutations
Occasionally, problems arise in cell division that cause cells to divide improperly. Failure of chromosomes to separate correctly in mitosis or meiosis is called nondisjunction. Should nondisjunction occur 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. 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 may produce chromosomal anomalies resulting in birth defects. In trisomy nondisjunction, cells contain an extra chromosome. In humans, this means that there are 47 total chromosomes instead of 46. Trisomy is seen in Down syndrome where chromosome 21 has an additional or partial chromosome. Nondisjunction may also produce abnormalities in sex chromosomes. Monosomy is a type of nondisjunction in which only one chromosome is present. Females with Turner syndrome have only one X sex chromosome. Males with XYY syndrome have an extra Y sex chromosome. Nondisjunction in sex chromosomes typically has less severe consequences than nondisjunction in autosomal chromosomes (non-sex chromosomes).
Chromosome mutations can impact both homologous chromosomes 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. This type of mutation occurs more often between non-homologous chromosomes and can be reciprocal (gene exchange between two chromosomes) or non-reciprocal (only one chromosome receives a new gene segment).