Homologous Chromosomes Definition
Homologous chromosomes are a pair of DNA molecules which contain information for the same genes, though each homologous chromosome may carry different alleles. A diploid organism (like a human) carries two copies of each gene, one on each part of this pair of homologous chromosomes.
Homologous chromosomes provide a number of functions to organisms, mainly in providing genetic variety. This genetic variety generally increases a species’ chance of surviving and adapting to changing environmental conditions. Homologous chromosomes typically have the same structure, because they contain genes coding for the same protein products. While the alleles for the genes may differ slightly by a few nucleotides, they are typically around the same length and form the same shape when condensed into a chromosome.
Homologous Chromosomes Function
Homologous chromosomes have the same function as any chromosome, they contain codons which specify the sequence of amino acids within a protein. Each codon consists of a three-nucleotide sequence, which can be matched to an amino acid. To create large and complex proteins, genes must contain many thousands of nucleotides. Each protein then has a unique and specific function, which contributes to the actions of the entire cell.
Diploid organisms carry two copies of each gene for every protein they create. This allows for a number of different possibilities depending on the function of the protein. Some alleles create normally functioning proteins, some create non-functioning proteins, and others produce novel or more efficient proteins. With homologous chromosomes, two versions of each protein are created. This means that an organism can have many levels of protein functionality, from totally non-functional to hyper-functional. In combination with the many thousands of proteins an organism uses and the environment, this creates enormous variety in animals with homologous chromosomes.
Further, homologous chromosomes may engage in the process of homologous recombination during the process of meiosis. This random process swaps two parts of a pair of homologous chromosomes which contain the same genes. This process adds even more variety, as it recombines the arrangement of genes you received from your parents.
Homologous Chromosomes Structure
Below is a picture of a karyotype, which is a graphic of sorted images showing every chromosome within a cell. Like the one below, most karyotypes show the homologous chromosomes next to each other. Realistically, unless they are lined up during cell division, these homologous chromosomes are randomly distributed throughout the nucleus.
If you look at the image above, you will see 22 pairs of homologous chromosomes as well as the sex chromosomes X and Y. If you look at any one of the pairs, you will notice the homologous chromosomes are always the same basic length and size. Look closer and you will notice that the dark banding on the homologous chromosomes seems to match. This is because the chromosomes have the same basic structure, changing only at a few nucleotides. When the dark dye binds to the chromosome it prefers certain locations based on the molecular structure. Each of the homologous chromosomes is the same, so the dye stains them the same.
Homologous chromosomes can only be identified this way when they are condensed. As chromatin, the individual molecules unwind into a loose structure that cannot be separated. Though some of the homologous chromosomes look like slightly different shapes in the above karyotype, they were likely being bent and pushed around by the cell membrane or other cellular components.
Homologous Chromosomes vs Sister Chromatids
Homologous chromosomes are often confused with the similar term, sister chromatids. Sister chromatids are formed when DNA is copied. DNA is made of two complimentary strands, which wrap around each other in a helix formation. As proteins replicate the DNA they split the strands apart, and match new nucleotides to each side. The new strands grow, until two newly formed strands of DNA are completed. These sister chromatids will be fully separated during mitosis.
In contrast, new homologous chromosomes are created during meiosis when duplicated chromosomes are created and separated into individual gametes. When two gametes fuse together, these homologous chromosomes will contribute the maternal and paternal alleles for each gene. When this organism matures and undergoes meiosis to create gametes, these homologous chromosomes will be rearranged, recombined, and repackaged into unique genetic combinations.