Homologous Chromosomes vs Sister Chromatids
All animals carry their genetic information in chromosomes and have characteristic number of chromosomes in their cells. Also, they vary widely in their size, the location of centromere, staining properties, the relative length of the two arms on either side of the centromere, and constricted sites along the arms. The number of chromosomes in organisms is basically determined by counting the haploid (n) number of chromosomes. Each chromosome is composed of a single DNA molecule. Usually chromosomes exist as homologous pairs. Each homologous chromosome contains a maternal and a paternal copy. A single chromosome in a homologous pair is referred to as a homologue.
Sister chromatids are produced only when a single chromosome is replicated into two copies of the same chromosome. Hence the sister chromatids are seen only in the replication phase. Each homologue contains two sister chromatids, which are being held together by adhesive proteins called cohesions at the centromere of the chromosome. A sister chromatid contains a single DNA, which is identical to the DNA copy of the other sister chromatid in the same homologue. Sister chromatids are synthesized during the ‘S’ phase of interphase, and are separated from each other during the mitosis. In some species, sister chromatids serve as the templates of DNA repair.
Homologous chromosomes are chromosome pairs with a similar length, centromere position and staining patterns. Each homologue of homologous chromosome pair is inherited either by paternally or maternally. Even though they are similar, these chromosomes are not identical as they are inherited from two different individuals. Each homologous chromosome contains two chromosomes, and they do not stick together. But when the replication process starts, the homologous pairs replicate itself and produce two identical DNA molecules. As they become more condensed, they become visible as two strands called chromatids. Each homologous chromosome contains four sister chromatids.
What is the difference between Homologous Chromosomes and Sister Chromatids?
• Homologous chromosomes contain two similar chromosomes, and each chromosome in the pair contains two sister chromatids.
• Ater DNA replication, two sister chromatids in a single chromosome are held together at their centromeres by cohesive proteins, whereas homologous chromosomes do not stick together at their centromeres.
• Homologous chromosomes are made up of both maternal and paternal copies of the same chromosome, whereas sister chromatids in a single chromosome can be either a maternal or a paternal copy.
• Homologous chromosomes are always seen, whereas sister chromatids are only visible during the replication phases.
• Homologous chromosome pair contains four DNA strands while one sister chromatid is composed of a single DNA strand.
• Unlike the two sister chromatids in a homologue, homologous chromosome pair is not identical.