Difference Between Homologous Chromosomes and Sister Chromatids

Main Difference – Homologous Chromosomes vs Sister Chromatids

Most organisms have DNA as their genetic material. Diploid organisms such as humans bear two copies of similar chromosomes. These are known as homologous chromosomes. During metaphase I of meiosis I, homologous chromosomes appear in pairs. Sister chromatids are the two exact copies which are formed by the replication of one DNA molecule at the S phase of the interphase. The key difference between homologous chromosomes and sister chromatids is that homologous chromosomes may not carry identical information all the time whereas sister chromatids carry identical information all the time.

Key Areas Covered

1. What are Homologous Chromosomes
      – Definition, Characteristics
2. What are Sister Chromatids
      – Definition, Characteristics
3. What is the Difference Between Homologous Chromosomes and Sister Chromatids
      – Comparison of Key Differences

What are Homologous Chromosomes

Homologous chromosomes are the chromosomes pairing in metaphase I of meiosis. One chromosome in the pair bears maternal origin while the other chromosome bears the paternal origin. One chromosome of the homologous pair is referred to as a homologue. Chromosome length and their positions of the centromeres are the same within the pair. Each copy contains an appropriate number of same genes, arranged in the same order (same loci). Hence, banding pattern of each chromosome in the pair appears the same. However, the same locus may contain either the same allele or a different allele on both chromosomes. Thus, an individual may be homozygous or heterozygous for a particular character. Humans have 22 homologous pairs of autosomes and 2 sex chromosomes . Female sex chromosomes, X and X are homologous while male sex chromosomes, X and Y are not truly homologous. X and Y differ from their size and genetic composition. The pairing of homologous chromosomes allows random segregation of genetic material. Homologous chromosomes exchange their DNA by recombination. Recombination at sexual reproduction leads to genetic variations in the offspring which consequently facilitates the evolution. Homologous chromosomes separate during anaphase I of meiosis I. Four haploid daughter cells are produced at the end of the meiosis. Chromosomal abnormalities such as trisomy and monosomy can occur due to the nondisjunction of homologous chromosomes.

Figure 1: Homologous Chromosomes

What are Sister Chromatids

Sister chromatids are two identical chromatids resulting in DNA replication during the S phase of interphase. They are joined together by the centromere. A sister chromatid is a one-half of a replicated chromosome. Hence, each replicated chromosome is composed of two sister chromatids.  Sister chromatids are identical to each other; one of them is the exact replica of the other. The exception is crossing over at prophase I of meiosis I. Chromosomal crossover exchanges genetic material over homologous chromosomes. Sister chromatids separate from each other during anaphase of mitosis and the anaphase II of meiosis II.

Figure 2: Sister Chromatids

Difference Between Homologous Chromosomes and Sister Chromatids

Content

Homologous Chromosomes: Homologous chromosomes are made up of both maternal and paternal chromosomes.

Sister Chromatids: Sister chromatids are made up of either a maternal or paternal chromosome.

Genetic Composition

Homologous Chromosomes: Homologous chromosomes may contain same or different alleles of the same gene. Thus, the gene sequence is not identical at all the times.

Sister Chromatids: Sister chromatids contain identical gene sequence throughout the chromatids except in the chromosomal crossover.

Appearance

Homologous Chromosomes: Homologous chromosomes appear in the metaphase I of meiosis I.

Sister Chromatids: Sister chromatids are formed during the DNA replication in the S phase of interphase.

Joining

Homologous Chromosomes: Homologous chromosomes do not stick together. They exist in pairs.

Sister Chromatids: Sister chromatids are joined together by their centromere.

Number of DNA Strands

Homologous Chromosomes: Homologous chromosomes are composed of four DNA strands.

Sister Chromatids: Single sister chromatid is composed of a single DNA strand.

Separation

Homologous Chromosomes: Homologous chromosomes are segregated during the anaphase I of meiosis I.

Sister Chromatids: Sister chromatids are separated from their centromere during anaphase II of meiosis II and the anaphase of mitosis.

Function

Homologous Chromosomes: Homologous chromosomes allow random segregation of chromosomes and genetic recombination during metaphase I.

Sister Chromatids: Sister chromatids allow random segregation of chromatids and chromosomal crossover during metaphase II of meiosis and metaphase of mitosis.

Conclusion

Homologous chromosomes are composed of both maternal and paternal chromosomes. Therefore, different alleles of the same gene can be found most of the times. On the other hand, sister chromatids are composed of same allele of a gene in both strands since they are synthesized by the DNA replication of a strand. Thus, the key difference between homologous chromosomes and sister chromatids is in their genetic composition.

Reference:
1. “Homologous Chromosomes”. Pearson – The Biology Place. Accessed 12 Feb. 2017
2. “Homologous chromosome”. Wikipedia, the free encyclopedia. 2017,  Accessed 12 Feb. 2017
3. Mccarthy E. M. “Sister chromatids”. Online Biology Dictionary. Accessed 17 Feb. 2017

Image Courtesy:
1. “PloSBiol3.5.Fig7ChromosomesAluFish.”. By Bolzer et al., (2005) Three-Dimentional Maps of all Human Male Fibroblast Nuclei and Prometaphase Rossttes. PloS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157, Figure 7a (CC-BY-2.5) via Commons Wikimedia
2. “HR in meiosis”. By Emw – Own work (CC-BY-SA-3.0) via Commons Wikimedia