Homologous chromosomes and sister chromatids are two different chromosome relationships found during the cell cycle. Homologous chromosomes occur as pairs in a diploid cell. One chromosome is from the maternal side and another from the paternal side. They carry the same genes at corresponding positions, but the alleles of these genes may be different.
Sister chromatids are formed during DNA replication (S phase). An individual chromosome makes its copy and forms two sister chromatids. These remain attached by cohesion, particularly around the centromere, and are copies of the same chromosome.
A replicated homologous pair therefore consists of four chromatids, with two sister chromatids present in each homolog.
During meiosis I, homologous chromosomes pair with each other and are later separated. At this stage, the sister chromatids remain together. Sister chromatids separate during meiosis II. In mitosis, separation of sister chromatids occurs during anaphase.
Homologous Chromosomes vs Sister Chromatids
| Basis | Homologous Chromosomes | Sister Chromatids |
|---|---|---|
| Formation | They are two corresponding chromosomes present in a diploid cell. One is maternal and another is paternal. | They are formed after DNA replication during S phase. One chromosome gets copied into two sister chromatids. |
| Relationship | They are separate chromosomes forming a homologous pair. | They are two copies of one replicated chromosome. |
| Genetic material | They contain the same genes at corresponding loci, but alleles may be different. Their DNA is not necessarily identical. | They are initially copies of the same chromosome. After crossing over in meiosis, they may no longer remain completely identical. |
| Parental origin | One chromosome comes from maternal parent and another from paternal parent. | Both are formed by replication of the same chromosome. No maternal-paternal relationship occurs between them. |
| Number of chromatids | Before replication, a homologous pair has two chromosomes. After replication, the pair contains four chromatids. | Two sister chromatids are present in each replicated chromosome. |
| Pairing in meiosis | They pair up during prophase I. This pairing is called “synapsis”. | They remain closely attached with each other, particularly around the centromere. |
| Crossing over | Crossing over takes place between the non-sister chromatids of homologous chromosomes. | Crossing over normally does not occur between the two sister chromatids of the same chromosome. |
| Separation in meiosis | Homologous chromosomes separate during anaphase I. | Sister chromatids remain together in meiosis I and separate during anaphase II. |
| During mitosis | Homologous chromosomes generally do not pair up as they do during meiosis I. | Sister chromatids are present and separate from each other during anaphase. |
| Centromere | Each homologous chromosome has its own centromere. | Sister chromatids remain joined, especially around the centromere, until their separation. |
Differences between Homologous Chromosomes & Sister Chromatids
The following are the differences between homologous chromosomes and sister chromatids.
- Formation- Homologous chromosomes are two corresponding chromosomes present in a diploid cell, one from maternal parent and another from paternal parent. Sister chromatids are produced after DNA replication. A chromosome gets copied during S phase and forms two sister chromatids.
- Relationship- Homologous chromosomes form a homologous pair. They are separate chromosomes. Sister chromatids, however, are the two parts of one replicated chromosome.
- Genetic material- Homologous chromosomes possess the same genes at corresponding loci, but alleles may be different. Their DNA is therefore not necessarily identical. Sister chromatids are initially copies of the same chromosome. After crossing over in meiosis, the chromatids may no longer remain completely identical.
- Parental origin- The homologous chromosomes are of different parental origin. One is maternal and another is paternal. Sister chromatids have no such maternal-paternal relationship, both are formed from replication of the same chromosome.
- Number of chromatids- Before replication, a homologous pair consists of two chromosomes. After replication, each chromosome has two sister chromatids. Four chromatids are then present in the replicated homologous pair.
- Pairing in meiosis- Homologous chromosomes pair up during prophase I. This pairing is called “synapsis”. Sister chromatids remain closely attached with each other by cohesin proteins, particularly at the centromere.
- Crossing over- Crossing over normally occurs between the non-sister chromatids of homologous chromosomes during prophase I. It does not normally occur between the two sister chromatids of the same chromosome.
- Separation during meiosis- Homologous chromosomes are separated first. This occurs during anaphase I. The sister chromatids stay together at this stage and are separated later, during anaphase II.
- During mitosis- Sister chromatids are also present in mitotic division. They separate from each other during anaphase. Homologous chromosomes generally do not pair up in mitosis as they do during meiosis I.
- Centromere- Each homologous chromosome has its own centromere. Sister chromatids of one replicated chromosome remain joined, especially around the centromere, until their separation takes place.
Similarities between Homologous Chromosomes and Sister Chromatids
The following are some of the similarities between homologous chromosomes and sister chromatids.
- Both are chromosomal structures and composed of DNA associated with chromosome proteins. Genetic information is carried in both.
- Genes are present in both. Homologous chromosomes contain the same genes at corresponding loci, whereas sister chromatids are copied from the same chromosome and contain the same set of genes.
- In a replicated diploid cell, both structures occur together. Each chromosome of the homologous pair contains two sister chromatids, making four chromatids in the replicated pair.
- Both become condensed chromosome material during cell division and are involved in chromosome movement.
- Both take part in meiosis. Homologous chromosomes are separated during first meiotic division. Sister chromatids remain together at this stage and their separation occurs in the second meiotic division.
- Centromeric regions and kinetochores take part in the segregation of both. Spindle microtubules act on these chromosome structures during their movement, although their orientation is different in meiosis I and meiosis II.
- Both are involved during genetic recombination in meiosis. Homologous chromosomes pair up in prophase I, and crossing over occurs between the non-sister chromatids belonging to these homologous chromosomes.
References
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK26840/
- Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. https://openstax.org/books/biology-2e/pages/12-3-laws-of-inheritance
- Cooper, G. M. (2000). The cell: A molecular approach (2nd ed.). Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK9901/
- Gilbert, S. F. (2000). Developmental biology (6th ed.). Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK10019/