chromatid
Summary
A chromatid is one of the two essentially identical DNA copies of a replicated chromosome. The pair, called sister chromatids, is joined at the centromere. Replication in S phase doubles the chromatids and DNA content but not the chromosome number, which is counted by centromeres.
Detail
Meaning: A chromatid is one DNA double helix, with its histones, within a replicated chromosome. DNA replication in S phase copies each chromosome, and the two copies stay attached at the centromere by cohesin proteins. Spindle fibers attach to kinetochores at the centromere. When sister chromatids separate in anaphase of mitosis or anaphase II of meiosis, each one is then counted as a chromosome.
On the MCAT: Expect counting questions. A human cell in G1 has 46 chromosomes, each a single chromatid. After S phase and through metaphase it still has 46 chromosomes but 92 chromatids, and its DNA content has doubled. After anaphase there are 92 separate chromosomes. Meiosis I separates homologous chromosomes, not sister chromatids, so cells entering meiosis II still have paired chromatids. Crossing over in prophase I occurs between nonsister chromatids of homologs, so the sisters are no longer identical afterward. Nondisjunction questions ask which chromatids or chromosomes failed to separate.
Common confusion: Sister chromatids are not homologous chromosomes. Sisters are copies of the same DNA, whereas homologs are one maternal and one paternal chromosome with the same genes but possibly different alleles. To count chromosomes, count centromeres. Replication adds chromatids, not chromosomes.
Sources
- Campbell Biology
- Molecular Biology of the Cell
- Biology (Raven and Johnson)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.