Chromatid is the condensed form of chromatin, which consists of DNA wound around histones. It is typically only seen during cell replication and division, as the genetic material must condense in order to be divided evenly between cells.
Sister chromatids consist of one chromatid and a replicated copy connected at the centromere. They are typically homozygous with similar traits, but may have slight differences due to mutation or crossing over (meiosis I). They only exist during cell reproduction and division. During mitosis and meiosis II, sister chromatids are pulled apart into separate daughter cells.
Homologous chromosomes are chromosome pairs that code for the same genes at similar loci, but have allelic variance as one chromosome is from the mother and the other from the father. During meiosis I, homologous chromosomes synapse and exchange genetic information when crossing over.
The tetrad is formed by two chromosomes (each chromosome consisting of two sister chromatids) grouped together. It only occurs in meiosis I, as homologous chromosomes do not synapse in mitosis or meiosis II.
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