Chromatid: Definition & Exchange

Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Chromatids are the copies of chromosomes that are made before mitosis and meiosis. In this lesson, learn how to tell sister chromatids apart from homologous chromosomes, and find out how DNA exchange between chromatids increases genetic diversity.

Definition

Cell division is essential for the life of a multicellular organism; after all, we all developed from a single cell that divided over and over again! When a cell divides, its nucleus has to divide, too. The two kinds of nuclear division processes in eukaryotic cells are called mitosis and meiosis.

When a chromosome is replicated before mitosis or meiosis, the two identical copies stay bound to each other at first and are called sister chromatids. A chromatid, then, is one copy of a duplicated chromosome.

Sister Chromatids versus Homologous Chromosomes

These terms can be a bit confusing, so it's important to keep them straight in your mind.

Homologous chromosomes are the two different copies of a chromosome that you got from your mom and from your dad. For example, you have two copies of chromosome 3 in all of your cells, and they have the same genes on them. The homologous chromosomes are almost identical, but not quite. Due to natural genetic variation, the one you got from your mom is slightly different than the one you got from your dad. Homologous chromosomes have different alleles of some of their genes.

Sister chromatids, in contrast, should be identical. Since they arise from duplication of one chromosome (for example, the chromosome 3 that you got from your mother), they are like photocopies, with the same alleles and everything. Of course, there may be minor errors that occur by chance during DNA replication. But otherwise, the sister chromatids are exactly the same.

In this diagram, the homologous chromosomes are labeled in different colors: red and blue. You can see that they have the same genes by the banding pattern that is shown on them. And as you can see, the sister chromatids are exact copies of each homologous chromosome.

A diagram showing homologous chromosomes and sister chromatids.
A diagram of homologous chromosomes, sister chromatids and recombination.

But wait... what is going on with the different-colored pieces of chromosome on two of the chromatids? That is a depiction of the homologous recombination that occurs between chromatids during meiosis.

Homologous Recombination and Exchange of Genetic Material

When it comes to evolution, genetic diversity is a very good thing. In meiosis - the nuclear division process that produces our haploid gamete (sperm and egg) cells - genetic diversity is increased in two major ways. First, the homologous chromosomes segregate randomly into the daughter cells, so there are many possible combinations of maternal and paternal chromosomes. Homologous recombination is the second way that genetic diversity is increased. Here's how it works.

During prophase of meiosis, the replicated homologous chromosomes find each other and group together. The groups are called tetrads because they are made up of four chromatids altogether. The chromatids in each tetrad are lined up next to each other so that the same genes are next to each other.

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