Short answer
A chromosome is an organized DNA-containing structure. After its DNA has been copied, the two associated copies are called sister chromatids. While they remain joined, the pair is conventionally counted as one replicated chromosome. When the sisters separate during division, each becomes a daughter chromosome. 1 2 3
The familiar X-shaped diagram usually shows one replicated chromosome made of two sister chromatids. Each side of the X is one chromatid; each arm is not a separate chromosome. The stage of the cell cycle is essential to interpreting the picture. 2 3
On this page
At a glance
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| Question | Chromosome | Chromatid |
|---|---|---|
| What does the term emphasize? | The DNA-containing unit being organized and distributed. | One of the copies in a replicated chromosome. |
| Before DNA replication | An unreplicated chromosome has one DNA molecule. | There is no joined pair of sister chromatids yet. |
| After replication, before separation | One replicated chromosome consists of two sisters. | Each sister contains one copied DNA molecule with associated proteins. |
| Relationship between sisters | Both belong to the same replicated chromosome. | They are copies produced by replication. |
| What happens at separation? | Each separated sister is now a chromosome. | The separated unit is called a daughter chromosome. |
| Does an X show two chromosomes? | Normally no, in the standard cell-cycle diagram. | It normally shows two chromatids. |
Sources: chromosome organization 1; chromatid terminology 2; replication and separation 3.
What each thing is
A chromosome is a DNA-containing structure with associated proteins. In human nuclei, long DNA molecules are packaged around histones and organized into chromosomes. Chromosomes become especially compact during division, but they exist throughout the cell cycle; they do not appear from nothing when an X becomes visible. 1 3
A chromatid is one of the two sister copies produced when a chromosome’s DNA is replicated. The sisters remain associated until the cell separates them. Introductory diagrams emphasize the centromere, the chromosome region involved in their attachment and in organizing their movement during division. 2 3
The relationship is therefore tied to an event: replication creates sister copies; later separation changes how those copies are named and counted.
Key differences
Copying DNA does not immediately double the chromosome count. During S phase, an unreplicated chromosome becomes a replicated chromosome containing two sister chromatids. The amount of DNA has doubled, but the associated pair is still one chromosome under the usual counting convention. 3
Separation changes the count within the dividing cell. During mitotic anaphase, sister chromatids separate and are called chromosomes. Those daughter chromosomes move toward opposite poles, after which the cell ordinarily completes division. A count made across the entire anaphase cell differs from a count made in either resulting daughter nucleus. 3
A chromatid is not half the information. Each sister originates as a copy of the chromosome’s DNA. Splitting an X down the middle does not divide its genes into two complementary half-lists. Each daughter nucleus needs a corresponding chromosome complement. 2 3
How to tell them apart
For a diagram, ask three questions: has DNA replication occurred, are the sister copies still associated, and has separation begun? Identify the stage before counting visible lines or shapes.
In a standard metaphase drawing, two sister chromatids form one replicated chromosome. In an anaphase drawing, separated sisters are daughter chromosomes. A long uncondensed DNA-containing structure can also be a chromosome, so visibility as an X is not the definition. 3
These rules interpret labeled biological diagrams. A chromosome’s identity or the precise stage of an unfamiliar microscopy image may require more information than its outline alone provides.
Where they overlap
Chromatids and chromosomes describe the same physical material at different stages and organizational levels. A chromatid is part of a replicated chromosome; after separation, that unit continues as a chromosome. The terms are connected descriptions rather than separate ingredients like DNA and protein. 2 3
Both contain genes and other DNA. Calling something a chromatid does not mean its DNA is less complete or biologically different merely because it is awaiting separation.
Edge cases
Sister chromatids are not homologous chromosomes. Homologs are the corresponding chromosomes in a diploid set, commonly one inherited from each parent. Each homolog can replicate to make its own pair of sisters. A paired set of replicated homologs therefore contains two chromosomes and four chromatids. 4
Sisters are not guaranteed to remain identical in every context. Replication normally begins with matching copies. During meiosis, crossing over between nonsister chromatids of homologous chromosomes can create differences between the sisters. “Identical copies” describes their ordinary origin, not an exceptionless lifelong condition. 4
Haploid does not mean unreplicated. After meiosis I, a nucleus can contain one chromosome set while each chromosome still has two sisters. Chromosome-set count and replication state are separate questions. 4
Why the distinction exists
Cells must copy genetic material and then distribute it accurately. The vocabulary lets us describe the difference between having two associated copies ready for separation and having two independent chromosome units moving to their destinations. Without the chromatid term, it is easy to confuse DNA duplication with a change in chromosome sets.
Examples and common misconceptions
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| Description | Interpretation |
|---|---|
| One X-shaped chromosome before sister separation | One chromosome, two chromatids. |
| Two replicated homologs paired during meiosis I | Two chromosomes, four chromatids. |
| The two former sisters after mitotic separation | Two daughter chromosomes, destined for opposite nuclei. |
| A haploid nucleus after meiosis I | One chromosome set, still replicated. |
These are standard textbook stages rather than counts for every unusual cell-division event. 3 4
Sources
Sources checked October 3, 2026.
- National Human Genome Research Institute — Chromosomes Fact Sheet. Chromosomal DNA and protein packaging.
- National Human Genome Research Institute — Chromatid. Sister-copy terminology.
- OpenStax, Biology 2e — The Cell Cycle. S phase, mitosis, and daughter chromosomes after separation.
- OpenStax, Biology 2e — The Process of Meiosis. Homologs, crossing over, and replicated haploid products.