Chromatids are the two identical copies of a single chromosome made during DNA replication. They stay joined at the centromere as sister chromatids until the cell pulls them apart during cell division.
A chromatid is one copy of a duplicated chromosome. When a cell copies its DNA during S phase, each chromosome ends up as two identical strands joined together at a region called the centromere. Those two joined copies are called sister chromatids. Think of a chromosome before replication as a single sticky note, and after replication as two identical sticky notes stuck together in the middle.
Those sister chromatids don't stay attached forever. During cell division the cell lines them up and then yanks them apart so each new cell gets exactly one copy. In meiosis, things get more interesting: double-strand breaks in chromatids can be repaired by swapping pieces between homologous nonsister chromatids, which is crossing over. That's how you get genetic variation in gametes.
Chromatids live in Unit 4 (Cell Communication and Cell Cycle), specifically topic 4.6, Regulation of the Cell Cycle. They matter because the whole point of cell cycle checkpoints (learning objective AP Bio 4.6.A) is making sure chromatids are copied correctly and separated evenly. If a checkpoint fails and chromatids don't split right, you get cells with the wrong number of chromosomes. That ties straight into AP Bio 4.6.B, where disruptions to the cell cycle can lead to cancer or apoptosis. So when the exam asks about genome integrity, chromatids are usually the thing being protected.
Sister Chromatids and the Centromere (Unit 4)
Sister chromatids are the two identical copies; the centromere is the spot that holds them together. The cell grabs the centromere region to line chromatids up and pull them apart, so you can't talk about one without the other.
Anaphase (Unit 4)
Anaphase is the exact moment sister chromatids get separated and dragged to opposite poles. If a question describes chromosomes 'aligned but unable to separate,' the cell is stuck before anaphase because the chromatids haven't been released.
Homologous Chromosomes and Crossing Over (Unit 5 / Meiosis)
In meiosis, double-strand breaks in chromatids get repaired by swapping genetic material between homologous nonsister chromatids, which is crossing over. This is the engine of genetic variation, linking chromatids directly to heredity in Unit 5.
Cancer and Apoptosis (Unit 4)
When checkpoints fail to verify that chromatids are properly attached and copied, division goes wrong. The cell either self-destructs through apoptosis or keeps dividing with damaged DNA, which is how cancer starts.
Chromatids show up most often inside cell cycle and meiosis questions rather than as a standalone vocab term. Expect MCQ stems describing a cell that 'arrests at metaphase with chromosomes aligned but unable to separate,' which points to a problem with the protein machinery (like APC/C) that triggers sister chromatid separation. A released 2022 Long FRQ used chromatids to set up crossing over, asking you to connect double-strand breaks repaired between homologous nonsister chromatids to genetic recombination. A 2024 free-response item tied crossing over in meiosis I to proper chromosome alignment and segregation. So on the exam you mostly need to DO two things: explain how chromatids separate and why that matters for getting the right chromosome count, and explain how exchanges between nonsister chromatids generate variation.
Sister chromatids are two identical copies of the SAME chromosome, joined at the centromere. Homologous chromosomes are two SIMILAR but non-identical chromosomes, one from each parent, that carry the same genes in possibly different versions. Sister chromatids separate in mitosis and meiosis II; homologous chromosomes separate in meiosis I.
Chromatids are the duplicated copies of a chromosome made during S phase, and two copies of the same chromosome joined at the centromere are called sister chromatids.
Sister chromatids stay attached at the centromere until anaphase, when the cell pulls them to opposite poles so each new cell gets one copy.
Cell cycle checkpoints (AP Bio 4.6.A) exist partly to verify chromatids are copied and attached correctly before division proceeds.
If chromatid separation goes wrong, the result can be cancer or apoptosis (AP Bio 4.6.B).
In meiosis, crossing over swaps genetic material between homologous nonsister chromatids, which creates genetic variation in gametes.
Chromatids are the duplicated copies of a chromosome produced during DNA replication in S phase. Two identical copies of the same chromosome joined at the centromere are called sister chromatids, and they separate during cell division.
No. Sister chromatids are two identical copies of one chromosome held together at the centromere, while homologous chromosomes are a matched pair (one from each parent) that carry the same genes but can have different alleles. Sister chromatids separate in anaphase of mitosis and meiosis II; homologous chromosomes separate in anaphase I of meiosis.
They separate during anaphase, after the cell confirms at a checkpoint that everything is properly attached. The anaphase-promoting complex (APC/C) triggers the release, which is why a mutation in APC/C can leave cells stuck at metaphase with chromosomes aligned but unable to split.
During meiosis, double-strand breaks form in chromatids and get repaired by exchanging genetic material between homologous nonsister chromatids. That exchange is crossing over, and it's a major source of genetic variation, which is exactly how the 2022 Long FRQ framed it.
Checkpoints make sure chromatids are copied and separated correctly. If those controls fail and chromatids don't divide evenly, the cell can either undergo apoptosis or keep dividing with damaged DNA, which is one of the pathways toward cancer (AP Bio 4.6.B).