Sister chromatids are two identical copies of a single chromosome, made during S phase and held together at the centromere. They separate during anaphase of mitosis and anaphase II of meiosis, ensuring each new cell gets a complete chromosome copy.
A sister chromatid is one of two identical copies of a chromosome that get connected at a region called the centromere. They start out as a single chromosome, then during the S phase of interphase, the DNA is copied. After copying, you have two attached copies, and each copy is a sister chromatid. Think of them as photocopies of the same page, stapled together until it's time to hand one to each new cell.
The key word is identical. Sister chromatids carry the same genes in the same order, which is different from homologous chromosomes (those are matching chromosomes from different parents that carry the same genes but possibly different versions). Sister chromatids stay joined through prophase and metaphase, then split apart during anaphase. In mitosis, sisters separate to make two genetically identical cells. In meiosis, the homologous pairs separate first (meiosis I), and the sister chromatids don't split until anaphase II.
Sister chromatids sit at the center of two big AP topics. In Topic 4.6 (Regulation of the Cell Cycle), the whole point of checkpoints is to make sure DNA is fully and correctly copied before sisters get separated, which connects to learning objective AP Bio 4.6.A and 4.6.B. If that copying or splitting goes wrong, you can get cancer or apoptosis. In Topic 5.1 (Meiosis), tracking when sister chromatids separate versus when homologous chromosomes separate is exactly how you tell meiosis I from meiosis II, which ties into AP Bio 5.1.A and 5.1.B. Getting the timing right is the difference between a haploid gamete and a genetic mistake.
Keep studying AP Biology Unit 5
Chromosome (Units 4-5)
A chromosome becomes two sister chromatids the moment it's copied in S phase. Before replication it's one chromatid, after replication it's two identical sisters joined at the centromere, and once they split they each count as a full chromosome again.
Anaphase I (Unit 5)
This is the trap. In anaphase I, homologous chromosomes separate but sister chromatids stay glued together. Sisters don't split until anaphase II, so a cell with homologs at opposite poles and sisters still attached is in meiosis I, not mitosis.
Interphase (Unit 4)
Sister chromatids don't exist until S phase of interphase does its job. S phase is the copying step, so a 2024 FRQ even asked you to describe its function. No S phase, no sister chromatids, no successful cell division.
Gametes (Unit 5)
Correctly separating sister chromatids in anaphase II is what gives each gamete exactly one copy of each chromosome. Mess up that separation and the gamete ends up with the wrong number of chromosomes.
On the multiple-choice section, the classic stem describes a cell where homologous chromosomes are at opposite poles but sister chromatids are still attached at their centromeres, and asks you to name the stage. The answer is anaphase I (or metaphase/anaphase of meiosis I), because sisters splitting is a meiosis II or mitosis event. Another common stem points to the shared spindle mechanism that pulls either sisters (mitosis, meiosis II) or homologs (meiosis I) toward opposite poles. On FRQs, the 2022 Long FRQ Q2 distinguished homologous nonsister chromatids (where crossing over happens) from sister chromatids, and the 2024 LRFRQ Q1 asked you to describe the function of S phase, where sisters are made. You need to be able to say WHEN sisters form, WHEN they separate, and WHY they're identical.
Sister chromatids are two identical copies of the SAME chromosome joined at one centromere. Homologous chromosomes are two different chromosomes (one from each parent) that carry the same genes but possibly different alleles, and they each have their own centromere. Sisters are clones; homologs are matched but not identical. Sisters separate in mitosis and meiosis II, homologs separate in meiosis I.
Sister chromatids are two identical copies of one chromosome, joined at the centromere, and they're created during S phase of interphase.
Sister chromatids separate during anaphase of mitosis and during anaphase II of meiosis, not during meiosis I.
In meiosis I, homologous chromosomes separate while sister chromatids stay attached, which is the single most-tested distinction on this topic.
Sister chromatids are identical to each other, while homologous chromosomes carry the same genes but can have different alleles.
Cell cycle checkpoints (Topic 4.6) make sure DNA is fully copied before sister chromatids are pulled apart, and failure here can lead to cancer or apoptosis.
Sister chromatids are two identical copies of a single chromosome, made during S phase of interphase and held together at the centromere. They separate during anaphase of mitosis and anaphase II of meiosis so each new cell gets a complete copy.
No. In meiosis I, homologous chromosomes separate while sister chromatids stay attached at their centromeres. Sister chromatids don't split until anaphase II. This is the most common test trap on the topic.
Sister chromatids are identical copies of the same chromosome joined at one centromere, like clones. Homologous chromosomes are two separate chromosomes (one from each parent) with the same genes but possibly different alleles, each with its own centromere.
They're created during S phase of interphase, when the cell copies its DNA. Before S phase a chromosome is one chromatid; after S phase it's two identical sister chromatids attached at the centromere.
Crossing over swaps genetic material between homologous nonsister chromatids during prophase I, which creates genetic diversity. The 2022 FRQ used this exact distinction. Sister chromatids are identical, so swapping between them wouldn't change anything.