Anaphase: Sister Chromatid Separation

TL;DR

During anaphase, the replicated sister chromatids, which were lined up at the metaphase plate, finally separate. The centromere holding them together divides, and each now-individual chromosome is pulled to opposite poles of the cell. This ensures each new daughter cell gets a complete and identical set of chromosomes.

1. The Mental Model

Imagine two identical twin siblings holding hands (sister chromatids joined at the centromere). During anaphase, their hands let go, and they are pulled to opposite sides of a room.

2. The Core Material

Anaphase is a crucial stage in mitosis where the replicated genetic material is precisely divided between the future daughter cells. Prior to this, in metaphase, the chromosomes (each still consisting of two sister chromatids) aligned on the metaphase plate.

Sister Chromatid Separation

The defining event of anaphase is the separation of sister chromatids. This happens when the centromere divides in two. Remember, the centromere is what previously joined the two sister chromatids. Once it divides, the sister chromatids are no longer joined.

Movement to Poles

The spindle fibres (protein microtubules), which were attached to the centromeres, begin to shorten. This shortening action pulls the separated sister chromatids towards opposite poles of the cell. Once separated, these structures are no longer called "sister chromatids" but are now considered individual chromosomes.

It’s important to note the change in terminology: what was one chromosome made of two chromatids becomes two individual chromosomes once they separate. If a human cell had 46 chromosomes (each made of two chromatids) before anaphase, it temporarily has 92 individual chromosomes moving to poles during anaphase.

graph TD
    A["Chromosomes aligned at Metaphase Plate (each 2 sister chromatids)"] --> B["Centromeres Divide"]
    B --> C["Sister Chromatids Separate"]
    C --> D["Spindle Fibres Shorten"]
    D --> E["Separated Sister Chromatids (now individual Chromosomes) pulled to opposite poles"]

3. Worked Example

Let's consider the simplified animal cell from your source material, which has four chromosomes.

1. Before Anaphase: In metaphase, you'd see four chromosomes, each made of two sister chromatids, lined up at the equator. This means there are 8 DNA molecules in total.
2. Start of Anaphase: The centromeres of these four chromosomes divide.
3. During Anaphase: The eight sister chromatids (four from each side of the original chromosomes) instantaneously become individual chromosomes. The spindle fibres shorten, pulling four chromosomes towards one pole and the other four chromosomes towards the opposite pole. At this stage, there are temporarily 8 chromosomes in total moving to the poles.

4. Key Takeaways

• The primary event of anaphase is the division of the centromere, leading to sister chromatid separation.
• Spindle fibres shorten and pull the separated sister chromatids to opposite poles.
• Once separated, the sister chromatids are considered individual chromosomes.
• Anaphase ensures equal distribution of genetic material to resulting daughter cells.
• This stage temporarily doubles the chromosome count within the cell before it divides.

Common Mistakes to Avoid:
- Don't confuse "sister chromatids" with "homologous chromosomes"—they are distinct concepts.
- Don't forget that separated chromatids are now called chromosomes; it's a critical terminology change.
- Don't think the entire cell divides in anaphase; that happens later in cytokinesis.
- Don't assume chromosomes simply float to the poles; they are actively pulled by shortening spindle fibres.

5. Now Try It

Draw a simple diagram showing an animal cell with six chromosomes (each made of two sister chromatids) entering anaphase. Label the centromeres, sister chromatids (before and after separation), spindle fibres, and the poles. What is the total count of individual chromosomes moving towards each pole during anaphase in your diagram? Success looks like clearly illustrating the separation and movement, with the correct count of chromosomes at each pole.