How many sister chromatids in a chromosome

Nature Reviews Molecular Cell Biology 2, 72 All rights reserved. Prophase is the first stage in mitosis, occurring after the conclusion of the G 2 portion of interphase. During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase.

Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere , these structures now appear as X-shaped bodies when viewed under a microscope. Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold the sister chromatids together, whereas condensin forms rings that coil the chromosomes into highly compact forms.

The mitotic spindle also begins to develop during prophase. As the cell's two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart. When prophase is complete, the cell enters prometaphase — the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles.

As a result, the spindle microtubules now have direct access to the genetic material of the cell. As prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Every chromosome has at least two microtubules extending from its kinetochore — with at least one microtubule connected to each pole.

At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth. In addition, the spindle is now complete, and three groups of spindle microtubules are apparent. Kinetochore microtubules attach the chromosomes to the spindle pole; interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole; and astral microtubules extend from the spindle pole to the cell membrane.

Metaphase leads to anaphase , during which each chromosome's sister chromatids separate and move to opposite poles of the cell. Enzymatic breakdown of cohesin — which linked the sister chromatids together during prophase — causes this separation to occur. Upon separation, every chromatid becomes an independent chromosome. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. More specifically, in the first part of anaphase — sometimes called anaphase A — the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles.

Then, in the second part of anaphase — sometimes called anaphase B — the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes Figure 2. Figure 2: Types of microtubules involved in mitosis During mitosis, several types of microtubules are active.

The motor proteins associated with the interpolar microtubules drive the assembly of the spindle. Note the other types of microtubules involved in anchoring the spindle pole and pulling apart the sister chromatids. Figure Detail. Cytokinesis is the physical process that finally splits the parent cell into two identical daughter cells. During cytokinesis, the cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow. The position of the furrow depends on the position of the astral and interpolar microtubules during anaphase.

The cleavage furrow forms because of the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments move past each other, the contractile ring becomes smaller, akin to pulling a drawstring at the top of a purse. When the ring reaches its smallest point, the cleavage furrow completely bisects the cell at its center, resulting in two separate daughter cells of equal size Figure 3.

It is only when sister chromatids separate — a step signaling that anaphase has begun — that each chromatid is considered a separate, individual chromosome. Now that the sister chromatids have separated, each chromatid is also considered a chromosome.

During anaphase, we now have a total of 16 chromosomes and 16 chromatids — in short, each chromatid is now a chromosome. Similarly, in humans, there are 92 chromosomes present and 92 chromatids during anaphase.

These numbers remain the same during telophase. It is only after the end of mitosis — when the dividing cells have fully separated and the membranes have reformed — that the normal chromosome number is restored to the cell.

Below is a table summarizing the chromosome and chromatid number during mitosis in humans:. The chromosome and chromatid count during meiosis works a bit differently.

Recall that there are two divisions during meiosis: meiosis I and meiosis II. The genetic material of the cell is duplicated during S phase of interphase just as it was with mitosis resulting in 46 chromosomes and 92 chromatids during Prophase I and Metaphase I. However, these chromosomes are not arranged in the same way as they were during mitosis. Rather than each chromosome lining up individually across the center of the cell, homologous pairs of chromosomes line up together forming tetrads , also known as bivalents :.

Here, the homologous chromosome pairs have been color coded:. When anaphase I begins, you may expect the chromosome number to change, but it does not. Remember — it is only after the sister chromatids separate that the chromosome number changes. Since anaphase I only separates the homologous chromosomes, neither the chromosome number nor the chromatid number changes during anaphase. Visualized below:. As you can see, the separation of homologous chromosomes does not change the chromosome number or the chromatid number.

There are still 8 chromosomes and 16 chromatids. The mitotic spindle is made of many long proteins called microtubules, which are attached to a chromosome at one end and to the pole of a cell at the other end. The sister chromatids are separated simultaneously at their centromeres. The separated chromosomes are then pulled by the spindle to opposite poles of the cell. Anaphase ensures that each daughter cell receives an identical set of chromosomes, and it is followed by the fifth and final phase of mitosis, known as telophase.

Further Exploration Concept Links for further exploration cell division chromosome centromere meiosis DNA cytokinesis spindle fibers metaphase telophase. Related Concepts 9.