Chromosome movements are highly organized

The next three phases of mitosis—prometaphase, metaphase, and anaphase—are the phases during which chromosomes actually move (Figure 9.8). During these phases, the centromeres holding the two chromatids together separate, and the former sister chromatids move away from each other in opposite directions.

prometaphase. Prometaphase is marked by the disappearance of the nuclear envelope. The material of the enve lope remains in the cytoplasm, however, to be reassembled when the daughter nuclei re-form. In prometaphase, the chromosomes begin to move toward the poles, but this movement is counteracted by two factors:

► A repulsive force from the poles pushes the chromosomes toward the middle region, or equatorial plate (metaphase plate), of the cell.

► The two chromatids are still held together at the centromere by cohesin.

Thus, during prometaphase chromosomes appear to move aimlessly back and forth between the poles and the middle of the spindle. Gradually, the centromeres approach the equatorial plate.

metaphase. The cell is said to be in metaphase when all the centromeres arrive at the equatorial plate. Metaphase is the best time to see the sizes and shapes of chromosomes

Metaphase

Anaphase

Telophase

Metaphase

Equatorial

(metaphase)

plate

Anaphase

Daughter chromosomes

Telophase

Equatorial

(metaphase)

plate

Daughter chromosomes

The centromeres (regions connecting paired chromatids) become aligned in a plane at the cell's equator.

The paired sister chromatids separate, and the new daughter chromosomes begin to move toward the poles.

The daughter chromosomes reach the poles. Telophase passes into the next interphase as the nuclear envelopes and nucleoli re-form and the chromatin becomes diffuse.

because they are maximally condensed. The chromatids are now clearly connected to one pole or the other by micro-tubules. At the end of metaphase, all of the chromatid pairs separate simultaneously.

This separation occurs because the cohesin holding the sister chromatids together is hydrolyzed by a specific protease, appropriately called separase. Until this point, separase has been present but inactive, because it has been bound to an inhibitory subunit called securin. Once all the chromatids are connected to the spindle, securin is hydrolyzed, allowing separase to catalyze cohesin breakdown (Figure 9.9). In this way, chromosome alignment is connected to chromatid separation. This process, called the spindle checkpoint, apparently senses whether there are any kinetochores that are unattached to the spindle. If there are, securin breakdown is blocked, and the sister chromatids stay together.

anaphase. Separation of the chromatids marks the beginning of anaphase, during which the two sister chromatids move to opposite ends of the spindle. Each chromatid contains one double-stranded DNA molecule and is now referred to as a daughter chromosome.

What propels this highly organized mass migration is not clear. Two things seem to move the chromosomes along.

Prophase

Metaphase

Chromatids

Anaphase

Daughter chromosomes

Prophase

Metaphase

Chromatids

Anaphase

Daughter chromosomes

Plasma Membrane Chromosome

2t At metaphase, most cohesin is removed, except for some at the centromere.

^ At anaphase, securin, an inhibitory subunit of separase, is hydro-lyzed. Separase hydrolyzes the remaining cohesin.

9.9 Molecular Biology of Chromatid Attachment and Separation

Cohesin holds sister chromatids together. Separase hydrolyzes cohesin at the onset of anaphase.

2t At metaphase, most cohesin is removed, except for some at the centromere.

^ At anaphase, securin, an inhibitory subunit of separase, is hydro-lyzed. Separase hydrolyzes the remaining cohesin.

9.9 Molecular Biology of Chromatid Attachment and Separation

Cohesin holds sister chromatids together. Separase hydrolyzes cohesin at the onset of anaphase.

First, at the kinetochores are proteins that act as "molecular motors." These proteins, called cytoplasmic dynein, have the ability to hydrolyze ATP to ADP and phosphate, thus releasing energy to move the chromosomes along the microtubules toward the poles. These motor proteins account for about 75 percent of the force of motion. Second, the kineto-chore microtubules shorten from the poles, drawing the chromosomes toward them. This shortening accounts for about 25 percent of the motion.

During anaphase the poles of the spindle are pushed farther apart, doubling the distance between them. The distance between poles increases because the overlapping polar mi-crotubules extending from opposite ends of the spindle contain motor proteins that cause them to slide past each other, in much the same way that microtubules slide in cilia and flagella (see Figure 4.24a). This polar separation further separates one set of daughter chromosomes from the other.

The movements of chromosomes are slow, even in cellular terms. At about 1 pm per minute, it takes about 10-60 minutes for them to complete their journey to the poles. This speed is equivalent to a person taking 9 million years to travel across the United States! This slow speed may ensure that the chromosomes segregate accurately.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • shannon
    What are daughter chromosomes?
    8 years ago

Post a comment