Prophase I

The main features of prophase I are as follows: (1) The chromosomes coil, becoming shorter and thicker as they do so, and their two-stranded nature becomes apparent; the chromosomes also become aligned in pairs. (2) The nuclear envelope and the nucleolus disassociate. (3) Parts of each closely associated pair of chromosomes may be exchanged with each other, and then the chromosomes separate (Fig. 12.2; see also Fig. 12.4A, B).

As with prophase of mitosis, the beginning of this phase is marked by the appearance of chromosomes as faint threads in the nucleus. These threads gradually coil like a spring so that they become as much as 100 times shorter in length, and correspondingly, the coil becomes obviously thicker than the thread alone. As the chromosomes become shorter and thicker, they become aligned in pairs, and eventually, two parallel threads, the chromatids, can be distinguished for each chromosome. Each pair of chromosomes, therefore, has four chromatids. The four chromatids of a homologous pair each have their own centromere (a dense, constricted area of a chromosome to which a spindle fiber, consisting of microtubules having the appearance of a fine thread, becomes attached; see Figs. 12.2 and 12.4B).

As prophase I progresses, parts of the chromatids of the homologous chromosomes may break and be exchanged with each other. Depending on the length of the chromosomes, this exchange may occur at one to several points throughout the length of the paired homologous chromosomes, or if the chromosomes are short, it may not occur at all. The evidence for this exchange of parts appears a little later in prophase I, when the chromosomes of each homologous pair appear to push each other apart. It can then be seen that adjacent chromatids have crossed over and are sticking together at one to several points.

Immediately following crossing-over, there is an exchange of some of the DNA contributed by the two parents, which is the basis for some of the variability seen in the offspring. An X-shaped figure called a chiasma (plural: chiasmata) results from each crossover; the relative positions of the chiasmata provide a basis for the study of where various genes are located on the chromosomes (Fig. 12.3).

After the chiasmata have appeared, the chromatids slowly separate, each remaining the same length as it was originally and with the same amount of DNA but now possessing "traded" material. The process is something like exchanging three fenders and the chrome grille of a red car with those of an otherwise identical blue car that has a black grille. When you were through with the exchange, you still would have structurally identical cars, but each would have a different combination of fender colors and grilles.

Chapter 12

mitosis mitosis

Prophase Meiosis

early prophase early prophase I

Figure 12.2 A comparison of mitosis and meiosis.

early prophase early prophase I

c c late prophase late prophase I

late prophase late prophase I

metaphase metaphase I

metaphase metaphase I

Figure 12.2 A comparison of mitosis and meiosis.

By the end of prophase I, the nuclear envelope and the nucleolus have disassociated and disappeared, and spindle fibers (microtubules having the appearance of fine threads) are beginning to form. As in mitosis, some spindle fibers are attached to the centromeres of the chromosomes, while others extend in arcs from pole to pole.

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

  • hanna-mari
    What are the two main features of prophase?
    8 years ago
  • celendine
    What phase do chromosomes become faint in?
    8 years ago
  • Jonas
    Why do the chromosomes appear thicker than in prophase?
    8 years ago

Post a comment