Question

The individual chromosomes become visible with a light microscope during which stage of mitosis? a. prophase b. prometaphase c. metaphase d. anaphase

Short answer

Answer: b. prometaphase

Step-by-step solution

Overview of mitosis stages

To answer the question, let's briefly go over each stage of mitosis. This will help with understanding when chromosomes become visible with a light microscope.

Prophase

During prophase, chromosomes start to condense, becoming shorter and thicker. The nuclear envelope starts to disintegrate, and the nucleolus disappears. The spindle fibers, which are responsible for aligning and separating the chromosomes, begin to form.

Prometaphase

At this stage, the nuclear envelope completely disintegrates, allowing spindle fibers to interact with the chromosomes. The chromosomes condense further and become fully visible under a light microscope. Each chromosome has two sister chromatids joined at the centromere, and the spindle fibers attach to these centromeres via kinetochore proteins.

Metaphase

Metaphase is marked by the alignment of chromosomes at the metaphase plate (equator) of the cell. The fully condensed chromosomes are still visible during this stage, and the spindle fibers exert equal and opposite forces at each centromere, mimicking a tug-of-war between sister chromatids.

Anaphase

During anaphase, sister chromatids separate and are pulled towards opposite poles of the cell by the spindle fibers. The cell begins to elongate, and the chromosomes start to move further away from the metaphase plate.

Conclusion

The individual chromosomes become fully visible under a light microscope during the prometaphase. During this stage, chromosomes condense and the nuclear envelope disintegrates, allowing spindle fibers to interact. Therefore, the correct answer is: b. prometaphase

Key concepts

Chromosome Visibility

Chromosomes are the carriers of genetic information and play a crucial role in cell division. At the start of mitosis, chromosomes are not visible under a light microscope as they are in a less condensed form called chromatin. However, as mitosis progresses, these chromosomes begin to condense and become more compact.

This condensation process is essential because it allows chromosomes to be easily separated and distributed to daughter cells. The stage at which chromosomes first become visible under a light microscope is during the prometaphase, as they have condensed enough to be seen distinctly. Prior to this, in prophase, they start condensing but are not yet fully visible. By prometaphase, chromosomes have become sufficiently condensed, allowing for clear visibility and interaction with spindle fibers.

Prophase

Prophase is the first stage of mitosis and marks the beginning of chromosome condensation. During this stage:

• Chromosomes start becoming shorter and thicker as they condense.
• The nuclear envelope begins to break down but has not completely disappeared.
• The nucleolus, a structure within the nucleus, disappears.

This stage sets the stage for subsequent mitosis events, providing the initial steps of chromosomal preparation necessary for dividing genetic material accurately. Despite the condensation during prophase, the chromosomes are not yet fully visible with a light microscope because they are still condensing.

Prometaphase

Prometaphase is the follow-up to prophase and it involves crucial changes that facilitate chromosome interaction.

During this stage:

• The nuclear envelope completely disintegrates, permitting the chromosome interaction with the mitotic spindle.
• The chromosomes condense further to become fully visible under the light microscope.
• Each chromosome is seen as having two sister chromatids held together at a central region known as the centromere.
• Spindle fibers attach to the chromosomes at specialized structures called kinetochores located at each centromere.

As a result of these changes, prometaphase is a critical phase where chromosome alignment and eventual separation begin to take shape.

Metaphase

The key characteristic of metaphase is the alignment of chromosomes at the cell's equatorial plane, known as the metaphase plate.

• Chromosomes are maximally condensed and remain clearly visible during metaphase.
• Equally opposing forces from spindle fibers act on centromeres, positioning chromosomes appropriately.
• This layout ensures that each daughter cell will receive one copy of each chromosome upon division.

Metaphase effectively serves as the checkpoint for accurate chromosomal alignment before proceeding to separate the sister chromatids.

Anaphase

Anaphase is a pivotal stage where the actual separation of sister chromatids occurs.

• Spindle fibers shorten, pulling sister chromatids apart towards opposite poles of the cell.
• As they move apart, the cell begins to elongate, ensuring that each pole receives identical chromosomes.
• This movement is driven by the shortening of microtubules that make up the spindle fibers.

Anaphase underscores the distribution of genetic material to future daughter cells, leading to the next stage that finalizes cell division.

Spindle Fibers

Spindle fibers are essential structures in mitosis, comprising microtubules that originate from the centrosomes.

• They form the mitotic spindle, a crucial apparatus for chromosome movement and segregation.
• In prometaphase, spindle fibers attach to the kinetochores at centromeres, allowing them to maneuver chromosomes.
• During anaphase, these fibers shorten to pull sister chromatids apart effectively.

Beyond dividing genetic material, spindle fibers help in cell elongation, facilitating the physical separation into two cells.

Sister Chromatids

Sister chromatids are identical copies of a single chromosome connected together at the centromere.

• They are formed during DNA replication in the S phase of the cell cycle.
• Throughout prophase and prometaphase, they remain cohesively held at the centromere.
• In anaphase, sister chromatids are separated into individual chromosomes and drawn to opposite poles of the cell.

Understanding sister chromatids is fundamental to comprehending genetic duplication and distribution during mitosis, ensuring genetic consistency in daughter cells.