7.3: Mitotic Phase - Mitosis and Cytokinesis (Vocabulary Flashcards)

Forms of DNA

In a non-dividing eukaryotic cell, nuclear DNA exists as chromatin, a grainy material. When a cell is about to divide and its DNA has replicated, the DNA condenses and coils into the familiar X-shaped chromosome. Because replication has occurred, each chromosome consists of two identical copies called sister chromatids. The two sister chromatids are joined at a region called the centromere. Kinetochores, protein structures at the centromere, serve as attachment points for spindle fibers during mitosis (not shown in the image).
The process of the nucleus dividing is called mitosis. During mitosis, the two sister chromatids that makeup each chromosome separate and move to opposite poles of the cell. Mitosis is described in four phases: prophase, metaphase, anaphase, and telophase.
Before replication, chromosomes exist as single chromatids. After replication, each chromosome has two identical sister chromatids held together at the centromere. This relationship can be summarized as:
\text{Before replication: } \text{1 chromatid per chromosome}
\text{After replication: } \text{2 sister chromatids per chromosome}
The nuclear envelope encloses the nucleus; during mitosis, it breaks down to allow spindle fibers access to the chromosomes. Spindles are composed of microtubules and other proteins.
Centrosomes (containing centrioles in animal cells) organize the spindle apparatus. Centrioles move toward opposite poles to help establish the bipolar spindle.

Mitosis: Overview

Mitosis is the phase of the eukaryotic cell cycle that occurs after DNA replication and before cytokinesis, resulting in two nuclei that contain identical genetic material. It ensures that each daughter cell will receive an identical set of chromosomes.
Mitosis is divided into four substages: prophase, metaphase, anaphase, and telophase. Prometaphase is often described as a related stage that occurs after prophase where spindle attachment to kinetochores becomes more established.

Prophase (and Prometaphase)

Prophase is the first and longest phase of mitosis. During prophase, chromatin condenses into visible chromosomes.
The nuclear envelope breaks down, allowing spindle fibers to interact with chromosomes.
In animal cells, centrioles near the nucleus begin to separate and move toward opposite poles. As centrioles move apart, the spindle begins to form between them.
The spindle (blue in the referenced image) is composed of microtubule fibers that will attach to chromosomes.
Prometaphase is the later stage of prophase where spindle fibers attach to the kinetochores of centromeres on the sister chromatids, enabling subsequent alignment and separation.

Metaphase

During metaphase, spindle fibers attach to the centromeres of the sister chromatids. Chromosomes align along the equator, or the center, of the cell.
Each chromosome is held in place at the equator by microtubules attached to kinetochores from opposite poles.
Some spindles may not attach to kinetochores; these non-kinetochore spindles help elongate the cell by pushing poles apart.
At this stage, every chromosome is held in place by attachments from two spindles, one from each pole.

Anaphase

In anaphase, the centromeres divide and the sister chromatids separate, becoming individual chromosomes.
The chromosomes are pulled toward opposite poles as the spindle fibers shorten (depolymerization of microtubules at the kinetochore ends). This process is often described as reeling in a fish by shortening the fishing line.
As a result, each pole ends up with a complete set of chromosomes.

Telophase

The chromosomes arrive at the poles and begin to de-condense, relaxing back into a thread-like chromatin form.
The mitotic spindles are depolymerized into tubulin monomers that will be reused for cytoskeletal components in the daughter cells.
Nuclear envelopes reform around the chromosomes, and nucleosomes reappear within the newly forming nuclei.

Cytokinesis

Cytokinesis is the final stage of cell division, dividing the cytoplasm to form two separate daughter cells.
In animal cells, a contractile actin ring creates a cleavage furrow that constricts the cell membrane at the cell’s equator, ultimately splitting the cytoplasm.
In plant cells, vesicles coalesce at the center to form a cell plate, which develops into a separating cell membrane and, eventually, a separating cell wall between the two daughter cells.
The process results in two genetically identical daughter cells (assuming no mutations occurred during DNA replication).

Key terms and concepts

Chromatin: uncondensed form of DNA during interphase.
Chromosome: condensed DNA that appears X-shaped during mitosis; consists of two sister chromatids held at the centromere.
Sister chromatids: two identical copies of a chromosome connected at the centromere.
Centromere: region of a chromosome where sister chromatids are held together and where kinetochores attach.
Kinetochores: protein structures at the centromere that attach to spindle microtubules.
Spindle: microtubule-based structure that segregates chromosomes during mitosis.
Centrosome: organizing center for the spindle; in animal cells it contains centrioles.
Cleavage furrow: indentation formed during cytokinesis in animal cells due to contractile actin filaments.
Cell plate: precursor to the separating cell wall and membrane in plant cytokinesis.

Review questions (based on the transcript)

Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.
Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.
Explain what happens during cytokinesis in an animal cell.
What are the main differences between mitosis and cytokinesis?
The familiar X-shaped chromosome represents:
A. How DNA always looks in eukaryotic cells
B. How DNA in eukaryotic cells looks once it is replicated and the cell is about to divide
C. Female sex chromosomes only
D. How DNA appears immediately after cytokinesis
Which of the following is not part of a chromosome in eukaryotic cells?
A. Centriole
B. Centromere
C. Chromatid
D. DNA
What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?
Put the following processes in order of when they occur during cell division, from first to last:
A. separation of sister chromatids
B. DNA replication
C. cytokinesis
D. lining up of chromosomes in the center of the cell
E. condensation and coiling of DNA into a chromosome
Why do you think the nuclear envelope breaks down at the start of mitosis?
What are the fibers made of microtubules that attach to the centromeres during mitosis called?
True or False: Chromosomes begin to uncoil during anaphase.
True or False: During cytokinesis in animal cells, sister chromatids line up along the equator of the cell.
True or False: After mitosis, the result is typically two daughter cells with identical DNA to each other.

Connections to broader concepts

Mitosis ensures equal distribution of a complete set of genetic material to two daughter nuclei, which is essential for growth, tissue repair, and asexual reproduction in many organisms.
Errors in spindle attachment, chromosome cohesion, or cytokinesis can lead to aneuploidy or other genetic abnormalities, highlighting the importance of precise regulation of the mitotic machinery.
The differences between animal and plant cytokinesis reflect structural differences in cell architecture (presence/absence of a rigid cell wall) and show how cellular context shapes the mechanics of division.

Quick recall and formulas (conceptual)

After replication: each chromosome consists of two sister chromatids held at the centromere. A minimal representation:
\text{Chromosome}{\text{post-replication}} = \text{Sister Chromatid}1 + \text{Sister Chromatid}_2 \quad \text{(held at centromere)}
The four mitotic phases can be summarized as a sequence: \text{Prophase} \rightarrow \text{Prometaphase} \rightarrow \text{Metaphase} \rightarrow \text{Anaphase} \rightarrow \text{Telophase} \rightarrow \text{Cytokinesis}.
In cytokinesis, the cytoplasm splits to form two daughter cells, which in animal cells is driven by a contractile actin ring forming a cleavage furrow, and in plant cells by a cell plate formation.

Attributions (context from the source)

The content is adapted from CK-12’s 7.3: Mitotic Phase - Mitosis and Cytokinesis, including figures and descriptions of prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis in both animal and plant cells.