The Cell Cycle & Mitosis: Sixth Grade Science
From a single microscopic fertilized egg cell, trillions of specialized cellular descendants multiply through precise rounds of division to build, repair, and maintain living organisms.
The Continuity of Life Through Cell Division
If you scrape your knee on a pavement sidewalk, your body loses thousands of skin cells. Yet within days, new tissue forms, knitting the wound together until the scrape vanishes. Similarly, a tiny acorn planted in soil gradually multiplies into a towering oak tree weighing several tons. How do living organisms grow larger, heal injuries, and replace worn-out tissues? The answer lies in cell division. Cell division is the biological process by which a parent cell divides to generate two or more daughter cells. In eukaryotic somatic (body) cells, this division is governed by an exquisitely coordinated sequence of events known as the cell cycle and mitosis, ensuring that every newly created daughter cell receives an exact, identical copy of the parent cell's genetic blueprints.
THE EUKARYOTIC CELL CYCLE
MITOSIS (M Phase: ~10%)
Prophase
Metaphase
Anaphase
Telophase
+ Cytokinesis
^
|
[ G2 Phase ] ---------+--------- [ G1 Phase ]
Growth & Final Primary Growth &
Prep for Division Normal Metabolism
^ |
\ /
'------- [ S Phase ] --------'
DNA Synthesis
(Replication!)
<===== INTERPHASE (~90%) =====>
The Eukaryotic Cell Cycle: An Overview
The cell cycle is an orderly, repeating sequence of growth, DNA replication, and cellular division that characterizes the life of a eukaryotic cell. The cell cycle is divided into two major phases: Interphase (which accounts for roughly 90 percent of the cell's lifespan) and the M Phase (Mitosis and Cytokinesis, accounting for the remaining 10 percent).
Interphase: Growth and Preparation
Interphase is an active, metabolic preparatory epoch divided into three sub-phases:
1. G1 Phase (First Gap / Growth): Directly following division, the newly formed daughter cell enters G1. The cell increases in physical size, manufactures new proteins, synthesizes additional organelles (such as mitochondria and ribosomes), and conducts its routine metabolic duties.
2. S Phase (Synthesis): During this critical stage, the cell replicates its entire nuclear genome. Every single chromosome is copied precisely, ensuring that two identical genetic duplicates exist before division begins.
3. G2 Phase (Second Gap / Growth): The cell undergoes a final period of rapid growth and protein synthesis, manufacturing specialized proteins (such as tubulin) needed to assemble the mitotic spindle apparatus. The cell checks its replicated DNA for errors before entering mitosis.
The Organization of Genetic Material
To understand how chromosomes divide, we must examine their physical structure:
- Chromatin: During interphase, DNA is uncoiled and diffuse, looking like loose tangled yarn under the microscope, allowing RNA polymerase enzymes to access genes for transcription.
- Chromosome: When preparing to divide, chromatin fibers coil, loop, and condense tightly around spool-like histone proteins into compact, visible X-shaped structures called chromosomes.
- Sister Chromatids: Following replication in the S phase, each chromosome consists of two identical copies called sister chromatids.
- Centromere: A constricted region where two sister chromatids are held together. On each side of the centromere sits a specialized protein complex called the kinetochore, where spindle fibers attach during mitosis.
STRUCTURE OF A REPLICATED CHROMOSOME
Sister Chromatid Sister Chromatid
\ /
\ /
\ /
\ /
'--[ ]--' <--- Centromere (with Kinetochores)
/ \
/ \
/ \
/ \
Identical copies of DNA!
The Four Stages of Mitosis (PMAT)
Mitosis is the division of the cell's nucleus, partitioned into four continuous, sequential stages easily remembered by the mnemonic PMAT: Prophase, Metaphase, Anaphase, and Telophase.
THE FOUR PHASES OF MITOSIS
PROPHASE METAPHASE ANAPHASE TELOPHASE
Chromosomes condense; Chromosomes line up Sister chromatids pull Two nuclei form;
Spindle forms. along equator. apart to poles. Cell begins cleavage.
.--------. .--------. .--------. .---..---.
/ /\ /\ \ / | \ / < > \ / || \
| (XX) (XX) | | ----(XX)---- | | (<) (>) | | (O) || (O) |
\ \/ \/ / \ | / \ < > / \ || /
'--------' '--------' '--------' '---''---'
Stage 1: Prophase
Prophase is the first and longest stage of mitosis, during which significant structural preparations occur:
- Chromatin fibers tightly condense into visible, individual chromosomes, each consisting of paired sister chromatids joined at the centromere.
- The nucleolus disappears from view.
- The nuclear envelope fragments and breaks down into small vesicles, exposing chromosomes to the cytoplasm.
- In animal cells, paired centrosomes (containing centrioles) begin migrating toward opposite poles of the cell, projecting microtubule filaments called spindle fibers to form the mitotic spindle.
- Spindle fibers attach to the kinetochores on both sides of each chromosome's centromere.
Stage 2: Metaphase
Metaphase (meaning middle phase) is characterized by precise alignment:
- The mitotic spindle fibers pull and tug the chromosomes until all chromosomes are aligned in a single file along the imaginary equator of the cell, known as the metaphase plate (or equatorial plane).
- A vital cell cycle checkpoint—the M-checkpoint (Spindle Assembly Checkpoint)—occurs here: the cell pauses until it verifies that every single kinetochore on every chromosome is securely tethered to spindle fibers from opposite poles. This prevents non-disjunction errors where daughter cells inherit incorrect numbers of chromosomes.
Stage 3: Anaphase
Anaphase (meaning upward or backward phase) is the shortest, most dramatic phase of mitosis:
- Enzymes cleave the cohesin proteins holding the sister chromatids together at the centromere.
- The sister chromatids suddenly separate, each now considered an individual, full-fledged daughter chromosome.
- Kinetochore spindle fibers shorten (depolymerize), pulling the daughter chromosomes rapidly toward opposite poles of the cell, centromeres leading the way in a V-shape.
- Simultaneously, non-kinetochore spindle fibers slide past each other and elongate, physically pushing the poles farther apart and elongating the entire cell.
Stage 4: Telophase
Telophase (meaning end phase) is essentially the reverse of prophase:
- The daughter chromosomes arrive at the opposite poles of the elongated cell and begin uncoiling, relaxing back into diffuse, thread-like chromatin.
- A new nuclear envelope reforms around each of the two sets of chromosomes, utilizing fragments of the parent cell's nuclear membrane.
- Nucleoli reappear within each newly formed nucleus.
- The mitotic spindle apparatus depolymerizes and breaks down. At this point, karyokinesis (nuclear division) is officially complete: the cell possesses two identical, genetically complete nuclei.
Cytokinesis: Dividing the Cytoplasm
While mitosis divides the genetic material within the nucleus, cytokinesis divides the cytoplasm and organelles, physically cleaving the single parent cell into two separate, independent daughter cells. Cytokinesis typically begins during late anaphase or telophase and finishes shortly after telophase ends.
CYTOKINESIS: ANIMAL VS. PLANT CELLS
ANIMAL CELL (Cleavage Furrow) PLANT CELL (Cell Plate)
Contractile Ring Cell Plate Vesicles
v v v v
.---. .---. +----+......+----+
/ (O) \ / (O) \ | (O)|:::::::|(O) |
| >< | | |:::::::| |
\ (O) / \ (O) / +----+......+----+
'---' '---' Rigid Wall prevents
Pinches from outside in! pinching; builds wall!
Cytokinesis in Animal Cells: The Cleavage Furrow
Because animal cells lack rigid cell walls and are bound only by a flexible plasma membrane, cytokinesis occurs through cleavage. A shallow groove called a cleavage furrow appears along the cell equator. Beneath the plasma membrane along this furrow, a contractile ring composed of actin microfilaments and myosin motor proteins forms. The ring contracts like a microscopic drawstring, pinching the plasma membrane inward until the cell is cleaved into two distinct daughter cells.
Cytokinesis in Plant Cells: The Cell Plate
Plant cells cannot pinch in half because they are encased in rigid, inflexible cellulose cell walls. Instead, cytokinesis in plants proceeds from the inside out:
- During telophase, the Golgi apparatus packages cellulose and pectin pectates into transport vesicles that travel along microtubules to the cell equator.
- The vesicles coalesce and fuse together to form a flattened disc called the cell plate.
- As more vesicles fuse, the cell plate expands outward until its edges merge with the existing parent cell walls and plasma membrane.
- The contents of the vesicles form a new primary cell wall and middle lamella, dividing the plant cell into two separate daughter cells.
Regulation of the Cell Cycle and Cancer
Cell division is not an unregulated, runaway process. In a healthy multicellular organism, cell division is controlled by molecular checkpoints:
- G1 Checkpoint: Determines if the cell is large enough, has sufficient energy reserves, and has intact DNA before committing to DNA replication.
- G2 Checkpoint: Ensures that all chromosomes were replicated completely and accurately without DNA damage before permitting mitosis.
- M Checkpoint: Verifies spindle attachment to all kinetochores at metaphase before triggering anaphase.
If a cell's DNA acquires mutations in regulatory genes (such as proto-oncogenes or tumor suppressor genes like p53), the cell may ignore checkpoints and divide uncontrollably. Runaway, unregulated cell division is the biological definition of cancer, leading to the formation of abnormal cell masses called tumors that can invade healthy tissues.
Exercises and Solutions
Exercise 1: Describe the three sub-phases of interphase, and explain why DNA replication in the S phase is critical for the success of mitosis.
Solution 1: Interphase consists of: (1) G1 phase (primary growth and organelle production), (2) S phase (synthesis, where DNA replication occurs), and (3) G2 phase (final growth and synthesis of division proteins like spindle fibers). DNA replication during the S phase is critical because each daughter cell requires a complete, identical set of genetic blueprints; without replicating DNA prior to division, the two resulting cells would receive only half the necessary genetic instructions, leading to non-viable cells.
Exercise 2: List the four stages of mitosis in correct sequential order, and summarize the key chromosomal event that occurs during each stage.
Solution 2: The four stages in order are: (1) Prophase, where chromatin condenses into visible, paired sister chromatid chromosomes and the nuclear envelope breaks down; (2) Metaphase, where spindle fibers align the chromosomes in a single file along the cell's equatorial metaphase plate; (3) Anaphase, where centromeres separate and sister chromatids are pulled apart to opposite poles as individual daughter chromosomes; and (4) Telophase, where chromosomes arrive at the poles, decondense back into chromatin, and two new nuclear envelopes reform.
Exercise 3: Contrast cytokinesis in animal cells with cytokinesis in plant cells, explaining why their mechanisms differ structurally.
Solution 3: In animal cells, cytokinesis occurs through a cleavage furrow, where a contractile ring of actin and myosin pinches the flexible plasma membrane inward from the outside until the cell divides. In plant cells, the presence of a rigid cellulose cell wall prevents pinching; instead, plant cells divide from the inside out by assembling a cell plate made of Golgi vesicles along the equator that fuse to construct a new dividing cell wall and plasma membrane between the daughter cells.