Biology (9610) | Cells divide by binary fission and mitosis

Cells Divide by Binary Fission and Mitosis: Study Notes (Biology 9610)

Welcome! This chapter is fundamental to understanding how life works, specifically how organisms grow, repair damage, and sometimes, how disease like cancer begins. Cells dividing is the core mechanism of life.

We will explore two main ways cells split: the simple process used by bacteria (binary fission) and the complex, tightly controlled process used by our own body cells (mitosis). Understanding these processes is key to grasping the concepts of growth and disease within the section Biological Systems and Disease.

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1. The Eukaryotic Cell Cycle (3.2.10.1)

The Cell Cycle is the highly regulated sequence of events that a eukaryotic cell goes through from one division to the next. It’s like a cell’s life timeline, broken into two major phases: Interphase (preparation) and Mitotic phase (division).

Interphase: The Preparation Phase

Interphase is the longest part of the cell cycle, where the cell prepares for division. Think of it as the cell doing all its chores and doubling up on supplies before a major trip.

Interphase is divided into three distinct sub-phases:

1. G₁ Phase (Gap 1):
   • The cell grows significantly in size.
   • It increases its production of proteins and enzymes.
   • It increases the number of organelles, such as mitochondria and ribosomes.
   • Memory Aid: G₁ stands for Growth or Gap 1.
2. S Phase (Synthesis):
   • This is the critical stage where the DNA is replicated.
   • Every chromosome is duplicated, resulting in two identical copies (sister chromatids) joined at the centromere.
   • If DNA replication fails, the cell cannot divide correctly.
3. G₂ Phase (Gap 2):
   • The cell continues rapid growth and protein synthesis.
   • It checks the duplicated DNA for any errors or damage that occurred during the S phase.
   • The cell ensures all necessary components for mitosis (like spindle proteins) are ready.

2. Mitosis: Division for Growth and Repair (3.2.10.2)

Mitosis is the process by which a single eukaryotic cell divides to produce two genetically identical daughter cells. It is used for:

• Growth (e.g., increasing height).
• Repair (e.g., healing a cut).
• Asexual Reproduction (in some organisms).

The Four Stages of Mitosis (PMAT)

Don't worry if these stages seem confusing at first! Just focus on what the chromosomes are doing in each step. A great mnemonic is PMAT.

Prerequisite Concept: Chromosomes

Before mitosis begins (during Interphase), DNA exists as loosely organised chromatin. Once replicated in S phase, the DNA condenses into visible, X-shaped structures called chromosomes. Each X-shaped chromosome consists of two identical strands, called sister chromatids, joined at the centromere.

1. P - Prophase
• The chromatin condenses and coils tightly, becoming visible as discrete chromosomes.
• The nuclear envelope (membrane) breaks down.
• The centrioles move to opposite poles of the cell, and the spindle fibres (made of microtubules) begin to form.
2. M - Metaphase
• The chromosomes line up along the centre of the cell—this central line is called the metaphase plate (or equator).
• Each chromosome attaches to the spindle fibres via its centromere. The spindle fibres ensure they are perfectly aligned for the next step.
3. A - Anaphase
• This is the dramatic separation stage!
• The centromeres divide, separating the sister chromatids.
• The spindle fibres pull these now-separate chromatids (now considered individual chromosomes) towards opposite poles of the cell.
• Key point: Anaphase ensures each new cell gets an identical set of genetic information.
4. T - Telophase
• The chromosomes arrive at the poles and decondense (unravel) back into chromatin.
• New nuclear envelopes form around the two sets of chromosomes.
• The spindle fibres disappear.

Cytokinesis

After telophase, the cell’s cytoplasm divides in a process called cytokinesis. This results in the final separation of the cell into two identical daughter cells.

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3. Binary Fission: Prokaryotic Division (3.2.10.3)

Binary fission is the method of asexual reproduction used by prokaryotic cells (like bacteria). It is much simpler and faster than mitosis because prokaryotes lack a nucleus and membrane-bound organelles.

Steps of Binary Fission

Prokaryotic cell division happens as follows:

1. Replication of Genetic Material:
   • The single, large, circular DNA molecule replicates.
   • Any smaller, extra-chromosomal DNA molecules (plasmids) also replicate.
2. Cell Elongation and Separation:
   • The cell grows and elongates.
   • The two copies of the circular DNA move to opposite poles of the cell.
3. Cytoplasm Division:
   • The cell membrane pinches inwards, and a new cell wall forms between the two DNA molecules.
   • This produces two daughter cells.

Each daughter cell receives one copy of the circular DNA. However, the number of plasmids inherited by each daughter cell may vary.

Horizontal Gene Transmission (Conjugation)

Sometimes, bacteria don't just pass DNA vertically (from parent to offspring). They can also transfer genetic material horizontally:

• Conjugation is the process where DNA (often plasmid DNA) is passed from one species of bacterium to another, usually through a connection tube called a pilus.
• This is very important for the rapid spread of genes, such as antibiotic resistance genes, within bacterial populations.

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4. Mitosis, Mutations, and Cancer (3.2.11.2)

Mitosis is normally a highly regulated process. When this regulation breaks down due to mutations, the result can be uncontrolled cell division, leading to cancer.

Control Mechanisms of the Cell Cycle

The cell cycle is controlled by specific genes that act as checkpoints to ensure DNA integrity and proper division:

1. Proto-oncogenes (The Accelerator):
   • These genes normally stimulate cell division, promoting growth and proliferation.
2. Tumour Suppressor Genes (The Brakes):
   • These genes normally slow cell division, repair damaged DNA, or induce programmed cell death (apoptosis) if the damage is too severe.

How Cancer Develops

Cancer is often caused by the accumulation of mutations in these regulatory genes, leading to a loss of control over cell division.

1. Mutated Proto-oncogene (Oncogene):

• A mutation turns a proto-oncogene into a hyperactive gene called an oncogene.
• The oncogene permanently "steps on the accelerator," causing the cell to divide too quickly, even when it shouldn't.

2. Mutated Tumour Suppressor Gene:

• A mutation inactivates the tumour suppressor gene.
• The cell loses its "brakes" and its ability to check for or repair DNA damage, allowing the rate of cell division to increase uncontrollably.

Tumours: Benign vs. Malignant

Uncontrolled cell division results in a mass of cells called a tumour:

• Benign Tumours: These tumours remain compact and are usually harmless. They stay in one place and do not invade surrounding tissues or spread to other parts of the body.
• Malignant Tumours (Cancer): These are aggressive. They grow rapidly, invade surrounding tissues, and can spread to other parts of the body via the bloodstream or lymphatic system (a process called metastasis).