Mitosis (AQA GCSE Biology Combined Science): Revision Notes
Mitosis
Mitosis is a type of cell division that creates two identical cells from one parent cell. This process is essential for growth and repair in living organisms.
What are chromosomes?
Your cells contain a nucleus which holds chromosomes. These chromosomes are made of DNA molecules that carry thousands of genes. In body cells, chromosomes come in pairs - this is called being diploid.
Understanding Diploid: The term "diploid" means having two complete sets of chromosomes. Humans have 46 chromosomes arranged in 23 pairs, making our body cells diploid. This is different from gametes (sex cells) which are haploid and contain only one set of chromosomes.
The cell cycle
The cell cycle has three main stages that happen in order:
Stage 1: Interphase
- This is the longest stage of the cell cycle
- The cell grows bigger
- The cell makes more structures like ribosomes and mitochondria
- Most importantly, the cell copies its DNA to make two identical copies of each chromosome
- The cell spends most of its life in this stage
Critical Point: DNA copying during interphase is absolutely essential for mitosis to work correctly. If DNA isn't copied properly, the daughter cells won't be genetically identical to the parent cell, which can lead to serious problems.
Stage 2: Mitosis
- One set of chromosomes gets pulled to each end of the cell
- The nucleus divides into two separate nuclei
Stage 3: Cell division
- This happens straight after mitosis
- The cell membrane and cytoplasm split
- Two identical cells are formed
Parent cell and daughter cells
The mitosis process follows a clear pattern that ensures genetic consistency:
Worked Example: Following Chromosomes Through Mitosis
Step 1: Parent cell preparation
- Parent cell is diploid (e.g., 46 chromosomes in humans)
- Each chromosome gets copied during interphase
Step 2: Division process
- Chromosomes line up in the middle of the cell
- One copy of each chromosome moves to each end
Step 3: Final result
- Two daughter cells are formed
- Each daughter cell has 46 chromosomes (same as parent)
- All cells are genetically identical
The original cell that divides is called the parent cell. The parent cell is diploid (has two sets of chromosomes). Before dividing, each chromosome is copied exactly. When the cell divides, it creates two daughter cells. Each daughter cell gets one copy of every chromosome, making the daughter cells genetically identical to each other and to the parent cell. The daughter cells are also diploid.
Key facts to remember
Here are three important points about mitosis that will help you remember the process:
Memory Aids for Mitosis:
- mi-To-sis makes Two cells - one parent cell becomes two daughter cells
- mi-tosis makes cells that are genetically Identical - all the cells have exactly the same DNA
- Diploid means Double - body cells have two sets of chromosomes
Why is mitosis important?
Mitosis serves several crucial functions in living organisms. The process is used to make new cells for:
- Growth - making your body bigger as you develop
- Repair - replacing damaged or dead cells
- Asexual reproduction - some organisms use mitosis to create offspring
Real-world Application: Every time you get a cut that heals, mitosis is working to replace the damaged skin cells. Similarly, as you grow from a child to an adult, mitosis is constantly creating new cells to increase your body size.
Key Points to Remember:
- Mitosis creates two genetically identical cells from one parent cell
- The cell cycle has three stages: interphase, mitosis, and cell division
- Interphase is the longest stage where DNA gets copied
- Daughter cells have the same number of chromosomes as the parent cell
- Mitosis is essential for growth and repair in your body