MYP Integrated Sciences Cell Division Notes

Why Do Cells Divide Instead of Just Growing Bigger?

1. The short answer is there are many fundamental problems that arise when cells get too large.
2. Many of which, get solved with cell division.

Larger cells have diffusion problems

1. Nutrients and oxygen must diffuse from the surface to the center, while waste products must diffuse from the center to the surface.
2. As cells get bigger, this distance increases.
3. The center of a large cell would be too far from the surface, making diffusion too slow.

Surface area doesn't keep up with volume

1. When a cell doubles in width, its surface area increases by 4 times but its volume increases by 8 times.
2. The cell needs more resources (volume) but has proportionally less membrane (surface area) to get them through.
3. This is known as the surface area to volume ratio (SA:V)

The nucleus can't control a huge cell

1. The nucleus produces instructions (mRNA) for the whole cell.
2. In a very large cell, these instructions take too long to reach distant parts.
3. The nucleus becomes overwhelmed trying to manage too much cytoplasm.

Dividing solves all these problems

1. Two small cells have more total surface area than one large cell of the same volume.
2. Each small cell has a short diffusion distance from surface to center.
3. Each nucleus only has to control a manageable amount of cytoplasm.

How Do Cells Know When It's Time to Divide?

1. Cells follow a controlled cycle known as the cell cycle, alternating between two phases: growth and division.
2. During interphase (growth phase), the cell gets larger and copies its DNA.
3. During division, the cell splits into two.
4. As a cell divides, it produces two small daughter cells.
5. Most of a cell's life is spent in interphase, doubling in size.
6. Once large enough, they divide again, producing small cells.
7. The cycle repeats: divide → grow → divide → grow.

Why can't cells divide and grow at the same time?

1. During growth (interphase), DNA must be unpacked and spread out so it can be copied.
2. During division on the other hand, DNA must be tightly packaged into chromosomes so it can be separated.
3. These two states are incompatible, DNA can't be both packed and unpacked simultaneously.

Cells divide when they receive the right signals

1. Growth factors - chemical signals from other cells.
2. Nutrient availability - cells need sufficient nutrients to support division.
3. Cell damage - damaged tissues release signals triggering nearby cells to divide.

Different cells divide at different rates

1. Frequently dividing cells: skin cells (every 2-3 weeks), intestinal lining cells (every 3-5 days), blood cells (continuously).
2. Rarely dividing cells: liver cells (only when needed for repair), muscle cells (rarely after childhood).
3. Never dividing cells: nerve cells and heart muscle cells (lost forever if damaged).

How Does a Cell Make Sure Each New Cell Gets the Same DNA?

1. Mitosis is the precise process that ensures accurate DNA copying and distribution.
2. This is key as any discrepancy will likely cause the cell to die or malfunction.

What Are the Stages of Mitosis?

1. Mitosis is divided into five distinct stages.
2. Each stage has specific events that must happen in order to successfully divide the cell.

1. Prophase

1. Chromosomes condense:
   1. They shorten and thicken through a coiling process.
   2. Each chromosome is a double structure because the DNA has been replicated.
   3. Chromosomes are still inside the nuclear membrane but becoming visible.
2. The spindle apparatus forms:
   1. Centrioles move to opposite poles of the cell (in animal cells).
   2. Spindle fibers begin forming between the poles.
3. The nuclear membrane breaks down:
   1. This allows spindle fibers to access the chromosomes.

2. Metaphase

1. Chromosomes line up at the center:
   1. The nuclear membrane has disappeared.
   2. Chromosomes are pulled into a line at the equator.
   3. Spindle fibers link the chromosomes to the poles of the cell.
   4. Each chromosome still contains two identical DNA molecules.

3. Anaphase

1. Chromosomes are pulled apart:
   1. Spindle fibers pull each chromosome apart.
   2. The identical DNA molecules separate and move to opposite poles.
   3. Each chromosome now contains just one DNA molecule.

4. Telophase

1. Nuclear membranes reform:
   1. Chromosomes reach the poles.
   2. Nuclear membranes form around each set of chromosomes.
   3. The DNA uncoils and spreads out.
   4. The cell can now divide into two identical cells.

5. Cytokinesis

1. The cytoplasm divides:
   1. In animal cells: A cleavage furrow forms and the membrane pinches inward until the cell splits.
   2. In plant cells: A cell plate forms down the middle, building new cell wall material until two cells are separated.
2. Two daughter cells are produced:
   1. Each has an identical nucleus with the same DNA.
   2. The cells are now in interphase where cell activities happen and the cells start to enlarge.

Note

• Although mitosis is an incredibly precise process, errors can slip through.
• When this happens, sometimes:
  • Both copies of a chromosome go to the same daughter cell (non-disjunction).
  • One cell ends up with an extra chromosome (Down Syndrome is caused by having an extra 21st chromosome)
  • Cells with wrong chromosome numbers usually die.

How Does Meiosis Create Gametes With Half the Usual DNA?

1. Meiosis and mitosis both involve cell division, but they solve different biological problems.
2. Mitosis is used for growth and repair, so it makes identical copies of cells with the same chromosome number.
3. Meiosis is used for sexual reproduction, so it must:
   1. halve the chromosome number (haploid gametes)
   2. generate genetic variation
4. Because of this, meiosis includes two divisions and has extra steps such as homologous pairing and crossing over that never occur in mitosis.

Why Does Meiosis Need Two Divisions?

1. Human body cells are diploid.
2. Gametes must be haploid so that fertilisation restores the original diploid number.
3. One division cannot achieve this reduction accurately, which is why meiosis splits chromosomes in two steps:
   1. Meiosis I halves the chromosome number
   2. Meiosis II separates chromatids to avoid duplication
4. This structure ensures every gamete carries one complete set of chromosomes, but in a new combination.

Meiosis I: Reducing Chromosome Number

1. Meiosis I is where the main difference from mitosis happens.
2. Instead of splitting sister chromatids, the cell splits homologous chromosome pairs.
3. Meiosis I also includes the two events that create most genetic variation: crossing over and independent assortment.
4. Once Meiosis I is complete, the cell is haploid, even though chromatids are still attached.

Prophase I

1. Chromosomes condense so they can be moved without breaking.
2. Homologous chromosomes pair up, which is essential because meiosis separates parental versions into different gametes.
3. Crossing over occurs.
4. Sections of DNA swap between homologues.
5. This produces new genetic combinations, increasing variation before the cell even divides.
6. Nuclear membrane breaks down and spindle fibres form.

Metaphase I

1. Homologous pairs randomly line up side-by-side along the centre.
2. This randomness generates independent assortment, which increases variation.
3. Spindle fibres attach to each homolog.

Anaphase I

1. Homologous chromosomes are pulled apart to opposite poles.
2. Sister chromatids remain together.
3. This is the exact moment the cell becomes haploid.

Telophase I and Cytokinesis

1. Chromosomes will reach opposite ends, and the nuclear membranes may briefly reform.
2. The cell divides into two haploid cells, with each chromosome still containing two chromatids, which will separate in Meiosis II.

Meiosis II: Separating Sister Chromatids

1. Meiosis II works like a mitosis-style division, but it starts from haploid cells, not diploid ones.
2. Because the chromatids stayed together during Meiosis I, they must now be separated so each gamete ends up with one chromatid per chromosome.
3. This step does not change chromosome number. It simply refines the distribution to prevent duplication errors.
4. Once Meiosis II finishes, the cell has completed the process of forming four unique haploid gametes.

Prophase II

1. Chromosomes re-condense if they had loosened.
2. Nuclear membranes break down again.
3. New spindle fibres form.
4. This division ensures chromatids are split evenly.

Metaphase II

1. Chromosomes line up single file at the centre.
2. Each still consists of two sister chromatids.
3. This stage looks similar to mitosis, but the starting cells are haploid, not diploid.

Anaphase II

1. Sister chromatids finally separate.
2. They move to opposite poles.
3. Each chromatid becomes an independent chromosome.
4. This ensures each gamete receives one copy of each chromosome.

Telophase II and Cytokinesis

1. Nuclear membranes reform around each set of chromatids.
2. The cells split.
3. Four genetically different haploid gametes are produced.
4. These differences arise from crossing over and independent assortment.

Why Does Meiosis Creates Variation?

1. Crossing over shuffles alleles within chromosomes.
2. Independent assortment shuffles whole chromosomes.
3. These processes operate before fertilisation even happens.
4. When a gamete meets another gamete, variation increases further through random fertilisation.
5. This continuous reshuffling helps populations adapt.