Cell cycle and division - Biology - HKDSE

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Study Notes: Cell Cycle and Division

Hey everyone! Welcome to your study notes for "Cell cycle and division". Don't worry if this topic seems a bit tricky at first. We're going to break it down together, step-by-step. Think about it: you started as a single cell, and now you're made of trillions of them! How did that happen? Through cell division! Understanding this process is key to understanding growth, healing, and even how life continues from one generation to the next. Let's get started!

1. The Importance of Cell Division

Why do cells need to divide? It's one of the most fundamental processes of life! Cell division is essential for:

Growth: A baby grows into an adult because its cells divide, increasing the total number of cells in the body.
Repair and Replacement: When you get a cut, cell division creates new skin cells to heal the wound. Your body is also constantly replacing old cells, like red blood cells.
Reproduction:
- For some simple organisms, dividing is their way of reproducing (asexual reproduction).
- For complex organisms like us, a special type of cell division produces sperm and egg cells (gametes) needed for sexual reproduction.

Key Takeaway

Cell division is how living things grow, heal, and reproduce. It's the process of one parent cell dividing to form two or more daughter cells.

2. The Cell Cycle: A Cell's Life Story

A cell doesn't just divide randomly. It goes through a carefully regulated sequence of events called the cell cycle. Think of it as a cell's "life story", from the moment it's formed until it divides itself.

The cell cycle has two main parts:

1. Interphase (The 'Getting Ready' Stage):

This is the longest part of the cycle. The cell isn't dividing yet; instead, it's busy growing and preparing for division. It's doing its normal job, like a muscle cell contracting or a pancreas cell making insulin.

Cell Growth: The cell gets bigger and makes more organelles (like mitochondria and ribosomes).
DNA Replication: This is super important! The cell makes an exact copy of its DNA. Before replication, a chromosome is a single strand. After, it consists of two identical sister chromatids joined together.

2. Cell Division (The 'Splitting' Stage):

This is when the cell actually divides. It happens in two steps:

Nuclear Division (Mitosis or Meiosis): The nucleus and the copied DNA inside it are divided.
Cytoplasmic Division (Cytokinesis): The cytoplasm splits, forming two separate cells.

Quick Review Box

Cell Cycle = Interphase + Cell Division
Interphase: Cell grows and copies its DNA.
Cell Division: Nucleus divides, then cytoplasm divides.

3. Nuclear Division Part 1: Mitosis (The Cloning Division)

Mitosis is the process that your body cells (somatic cells) use to divide. Its goal is to create two genetically identical daughter cells from one parent cell.

Analogy: Think of mitosis as a biological photocopy machine. It takes one cell and makes two perfect copies.

Why is Mitosis important?

It's responsible for growth (making more body cells) and repair (replacing damaged cells). It's also the basis of asexual reproduction in some organisms.

The Stages of Mitosis (Remember: PMAT)

Don't be intimidated by the names! We can use a simple mnemonic: P-M-A-T.

Prophase (Prepare)

The long, stringy DNA (chromatin) condenses and coils up into visible, X-shaped chromosomes. Each chromosome consists of two identical sister chromatids.
The membrane around the nucleus breaks down.
Spindle fibres (special protein threads) start to form.

Metaphase (Middle)

The chromosomes line up single file along the middle (the equator) of the cell.
The spindle fibres attach to the centre of each chromosome.

Anaphase (Apart)

The spindle fibres pull the sister chromatids apart.
The separated chromatids (now considered individual chromosomes) move to opposite ends (poles) of the cell.

Telophase (Two)

The chromosomes arrive at opposite poles and start to uncoil.
A new nuclear membrane forms around each of the two sets of chromosomes.
Now, there are two new, identical nuclei in one cell.

Right after Telophase, cytokinesis (cytoplasmic division) happens, splitting the cell into two, completing the division.

Key Takeaway for Mitosis

Mitosis produces two daughter cells that are genetically identical to the parent cell. Each daughter cell has the same number of chromosomes as the parent (it's diploid, written as 2n). This is essential for growth and repair.

4. Nuclear Division Part 2: Meiosis (The Reproduction Division)

Meiosis is a special type of division used to produce gametes (sperm and egg cells). Its goal is to create four genetically different daughter cells with half the number of chromosomes as the parent cell.

Analogy: Think of meiosis like shuffling a deck of cards and then dealing half to four different players. Each player gets a unique hand with half the number of cards.

Why is Meiosis important?

It's essential for sexual reproduction. By halving the chromosome number, it ensures that when a sperm fertilises an egg, the resulting zygote has the correct, full number of chromosomes. It also creates genetic variation, which is why siblings look different from each other!

The Stages of Meiosis

Meiosis is like "Mitosis x 2", but with some very important differences. It involves two rounds of division: Meiosis I and Meiosis II.

Meiosis I (The "Reduction Division" - Separating Homologous Pairs)

This is where the chromosome number gets halved. Remember, you get one set of chromosomes from your mum and one from your dad. These pairs are called homologous chromosomes.

Prophase I: Chromosomes condense. Crucially, homologous chromosomes pair up. This pairing is unique to meiosis!
Metaphase I: The pairs of homologous chromosomes line up in the middle of the cell. (Contrast with Mitosis: In mitosis, they line up single file!)
Anaphase I: Spindle fibres pull the homologous pairs apart. One chromosome from each pair goes to an opposite pole. The sister chromatids are NOT separated yet.
Telophase I & Cytokinesis: Two new cells are formed. Each cell now has only half the number of chromosomes as the original parent cell. These cells are now haploid (written as n).

Meiosis II (The "Mitotic-like Division" - Separating Sister Chromatids)

This second division is very similar to mitosis. The two haploid cells from Meiosis I divide again.

Prophase II: Chromosomes become visible in each of the two cells.
Metaphase II: Chromosomes line up single file in the middle of each cell.
Anaphase II: Spindle fibres pull the sister chromatids apart.
Telophase II & Cytokinesis: The cells divide, resulting in a total of four haploid daughter cells. Each one is genetically unique.

Did You Know?

The way the homologous pairs line up in Metaphase I is completely random. For humans, with 23 pairs of chromosomes, this random arrangement alone allows for over 8 million different combinations of chromosomes in the gametes! This is a major source of genetic variation.

Key Takeaway for Meiosis

Meiosis starts with one diploid (2n) cell and produces four genetically different haploid (n) daughter cells (gametes). It involves two rounds of division and the key event of pairing and separating homologous chromosomes.

5. Mitosis vs. Meiosis: The Ultimate Comparison

This is a very common exam topic! Let's put everything side-by-side to make it crystal clear.

Purpose:
Mitosis: Growth, repair, asexual reproduction.
Meiosis: Production of gametes for sexual reproduction.

Number of Divisions:
Mitosis: One.
Meiosis: Two (Meiosis I and Meiosis II).

Pairing of Homologous Chromosomes:
Mitosis: No, chromosomes line up single file.
Meiosis: Yes, in Prophase I and Metaphase I.

Number of Daughter Cells:
Mitosis: Two.
Meiosis: Four.

Chromosome Number of Daughter Cells:
Mitosis: Diploid (2n) - same as parent cell.
Meiosis: Haploid (n) - half of parent cell.

Genetic Makeup of Daughter Cells:
Mitosis: Genetically identical to parent cell ("clones").
Meiosis: Genetically different from parent cell and from each other.

Common Mistake to Avoid

Students often mix up what separates in Anaphase I vs. Anaphase II.

Anaphase I (Meiosis): Homologous chromosomes separate.
Anaphase (Mitosis) & Anaphase II (Meiosis): Sister chromatids separate.

You've made it to the end! This chapter covers the core processes of how life grows and continues. Go over the PMAT stages for both divisions, and make sure you are 100% clear on the differences in the comparison table. You can do this!