【Science: Grade 9】Continuity of Life 〜 The Baton Pass of Life 〜

Hello! In this chapter, we’re going to explore the "Continuity of Life." Ever wonder, "Why do children look like their parents?" or "How does our body grow?" Let’s uncover the mysteries surrounding our own lives.
Some of the terms might sound a bit tricky at first, but don't worry! We’ll take it one step at a time using relatable examples.

1. Growth of Organisms and Cell Division

We keep growing from the time we are babies because the "number of cells in our body is increasing." The process where one cell divides into two is called cell division.

Steps of Cell Division

Among types of cell division, the one involved in bodily growth is called mitosis. It follows these steps:

1. String-like structures called chromosomes appear inside the nucleus.
2. The chromosomes line up in the center of the cell.
3. The chromosomes split into two and move to opposite ends of the cell.
4. Two nuclei form, the cytoplasm splits, and two new cells are created.

【Key Point!】
Just before cell division, the chromosomes are copied (replicated). Because of this, the two resulting cells hold the exact same blueprint as the original cell.

💡 Fun Fact: Differences in Growth
In animals, cell division occurs throughout the whole body, but in plants, it happens actively only in specific areas called meristems (such as the tips of roots and stems). This is a popular target for questions on onion root observation tests!

2. How Organisms Reproduce (Reproduction)

The process by which organisms produce new individuals of the same species is called reproduction. It is broadly divided into two methods.

① Asexual Reproduction

A method where one parent produces new individuals on its own. No fertilization occurs.
・Fission: The body splits into two (e.g., amoebas, paramecia)
・Budding: The organism grows by producing a "bud" from part of its body (e.g., hydras, yeast)
・Vegetative Reproduction: New individuals grow from the stems or roots of a plant (e.g., potatoes, strawberry runners)

【Characteristics】
Because they simply copy the parent's blueprint, the offspring are clones with identical genes to the parent.

② Sexual Reproduction

A method where both a male and female are involved to produce new individuals. Special cells called gametes fuse together.

・Animals: Sperm + Egg = Zygote
・Plants: Sperm cell + Egg cell = Zygote

【Meiosis】
This is a special type of division that occurs only when producing gametes. The number of chromosomes is reduced by half. Why cut them in half?
Example: Father (half) + Mother (half) = Offspring (1 full set)
If we didn't cut them in half, the number of chromosomes would double every generation, which would be a huge problem! It’s a clever way to keep the balance of life in check.

★ Important!
In sexual reproduction, since the offspring receive half of their blueprint from each parent, they end up with unique traits (diversity) that differ from their parents. This diversity is a major strength for survival.

3. Principles of Heredity and Genes

The process by which a parent's traits (shape or characteristics) are passed on to their offspring is called heredity. This was scientifically unlocked by Mendel, who is famous for his research on pea plants.

Dominant and Recessive Traits

When you cross-breed opposing traits (e.g., round seeds vs. wrinkled seeds), the trait that appears in the offspring is called the dominant trait, and the one that is hidden is the recessive trait.

【Mechanism of Heredity (Model)】
If we label the "round" gene as "A" and the "wrinkled" gene as "a," the combinations for the pea plants look like this:

1. Parent: AA (Round) × aa (Wrinkled)
2. Gametes: A and a
3. Offspring (First Generation): Aa (All turn out Round because "A" is stronger.)
4. When you cross the offspring (Aa × Aa), the second generation results in the following ratio:

\( AA : Aa : aa = 1 : 2 : 1 \)
The physical characteristics (phenotype) result in a ratio of Round : Wrinkled = 3 : 1!

⚠️ Common Mistake
It's easy to mistakenly think that "recessive (wrinkled) traits are weak or flawed," but that isn't true at all! It just means it is "hidden and less likely to appear." If two recessive genes (aa) come together, the wrinkled trait will definitely show up.

The Essence of Genes: DNA

The core of genetic information found within chromosomes is called DNA (deoxyribonucleic acid). In recent years, studying this DNA has become essential for medical diagnosis and agricultural breeding.

🌟 Summary of This Chapter

1. Mitosis: Copying and increasing identical cells for growth.
2. Reproduction: Asexual reproduction creates clones; sexual reproduction creates a mix (diversity).
3. Meiosis: Reducing the number of chromosomes by half when making gametes.
4. Mendel's Laws: In the second generation, traits appear in a 3:1 ratio.
5. DNA: The master file of genetic information contained in chromosomes.

"The Continuity of Life" is like a miraculous relay race that has continued uninterrupted for billions of years. The DNA flowing through you is a part of that long history. Doesn't that make studying science a little more exciting?
Next time, we’ll use this knowledge of genetics to think about "Evolution!"