CORE Biology (9221) | Reproduction

Welcome to the Reproduction Chapter!

Hello future biologists! This chapter is super important because it connects everything we've learned about cells and DNA directly to how life continues. Reproduction is the process by which organisms create new individuals, ensuring that their species doesn't disappear.

Since this section is part of "Inheritance," we won't just look at anatomy; we will focus on how genetic information (DNA) is passed on and how different methods of reproduction affect the genetic variation in a population.

1. Two Ways to Make Life: Asexual vs. Sexual Reproduction

There are two fundamental strategies for reproduction in the living world. The main difference between them lies in how much genetic mixing happens.

1.1 Asexual Reproduction: The Cloning Method

Imagine you want to make an exact copy of a document. You use a photocopier! Asexual reproduction is similar—it creates exact genetic copies.

What is Asexual Reproduction?

• Only one parent is involved.
• There is no fusion of gametes (sex cells).
• The process relies on mitosis (simple cell division) to produce new cells.
• Offspring are genetically identical to the parent. They are called clones.

Quick Analogy: Asexual reproduction is like cloning a document on a photocopier. Every copy is exactly the same as the original.

Examples of Asexual Reproduction

• Bacteria: Divide rapidly by binary fission.
• Yeast: Reproduce by budding (a small outgrowth detaches and grows).
• Plants: Many plants reproduce asexually using stems, tubers, or runners (like strawberry plants sending out long stems that grow new plants). This is called vegetative propagation.

Advantages and Disadvantages of Asexual Reproduction

Advantages:

1. Speed: It’s very fast, allowing organisms to colonize new areas quickly.
2. Efficiency: Only one parent is needed (no need to find a mate).
3. Good Environment: If the environment is stable and perfect for the parent, all identical offspring will also thrive.

Disadvantages:

1. No Variation: The biggest problem! If the environment changes (e.g., a new disease appears), all genetically identical individuals will be vulnerable and could be wiped out.

1.2 Sexual Reproduction: The Mixing Method

Sexual reproduction is the mixing of genetic material from two different individuals to create new, unique offspring.

What is Sexual Reproduction?

• Usually involves two parents (male and female).
• It involves the production and fusion of specialised sex cells called gametes.
• The fusion of the male gamete and the female gamete is called fertilisation.
• Offspring are genetically varied (not identical to either parent).

Prerequisite Check: Gametes (like sperm and egg) are produced by a special type of cell division called meiosis, which halves the chromosome number. When they fuse during fertilization, the full number of chromosomes is restored.

The Key Step: Fertilisation

When the male and female gametes fuse, their nuclei combine. The resulting cell is called a zygote. This zygote then divides many times by mitosis to grow into a new, unique organism.

Advantages and Disadvantages of Sexual Reproduction

Advantages:

1. Genetic Variation: This is the huge benefit! Offspring are unique, giving the species a better chance of survival if the environment changes.
2. Evolution: Variation allows species to adapt and evolve over time, which is essential for long-term survival.

Disadvantages:

1. Time and Energy: Finding a mate and successfully reproducing takes a lot of time and energy.
2. Slower Rate: Reproduction is generally much slower than asexual reproduction.

2. Comparing Asexual and Sexual Reproduction

This is a popular comparison question in exams. Focus on the number of parents and the genetic outcome!

3. Sexual Reproduction in Flowering Plants

Plants use sexual reproduction to produce seeds, ensuring genetic mixing. The flower is the specialised organ for this process.

3.1 Structure of the Flower (The Reproductive Organs)

We only need to focus on the essential reproductive parts:

Female Parts (The Carpel or Pistil)

• Stigma: The sticky tip that receives the pollen.
• Style: The stalk connecting the stigma to the ovary.
• Ovary: Contains the ovules, which house the female gametes (egg cells). The ovary develops into the fruit after fertilisation.

Male Parts (The Stamen)

• Anther: Produces and stores the pollen grains (which contain the male gametes).
• Filament: The stalk that holds the anther up.

Memory Trick: Think of the Stamen as the MAN part. It contains the Anther.

3.2 The Process: Pollination

Pollination is the transfer of pollen (containing the male gamete) from the anther of one flower to the stigma of another (or the same) flower.

Types of Pollination

1. Self-pollination: Pollen moves from the anther to the stigma on the same flower or another flower on the same plant. (Lower genetic variation).
2. Cross-pollination: Pollen moves from the anther of one plant to the stigma of a different plant of the same species. (Higher genetic variation, preferred).

How Pollen is Moved

Plants have developed adaptations for two main methods of cross-pollination:

• Insect Pollination: Flowers are brightly coloured, scented, and produce nectar to attract animals (like bees). Pollen is often sticky.
• Wind Pollination: Flowers are typically dull, scentless, and small. They produce huge amounts of lightweight, dry pollen, and often have large, feathery stigmas to catch pollen floating in the air.

3.3 The Process: Fertilisation (in Plants)

After the stigma receives the pollen:

1. The pollen grain grows a tiny tube called the pollen tube down through the style toward the ovary.
2. The male gamete travels down the pollen tube and enters the ovule.
3. The male nucleus fuses with the female egg nucleus (fertilisation).

Result: The fertilised ovule develops into a seed. The ovary wall develops into the fruit (which helps disperse the seed).

4. Sexual Reproduction in Humans (Mammals)

While the specific anatomy is complex, for CORE Biology, we need to understand the fundamental process of gamete formation and fertilization, which links directly to inheritance.

4.1 Human Gametes

These are the specialized cells that carry half the genetic information (23 chromosomes).

• Male Gamete (Sperm): Small, streamlined cell with a tail for mobility (motile). Produced in the testes.
• Female Gamete (Egg or Ovum): Large cell containing nutrients, non-motile. Produced in the ovaries.

4.2 Fertilisation and Zygote Formation

Fertilisation is the internal fusion of the gametes, typically occurring within the oviduct (fallopian tube) of the female.

Step-by-Step Fertilisation:

1. Millions of sperm travel towards the egg cell.
2. Usually, only one sperm successfully penetrates the outer layers of the egg.
3. Once penetrated, the egg's surface changes to block all other sperm from entering (preventing polyploidy).
4. The nucleus of the sperm fuses with the nucleus of the egg.
5. This fusion restores the full set of chromosomes (46). The new cell is called a zygote.
6. The zygote immediately begins dividing by mitosis to form an embryo, eventually leading to a new, genetically unique individual.

Did you know? The sex of the baby is determined entirely by the sperm. The egg always carries an X chromosome, but the sperm can carry either an X or a Y chromosome. We will explore this more in the dedicated Inheritance chapter!