Meiosis, sexual and asexual reproduction

In a nutshell

Reproduction is necessary to keep producing new individuals in a species. Reproduction can be sexual, where male and female sex cells need to be fertilised to produce new offspring, or asexual, where only one parent is required and clones of the parent are produced. The fertilisation process can also be different for different types of organisms. Therefore, it is important to know how the reproductive system works and how fertilisation is carried out in them.

Sexual reproduction

Gametes

Reproduction is how a new organism is formed using the genetic material of its parents. Many animals use sexual reproduction between males and females to produce offspring. Males and females have different gametes. These are the sex cells that are fertilised to form a new organism. Males have sperm and females have ova (eggs).

Biology; Genetics; KS4 Year 10; Meiosis, sexual and asexual reproduction — 1. Acrosome, 2. Nucleus, 3. Cytoplasm, 4. Tail

Biology; Genetics; KS4 Year 10; Meiosis, sexual and asexual reproduction — 1. Cytoplasm, 2. Nucleus

Egg cells have certain adaptations to increase the chances of fertilisation.

Example

Egg cells have lots of cytoplasm to store nutrients. They also have a thick jelly coat to prevent more than one sperm fertilising each egg.

Sperm cells also have certain adaptations to increase the chances of fertilisation.

Example

A sperm cell has a tail that helps it swim towards the egg. It also has a lot of mitochondria to provide energy for swimming. The head of a sperm cell is called an acrosome which contains enzymes that will digest through the jelly coat of the egg.

Fertilisation in humans

Fertilisation occurs when the nuclei of the male and female gametes fuse and share their genetic information. This forms a new cell called a zygote, which grows into an embryo and eventually develops into a new organism.

In humans, fertilisation occurs in the female reproductive system. The sperm ejects its nucleus into the egg, and the two nuclei now fuse to share their genetic material and form a zygote. This cell travels to the uterus where it will grow into an embryo.

Before fertilisation, the sperm cell and ovum each have $23$ chromosomes. Upon fusion of the nuclei, the zygote nucleus now contains $46$ chromosomes.

However, fertilisation does not always mean that females carry the embryo.

Example

Male seahorses carry the embryo.

Meiosis

Meiosis is the process of forming four non-identical gametes from one cell. These only contain one copy of each chromosome rather than two, meaning that each cell has $23$ chromosomes. Meiosis is important for producing genetic variation in the gametes. The process of meiosis is as follows:

1.	Each chromosome in the nucleus is duplicated. There are now $92$ chromosomes in the cell. The chromosome pairs line up in the middle of the cell.
2.	The chromosome pairs are pulled apart so that one copy is pulled one end of the cell, and the other to the opposite end.
3.	The first division of the cell occurs to produce two diploid daughter cells. There are now $46$ chromosomes in each cell.
4.	The chromosomes line up in the middle again and the arms are pulled to opposite ends.
5.	The second division of the cell occurs to produce four haploid cells. There are now $23$ chromosomes in each cell.

Biology; Genetics; KS4 Year 10; Meiosis, sexual and asexual reproduction

Asexual reproduction

Reproduction does not always require two parents. In asexual reproduction, only one parent is required. In this way, the same genetic material is passed from parent to offspring without any variation - this produces a clone of the parent. Asexual reproduction is used by several organisms.

Examples

Fungi produce spores which contain the same genetic material.
Bacteria clone themselves.
Strawberries shoot off runners that can produce more strawberries.
Potatoes can produce tubers which can grow more potatoes.

Asexual reproduction uses mitosis to pass on its genetic information to offspring. This is the same process that organisms use to grow and replace cells that are damaged by producing clones of already existing cells.