Sexual and asexual reproduction and meiosis – GCSE Combined Science Biology

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Every animal and plant began as a single cell. New living things are made by reproduction. This lesson is about the two ways that can happen, and about the special cell division that makes sex cells.

This matters because it explains why you are not an exact copy of either parent. It also explains why a patch of identical plants can all be wiped out by one disease.

By the end you will be able to compare sexual and asexual reproduction. You will describe meiosis and fertilisation. You will also work out chromosome numbers.

Two types of reproduction

In sexual reproduction two gametes join in fertilisation. A gamete is a sex cell, one male and one female. The offspring get a mixture of genes from both parents, so they vary.

In asexual reproduction only one parent is needed. There are no gametes and no fertilisation. The offspring are clones with identical genes to the parent. They are made by mitosis, which is cell division that makes identical cells.

In animals the gametes are sperm and egg cells. In flowering plants they are pollen and egg cells. Think of photocopying a worksheet. Every copy is the same.

Meiosis

Gametes are made by meiosis, a special type of cell division. It happens in the reproductive organs, the ovaries and testes. The cell copies its DNA, then divides twice. This makes four gametes.

Each gamete has half the number of chromosomes of a normal body cell. Chromosomes are the thread-like structures that carry genes. In humans, a body cell has 46 chromosomes and a gamete has 23.

The gametes are all different from each other. The chromosomes are shuffled and shared out at random. Think of shuffling a pack of cards and then dealing out half.

Meiosis makes four daughter cells from one cell, and each is haploid, with a single set of chromosomes. The gametes are not identical to the parent cell or to each other.

Fertilisation

At fertilisation the nucleus of a sperm cell fuses with the nucleus of an egg cell. This makes a cell called a zygote.

The zygote has a full set of chromosomes again. In humans this is 23 + 23 = 46. Half come from each parent, so the zygote has a new mixture of genes.

The zygote divides by mitosis to make an embryo. The cells in the embryo then differentiate, which means they become specialised to form different tissues.

Comparing the two types

Sexual reproduction produces variation. If the environment changes, some offspring may survive, so the species can adapt through natural selection. But it takes time and energy to find a mate.

Asexual reproduction needs only one parent, so it is faster and uses less energy. Many identical offspring can be made quickly. But there is no variation, so one new disease can kill them all.

In a stable environment, asexual reproduction is a good way to make many successful copies.

Organisms that use both

Some organisms use both types. Malaria is caused by a protist, which is a very small organism with one cell. It reproduces asexually in the human host, and sexually in the mosquito.

Many fungi make spores by asexual reproduction, but they can reproduce sexually too. Many plants make seeds by sexual reproduction. They also reproduce asexually, such as strawberry plants making runners and daffodils making bulbs.

Using both types lets the organism gain the advantages of each.

Working with chromosome numbers

A gamete has half the number of chromosomes of a body cell. A zygote has double the number in a gamete.

A fruit fly has 8 chromosomes in a body cell, so a gamete has 4. A dog has 78 in a body cell, so a dog gamete has 39. If a gamete has 23 chromosomes, the zygote has 46.

Mitosis keeps the number the same. Meiosis halves it. Fertilisation doubles it again. Think of splitting a pack of 46 cards into two halves of 23, then joining two halves back.

Recap

Sexual reproduction: two gametes, so variation. Asexual: one parent, so clones.

Meiosis makes four different gametes, each with half the chromosomes.

Fertilisation restores the full number and mixes genes.

Asexual reproduction is faster but has no variation.