4. Reproduction

What you need to know...

- Body cells are diploid.
- The structures and sites of gamete production in plants and animals.
- The fertilisation of haploid gametes to produce a diploid zygote.

Notes

Reproduction for most single celled organisms is a relatively straightforward affair. Most of them simply divide. Things are more complex for multicellular organisms. In this topic we'll focus on the structures of sexual reproduction in plants and animals.

You already know that chromosomes consist of DNA which carries our genetic information. But in order for a species to be successful, this genetic information needs to be passed onto future generations. The vast majority of cells in a multicellular organism can be described as diploid. A diploid cell contains two copies of each of the chromosomes for that species. For example, a healthy human cell consists of 23 different chromosomes. A diploid human cell therefore has 46 chromosomes, or 23 pairs:

Image from Wikimedia Commons

In order for sexual reproduction to occur, two cells must fuse to form the new individual. If two diploid cells were to fuse the number of chromosomes in the offspring would be double that of the parents. For most species, particularly animal species, this would be fatal. Multicellular organisms therefore need to reduce the chromosome complement of the cells involved in sexual reproduction by half. Cells which have only one set of the species' chromosomes are described as being haploid. In humans these haploid cells are egg and sperm cells, but they are pollen and ovule in plants. Collectively, these types of cells are known as gametes. During sexual reproduction, two haploid gametes fuse to form a new individual. This process is known as fertilisation and the new diploid cell is called a zygote. This process is summarised in the diagram below. To simplify the diagram, the diploid number in the diagram is 4 and the haploid number is 2. In humans this is 46 and 23, whereas for corn it is 20 and 10.

Gamete production in plants

There are many different types of plants and a great variety of methods of reproduction in the plant kingdom. Many plants can reproduce asuxually, but many can also reproduce sexually. In order to reproduce sexually plants will need to produce haploid gametes which can fuse to form a diploid zygote. The organ of sexual reproduction for many plants is the flower. Gamete production and fertilisation occurs in flowers and we therefore need to explore the structure of flowers in some detail.

The diagram below shows the generalised structures found in most flowers. If you cut a flower in half length ways you should be able to identify most of these structures.

Each structure in a flower carries out a particular function which are shown in the table below.

The two structures which we are particularly interested in are the ovary and the anthers as these are the sites of gamete production. The 'female' gamete in plants are called ovules. These are produced and stored in the ovary. The ovary is also the site of fertilisation, so the 'male' gamete must travel here. The 'male' gamete in plants is found in the pollen grains. These are produced in the anther and are dispersed in various methods depending on the species of plant, such as by wind or insect, in a process called pollination. Once a pollen grain lands on the stigma of a flower the 'male' gamete travels down the style in a pollen tube to the ovary where it fuses with an ovule to form a diploid zygote. This diploid zygote then develops into a seed which is then dispersed from the plant by various methods depending on the species of plant, such as wind or animals, to avoid the offspring competing with the parent plant for resources.

There are some great videos on seed dispersal on the BBC website.

Gamete production in animals

By now, you'll most likely know that you weren't actually delivered by a stork or found in a cabbage patch...if not, you might need to have a conversation with someone about the birds and the bees...

So, assuming you understand the mechanics of sexual reproduction in mammals at least, we'll now have a look at the sites of gamete production in mammals. As you know, the haploid gametes in animals are egg cells and sperm cells. When these cells fuse they form a diploid zygote, but where do they come from in the first place? The organs of gamete production in animals are referred to generically as gonads. In mammals, sperm is produced in testes and egg cells are produced in ovaries. Although there are similarities between the production of both, there are obviously some differences also, so we'll look at them separately.

Image from Wikimedia Commons

The diagram above from Wikimedia Commons shows the structures of the human male reproductive system in cross-section in some detail. You don't need to know all these names, but if you happen to be male you might still be interested in the detail which is why I've left it in. You'll already know that the function of the penis in terms of reproduction is that it is inserted into the vagina of a female to allow the sperm cells to come into contact with an egg cell and for fertilisation to occur. But where do the sperm cells come from? Sperm cells are produced continuously throughout a male's life following puberty in the testes. The testes are the male mammalian gonads and are contained within the scrotum. You'll notice from the diagram above that the testis is connected to the tube which runs through the penis (the urethra) via several other tubes. Sperm cells which are produced in the testis pass through the epididymis to the vas deferens then onto the urethra. Before entering the urethra the sperm cells are mixed with semen from the seminal vesicle. Semen contains the fluid and nutrients necessary to allow sperm cells to survive their arrival in the vagina.

Image from Wikimedia Commons

The diagram above shows the main structures of the human female reproductive system. As we were saying, sperm cells are delivered to the vagina by the penis, but where do the egg cells they fuse with come from? The human female gonads are the ovaries. Unlike in males, mammalian females are born with all their egg cells already produced. Human females develop their haploid egg cells when they themselves are developing in their own mother's uterus. When human females reach puberty they begin the process of releasing their own eggs, known as ovulation, normally at the rate of one per month. If a female becomes pregnant they support the developing embryo in the uterus (womb). The uterus lining therefore thickens during the process of ovulation in preparation for pregnancy. If pregnancy is not achieved following ovulation the lining of the uterus wall breaks down and is released from the body via the vagina - this is what is known as "periods". Not all of the egg cells which a female is born with are released, many of them die over time. This is an interesting piece of research on the issue of lost egg cells.

Image from Wikimedia Commons

This last image is useful as it shows three things. It shows the process of ovulation, fertilisation and the very first stages of pregnancy known as implantation. Again, you don't need to know all of the detail in the diagram but given that this is how you started out in life I thought it best to leave the detail in there for your own interest. As you can see from the diagram ovulation involves a complex process within the ovary followed by the release of the egg cell (referred to as an oocyte in the diagram) in the fallopian tube, or oviduct. If sperm cells are present during or soon after ovulation then fertilisation can occur to form a diploid zygote. This diploid zygote can then progress to form a new individial...such as you!

We've only discussed mammalian gamete production here, and with a particular focus on humans. You could explore the reproductive strategies of other animals as well.