All organisms have the potential to reproduce. Reproduction is one of the 7 characteristics of life (8 if you study EdExcel IGCSE Biology…..) and of course it means the ability to produce new individuals. But over the 4 billion or so years life has been around on the planet, evolution has developed a myriad ways of producing new individuals. So as biologists are simple folk (well all the ones I work with are….), it makes sense to group different reproductive strategies together to make it easier to understand.
The major distinction between different ways of reproducing is to divide them into asexual and sexual reproduction. This works fairly well, although it seems to be a subject many GCSE students don’t understand too well. So here goes….
The first big idea to dispel is that the number of parents involved determines whether reproduction is sexual or asexual. Too often I am told that asexual involves one parent, sexual two. But although this works in most cases, sexual reproduction can happen with only one parent, for example in flowering plants that self-pollinate. So we need a different way of deciding whether reproduction is sexual or asexual.
And in fact a clear distinction does exist and it is to do with genetics. If the offspring produced are genetically identical to the parent (i.e. a clone) then it is an asexual form of reproduction. If the offspring produced are genetically different to the parents, then it is sexual.
Often sexual reproduction involves the process of fertilisation. This allows two parents to each contribute half their genetic material to their offspring thus generating individuals with new and unique genetic make ups. These specialised cells that contain half the genetic material are called gametes and as you all know, they are made by a special type of cell division called meiosis. Meiosis is vital for sexual reproduction as it produces cells that are haploid (one member of each pair of chromosomes) and all genetically unique.
So this diagram shows two humans each producing gametes by meiosis. The parents on the left will have 23 pairs of chromosomes and so each haploid gamete will have 23 individual chromosomes. Fertilisation restores the diploid number. Mitosis is then used to turn this single cell, the zygote into a multicellular embryo and then indeed into a new individual. (You will remember mitosis is a type of cell division that always produces genetically identical daughter cells)
The examples of asexual reproduction on the left all involve only this second type of cell division, mitosis. There are no gametes, no fertilisation and hence no genetic variation. The simplest type of asexual reproduction is shown as (A) and this is called binary fission. A single-celled organism can divide in two to produce two genetically identical daughter cells. Hydra (B) are a simple type of animal and they reproduce by budding. A new individual just grows off the side and when it is big enough, it drops off….. And many plants can reproduce asexually using a technique called vegetative propagation. The sweet potato plant in (C) can produce several offspring plants from each potato but as they are all clones of each other, this is definitely asexual reproduction.
Students do get confused with this topic so please ask me a question using the comment feature below the post. Keep revising hard!
In sexually reproducing organisms two types of cell division are needed. One is for the processes of growth, repair and asexual reproduction and it is called mitosis. Mitosis produces daughter cells that are diploid and genetically identical to the parent cell.
But when the organism wants to make gametes a different mechanism is needed. Gametes are not diploid like all the other body cells, but instead they only have one member of each homologous pair of chromosomes. In order to make a haploid daughter cell, a second type of cell division, meiosis, is needed.
You can see in the diagram above some of the key differences between mitosis and meiosis. Both start with diploid cells (2n) but whereas mitosis involves one round of division and produces two identical diploid daughter cells, meiosis is different. Meiosis has two rounds of division, called Meiosis I and Meiosis II. This results in four daughter cells and you can see that they are all haploid (n) cells. These cells develop into gametes (sperm and egg cells in humans) and so when they fuse together in fertilisation, the diploid number is restored.
Gametes are all genetically different to each other
Meiosis does not just produce haploid daughter cells. It also introduces genetic variation into the daughter cells so each is genetically unique. This means that random fertilisation will produce offspring that are all genetically different to either parent. How does this genetic variation in gametes come about?
Well to answer that, you need to understand a little bit more about how the chromosomes behave during meiosis. I am not going to talk through all the various stages of meiosis (life is too short and you can read the diagram below) but there is a key event that happens in meiosis that never happens in mitosis…..
It happens in prophase of the first meiotic division and is called synapsis. As the nuclear membrane is degenerating, the two members of a homologous pair of chromosomes line up alongside each other to form a structure called a bivalent (or tetrad)
In the first meiotic division, the two members of the homologous pair are pulled apart and separated. Because these bivalents attach and assort independently of each other, this means that this random assortment can produce many different gametes. A human cell with 23 pairs of chromosomes can produce 2^23 possible gametes just by random assortment.
But there is a second process called crossing over that happens during prophase 1 when the bivalents are formed. As you can see in the diagrams, small sections of chromatid can be swapped between the chromatids of one chromosome and with its homologous partner. This ensures that when the individual chromatids are separated in meiosis 2, each is different to each other. This multiplies up the genetic variation by several orders of magnitude. (see diagram below)
Now that is more detailed than you will need in an iGCSE exam, but it is good to understand where the genetic variation in gametes comes from. Let’s finish with something more simple – the differences between mitosis and meiosis.
One final point: please learn the spellings of these two types of cell division. Spelling is only penalised in exams when the meaning is lost and any intermediate spelling (e.g. meitosis or miosis) has no meaning! So if you are one of those people who finds spelling difficult, find a way of learning mitosis (produces identical diploid daughter cells and is used in growth) compared to meiosis (produces genetically different haploid cells and is used to make gametes)