0610 · 17.3Meiosis

Chromosome number and meiosis

0:00Most human body cells contain forty-six chromosomes. But when a sperm cell and an egg cell join, their chromosomes come together in one cell. So why does that cell not contain ninety-two chromosomes? The answer is meiosis, a type of cell division involved in producing gametes. By the end of this video, you will understand why meiosis halves the chromosome number and why the cells it produces are genetically different.

0:22First, we need a little background. Chromosomes are structures containing genetic information, including genes. A gamete is a sex cell, such as a sperm cell or an egg cell in humans. When gametes fuse during fertilisation, their genetic information comes together. That means the chromosome number in each gamete matters, because each contributes chromosomes to the new cell. In a diploid cell, chromosomes occur in pairs. Diploid means having two sets of chromosomes, with one set inherited from each parent.

0:50The chromosomes in a corresponding pair carry genes for the same characteristics, although they can carry different versions of those genes. For our explanation, imagine a cell with just six chromosomes, arranged as three pairs, rather than trying to draw all forty-six human chromosomes. Now compare that with a haploid cell. Haploid means having a single set of chromosomes, so there is one chromosome from each corresponding pair. In our simplified example, the diploid cell has six chromosomes, but a haploid cell has three.

1:18Notice that the haploid cell still has a complete set. It has not simply lost a random collection of chromosomes. This is the central idea of meiosis. Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells. It is involved in the production of gametes. The word reduction refers to the chromosome number, not to the size of the cell or the length of each chromosome.

1:43Think of the paired chromosomes as two corresponding sets of instructions. A diploid cell has two copies of each kind of chromosome, but a gamete needs only one of each kind. Meiosis separates the chromosomes of each pair so that the resulting cells are haploid. We do not need to learn the individual stages here. What matters is the change from two sets to one complete set. Why is that reduction necessary?

2:03Fertilisation combines the chromosomes from two gametes. If each human gamete contained forty-six chromosomes, their fusion would produce a cell with ninety-two. Instead, each normally contains twenty-three, so twenty-three from the sperm and twenty-three from the egg restore forty-six. Meiosis and fertilisation therefore balance one another, because one halves the chromosome number, while the other brings two sets together. Without that reduction before fertilisation, the chromosome number would double from one generation to the next.

2:32Halving it allows the normal diploid number to be maintained across generations. The gametes are haploid, but the fertilised egg is diploid. You will explore fertilisation itself in the reproduction videos. Here, it explains why producing haploid cells is so important.

Genetic variation and chromosome calculations

2:47Another important outcome is that meiosis produces genetically different cells. Remember that the two chromosomes in a pair can carry different versions of genes. Different gametes can receive different combinations of chromosomes, so their genetic information is not identical. The diagram shows this using colours to represent the chromosomes inherited from the two parents. Those colours are only a visual guide, not the real appearance of chromosomes. Both illustrated gametes contain three chromosomes, but their combinations differ.

3:14One contains the blue long chromosome, while the other contains the orange long chromosome, for example. Having the same chromosome number does not mean having exactly the same genetic information. That is the distinction to keep clear. Meiosis reduces the number of chromosome sets and produces genetic differences between the resulting cells. Let us work through a chromosome-number question. Suppose an animal has sixteen chromosomes in each diploid body cell, and you are asked how many chromosomes its gametes contain.

3:41First, identify that sixteen is the diploid number, meaning two sets. Meiosis reduces two sets to one, so divide sixteen by two. Each gamete therefore contains eight chromosomes. Now check that answer using fertilisation. One gamete contributes eight chromosomes and the other contributes eight, giving sixteen in the fertilised egg. That restores the original diploid number, so our answer makes biological sense. If the question instead gave a haploid number of eight and asked for the diploid number, we would double it.

4:10Always identify whether the starting number represents one set or two sets before calculating. A common mistake is saying that meiosis cuts every chromosome in half. That would suggest damaging the genetic instructions, which is not what reduction division means. The chromosome number is halved because the resulting cells receive one complete set instead of two. Another mistake is assuming that haploid always means twenty-three chromosomes. Twenty-three is the human example, and other species have different chromosome numbers.

4:37Also, do not confuse meiosis with mitosis. Mitosis maintains chromosome number and normally produces genetically identical cells, whereas meiosis halves chromosome number and produces genetically different cells. Avoid describing the products of meiosis as identical simply because they contain equal numbers of chromosomes. Number and genetic information are different things. Two books can have the same number of pages without containing exactly the same words.

Exam answer and recap

5:03Here is an exam-style question. State the role of meiosis in reproduction, and describe meiosis. The command word state asks for a direct point, while describe asks you to give the important features. A suitable answer is that meiosis is involved in the production of gametes. Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells. Let us bring the ideas together.

5:27Diploid cells have two sets of chromosomes, while haploid cells have one set. Meiosis halves the chromosome number from diploid to haploid and is involved in producing gametes. Fertilisation then combines two haploid sets to restore the diploid number. The cells produced by meiosis are genetically different, so equal chromosome numbers do not mean identical genetic information.