1. Introduction

A Havanese dog family

Let’s think for a moment about what happens on a cellular level when sexually reproducing organisms, like the dogs shown on the right, reproduce.

Cells, Chromosomes, and DNA

Genetic information is stored in long strings of DNA called chromosomes. Every dog has cells with 78 chromosomes. 39 of those chromosomes came from its father, and 39 from its mother.

When a dog reproduces, it can’t pass on all 78 of its chromosomes. Why not? Because if it did, and its mate also did, then after fertilization the offspring would have 78 + 78 = 156 chromosomes.

Every sexually reproducing species is faced with this same problem. Chromosome number has to be maintained throughout each reproduction cycle. So each dog, during reproduction, has to reduce its chromosome number in half to create sperm cells or egg cells with 39 chromosomes. When one dog sperm cell with 39 chromosomes fertilizes a dog egg cell with 39 chromosomes, a fertilized egg cell with 78 chromosomes is created. That fertilized egg has all the genetic information it needs to develop into an adult dog, which can repeat the cycle.

While “reducing chromosome number in half” sounds straightforward, it’s not. To pass their DNA onto their offspring, sexually reproducing eukaryotic organisms carry out a special type of cell division called meiosis that creates sex cells (sperm cells or egg cells) with half the chromosome number of body cells. As these sex cells (or gametes) are created, an enormous amount of shuffling of genes occurs, making meiosis a powerful engine for creating variation within sexually reproducing, eukaryotic organisms.

In what follows, we’ll start by looking at a cell’s chromosome number and its relationship to meiosis. Then we’ll turn our attention to how meiosis creates variation.

Meiosis evolved from (or with) mitosis and uses many of the same cellular mechanisms. If you need to review mitosis (which we covered in AP Bio Unit 4), consider listening to my Mitosis Rap, and/or redoing our mitosis tutorial before proceeding with what’s below. Also, if you want to watch my music video about meiosis before interacting with the material below, then click here for my Meiosis Song.

2. Chromosome number; alleles; somatic vs germ-line cells

Eukaryotic chromosomes are linear packages of DNA, with the DNA tightly coiled around various proteins. The cells in your body (excluding sperm cells or egg cells) have two sets of chromosomes: one inherited through your mother’s egg, and one through your father’s sperm. These two sets came together during fertilization, and all the cells of your body are descended from that one fertilized egg cell.

Humans have 23 pairs of chromosomes, for a total of 46. There’s nothing important or significant about the chromosome number of a species. More is not better. Chimpanzees have 48 chromosomes. We saw above that dogs have 78. Pineapples have 50. Fruit flies have 8. Notice that because there are two sets, the total chromosome number is always an even number.

The word that describes having two sets of chromosomes in each cell is diploid. Focus on the prefix di (as in dialogue, where two people speak). You can also represent this with a mathematical symbol. If n stands for each set of chromosomes, then a diploid cell has 2n chromosomes.

A human karyotype. Source: Public Library of Science through Wikipedia. Note that each chromosome has a matching (homologous) pair.

You can see this in the image on the right, which is called a karyotype. A karyotype shows the chromosomes in a human diploid cell, artificially paired up by size and staining pattern. In each of the pairs, one of the chromosomes is from the mother, and the other is from the father.

An essential point is that the chromosomes in each pair are not identical. They contain the same genes for the same proteins and RNAs in the same order, but the actual DNA sequences in each gene might be different.

The word for this kind of pair is homologous. It means “sharing the same pattern,” and it’s represented in more detail in the image of two homologous chromosomes on the left. As evolutionary biologist Richard Dawkins explains, it’s as if both of your parents gave you, as part of your inheritance, a 46-volume set of recipes, organized into 23 pairs. The recipe books from your mother would be numbered 1M, 2M, 3M, and so on (with “M” standing for “maternal”); the ones from your father would be numbered 1P, 2P, and 3P (with “P” standing for “paternal”). If you opened volumes 1M and 1P to the same page (page 73, for example), you’d find recipes for the same dish. Those recipes might be the same, but they can also differ. If this were a recipe for tomato sauce, your mother’s recipe might have a bit more onion. Your father’s recipe might have more oregano.

In the diagram on the left, the gene you inherited from your mother and father was the same for gene A, so both are represented by a capital A. It’s the same recipe for tomato sauce. But for gene B (a recipe for something else), what you inherited from your Mom (represented by “b”) is different from what you inherited from your Dad (represented by “B”). Biologically speaking the gene is the same, but you inherited two alternative versions, with distinct sequences of DNA nucleotides. Alternative versions of genes are called alleles, and we’ll see the consequences of differences in alleles when we study genetics in an upcoming module.

Credit: Testes and ovaries from Biorender.com. Other images from National Human Genome Research Institute

The diploid cells in your body fall into two categories. The vast majority of these cells are somatic cells. Somatic cells are all the cells in your body that are doing the job of keeping you alive. They’re the cells making up your brain, muscles, liver, kidney, skin, etc. These somatic cells were created through mitosis, during which a diploid cell cloned itself to create diploid daughter cells.

The only diploid cells that aren’t somatic cells are germ cells. Germ cells are specialized cells in the ovaries or testes that go through meiosis to create gametes (sperm cells or egg cells).

While our somatic and germ cells are diploid, our gametes have only one chromosome set. Think about it. We have two sets of chromosomes. Since we inherit half of our chromosomes from our mothers and the other half from our fathers, then we can only get one set from each parent. The gametes (sperm cells and egg cells) deliver that one set. A cell with one set of chromosomes is said to be haploid. Mathematically, this is represented by the symbol n. In humans, the haploid number of chromosomes (n) is 23 and the diploid number (2n) is 46. Fruit flies have four pairs of chromosomes, so their haploid number is 4. In dogs, with 39 pairs of chromosomes, the haploid number is 39.

During fertilization, haploid gametes fuse to create a diploid zygote (fertilized egg), which gives rise to all of the cells of the adult organism.

In the next tutorial, we’ll focus on the meiotic cell division that germ cells undergo to create gametes. But first, a few flashcards and a quiz about what you just read.

3. Flashcards: Key Meiosis Terms

[qdeck bold_text=”false” style=”min-height: 400px !important; width: 550px !important;” qrecord_id=”sciencemusicvideosMeister1961-Meiosis Key Terms (v2.0)”]

[h] Flashcards: Key Meiosis Terms



[q] What’s a chromosome?

[a] Chromosomes are linear packages of DNA, with the DNA tightly coiled around proteins. Chromosomes carry genetic information.

[q] Define gamete, and describe a gamete’s chromosomal condition.

[a] Gametes are sex cells: sperm cells and egg cells. Gametes are haploid: they have only one set of chromosomes.

[q] What does haploid mean?

[a] Haploid means one chromosome set.

[q] What does diploid mean?

[a] Diploid means two chromosome sets.

[q] What are homologous chromosomes?

[a] Homologous chromosomes are chromosomes that are matched in terms of size, banding pattern, and above all information. In each homologous pair, one member of the pair comes from the mother, and the other from the father. The genes in these homologs are the same, in the same order, but the actual genetic information (in the sequence of DNA) can vary. In other words, the genes are the same, but the alleles might vary.

[q] Define “somatic cell,” describe a somatic cell’s chromosomal condition, and give some examples.

[a] Somatic cells are body cells. They’re diploid (with two chromosome sets). Examples are any cell in the body (liver, heart, skin, pancreatic, muscle, etc.) except for gametes (egg or sperm cells), or the germ cells that produce egg cells or sperm cells.

[q] What’s a karyotype?

[a] A karyotype is an image that’s made by matching up chromosomes by size and banding pattern.

Karyotype of a human male

[q] What are germ cells? What’s their chromosomal condition? What kind of cell division do they undergo?

[a] Germ cells are the diploid cells in the testes or ovaries that go through meiosis to produce haploid gametes.

[q]In terms of maintaining chromosome number, why is meiosis necessary?

[a]In any sexually reproducing species, the chromosomes of the father are combined with the chromosomes of the mother to create offspring. To keep the chromosome number from doubling in each generation, each parent needs to create gametes with 1/2 the diploid number of chromosomes. That’s what meiosis does: it reduces chromosome number by half, from diploid to haploid.

[x] [restart]


4. Quiz: Basic Concepts of Meiosis

[qwiz style = “border: 2px solid black; width: 550px !important; min-height: 400px !important;” random=”true” qrecord_id=”sciencemusicvideosMeister1961-Meiosis Basic Concepts (v2.0)”]

[h] Meiosis: Basic Concepts


[q multiple_choice=”true”] The word for “two chromosome sets” is

[c]IGRpcG xvaWQ=[Qq]








[q multiple_choice=”true”] The somatic and germ cells of adult animals (including humans) have



[c]IHR3byBzZXRzIG9m IGNocm9tb3NvbWVz[Qq]








[q multiple_choice=”true”] The word for “one chromosome set” is



[c]IGhhcG xvaWQ=[Qq]






[q multiple_choice=”true”] If you wanted to use one word to refer to both sperm cells and egg cells, the best word to use would be







[c]IGdhbW V0ZXM=[Qq]




[q multiple_choice=”true”] Chromosomes with the same size, appearance, and pattern of genes, such as the ones shown here, are said to be





[c]IGhvbW9s b2dvdXM=[Qq]






[q multiple_choice=”true”] The cells that make up your kidneys, skin, and brain are

[c]IGRpcG xvaWQ=[Qq]








[q multiple_choice=”true”] Human somatic cells have ______ chromosomes.





[c]ID Q2[Qq]




[q multiple_choice=”true”] This image comes from a cell of a human male. Which of the terms below best describes the chromosomal condition of this cell?



[c]IGRpcG xvaWQ=[Qq]






[q multiple_choice=”true”] In human gametes, the chromosome number is



[c]ID Iz[Qq]




[q] In the diagram below, diploid germ cells are represented by the letter

[textentry single_char=”true”]
[c]IE E=





[q] In the diagram below, haploid gametes are represented by the letter

[textentry single_char=”true”]
[c]IE I=





[q] In the diagram below, diploid somatic cells are represented by the letter

[textentry single_char=”true”]
[c]IE M=





[q] In the diagram below, meiosis is represented by

[textentry single_char=”true”]
[c]IE Y=





[q] In the diagram below, fertilization is represented by the letter

[textentry single_char=”true”]
[c]IE k=





[q] These two chromosomes can be described as being [hangman].



[q] A cell with two sets of chromosomes is said to be [hangman]. By contrast, cells with one set of chromosomes (like sperm cells and egg cells) are said to be [hangman].





[q] The term describing both sperm and egg cells (shown in column “B”) is [hangman].



[q] The term [hangman] is used to describe the diploid cells that make up the tissues of the body.



[q] The term [hangman] is used to describe a diploid fertilized egg cell (shown at “J.”)






What’s next?

  1. Continue to Topics 5.1-5.2, Part 2: Meiosis 1 v. Meiosis 2 (the next tutorial in AP Bio Unit 5).
  2. Watch the Meiosis Music Video