1. Introduction

Meiosis is a special type of cell division 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 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. While the AP curriculum de-emphasizes the names of the phases of mitosis and meiosis (interphase, prophase, etc.), these phases are useful for talking about the process. I’ll be using them below, and if you need an easy review, consider listening to my Mitosis Rap and trying my mitosis quizzes 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.

A typical lab relating to meiosis and recombination is the Sordaria lab. Here’s a piece with my reflections on the underlying biology of this lab.

2. Understanding Chromosome Number

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. 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.

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

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

It’s essential to note 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.” As Richard Dawkins explains, it’s as if both of your parents gave you, as part of your inheritance, a 23 volume set of recipes. The recipe books from your mother would be numbered 1a, 2a, 3a, and so on; the ones from your father would be numbered 1b, 2b, and 3b. If you opened volumes 1a and 1b to page 73, 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. To return to cells and biology, the genes are the same, but there might be alternative versions of these genes, 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.

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.

In what follows, the meiotic cell division that germ cells undergo will be our focus. But first, a few flashcards about what you just read.

3. Flashcards: Key Meiosis Terms

[qdeck qrecord_id=”sciencemusicvideosMeister1961-meiosis: key meiosis terms”]

[h]Flashcards: Key Meiosis Terms

[i]Chromosomes and Chromosome Number in Sexually Reproducing Organisms

[start]

[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. The term encompasses both sperm cells and egg cells. Gametes are haploid: they have only one chromosome set.

[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 your mother, and the other from your father. The genes in these homologues 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 egg 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.

Human male karyotype, Wikipedia

[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.

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[/qdeck]

4. Quiz: Basic Concepts of Meiosis

[qwiz style = “border: 3px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-meiosis: basic concepts”]

[h]Meiosis: Basic Concepts

[i] Here’s how the quiz works:

  • Each question is multiple choice, but the entire quiz is like a series of flashcards.
  • If you get the question right, it comes off the deck.
  • If you get the question wrong, it goes to the bottom of the deck, so you can try it again.

[!!!!!!] question 1 +++++++++[/!!!!!!]

[q] The word for ‘two chromosome sets’ is

[c*] diploid

[c] haploid

[c] triploid

[f] Correct. ‘Diploid’ means ‘two chromosome sets.’

[f] No. ‘Haploid’ means ‘a single set of chromosomes.’ A prefix used to indicate ‘two’ is ‘di.’ Remember that the next time you see this question.

[f] No. ‘Triploid’ has the prefix ‘tri,’ which means ‘three.’ A prefix used to indicate ‘two’ is ‘di.’ Remember that the next time you see this question.

[!!!!!!] question 2+++++++++[/!!!!!!]

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

[c] one set of chromosomes

[c*] two sets of chromosomes

[c] three sets of chromosomes

[c] four sets of chromosomes.

[f] No. Think of it this way. In an adult animal, one set of chromosomes was contributed by the mother, the other by the father. If you add them up, you have how many sets?

[f] Exactly. Adult animals have two sets of chromosome, with one set contributed by the mother, and the other set contributed by the father.

[f] No. Think of it this way. In an adult animal, one set of chromosomes was contributed by the mother, the other by the father. If you add them up, you have how many sets?

[f] No. Think of it this way. In an adult animal, one set of chromosomes was contributed by the mother, the other by the father. If you add them up, you have how many sets?

[!!!!!!] question 3+++++++++[/!!!!!!]

[q] The word for ‘one chromosome set’ is

[c] diploid

[c*] haploid

[c] triploid

[f] No. ‘Diploid’ means ‘two chromosome sets.’ Here’s a hint. If you have one chromosome set, then you have half the number of chromosomes as an adult organism with two sets. The word ‘half’ begins with ‘H-A.’ Which of these choices also begins with ‘H-A?’

[f] Yes. ‘Haploid’ means ‘a single set of chromosomes.’

[f] No. ‘Triploid’ has the prefix ‘tri,’ which means ‘three.’ Here’s a hint. If you have one chromosome set, then you have half the number of chromosomes as an adult organism with two sets. The word ‘half’ begins with ‘H-A.’ Which of these choices also begins with ‘H-A?’

[!!!!!!] question 4+++++++++[/!!!!!!]

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

[c] diploids

[c] sexy cells

[c] ovary cells

[c*] gametes

[f] No. ‘Diploid’ means ‘two chromosome sets. Choose another term next time.

[f] No. ‘Sex cells’ might be a good response, but ‘sexy?’ Choose another word next time.

[f] No. Egg cells are made in ovaries, but there’s a single word that covers both sperm cells and egg cells. Make another choice next time you see this question.

[f] Excellent. The word gamete means ‘sex cell,’ and refers to both egg and sperm cells.

[!!!!!!] question 5+++++++++[/!!!!!!]

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

[c] synonymous

[c] identical

[c*] homologous

[c] similar

[f] No. ‘’Synonymous’ refers to words that have the same meaning. You’re looking for a more specialized term used mostly in genetics and evolutionary biology.

[f] No. A key point is that while these chromosomes are very similar, they are NOT identical. You’re looking for a somewhat specialized term used mostly in genetics and evolutionary biology.

[f] Correct. Chromosomes with the same size, appearance, and pattern of genes, are said to be ‘homologous.’

[f] No. These chromosomes are, indeed, similar. But you’re looking for a somewhat specialized term used mostly in genetics and evolutionary biology.

[!!!!!!] question 6+++++++++[/!!!!!!]

[q] The cells that make up your kidneys, skin, and brain are

[c*] diploid

[c] haploid

[c] triploid

[f] That’s right. All of these cells are somatic cells. Somatic cells are diploid, with two chromosome sets.

[f] No. The only cells in your body that are haploid are your gametes (sperm cells or egg cells, depending on your sex). Here’s a hint: Somatic cells are the cells that make up your body. They’re all descended from a zygote, which is formed when a haploid sperm fertilized a haploid egg. If you combine two haploid cells, the resulting cell is…

[f] No. ‘Triploid’ means three chromosome sets. Triploid tissue is important in plant reproduction, but you almost never see triploid tissue in animals. Here’s a hint: Somatic cells are the cells that make up your body. They’re all descended from a zygote, which is formed when a haploid sperm fertilized a haploid egg. If you combine two haploid cells, the resulting cell is…

[!!!!!!] question 7+++++++++[/!!!!!!]

[q] Human somatic cells have ______ chromosomes.

[c] 69

[c] 23

[c*] 46

[f] No. 69 would be equivalent to three chromosome sets. Somatic cells (the cells that make up the tissues of your body) have two chromosome sets. Do a little math, figure out the number of chromosomes in one chromosome set, multiply by two, and next time, choose a different answer.

[f] No. The only cells in your body that are have 23 chromosomes are your gametes (sperm cells or egg cells, depending on your sex). Here’s a hint: Somatic cells are the cells that make up your body. They’re all descended from a zygote, which is formed when a  sperm with 23 chromosomes fertilized an egg, also with 23 chromosomes. If you combine these two sets, what’s the total?

[f] That’s correct. Human somatic cells have 46 chromosomes.

[!!!!!!] question 8+++++++++[/!!!!!!]

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

[c] haploid

[c*] diploid

[c] triploid

[f] No. ‘Haploid’ means ‘one chromosome set.’ You can see that the chromosomes in this image are in matched pairs, which means that there are two chromosome sets. Next time, select a term that has the prefix that means ‘two,’ as in ‘disaccharide.’

[f] Yes. There are two chromosome sets, which means that the cell is diploid.

[f] No. ‘Triploid’ has the prefix ‘tri,’ which means ‘three.’ You can see that the chromosomes in this image are in matched pairs, which means that there are two chromosome sets. Next time, select a term that has the prefix that means ‘two,’ as in ‘disaccharide.’

[!!!!!!] question 9+++++++++[/!!!!!!]

[q] In human gametes, the chromosome number is

[c] 69

[c*] 23

[c] 46

[f] No. 69 would be equivalent to three chromosome sets. Gametes (sperm cells or egg cells) have one chromosome set. Do a little math and next time, choose a different answer.

[f] Excellent. Gametes have 23 chromosomes.

[f] No. 46 is the number of chromosomes found in somatic cells (the cells in all of your body tissues, except for sperm and egg cells). Cells with 46 chromosomes are diploid, meaning that they have two chromosome sets. Gametes have one chromosome set. Do a little math and next time, choose a different answer.

[x]
[restart]

[/qwiz]

 

Links

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