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--- chapter_01 [2024/08/26 20:05] – [Meiosis results in four non-identical daughter cells called gametes] mike
+++ chapter_01 [2026/10/25 00:59] (current) – [Meiosis results in four non-identical daughter cells called gametes] mike
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-<typo fs:x-large>Chapter 01. <nowiki>Mitosis and meiosis</nowiki>: a primer for genetics</typo>
+<- table_of_contents|Table of Contents^start|Start^chapter_02|Chapter 02 ->
+<typo fs:x-large>Chapter 01. <nowiki>Mitosis and meiosis</nowiki>: a %%primer%% for genetics</typo>
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 Ironically, mitosis and meiosis are also among the topics that biology students are least interested in, and as a result they are also among the topics that students understand the least. These topics are sometimes challenging for students to learn because they are indeed complex; there are a lot of unfamiliar terms used to describe all the different steps of these processes; and students do not always connect the relevance of learning mitosis and meiosis with biology they can directly observe.
-Our goal in this chapter is to learn (or review) just enough about mitosis and meiosis to support our learning of genetics, and to minimize jargon where possible. To meet this goal, we will omit many of the important cell biology details. However, students are encouraged to study and review mitosis and meiosis from a cell biology perspective to gain a deeper understanding of cell division. You should also revisit this chapter when you get confused about concepts in later chapters that relate to meiosis.
+Our goal in this chapter is to learn (or review) just enough about mitosis and meiosis to support our learning of genetics, and to minimize jargon where possible. To meet this goal, we will omit many of the important cell biology details. However, students are encouraged to study and review mitosis and meiosis from a cell biology perspective to gain a deeper understanding of cell division. You should also revisit this chapter when you get confused about concepts in later chapters that relate to meiosis. Finally, you should study [[appendix_a|Appendix A]], which goes into detail on the chromosome mechanics of meiosis using yeast as an example.
 ===== Understanding chromosomes =====
-When thinking about mitosis and meiosis from the perspective of genetics, we really only care about how the chromosomes behave. In somatic cells (i.e., any cell that is not a reproductive cell), chromosomes usually exist as pairs. Human cells, for example, contain 23 pairs of chromosomes. Another way to say this is that human cells have 46 total chromosomes. In humans, these chromosome pairs are named by number, from largest to smallest (1 through 23), except for the sex chromosomes, which are named "X" and "Y". Each chromosome pair consists of two homologous chromosomes, each of which comes from the two different parents of the individual. For instance, each human somatic cell has two of Chromosome 1, two of Chromosome 2, two of Chromosome 3, etc. The mother contributes one of the chromosomes in the pair, and the father contributes the other. Homologous chromosomes are highly similar to each other but are not identical.
+When thinking about mitosis and meiosis from the perspective of genetics, we really only care about how the chromosomes behave. In somatic cells (i.e., any cell that is not a reproductive cell), chromosomes usually exist as pairs. Human cells, for example, contain 23 pairs of chromosomes. Another way to say this is that human cells have 46 total chromosomes. In humans, these chromosome pairs are named by number, from largest to smallest (1 through 22), except for the sex chromosomes (the 23rd "pair"), which are named "X" and "Y". Each chromosome pair consists of two homologous chromosomes, each of which comes from the two different parents of the individual. For instance, each human somatic cell has two of Chromosome 1, two of Chromosome 2, two of Chromosome 3, etc. The mother contributes one of the chromosomes in the pair, and the father contributes the other. Homologous chromosomes are highly similar to each other but are not identical. The only exceptions are the sex chromosomes. Female humans have two X chromosomes that are homologous and pair with each other, but male humans have only one X chromosome; this X chromosome pairs with the much smaller Y (and not homologous) chromosome during meiosis.
-Without explaining any details about this right now, you probably have at least a vague idea that chromosomes contain genes (an idea we will talk about more in [[chapter_04|Chapter 04]]) and genes control traits (or phenotypes), and since two parents are different people with different traits, it follows that their chromosomes are not identical to each other. On the other hand, all humans have similar traits; that is, different individual humans are still very similar to each other (we all have the same general anatomy, our physiology generally works in the same way, etc). Therefore, it should also be pretty easy to imagine that even though homologous chromosomes from two parents are not identical, they must be very similar to each other.
+You probably already have at least a vague idea that chromosomes contain genes (an idea we will talk about more in [[chapter_04|Chapter 04]]) and genes control traits (or phenotypes), and since two parents are different people with different traits, it follows that their chromosomes are not identical to each other. On the other hand, all humans have similar traits; that is, different individual humans are still very similar to each other (we all have the same general anatomy, our physiology generally works in the same way, etc). Therefore, it should also be pretty easy to imagine that even though homologous chromosomes from two parents are not identical, they must be very similar to each other.
 ===== Mitosis results in two genetically identical daughter cells =====
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 </caption>
 </figure>
-From a genetics perspective, the goal of mitosis is to distribute replicas of each chromosome into two newly formed daughter cells, so that each daughter cell has the exact same chromosomal content as the parent cell. In other words, each daughter cell is a genetic clone of the parent cell. Prior to mitosis (in a stage of the cell cycle called interphase), cells will replicate their chromosomes; each chromosome becomes doubled, and we now call this replicated pair of chromosomes sister chromatids (in Fig. {{ref>Fig1}}, these duplicated sister chromatids are labeled as "duplicated chromatin"). Unlike homologous chromosomes, sister chromatids are almost always identical to each other. There are rare reasons why sister chromatids might be different from each other after mitosis, and while we will not consider this further in this chapter, it's worth wondering about:
+From a genetics perspective, the goal of mitosis is to distribute replicas of each chromosome into two newly formed daughter cells, so that each daughter cell has the exact same chromosomal content as the parent cell. In other words, each daughter cell is a genetic clone of the parent cell. Prior to mitosis (in a stage of the cell cycle called interphase), cells will replicate their chromosomes; each chromosome becomes doubled, and we now call this replicated pair of chromosomes sister chromatids (in Fig. {{ref>Fig1}}, these duplicated sister chromatids are labeled as "duplicated chromatin"). Unlike homologous chromosomes, sister chromatids are almost always identical to each other.
 After chromosomes replicate, there should be a total of two chromatids per chromosome, and four chromatids per homologous chromosome pair. For example, after chromosome replication there should be a total of four chromatids representing Chromosome 1; two sister chromatids that are identical to each other that represent the maternal contribution, and two sister chromatids (also identical to each other and similar but different from the first two) representing the paternal contribution. The two identical sister chromatids are held together by a protein called cohesin, which resembles a rubber band that holds the two sister chromatids together. The homologous chromosomes do not associate with each other.
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 {{ :meiosis.jpg?400 |}}
 <caption>
-A conceptual diagram of meiosis of a human cell. Like Fig. 1.1, this diagram only shows 4 chromosomes that comprise two homologous pairs that can be distinguished based on their size. The color coding is a little different to emphasize the maternal and paternal contributions; teal (maternal) and blue (paternal) represent one pair, and orange (maternal) and red (paternal) represent another pair. Compare how the chromosomes line up in metaphase of mitosis in Fig. 1.1 vs metaphase I in this figure. This figure also illustrates an important concept called crossing over, which we will discuss in Chapter 5. Source: [[https://www.genome.gov/genetics-glossary/Meiosis|National Human Genome Research Institute]]. Licensing: public domain.
+A conceptual diagram of meiosis of a human cell. Like Fig. 1.1, this diagram only shows 4 chromosomes that comprise two homologous pairs that can be distinguished based on their size. The color coding is a little different to emphasize the maternal and paternal contributions; teal (maternal) and blue (paternal) represent one pair, and orange (maternal) and red (paternal) represent another pair. Compare how the chromosomes line up in metaphase of mitosis in Fig. 1.1 vs metaphase I in this figure. This figure also illustrates an important concept called crossing over, which we will discuss in Chapter 5. Note that the daughter cells of the first division are labeled as 2n; this is in error (see also footnote in this chapter). Source: [[https://www.genome.gov/genetics-glossary/Meiosis|National Human Genome Research Institute]]. Licensing: public domain.
 </caption>
 </figure>
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   - In metaphase I when the tetrads are lined up in the middle of the cell, the orientation of the tetrad is random. Each dyad has an equal chance of facing one side of the spindle as the other.
-A final word about gametes and somatic cells. The process of sexual reproduction occurs when haploid gametes from opposite sexes fuse. This results in a new somatic cell (called the zygote) that is now diploid. This zygote can undergo mitosis. In some cases such as with yeast, gametes can undergo mitosis to produce additional haploid daughter cells, but in most examples in this book this does not happen - in general, gametes do not further divide.
+A final word about gametes and somatic cells: The process of sexual reproduction occurs when haploid gametes from opposite sexes fuse. This results in a new somatic cell (called the zygote) that is now diploid. This zygote can undergo mitosis. In some cases such as with yeast, gametes can undergo mitosis to produce additional haploid daughter cells, but in most examples in this book this does not happen - in general, gametes do not further divide.
-Meiosis is worth revisiting often as we explore genetics in subsequent chapters. Also, be careful of confusing prokaryotes (bacteria) with eukaryotes - only eukaryotes go through mitosis and meiosis, whereas bacteria divide by binary fission, which is a different type of cell division.
+Meiosis is worth revisiting often as we explore genetics in subsequent chapters. Also, be careful of confusing prokaryotes (bacteria) with eukaryotes - only eukaryotes go through mitosis and meiosis, whereas bacteria divide by binary fission, which is a different type of cell division. It is strongly recommended that the serious student at this point study [[appendix_a|Appendix A]], which goes into a deep dive on the details of the chromosome mechanics of meiosis.
 A final closing note: there is a lot of jargon (specialized vocabulary) associated with mitosis and meiosis. While you can't avoid memorizing and learning these new terms, don't make that your main focus. The most important part of learning about mitosis and meiosis is understanding how the chromosomes move. It's much better to know how chromosomes move and be unsure about the terms, rather than the other way around.
 ===== Questions and exercises =====