TNBGGA: The No Bullshit Guide to Genetic Analysis

Meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). is one of the most important and fundamental phenomena in all biology. All understanding of eukaryoticplugin-autotooltip__default plugin-autotooltip_bigeukaryote: organism whose cells have membrane bound organelles, including the nucleus. genetics must start with an understanding of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)., and all understanding of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). must start with an understanding of mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells..

Ironically, mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). 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 mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). with biology they can directly observe.

Our goal in this chapter is to learn (or review) just enough about mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). 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 mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). 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 meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)..

When thinking about mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). from the perspective of genetics, we really only care about how the chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. behave. In somatic cellsplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell. (i.e., any cell that is not a reproductive cell), chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. usually exist as pairs. Human cells, for example, contain 23 pairs of chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. Another way to say this is that human cells have 46 total chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. In humans, these chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. pairs are named by number, from largest to smallest (1 through 23), except for the sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex, which are named “X” and “Y”. Each chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. pair consists of two homologous chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes., each of which comes from the two different parents of the individual. For instance, each human somatic cellplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell. has two of Chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. 1, two of Chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. 2, two of Chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. 3, etc. The mother contributes one of the chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. in the pair, and the father contributes the other. Homologous chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. are highly similar to each other but are not identical.

Without explaining any details about this right now, you probably have at least a vague idea that chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. contain genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) (an idea we will talk about more in Chapter 04) and genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) control traits (or phenotypesplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism.), and since two parents are different people with different traits, it follows that their chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. 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 chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. from two parents are not identical, they must be very similar to each other.

At the beginning of the 20th century, microscopes allowed scientists to watch chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. in the nucleiplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes. (singular: nucleusplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes.) of dividing somatic cellsplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell.. The process of somatic cellsplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell. dividing is called mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. (Fig. 1).

From a genetics perspective, the goal of mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. is to distribute replicas of each chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. into two newly formed daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells., so that each daughter cellplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. has the exact same chromosomalplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. content as the parent cell. In other words, each daughter cellplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. is a genetic cloneplugin-autotooltip__default plugin-autotooltip_bigClone: Depending on the context, this word can have a few different meanings:

* In the context of genes, cloning means that the physical identity of a gene has been found, and the gene has been sequenced. * In the context of DNA, a cloned DNA fragment is one that has been inserted into some kind of of the parent cell. Prior to mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. (in a stage of the cell cycle called interphase), cells will replicate their chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.; each chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. becomes doubled, and we now call this replicated pair of chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. (in Fig. 1, these duplicated sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. are labeled as “duplicated chromatinplugin-autotooltip__default plugin-autotooltip_bigChromatin: the collective structure of all proteins and DNA in the nucleus of a eukaryotic cell. Nucleosomes are stacked in different configurations to form chromatin structures of different density. Chromatin structure is regulated by a variety of chromatin modifying enzymes.”). Unlike homologous chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes., sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. are almost always identical to each other.

After chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. replicate, there should be a total of two chromatidsplugin-autotooltip__default plugin-autotooltip_bigChromatid: a chromosome-like structure, two of which are formed after replication of a chromosome in both mitosis and meiosis. per chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., and four chromatidsplugin-autotooltip__default plugin-autotooltip_bigChromatid: a chromosome-like structure, two of which are formed after replication of a chromosome in both mitosis and meiosis. per homologous chromosomeplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. pair. For example, after chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. replicationplugin-autotooltip__default plugin-autotooltip_bigDNA replication: usually the process of starting with a dsDNA molecule and ending with two identical copies of that dsDNA molecule. In most cases, “replication” implies DNA replication. there should be a total of four chromatidsplugin-autotooltip__default plugin-autotooltip_bigChromatid: a chromosome-like structure, two of which are formed after replication of a chromosome in both mitosis and meiosis. representing Chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. 1; two sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. that are identical to each other that represent the maternal contribution, and two sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. (also identical to each other and similar but different from the first two) representing the paternal contribution. The two identical sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. are held together by a proteinplugin-autotooltip__default plugin-autotooltip_bigProtein: a molecule that is formed by the translation of messenger RNAs (mRNAs). Functions that proteins provide are what usually give organisms their phenotypes. called cohesin, which resembles a rubber band that holds the two sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. together. The homologous chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. do not associate with each other.

The stages of mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. may be familiar to you if you've learned about mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. or the cell cycle before. They are summarized in the order in which they occur as follows:

1. Prophase: The chromatinplugin-autotooltip__default plugin-autotooltip_bigChromatin: the collective structure of all proteins and DNA in the nucleus of a eukaryotic cell. Nucleosomes are stacked in different configurations to form chromatin structures of different density. Chromatin structure is regulated by a variety of chromatin modifying enzymes. of the chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. condense to form the familiar bar-shaped structures you may be familiar with, and the mitotic spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. (sometimes just called the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart.) starts to form;
2. Prometaphase: the nuclear membrane disassembles, and spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. fibers attach to a structure on the chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. called the centromereplugin-autotooltip__default plugin-autotooltip_bigCentromere: a locus on a chromosome that marks where spindle fibers attach during mitosis and meiosis.;
3. Metaphase: Chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (joined sister chromatidplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. pairs) are positioned in the middle of the cell by the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart.;
4. Anaphase: Sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. are pulled apart by the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. and move toward the two centrosomes;
5. Telophase and cytokinesis: Nucleiplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes. reform and the cell membrane pinches to form two new daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells..

In TNBGGA, we almost never care about the details of the cell biology. Therefore, we will not worry about the nucleusplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes. past this point, and we will not be concerned with how the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. works, either, except to generally understand that the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. is a rope-like structure that attaches to centromeresplugin-autotooltip__default plugin-autotooltip_bigCentromere: a locus on a chromosome that marks where spindle fibers attach during mitosis and meiosis. and pulls chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. apart during cell division. Most drawings in TNBGGA will omit the nucleusplugin-autotooltip__default plugin-autotooltip_bigNucleus: in eukaryotes, the membrane-bound organelle in cells that contains the chromosomes. and simplify the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. to make it easier to focus just on what the chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. are doing. However, serious students of biology will want to understand mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. from various perspectives. For instance, thinking about mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. through the lens of genetics helps you understand how chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. move around and is therefore helpful in thinking about how genetic diseases might be inherited. But thinking about mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. through the lens of cell biology will help you understand different aspects of cancer biology and how cancer biologists think of ways to treat cancer.

Cells that divide to form germ cellsplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. (also called gametesplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes.) such as oocytes (eggs) or sperm undergo a very different kind of cell division called meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). (Fig. 2). Similar to mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells., chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. replicationplugin-autotooltip__default plugin-autotooltip_bigDNA replication: usually the process of starting with a dsDNA molecule and ending with two identical copies of that dsDNA molecule. In most cases, “replication” implies DNA replication. happens before meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). during interphase. The pair of sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. are also held together after replicationplugin-autotooltip__default plugin-autotooltip_bigDNA replication: usually the process of starting with a dsDNA molecule and ending with two identical copies of that dsDNA molecule. In most cases, “replication” implies DNA replication., and that structure is sometimes referred to as a dyad. However, meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). differs from mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. in two fundamental ways:

1. In meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)., there are two rounds of chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. segregation and cell division for every one round of chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. replicationplugin-autotooltip__default plugin-autotooltip_bigDNA replication: usually the process of starting with a dsDNA molecule and ending with two identical copies of that dsDNA molecule. In most cases, “replication” implies DNA replication.. These two rounds are called meiosis Iplugin-autotooltip__default plugin-autotooltip_bigMeiosis I: the first cell division of meiosis. Crossing over occurs during Meiosis I. and meiosis IIplugin-autotooltip__default plugin-autotooltip_bigMeiosis II: the second cell division of meiosis.. In each round, the cell goes through the same steps as mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells.; meiosis Iplugin-autotooltip__default plugin-autotooltip_bigMeiosis I: the first cell division of meiosis. Crossing over occurs during Meiosis I. includes prophase I, prometaphase I, etc., and meiosis IIplugin-autotooltip__default plugin-autotooltip_bigMeiosis II: the second cell division of meiosis. includes prophase II, prometaphase II, etc. This means that at the end of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). there are four daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. produced, whereas there are only two daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. produced after mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells.. In addition, it means that each gameteplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. receives only one of the two homologous chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. in a pair.
2. During prophase I, the dyads pair with one another. This structure where four chromatidsplugin-autotooltip__default plugin-autotooltip_bigChromatid: a chromosome-like structure, two of which are formed after replication of a chromosome in both mitosis and meiosis. are all paired together has a few different names. In cell biology, it is called the synaptonemal complex, and the process of forming this structure is called synapsis. In genetics, we usually refer to this structure instead as the tetradplugin-autotooltip__default plugin-autotooltip_bigTetrad: In the context of yeast genetics, a tetrad refers to the four ascospores from a single ascus, which represents the four products of a single meiotic event. In the context of meiosis in general, a tetrad refers to the four chromatids from a set of replicated homologous chromosomes lined up at metaphase I., and this is the term we will use in this book. Essentially, it is the tetradsplugin-autotooltip__default plugin-autotooltip_bigTetrad: In the context of yeast genetics, a tetrad refers to the four ascospores from a single ascus, which represents the four products of a single meiotic event. In the context of meiosis in general, a tetrad refers to the four chromatids from a set of replicated homologous chromosomes lined up at metaphase I. that are lined up in the middle of the cell during metaphase I. In anaphaseplugin-autotooltip__default plugin-autotooltip_bigAnaphase: a phase of mitosis or meiosis during which chromosomes start to be pulled apart. I, the dyads separate from each other. The sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigSister chromatids: a pair of chromosome-like structures that contain two identical dsDNA molecules that are the result of DNA replication. separate during the second meiotic division during anaphaseplugin-autotooltip__default plugin-autotooltip_bigAnaphase: a phase of mitosis or meiosis during which chromosomes start to be pulled apart. II. This difference in alignment is probably the most important thing to know when comparing mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)..

At the end of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)., there is half as much chromosomalplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. content in the four daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. compared to the mother cellplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells.. Generally speaking, we say that the mother cellplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. is diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent.; the two daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. at the end of mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. are also diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent.; and the four daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells. at the end of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). are haploidplugin-autotooltip__default plugin-autotooltip_bigHaploid: a term that describes a cell or organism that has only one copy of genetic information. Haploid cells typically arise from meiosis (or mitosis of a haploid mother cell). ¹⁾. The terms diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent. and haploidplugin-autotooltip__default plugin-autotooltip_bigHaploid: a term that describes a cell or organism that has only one copy of genetic information. Haploid cells typically arise from meiosis (or mitosis of a haploid mother cell). refer to the number of copies of genetic information that are contained in the cell. The etymology (origin of these words) is Greek - “haplo” meaning single, “diplo” meaning double, and “ploidy” meaning “likeness”. A diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent. somatic cellplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell. contains two copies of genetic information, but a haploidplugin-autotooltip__default plugin-autotooltip_bigHaploid: a term that describes a cell or organism that has only one copy of genetic information. Haploid cells typically arise from meiosis (or mitosis of a haploid mother cell). gameteplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. contains just one copy. In fact, the word meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). also comes from a Greek word that means “lessening” - it is a process that decreases the amount of genetic information in a cell. Geneticists use the symbols “1n” to indicate haploidplugin-autotooltip__default plugin-autotooltip_bigHaploid: a term that describes a cell or organism that has only one copy of genetic information. Haploid cells typically arise from meiosis (or mitosis of a haploid mother cell). and “2n” to indicate diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent.. In humans, we would say that n=23; the value for “n” tells you how many pairs of homologous chromosomesplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. there are.

There are two other important features of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs).:

1. In prophase I when the tetradplugin-autotooltip__default plugin-autotooltip_bigTetrad: In the context of yeast genetics, a tetrad refers to the four ascospores from a single ascus, which represents the four products of a single meiotic event. In the context of meiosis in general, a tetrad refers to the four chromatids from a set of replicated homologous chromosomes lined up at metaphase I. is forming, non-sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigNon-sister chromatids: Chromatids that are derived from two different chromosomes in a homologous chromosome pair during meiosis I. See also: sister chromatids. can exchange portions of their material with each other. This is called crossing overplugin-autotooltip__default plugin-autotooltip_bigCrossing over: an event where non-sister chromatids exchange material with each other during meiosis I.. We will ignore this for now but this is very important; more on this in Chapter 5.
2. In metaphase I when the tetradsplugin-autotooltip__default plugin-autotooltip_bigTetrad: In the context of yeast genetics, a tetrad refers to the four ascospores from a single ascus, which represents the four products of a single meiotic event. In the context of meiosis in general, a tetrad refers to the four chromatids from a set of replicated homologous chromosomes lined up at metaphase I. are lined up in the middle of the cell, the orientation of the tetradplugin-autotooltip__default plugin-autotooltip_bigTetrad: In the context of yeast genetics, a tetrad refers to the four ascospores from a single ascus, which represents the four products of a single meiotic event. In the context of meiosis in general, a tetrad refers to the four chromatids from a set of replicated homologous chromosomes lined up at metaphase I. is random. Each dyad has an equal chance of facing one side of the spindleplugin-autotooltip__default plugin-autotooltip_bigSpindle: a rope-like structure that forms during mitosis or meiosis and pulls chromosomes apart. as the other.

A final word about gametesplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. and somatic cellsplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell.. The process of sexual reproduction occurs when haploidplugin-autotooltip__default plugin-autotooltip_bigHaploid: a term that describes a cell or organism that has only one copy of genetic information. Haploid cells typically arise from meiosis (or mitosis of a haploid mother cell). gametesplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. from opposite sexes fuse. This results in a new somatic cellplugin-autotooltip__default plugin-autotooltip_bigSomatic cell: any cell in a multicellular organism that is not a reproductive (germ) cell. (called the zygoteplugin-autotooltip__default plugin-autotooltip_bigZygote: a cell formed by the fertilization of an oocyte by a sperm.) that is now diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent.. This zygoteplugin-autotooltip__default plugin-autotooltip_bigZygote: a cell formed by the fertilization of an oocyte by a sperm. can undergo mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells.. In some cases such as with yeastplugin-autotooltip__default plugin-autotooltip_bigYeast: in this book, refers to Saccharomyces cerevisiae, a single-celled eukaryotic microbe used as a model genetic organism. See Chapter 02, gametesplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. can undergo mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. to produce additional haploidplugin-autotooltip__default plugin-autotooltip_bigHaploid: a term that describes a cell or organism that has only one copy of genetic information. Haploid cells typically arise from meiosis (or mitosis of a haploid mother cell). daughter cellsplugin-autotooltip__default plugin-autotooltip_bigMother/daughter cells: in cell division, a mother cell divides to form two daughter cells., but in most examples in this book this does not happen - in general, gametesplugin-autotooltip__default plugin-autotooltip_bigGamete: a specialized (usually haploid) cell used for sexual reproduction. Eggs (oocytes) and sperm are gametes. do not further divide.

Meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). is worth revisiting often as we explore genetics in subsequent chapters. Also, be careful of confusing prokaryotesplugin-autotooltip__default plugin-autotooltip_bigProkaryote: an organism that does not have membrane bound organelles. In this book prokaryotes refer to bacteria. (bacteriaplugin-autotooltip__default plugin-autotooltip_bigBacteria: Single-celled organisms that also utilize DNA and the standard genetic code as all organisms on earth, but unlike eukaryotes do not have intracellular membranes and membrane-bound organelles. In this book we use bacteria and prokaryote interchangeably.) with eukaryotesplugin-autotooltip__default plugin-autotooltip_bigeukaryote: organism whose cells have membrane bound organelles, including the nucleus. - only eukaryotesplugin-autotooltip__default plugin-autotooltip_bigeukaryote: organism whose cells have membrane bound organelles, including the nucleus. go through mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)., whereas bacteriaplugin-autotooltip__default plugin-autotooltip_bigBacteria: Single-celled organisms that also utilize DNA and the standard genetic code as all organisms on earth, but unlike eukaryotes do not have intracellular membranes and membrane-bound organelles. In this book we use bacteria and prokaryote interchangeably. divide by binary fission, which is a different type of cell division.

A final closing note: there is a lot of jargon (specialized vocabulary) associated with mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs).. 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 mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). is understanding how the chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. move. It's much better to know how chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. move and be unsure about the terms, rather than the other way around.

Exercise 1: After reading this chapter and thinking about it, get some colored pens and some paper. Try to draw mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. of a diploidplugin-autotooltip__default plugin-autotooltip_bigDiploid: a term that describes a cell or organism that has two copies of similar genetic information, usually obtaining one copy from a male parent and the other copy from a female parent. cell that has two pairs of chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (n=2). Focus just on the replicationplugin-autotooltip__default plugin-autotooltip_bigDNA replication: usually the process of starting with a dsDNA molecule and ending with two identical copies of that dsDNA molecule. In most cases, “replication” implies DNA replication. and movement of chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. Don't just copy it from Figure 1 - try to draw it based on your understanding of what the end goal is of mitosisplugin-autotooltip__default plugin-autotooltip_bigMitosis: a type of cell division that generates two genetically identical daughter cells. and how you think it works. In your drawing, make sure there is an easy way to distinguish between different chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (by using different sizes, colors, etc.). After you complete your drawing, compare it to the illustration here - how close were you? Note where you made mistakes and try again from scratch. Keep at it until you can get it perfect.

Exercise 2: Repeat Exercise 1 except with meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)..