In this Investigation, we wish to return to a discussion of chromosomes. We will begin by discussing chromosome structure and the difference between chromatids and chromosomes. We will also discuss the steps of mitosis and then will do have a quick overview of meiosis and sexual reproduction.

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• In this slide, we once again show a human female and male karyotype. Recall that a karyotype is created by arranging chromosomes photographed through a light microscope. They are numbered and arranged by size. Chromosomes 1 through 22 are called autosomes because they look the same in females and males. The 23rd chromosome pair are referred to as the sex chromosomes. These are different between females and males. Females have two “X” chromosomes whereas males have one X chromosome and one much smaller “Y” chromosome.

• While a human has a total of 46 chromosomes, there are in the form of chromosome pairs. One of the two chromosomes in every chromosome pair is from the individual’s mother and the other from their father.

• In the disorder known as Downs Syndrome, there are three 21st chromosomes rather than only two.

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• This slide shows an overview of the individual steps in the process of mitosis. Mitosis is the mechanism by which body cells (somatic cells), that is all cells in the body except for sex cells, divide. It is by mitosis that an individual grows from a fertilized egg (zygote) to an embryo, fetus to an adult individual. In addition, it is through mitosis that tissues are replaced or repaired during the life of an organism.

• The central part of this slide shows a model of the steps in mitosis. These include interphase, prophase, metaphase, anaphase, telophase, and cytokinesis. This model will allow us to follow the movement of genetic material within the cell during the steps of mitosis. The round micrographs to the left and right of the model show the images of these phases of mitosis that students will see in the lab.

• In short, mitosis is a process by which a single cell divides to form two genetically identical “daughter” cells. The term “daughter” has absolutely nothing to do with the sex of a cell. It simply refers to the two newly formed cells as “children” of the original.

Note: The teacher may which to emphasize that, while we often refer to humans during this discussion, the process of mitosis and meiosis occurs in all plants and animals. The individual phases of mitosis will be examined in greater detail below.

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Note: For this series of slides depicting the steps or phases of mitosis, we will show the microscopic view of the phase that students will see in the lab, the model/drawing of the phase, and, at the bottom, the form of the genetic material of the phase.

• This slide focuses on interphase. Interphase is the phase that a cell spends most of its time in. It is essentially the phase between cell divisions. During this phase, the cell metabolizes and performs its function in the body. A nuclear membrane is present and the DNA is not organized into neat chromosome structures but is rather unorganized and performing its function in transcription, serving as a template for new mRNA molecules to be used in the cytoplasm to make proteins for cell growth, function, and maintenance.

• Notice the two centrioles shown in the model. These structures will become exceptionally important as mitosis proceeds.

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• This slide shows the general structure of chromosomes before and after DNA is replicated during interphase. Before DNA replication, chromosomes are arranged in pairs: one chromosome derived from the mother, one derived from the father. Illustrated here is one pair of chromosomes. Technically DNA at this stage is called chromatin and does not resemble the X-shape seen during mitosis.

• During replication, each chromosome is replicated and the replicated DNA remains joined to the original chromosome. A complex of protein molecules called cohesin bonds DNA strands together. Ultimately this produces a chromosome with two “arms” or chromatids. These are called “sister chromatids.”

• Between DNA replication and the beginning of mitosis, the substances in the cell begin to condense the DNA and it takes on the X-shaped appearance many people associate with chromosomes. During this process, the cohesin detaches along the DNA strands except near the centromere. A protein called condensin acts to condense the chromatids. The action of both proteins produces an “X” shaped chromosome.

• Each “X” is a chromosome that consists of two chromatids. These chromatids are identical. One “X” is the replicated maternal chromosome. The other “X” is the replicated paternal chromosome.

• Sister chromatids of the original chromosomes are bound together at the centromere. Kinetochores associate with the centromeres and will help later in mitosis. Prior to DNA replication there would be 46 chromosomes. Each chromosome has a single chromatid. After DNA replication, there would now be 92 chromatids.

• Notice the colored bands on this cartoon. They are labeled as alleles or genes. All the bands are genes as they signify a specific stretch of coiled DNA that codes for a specific protein. Alleles refer to the idea that the genes are not necessarily exactly the same on the maternal and paternal chromatid. For a gene that codes for eye color, for example, one chromosome may code for blue eyes while the other codes for brown eyes. During meiosis (more on meiosis later), when the chromatids separate during the formation of sex cells (egg or sperm), the eye color of the new individual will depend on the allele on the chromatid that is inherited.

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• This slide focuses on prophase. At prophase, the cell prepares for division. The nuclear membrane dissolves and the genetic material “condenses” into clearly visible chromosome structures. 

• At prophase, the centrioles begin forming fibrous structures composed of the protein tubulin called spindle fibers. These will soon attach to the kinetochores of the chromosomes as we move to the next phase in mitosis.

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• This slide focuses on metaphase. At metaphase, the chromosomes move to the center of the cell and line up along the equatorial plate. The equatorial plate is a reference point referring to the center of the cell. One of the centrioles migrates to each “pole” of the cell and extends spindle fibers to the kinetochore of each chromatid.

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• This slide focuses on anaphase. At anaphase, the spindle fibers pull the chromosomes (sister chromatids) apart into daughter chromosomes. These are pulled towards the centrioles at either pole of the cell. This step guarantees that half of the genetic material will end up in each of the two daughter cells that will soon form.

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• This slide focuses on telophase. At telophase, the nuclear membrane reforms, encasing the recently divided chromosomes. The spindle fibers disappear and the genetic material is present as daughter chromosomes.

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• Cytokinesis is the actual physical separation of the two new daughter cells. These cells will reinter interphase and stay there until it is time to divide again. At that point, the cell will enter prophase again and undergo another round of mitosis.

• We will discuss cell division again in the next Cell, Cell Cycle and Cancer. At that time, we will see how certain mutations can cause cancer cells to uncontrollably undergo round after round of mitosis.

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• This slide shows the process of meiosis. Meiosis only occurs in the testis and ovaries and forms sex cells (egg and sperm). The purpose of sex cells is to mix the genetic information from the mother and father into a new individual. This is called sexual reproduction.

• When discussing mitosis, meiosis, and sexual reproduction; scientists use the terms 2n and n to reference the number of chromosomes in a cell. In humans, somatic or body cells are diploid (2n) which means they have 46 chromosomes or 23 pairs of chromosomes (2n). Human sex cells (sperm and eggs) are haploid (n) or have 23 single chromosomes.

• To make sure the cells of a new individual have only 2n chromosomes, each sex cell must contain only half this amount or n. In order to form sex cells (n) from a 2n cell, the amount of genetic material must be cut in half. Meiosis is the process that causes this reduction of genetic material in sex cells.

• We begin with a 2n cell in the testis or ovary. This cell divides similar to the steps we discussed for mitosis: interphase, prophase, metaphase, anaphase, telophase and cytokinesis. In meiosis this sequences of phases is called Meiosis I and the individual phases are called prophase I, metaphase I and so forth. As was the case for mitosis, this results in two identical 2n daughter cells.

• Unlike in mitosis, after Meiosis I, the DNA does not replicate at interphase. Rather, the cells immediately enter a second round of cell division (Meiosis II). This results in four sex cells from the original cell, all of which contain only one chromatid of each chromosome pair (n). For humans, this means that sperm and egg cells contain only 23 chromosomes rather than the 46 chromosomes found in each body cell (2n).

• The enlarged drawings of chromosome pairs (2n) and chromatid (n) are also shown in this slide.

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• When two sex cells fuse during sexual reproduction, a full complement of genetic material (2n) is reestablished in the fertilized egg, also known as a zygote. Thus, meiosis reduces the body cell (2n) to a sex cell (n) and sexual reproduction forms a new 2n cell with half the genetic material derived from the mother and half from the father.

• As shown in this slide, the zygote then may undergo repeated rounds of mitosis, forming more and more 2n cells. As mitosis continues, the new mass of cells forms into an embryo and then a fetus. Each embryo and fetus is therefore a unique genetic individual – related to but not identical to either of its parents.

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