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Bio 106 LAB TOPIC: Cell Division & Inheritance (Revised Summer 2022)

Objectives

1. Identify, describe, and sketch the stages of mitosis in a prepared onion, Allium, root tip slide.

2. Distinguish between nuclear division and cytokinesis.

3. Distinguish between mitosis and meiosis

4. Discuss how meiosis results in genetically unique cells

5. Discuss how nondisjunction can result in aneuploidy

6. Analyze a karyotype

Things you should be able to explain to someone else after this lab:

Mitosis Spindle apparatus

Chromosome Spindle equator

Sister chromatids Cytokinesis

Centromere Cleavage furrow

Fertilization Cell plate

Interphase Meiosis

Daughter cell Gamete

Crossing over Random alignment

Diploid Haploid

Somatic cell Aneuploidy

Karyotype

There are many facts and processes encountered in the course of studying biology. Few are more

important, and marvelous, than the division of one cell into two cells, each with identical genetic

material from the "mother" cell. This process, called mitosis, allows for asexual reproduction in

protists, the development of an entire organism from one cell in plants, animals and fungi, and

continued growth and cell replacement during the life span of any multicellular organism. In

multicellular organisms, mitosis occurs in somatic or body cells. All of the cells in your body are

somatic except for sperm and eggs. Somatic cells are diploid, which means they have two of each

type of chromosome. A chromosome is a long piece of DNA wrapped around histone proteins. As a

human, each of your somatic cells has 46 chromosomes, two copies of 23 different chromosomes.

Originally, one set of chromosomes came from your mother, and one came from your father, when

the sperm and egg joined at fertilization. The number of chromosomes in a diploid cell differs

between organisms.

Sperm and egg are examples of gametes, reproductive cells that form through a special type of cell

division called meiosis. Meiosis is a reductive division used by sexually reproducing multi-cellular

organisms to create haploid cells (gametes) that only contain one copy of each type of chromosome.

Both human sperm and eggs each have 23 chromosomes total, and when the sperm and egg join in

fertilization the diploid chromosome number (46) is restored.

The actual events of mitosis and meiosis are well catalogued, and you will review them during this

lab. Both processes have many similarities, but they also have some important differences. Cell

division is actually two separate processes: nuclear division and division of the cytoplasm. The

division of the nucleus within a cell, called mitosis or meiosis precedes the division of the cytoplasm.

In nuclear division the chromosomes within the nucleus are precisely separated into new nuclei.

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Bio 106 LAB TOPIC: Cell Division & Inheritance (Revised Summer 2022) Objectives

Identify, describe, and sketch the stages of mitosis in a prepared onion, Allium , root tip slide.
Distinguish between nuclear division and cytokinesis.
Distinguish between mitosis and meiosis
Discuss how meiosis results in genetically unique cells
Discuss how nondisjunction can result in aneuploidy
Analyze a karyotype Things you should be able to explain to someone else after this lab: Mitosis Spindle apparatus Chromosome Spindle equator Sister chromatids Cytokinesis Centromere Cleavage furrow Fertilization Cell plate Interphase Meiosis Daughter cell Gamete Crossing over Random alignment Diploid Haploid Somatic cell Aneuploidy Karyotype There are many facts and processes encountered in the course of studying biology. Few are more important, and marvelous, than the division of one cell into two cells, each with identical genetic material from the "mother" cell. This process, called mitosis, allows for asexual reproduction in protists, the development of an entire organism from one cell in plants, animals and fungi, and continued growth and cell replacement during the life span of any multicellular organism. In multicellular organisms, mitosis occurs in somatic or body cells. All of the cells in your body are somatic except for sperm and eggs. Somatic cells are diploid , which means they have two of each type of chromosome. A chromosome is a long piece of DNA wrapped around histone proteins. As a human, each of your somatic cells has 46 chromosomes, two copies of 23 different chromosomes. Originally, one set of chromosomes came from your mother, and one came from your father, when the sperm and egg joined at fertilization. The number of chromosomes in a diploid cell differs between organisms. Sperm and egg are examples of gametes, reproductive cells that form through a special type of cell division called meiosis. Meiosis is a reductive division used by sexually reproducing multi-cellular organisms to create haploid cells (gametes) that only contain one copy of each type of chromosome. Both human sperm and eggs each have 23 chromosomes total, and when the sperm and egg join in fertilization the diploid chromosome number (46) is restored. The actual events of mitosis and meiosis are well catalogued, and you will review them during this lab. Both processes have many similarities, but they also have some important differences. Cell division is actually two separate processes: nuclear division and division of the cytoplasm. The division of the nucleus within a cell, called mitosis or meiosis precedes the division of the cytoplasm. In nuclear division the chromosomes within the nucleus are precisely separated into new nuclei.

Cytoplasmic division, called cytokinesis , begins at the end of nuclear division. Cytokinesis is not usually as exact a process as nuclear division. Cytokinesis also differs in plant and animal cells. Animal cells undergo cleavage furrow pinching along the center of the cell. Plant cells undergo cell plate formation dividing the cell with new cell wall material. The picture below shows a generalized schematic of the cell cycle. Figure 1: Generalized cell cycle diagram You will note in Figure 1 that the cell spends 85-90% of its life in interphase. Cell division is not occurring during this phase of the cell cycle. Interphase was previously considered a "resting" stage, but through further study with new techniques, has proven to be vital in determining the future of the cell. It is during interphase that a cell is directed—or not directed—to begin the division process. Furthermore, the DNA is copied in interphase through the process of replication. Once the DNA is replicated, the two strands stay connected at the centromere, forming sister chromatids or one duplicated chromosome. The sister chromatids will be separated into new nuclei through nuclear division. See the illustration on the next page. Figure 2: Sister chromatids or a duplicated chromosome separating into two unduplicated chromosomes

Metaphase Anaphase Telophase MEIOSIS Through the process of meiosis, germ cells create gametes. Meiosis, unlike mitosis, involves two nuclear and cytoplasmic divisions, meiosis I and meiosis II. The chromosomes interact differently in meiosis creating homologous chromosome pairs (see Figure 3). The duplicated sister chromatids still form after replication, but then they match up with their homologous counterpart. Homologous chromosomes are identical in size, shape, and genetic information. Originally, one of these sister chromatids came from the organism’s mother, and one came from the organism’s father. In meiosis I , the homologous chromosomes split, but the sister chromatids remain intact. In meiosis II , sister chromatids split as in mitosis. The picture below shows a pair of homologous chromosomes.

Figure 3: Homologous chromosomes paired up in meiosis I. The paired pink strands represent one chromosome, comprised of sister chromatids, and the blue represents the other homologous chromosome with its sister chromatids. With the sister chromatids in such close contact, a process called crossing over can occur during prophase I. In crossing over, pieces of the homologous chromosomes switch places. This results in hybrid chromosomes and increases genetic diversity, as illustrated in Figure 4. Figure 4: Crossing over at Prophase I results in two hybrid chromosomes Another process that increases genetic diversity occurs during metaphase I. When the homologous chromosomes line up on the spindle equator, it is random which microtubule attaches to which sister chromatid. When the homologous chromosomes separate in anaphase I and the daughter cells form at the end of meiosis I there will be a new combination of maternal and paternal in each cell. The picture below illustrates random alignment. Note that there are four possible outcomes of the gamete genetic material with only two types of chromosomes. With humans, who have 46 chromosomes, there are more than 8 million possibilities.

Access this website to reinforce your knowledge of meiosis using this interactive website: http://www.cellsalive.com/meiosis.htm ACTIVITY 3: INVESTIGATE MEIOSIS To learn more about meiosis, access this website: http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html You will see a website that looks like this: Work through the first four sections in the table of contents in the order presented [Reproduction, Chromosomes in a diploid cell, Meiosis I, Meiosis II] and answer the questions below. (At the end of this interactive website, there is a "Test yourself" option with 10 questions. This "Test yourself" part is optional.) Reproduction: As you read about reproduction, define the following terms:

asexual reproduction
sexual reproduction
gametes
haploid
fertilization
zygote
diploid Chromosomes in a diploid cell (including a karyotyping activity): Questions:

For humans, what is 2n (the diploid set of chromosomes number)? 2n =
How many pairs of autosomes?
How many pairs of sex chromosomes?
What combination of sex chromosomes determines the female sex?
What combination of sex chromosomes determines the male sex?
If a normal human gamete has 23 chromosomes, how many chromosomes would an n+ human gamete contain? An n-1 human gamete?

Now you will play the role of a cytogeneticist , an individual who studies genetics specifically focusing on the structure and functions of chromosomes. You are asked to construct a karyotype. A karyotype is an organized picture of a person’s chromosomes matching homologous pairs based on size. A karyotype is a tool used to identify if someone has a chromosomal abnormality, either an extra chromosome or a missing chromosome. Aneuploidy is the term that describes an individual with the wrong number of chromosomes. How does aneuploidy occur? In either Meiosis I or Meiosis II through nondisjunction the spindle does not separate the chromosomes properly resulting in some daughter cells that have an extra chromosome (n+1) and some daughter cells that are missing a chromosome (n-1). Fertilization is a random act that restores the diploid number of chromosomes. There is no control over which egg joins with which sperm in fertilization. If an n sperm joins with an n+1 egg the zygote will be 2n+1 having two copies of 22 types of chromosomes and three copies of the 23 rd^ chromosome, or 47 chromosomes total. This person is called trisomic. If an n sperm joins with an n-1 egg the zygote will be 2n-1 with two copies of 22 types of chromosomes and only one copy of the 23rd^ chromosome, 45 chromosomes total. This individual would be monosomic. Click on the link for "Karyotype." This will open a new tab where you will complete a karyotyping activity. "This exercise is a simulation of human karyotyping using digital images of chromosomes from actual human genetic studies. You will be arranging chromosomes into a completed karyotype and interpreting your findings just as if you were working in a genetic analysis program at a hospital or clinic. Karyotype analyses are performed over 400,000 times per year in the U.S. and Canada. Imagine that you were performing these analyses for real people, and that your conclusions would drastically affect their lives." Read about G-banding but ignore the "Your assignment" instructions (you will complete the karyotypes and fill out the table below, but you don't have to do an internet search as the website instructs). Click on the "PATIENT HISTORIES" tab, and then click on "Complete Patient A's Karyotype." Do the same for Patients B & C, then fill out the table below:

If cells in interphase are subjected to colchicine, a drug that interferes with the functioning of the microtubules in the spindle apparatus, at which stage will mitosis be stopped? Why?
Why must the cell duplicate the DNA before entering mitosis or meiosis? Explain. LAB CLEAN UP CHECK LIST Microscopes are correctly returned to the cabinets

Cords are wrapped around the arm
Stage is lowered
On scan objective
Onion slides are returned to the blue box
Computer is turned off

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