The life cycle of a typical cell is roughly divided into two phases, Interphase and Cell Division. Interphase is a time of growth and synthesis and is further divided in to G1, S and G2 stages. G stands for “growth”. During G1, the cell grows and makes materials needed to replicate its DNA. DNA is then replicated during the S or “synthesis” phase. Additional growth occurs during G2 as the cell prepares for cell division. Cell division encompasses both nuclear division (Mitosis) and cytoplasm division (Cytokinesis). See Fig 8.6 on p 125 in your book.

Mitosis allows cells in our body to regenerate (e.g., replacement of shed skin cells) and to repair damaged tissue, since mitosis produces daughter cells that are identical to the parent cell. Before beginning this exercise, you will want to review Mitosis in our online lecture and examine the diagrams in Fig 8.7 on pp 126-127 and Fig 8.8 on p 128, which illustrates some differences between animal and plant cells. As you can see, each phase of the cell cycle can be recognized based on the appearance the chromosomes and nucleus. In this first exercise, you will need to be able to identify the various phases of the cell in onion cells. Below is an example of what you will be looking at:

Phases of the cell in onion cells

From: Ackroyd, Miller and Sauriol. No date. Mitotic cell division in onion root tip, Allium cepa.

Slide # Phase Description
1-2 Interphase Chromosomes are not visible in the nucleus (gray circle).
3-5 Prophase Chromosomes have become visible; although not apparent, the nuclear membrane is breaking down.
6 Metaphase Chromosomes have aligned along the equator (mid-line).
7-8 Anaphase Sister chromatids have separated and are being pulled to opposite poles.
9-10 Telophase Chromosomes are clumped at opposite poles, the nucleus has reformed and a cell plate is forming.

Note that is hard to determine exactly when one phase ends and the next one starts. Slide #7, for example is early anaphase and could also be considered late metaphase. Slide #9 is early telophase, but might also be considered late anaphase.

See your Unit 4 Experiment Answer Sheet for the instructions and the photographs you will use to complete this exercise.

Meiosis – Introduction

Meiosis is the process by which gametes (eggs and sperm) are produced from a germ cell. This type of cell division is unlike mitosis in that the cells produced are genetically different than the parent cell and the resulting cells contain only half the number of chromosomes (haploid). In this exercise, you will need to demonstrate your understanding of Meiosis and the role it plays in sexually reproducing organisms. Review this unit’s reading in your book (pp 131-141) and this unit’s online lecture before completing this exercise. You will need to review the following animation, so be sure that you are able to open and view it:

McGraw-Hill Higher Education. 2006. How Meiosis Works (Links to an external site.)

When you are ready, open the Unit 4 Experiment Answer Sheet and answer the questions for this exercise.

Karyotyping – Introduction

Review pp 130-121, pp 138-140 and our online lecture this unit before beginning this exercise. Many genetic disorders arise as a result of errors during Meiosis and the formation of gametes (= egg and sperm). Frequently, these errors are due to non-disjunction, in which chromosomes fail to separate correctly during Anaphase. When this happens, a daughter cell ends up with either an extra chromosome or it is missing a chromosome. These genetic disorders can be detecting using a screening process called a karyotype. In this exercise, you will learn how a karyotype is generated and used to diagnose the presence or absence of a disorder.

You will use the following website to complete this exercise:

The Biology Project. 1998. Karyotyping Activity (Links to an external site.)

Be sure to carefully read through the Introduction material on the first page of the website before starting this exercise. Follow the instructions carefully and enter the necessary information in your Unit 4 Experiment Answer Sheet.