Lesson 1 — Cell Division and Chromosomes


Lab: Human Karyotype Lab

A karyotype is an image of the set of chromosomes present in a somatic cell. Often, karyotypes are prepared from pictures of stained chromosomes taken of cells that just began mitosis (cell division) so that the size, shape, and number of chromosomes can be studied easily. In this lab simulation, you will prepare a human karyotype and answer several questions about it and other prepared karyotypes.


Introduction

Chromosomes are rod-shaped structures composed of protein and DNA. Each organism has a characteristic number of chromosomes in its somatic cells. Somatic cells (body cells) are any cells other than eggs and sperm. Every somatic cell contains two sets of 23 chromosomes for a total of 46 chromosomes.

One set of chromosomes comes from an individual’s mother, and the other set comes from the father. During most of the live of a cell, its DNA remains in loosely coiled structures called chromatins. Only when mitosis (cell division) occurs do chromatins condense into tightly coiled chromosomes.

Chromosomes are most clearly visible during the metaphase stage of mitosis. Because of this, photomicrographs of cells in metaphase are used to construct karyotypes. Karyotypes are usually made using an individual’s white blood cells. Chemicals are added to the cells to induce mitosis and stop the division in metaphase. The chromosomes are stained and photographed. Scientists copy the image of each chromosome and arrange the chromosomes into pairs according to their shape and size.

After a karyotype has been assembled, scientists can examine the karyotype for signs of a chromosomal disorder. Often, chromosomal disorders  are caused by an error known as nondisjunction, which occurs when chromosomes fail to separate during meiosis (the formation of sex cells). Because the chromosomes did not separate, the resulting sex cells can have two copies of the chromosome instead of one or none at all. Then, when one sex cell joins with another sex cell, the resulting zygote has three copies of a chromosome instead of only two or a single copy of the chromosome.


Problem

Your task is to construct a human karyotype and analyze the result.


Materials

For this simulation, you require access to the Internet and a word processing program to record your results.


Procedure
  1. Open the following link: Human Karyotyping Simulation.

  2. Open the Exploration Guide for students. In this investigation, you will follow the instructions listed in the exploration guide for the parts titled
    • Identifying Human Chromosomes
    • Genetic Abnormalities

  3. As you read and follow the instructions, be sure you are able to answer the questions listed for yourself. 

NOTE:  Each time it asks you to click 'Copy Screen', you have the option of omitting this step. You will be wise to paste the results into your own notes for studying later.

Assessment Questions

Complete the assessment questions and check your answers in the simulation.



Discuss

 
© Getty Images

Cell division as introduced in this lesson can be dramatic and amazing. From a tiny sperm and egg comes a complete new person. However, we have come to accept less dramatic growth or repair after we have reached maturity. You are thankful that your bone mended after that break, or your skin closed after a cut; however, if you lost a finger or an ear, you would accept that very little can be done to replace it.


That may not be the case now. Science looks constantly for ways to extend natural re-growth or repair. Re-growth of finger tips has been reported with the addition of ground-up pig bladder (a source of stem cells). Perhaps more amazing is our experimentation with growing human tissue or organs inside animals for transplant into the desired location on humans. If you are interested in reading further about growing artificial human tissue, click the link to read the article on Artificial Liver 'Could be Grown' 

from BBC. WARNING: One image is of a human ear growing on the back of a mouse in preparation for the transplant.


As promising as these developments are, they produce significant ethical questions. Should scientists be experimenting with tissue or organ re-growth using animals? Discuss this question with your friends and family.