GENETICS LESSON II: Chromosome Structure and Mitosis

Key Concepts:
Organization of genes on chromosomes
Replication of chromosomes
Mitosis

Materials Needed:

• Introduction:
  Toilet paper
  Yarn
  Steelwool
  
• Video station:
  Video -(School provides VCR)
  
• Karyotype station:
  Karyotype Pictures (normal and abnormal)
  
• Mitosis flip book station:
  Cards with pictures of stages of mitosis
  Colored pencils
  Blank flip book worksheets
  Scissors

Introduction

Remind students that last week we discussed how many of our phenotypes (i.e.: rolling tongue, eye color, hair color) are determined by the genes we inherit from our mother and father, which is called our genotype. Genes are located in all cells of the body.

Point out to the students that they started as one single cell. Draw the cell on the board. Ask them how many cells they have now (about a trillion). Ask them how they grew up if they started with one cell. Draw two cells on the board that are the result of cell division. Ask them how many cells there would be if both of those two divided. Go on to at least 8 cells so that the students understand that the pattern is 2,4,8,16, not 2,4,6,8!

In what region of the cell are genes found? The students often answer that genes are in the nucleus, on chromomosomes, or in DNA. As they answer, draw on the board and briefly explain that genes are made of DNA, are organized into chromosomes, and the chromosomes are in the nucleus. DNA stands for deoxyribonucleic acid and we will be discussing it next week. Chromosomes are made up of very compact DNA that you can see with powerful microscopes when the cells are dividing. A brillo-pad/steel wool prop is useful to show how the same amount of DNA takes up a lot less space when folded up rather than stretched out.

Go over the stages of mitosis. If a model is not present in the classroom it is helpful to draw some or all of it on the board before class to save time. Explain that before the cell divides it has to make another copy of all of its genes. Show one chromosome becoming two (may explain that really the DNA would be all stretched out at this time and not in its brillo pad form). Mitosis consists of 4 phases; Prophase, Metaphase, Anaphase, and Telophase. During prophase, the DNA in the nucleus condenses to form chromosomes and the centrioles move to opposite ends of the cell. Spindle fibers form and move toward the chromosomes. Eventually the nuclear membrane breaks down and the spindle fibers are able to attach to the chromosomes. During metaphase, the chromosomes align at the middle (equatorial plane) of the cell. During anaphase, the sister chromatids are pulled apart and separate into two independent daughter chromosomes. During telophase, the chromosomes decondense, and nuclear membranes reform around both new nuclei.

The students will usually also know about cytokinesis and interphase. Technically cytokinesis is not a stage of mitosis, which only refers to nuclear division. Explain that cytokinesis usually occurs, and it often begins sometime during anaphase and ends after telophase. Interphase is also not technically a stage of mitosis, but it is a time that cells may grow, rest, and prepare for further division.

Note: the correct terminology of chromatid and chromosome need not be used in order to prevent confusion.

Activity: A simple demonstration of mitosis makes this class fun as well as interesting. Explain that for simplicity, you’re only going to demonstrate what happens with one pair of chromosomes, but that all 46 pairs of human chromosomes undergo this process in a coordinated manner. You will need 4 volunteers for this demonstration. Have one individual act as chromosome that makes a copy of itself. Bring another student up to act as this newly replicated chromosome (Prophase). Point out that they then line up at the metaphase plate or middle of the nucleus (Metaphase). Use toilet paper to tie the "chromosomes" together, mention that this structure is the centromere. Have another 2 students stand at opposite ends of the room pretending to be centrioles. Give them each a piece of yarn to represent spindle fibers with which they tie around the student at their respective end. Have the 2 "centriole" students pull, as they move towards opposite ends of the room (Anaphase and Telophase) . Make sure that the 2 "chromosome" students know that they should move along, and not try to resist the tugging. As the two chromosomes move apart, the toilet paper centromere will split. Mention that with this nuclear division each chromosome is now in a new cell.

Workstation instructions:
Split the students into three groups that will rotate around each of three workstations.

1. Karyotype Analysis / Gene Guessing
A karyotype is the display of a person’s chromosomes. Because each cell is supposed to have the same set of chromosomes, in theory, a karyotype could be made from any cell in metaphase. Karyotypes from adults can be made easily from cheek cells or white blood cells. The karyotype can also be determined during fetal development by studying cells removed from the amniotic fluid during amniocentesis. After the cells have been stained and squashed, the individual chromosomes are apparent and can be photographed. The chromosomes can be cut, matched in pairs, and arranged in a chart according to size and shape.

Humans have 46 chromosomes, 22 pairs of autosomes, and 1 pair of sex chromosomes that determines whether one is male or female. Talk about why you have pairs of chromosomes and what is means to have XX or XY chromosomes.

Distribute pictures of normal chromosome spreads and have the students examine and count the chromosomes. Ask the students whether they think that theirs is from a male or female. After giving them time to wonder and discuss, point out features of the karyotypes (number of chromosomes, arranged in pairs, numbered according to size, have different banding patterns and centromere locations) including XX and XY. Then have each student figure out whether theirs is male or female. (could discuss numbers in other species).

Next, distribute pictures of abnormal chromosome alignments. Ask the students to note any differences. There are karyotypes from individuals with Down syndrome (trisomy 21), Kleinfelter's syndrome (XXY), Turner syndrome (X0), Edwards Syndrome (trisomy 18), Patau syndrome (trisomy 13), and various translocations and deletions. Discuss the syndromes with the students including the consequence of having more than 2 copies of a set of genes. Explain that most trisomies are lethal.

You might discuss the role of genetic counselors: to decipher, based on a karyotype, whether an individual is female or male, and whether there are any chromosomal abnormalities. They inform the parents about the developmental consequences for a baby born with the abnormality.

2. Videotape and Chromosome Structure
The video is called “Mitosis and Genetics” (Barr Films, CA). It is 16 minutes but we usually show the first 4 minutes. It starts with micro-time-lapse cinematography of cells undergoing mitosis (chromosome condensation, stages of mitosis, and cytokinesis). A scientist then describes how mitosis is a process occurring in all living organisms. A fun moment is when you watch a man eating a banana while he watches a gorilla at a zoo also eating a banana. The similarities are striking. You can watch the full 4 minutes and then discuss, or pause the tape as you go and talk about what’s happening. This does a better job of keeping the kids’ attention.

Suggestions for talking points:

* While observing mitosis, ask the students what they’re seeing and point out the chromosomes. Ask what’s going on in the cell and go over the hand motions for mitosis. (**definitely make this part of the initial presentation – the kids loved it, and teachers commented favorably). Can stop at various points and ask them what stage the cell is at (use hand motions). As the chromosomes begins to go toward the poles, ask where the new daughter nuclei will be – you can bring up the activity with the strings and toilet paper and ask where the anchor points are, in the cell that you’re looking at.

* Pause the video when the cell starts dividing and point out the pinching off at the metaphase plate. Liken it to tightening a belt around the cell until it splits in two.

* Point out the camera on the microscope for taking pictures for the video.

* At one point the narrator will say how many cells begin and end mitosis each second. Pause video. Talk about which cell types divide more (skin and blood cells = high division, brain = not so fast).

* When it starts with the human/gorilla comparison, pause it and ask what they think the difference is between humans and gorillas (1%). Tell them to listen for the real number. Could talk about evolution and how scientists are comparing the genetic differences between humans and gorillas to understand what made us different and what we have in common.

3. Mitosis flip book
Give each student a blank piece of paper with spaces for drawing the stages of mitosis. Ask the kids to draw each stage. Encourange them to think about the stages as they draw by discussing each stage with them and helping them decide together what to draw. At the end, cut out the pieces and staple them together. If there is not enough time, the students can either take them home to finish, or the volunteer can collect them and assemble them later or leave them with the classroom teacher.