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3.1: Cells and Chromosomes

Difficulty Level: At Grade Created by: CK-12
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How does the individual develop certain characteristics?

We can identify a species by a list of unique traits that are shared by all members of that species but not shared by members of other species. Variations of those same traits also can help us distinguish individuals within the species from one another. Anyone who studies genetics is interested in the biological causes of traits and variations. Geneticists ask questions such as, “Why does Paul have blue eyes when his mother and father both have brown eyes? What color might the kittens be if a black cat and a gray cat reproduced?” Notice that the first question requires an explanation. It is a question about something that has happened. The second question requires a prediction. It asks about something that may happen or is going to happen. Geneticists solve problems that help them explain and predict the inheritance of traits and variations. (Notice the last sentence had six words that were italicized. Those six, italicized words have special meaning to scientists. Make sure you know the meanings of all these words!)

“Most of modern genetics is nothing more than a search for variation. Some of the differences can be seen with the naked eye. Others need more sophisticated methods of molecular biology.”

-The Language of Genes,

Steve Jones

To solve problems that explain and predict traits and variations, you have to know some things about cells. All living organisms are composed of cells. Cells work like little factories doing all the jobs inside your body that are needed to keep your body functioning. Your body is made up of many different kinds of cells such as skin cells, muscle cells, and nerve cells. Some cells look like squashed bricks, some look like doughnuts, and many have irregular shapes. However, every cell, no matter what its job, has the same basic parts.

All human cells have an outer border that is the boundary of the cell. This boundary is called the cell membrane. A liquid material called cytoplasm is inside the cell membrane. There is a large structure suspended in the cytoplasm called the nucleus. The nucleus is the part of a cell that contains the genetic information. The nucleus is surrounded by a nuclear membrane that, like the cell membrane, makes a boundary around the nucleus. With few exceptions, every cell in your body has a nucleus. Your mature red blood cells and the cells in the lenses of your eyes do not have nuclei. The cells that give rise to, or produce, your red blood cells and the cells in the lenses have nuclei. However, the mature cells do not.

Figure 2.1 Using a microscope, you can tell some cells apart by their appearance. How cells look often tells something about what they do.

Did You Know?

You have 60 trillion cells in your body. While some of your cells will be with you your entire life, other cells have a shorter life span and must make copies of themselves. Many of your body cells make copies of themselves every day, such as red blood cells.

Genetics in the News What is the news in genetics today? Which new discoveries are being made in the areas of genetics in human health? In agriculture, to develop new foods? In productivity of food, to increase quality, improve storage, or quantity? Find a news article related to genetics. Attach the article to a summary sheet on which you have included:

  • title;
  • source and date;
  • summary of contents;
  • your opinion of the article, supported by reasons;
  • explanation of how this relates to you; and
  • what more you would like to know.

Share your information with the class by creating a bulletin board or class resource file.

Inside the nuclear membrane are chromosomes. Chromosomes are the cell parts that carry the genes. The genes give us the traits and variations that set us apart as individuals and as a species. Geneticists are constantly trying to learn more about chromosomes and the genes they carry. They study what chromosomes are made of and how they work. Knowing about the structure and function of chromosomes allows us to solve problems about continuity and diversity.

What do we know about chromosomes? We know that chromosomes occur in pairs. We also know that all the individuals of the same species usually have the same number of chromosomes. The chromosomes are located in the nucleus of the cell. The sets of chromosomes in individuals of the same species also look the same in a pair-by-pair comparison. That means that there will be the same number of short pairs and long pairs of chromosomes.

Having the same number and kinds of chromosomes is the reason species have continuity. Chromosome pairs possess information about certain traits, such as hair color, eye color, blood type, the number of fingers and toes, and many other characteristics. Humans have 23 kinds of chromosomes. We have 2 of each kind of chromosome. So humans have a total of 46 chromosomes.

Looking at Chromosomes

How can geneticists see chromosomes? What do they look like? The life cycle of a cell can vary from a few hours to many years depending on the cell. We know that throughout the cell's life chromosomes take on different appearances. Using a microscope you can see chromosomes when a cell is preparing to divide. The chromosomes become compact when they prepare to divide. When this happens within a single chromosome, a point of constriction can be seen. Before the cell divides, the chromosomes replicate. However, the two resulting chromosomes remain attached at the point of constriction. As a result, the replicated chromosome looks like an X.

Figure 2.2 In all species, except very primitive ones such as bacteria, chromosomes are located inside the nucleus of a cell. Although the number of chromosomes may vary between species, the individuals within a species usually have the same number of chromosomes. The number of chromosomes is a trait that distinguishes one species from another.

Figure 2.3 Chromosomes can be distinguished by their arm lengths and by the point where the two chromosome strands meet.

Did You Know?

Cells do the most amazing things: They can recognize their neighbors. They can tell the difference between cells like themselves and other cells. They can stick close to their neighbors or keep to themselves. They can send messages to each other-coded messages that only certain cells can read.

The best way to distinguish one chromosome from another is to stain them. In fact, chromosomes got their name because they can be stained. The word chromosome comes from two Greek words meaning color (chroma) and body (soma). Different parts of the chromosome arms absorb dye differently so the chromosomes look like they have horizontal stripes. Chromosomes that are dyed so that they show these stripes are called banded chromosomes.

When we examine chromosomes, we see that the “arms” of different chromosomes are different lengths. Also, the meeting point of the arms is not always in the center. We also can see that the chromosomes can be matched. Notice that each chromosome is double. This process of doubling is called replication. Replication will be discussed later.

Figure 2.4 (a) All 46 chromosomes in humans are found inside a cell's nucleus. (b) To better study chromosomes, scientists have organized and numbered them by size. Karyotypes allow for close study of chromosomes.

A karyotype is a portrait of the chromosomes of a cell. Look at Figure 2.4. The first picture (a) illustrates all of the chromosomes that are in the nucleus of a human cell. The second picture (b) is a karyotype. To make a karyotype, the cell is injected with dye when the chromosome arms are most compact and easy to see. Then the nucleus is squashed so the chromosomes spread out. Next, a photograph is taken through the microscope. Then the individual chromosomes are cut out and arranged. Notice that the chromosomes are paired and ordered from longest to shortest. When chromosomes are the same lengths, they are arranged by the length of the arm. Using this standard arrangement makes it easier to talk about chromosomes because we can identify them by their number.

There is one exception to the rule that chromosomes are paired. While 22 pairs of chromosomes match, the 23 rd pair may or may not match each other. This pair of chromosomes makes up the sex chromosomes. If a person has a matched pair of sex chromosomes, the person is a female. Both of the chromosomes look very much like the letter X. If the person has one chromosome that looks like an X and the other that does not look like an X, that person is a male. The chromosome that does not look like an X is called a Y chromosome. Some people think this unpaired chromosome looks like an upside-down letter Y. Females normally have XX chromosomes, and males have XY chromosomes. The sex chromosomes usually are placed last in a karyotype.

In the next section, we'll look more closely at chromosomes and what they are made of.

Activity 2-1: Karyotyping-A Chromosome Portrait


What can magnified photographs of your chromosomes tell you? A karyotype is a picture of stained chromosomes arranged in a standard order. Karyotyping is important in diagnosing, learning about, and explaining approximately 100 genetic diseases. The chromosomes are easiest to see when the cells are dividing. In this activity you receive an illustration of chromosomes to cut out and arrange in order on a karyotype sheet. Then you can tell if the chromosomes are from a male or a female. In a karyotype the chromosomes are paired and ordered from the longest to the shortest. Each duplicated chromosome is held together by a structure known as a centromere. When chromosomes are the same lengths, they are arranged by length of the arm. Each duplicated chromosome is made up of two sister chromatids that are joined together at the centromere. Humans have 22 pairs of matching chromosomes. The 23 rd pair of chromosomes may or may not match. This pair of chromosomes is called the sex chromosomes. Two X chromosomes represent a female (XX), while an X and Y together represent a male (XY). In a karyotype, the sex chromosomes are placed last.


  • Scissors
  • Ruler (metric)
  • Glue or tape
  • Resources 1 or 2, and 3
  • Activity Report


Step 1 Using Resource 1 or 2, cut out all the chromosomes.

Step 2 Using Resource 3, arrange the chromosomes in order of size from the longest to the shortest. Glue them in order of size onto Resource 3 (the human karyotype form sheet). Answer the questions on the Activity Report.

The person who makes karyotypes is called a cytotechnologist. (Cyto is the Greek word for cell, and technologist comes from the Greek word meaning art.) In Activity 2-1 you did some of the things a cytotechnologist does. Find out what some of the jobs of a cytotechnologist are. Do you think you would want to be a cytotechnologist? If not, why do you think someone else might want to be a cytotechnologist? In a paragraph, explain why or why not you might want to be a cytotechnologist.

Review Questions

  1. Explain three facts that scientists know about chromosomes.
  2. Chromosomes exist in pairs with one exception. Describe that exception.
  3. What is a karyotype? How is it made? How is a karyotype useful in the study of genetics?

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Date Created:
Feb 23, 2012
Last Modified:
Jul 13, 2016
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