How are traits passed from one generation to the next generation?
Remember that one of DNA's most important jobs is to replicate-make a copy of itself. In making copies of itself, DNA can pass from one generation of a family to the next. Genetics is the study of variations that are passed from one generation of a species to another. Before we study why certain variations are passed along in a species, you need to learn how genes are passed along during the reproductive process.
A cell cycle includes the stages beginning when a cell starts dividing and ending with two new cells. The cell cycle lasts about 18-24 hours. It takes about 6-8 hours for DNA to copy (replicate) itself and about 1 hour for the nucleus and chromosomes to divide. The rest of the time the cell is growing and producing proteins. You will find out about protein synthesis in the next section.
Like sentences make up a paragraph, genes make up chromosomes. Every cell in the human body has a complete set of 46 chromosomes, on which an estimated 100,000 genes are located. Genes are responsible for an individual having the traits he or she has. For example, every human being has a gene for blood type that is located on Chromosome 9. When your body grows by making new cells, and when your body makes new cells to repair an injury, all of the new cells must have 46 chromosomes-your 46 chromosomes specific to you. Every cell has a complete set of chromosomes and genes. When cells divide, the chromosomes also must divide in a very regular way during a process called mitosis. Mitosis is the process in which the cell divides, producing two new cells, each with the same number and exact type of chromosomes as the parent cell.
Geneticists know that the traits and variations passed from one generation of a family to another are contained in the chromosomes. They also know that 46 chromosomes must be passed from human parents to their children, so that each new generation has a complete set of chromosomes and genes. Why is this important? It is important because genes possess instructions for traits that make us human-instructions for building a person. If instructions are lost, the person may not develop properly. Each pair of chromosomes of an offspring consists of one chromosome from the mother and one chromosome from the father. This happens because gamete cells first go through a different division process called meiosis. Meiosis happens before fertilization occurs.
Figure 4.1 When a cell divides to produce new cells for the body, the nucleus splits into two, so that each daughter cell gets an identical set of chromosomes. Details of the process are in Activity 4-1.
As you learned, mitosis is the process in which the cell divides, producing two new cells. Each of the new cells has the same number and exact type of chromosomes as the parent cell had. Cell division provides for growth or replacement of worn-out cells throughout your body. In mitosis the set of chromosomes divides into two identical sets. Then one cell nucleus becomes two cell nuclei, each containing an identical set of chromosomes. When the cell divides into two, one set of chromosomes goes to each daughter cell so that each new daughter cell has an identical set of chromosomes.
Not all cells are actively cycling (dividing) at once. Many cells are resting. The lives of cells vary a lot depending on their function. Red blood cells live for about 120 days, while skin cells live only a few days. The nerve cells in your brain last your whole life and do not divide.
Activity 4-1: Cell Division-Double or Nothing
Many-celled organisms, including you, begin as a single cell. What is the process of producing millions of cells from a single cell? In this activity you explore how a cell reproduces (divides) to form two new cells. Mitosis is a continuous and orderly process that occurs in your body's somatic cells. In this activity you model each stage of mitosis using pipe cleaners to represent chromosomes.
- Activity Report
- Colored crayons or colored pens or pencils (same colors as pipe cleaners if possible)
- 2 large paper plates
- 8 pipe cleaners (2 long of color A, 2 long of color B, 2 short of color A, and 2 short of color B)
Step 1 Gather four pairs of pipe cleaners to represent the chromosomes. Remember that your somatic cells have 23 pairs of chromosomes. In this activity we follow two pairs of chromosomes through the process of mitosis.
Step 2 Arrange your pipe cleaners so each pipe cleaner in the pair is of the same length and color. Twist each pair together by one turn at the midpoint. Each pair of pipe cleaners represents a duplicated chromosome. The two different colors indicate that one chromosome came from the father and one chromosome came from the mother. Duplicate chromosomes are formed through the process of DNA replication, which occurs before mitosis begins.
Figure 4.2 One long and one short pair, color A. One long and one short pair, color B.
Step 3 Take two plates and place one plate on top of the other. The plates represent a cell. Put your chromosomes on the top plate. Using the crayons, draw a picture of the chromosomes on your Activity Report.
This phase (prophase) can be recognized when the double chromosomes are visible and can be observed under the microscope as distinct bodies.
Step 4 Line up the double chromosomes along a line that divides the top plate into two halves. Using crayons or colored pencils, draw a picture of the chromosomes on your Activity Report.
During this phase (metaphase), the chromosomes line up in the middle of the cell.
Step 5 Now separate each double chromosome by untwisting them. Leave them side by side on the midline that runs through the center of the plate. Next, move one single, chromosome of each pair to the left of the plate and one to the right. Each chromosome is now a single chromosome, and each side of the plate should have two long and two Short, single chromosomes.
This phase of mitosis (anaphase) occurs when double chromosomes separate into two single chromosomes that move to opposite sides of the cell.
Step 6 Now it is time for the cell to divide into two daughter cells. You represent this step by bringing out the second plate and moving one set of single chromosomes to it. Each newly formed daughter cell has an identical set of chromosomes-two short and two long chromosomes.
In this phase of mitosis (telophase), the cytoplasm divides, resulting in two daughter cells. Each newly formed daughter cell has a nucleus containing a complete set of chromosomes-two copies of the 23 different chromosomes found in the cell. On your Activity Report, record a colored drawing of the chromosomes placed on each plate.
Step 7 In the next phase (interphase), the chromosomes lose their compact appearance. The chromosomes replicate their DNA so that each single chromosome becomes a duplicated chromosome as you did in Step 2 of the activity.
Step 8 Discuss with your partner the questions on the Activity Report. Then record your responses.
In animals, meiosis produces gamete cells. The gamete cells are the cells that are used to reproduce. Meiosis is more complex than mitosis. In meiosis, the chromosomes make copies of themselves as they did in mitosis, but the original cell divides two times. For every cell that begins the process, four new cells are produced. However, the chromosomes have replicated themselves only once. So the four new cells each have half the number of chromosomes of the species. The process of meiosis guarantees that each of these four cells receives one chromosome from each pair of chromosomes. That means that each of these cells has all of the chromosomes and genes that are typical for the species. But each cell has only one copy of each of the chromosomes and genes instead of the two copies that are in all of the other cells of the body. These special cells with half the number of chromosomes are called gamete cells. Gamete cells in males are called sperm. Gamete cells in females are called ova (eggs). All four cells produced by meiosis in males become sperm. In females, although four cells are produced, only one becomes an ovum (singular form of ova, or one egg).
Women are born with all the ova they will ever have for their reproductive lives. Men, however, once they start producing sperm during puberty, continue to produce millions of new sperm every day.
Human gamete cells produced by meiosis have 23 chromosomes. The event of an egg cell or ovum combining with a sperm cell is fertilization. In humans when an ovum and sperm join, each cell contributes 23 chromosomes. So the fertilized egg has the normal number of 46 chromosomes-half come from the mother and half come from the father. The fertilized egg is a zygote. The zygote goes through round after round of mitosis to produce a fetus, and eventually a baby.
Through meiosis, chromosomes are sorted in a way that separates each pair of chromosomes. Through fertilization, they are recombined into new pairs. This is important for two reasons. First, the number of chromosomes per body cell (46) remains the same from one generation to the next (genetic continuity). Second, because each individual has his or her own unique set of chromosomes, there is greater human variety.
Figure 4.3 Meiosis: (a) The first cell has 2 pairs of duplicated chromosomes. (b) After the first division, each cell receives one duplicated chromosome from each pair. (c) After the second division, each cell receives one copy of each chromosome.
Figure 4.4 When human sex cells combine to form a fertilized egg, they each bring 23 chromosomes that join to create a new individual. This figure shows 2 of the 23 chromosomes from the father and 2 from the mother.
Activity 4-2: Meiosis and Fertilization
What is meiosis? Why is it important? How is it different from mitosis? In this activity you explore how chromosomes sort and recombine through meiosis and fertilization. Meiosis occurs only in the reproductive organs to produce gametes (sperm or eggs).
- Data Sheets 1 and 2
- Activity Report
- Crayons or colored pens or pencils (same colors as pipe cleaners if possible)
- 4 large paper plates
- 4 pairs of pipe cleaners to represent 2 pairs of double chromosomes (2 different lengths and 2 different colors)
Step 1 Work in pairs. In this part of the activity, you model what happens during meiosis. You should work on a flat surface. Remember that you have 23 pairs of chromosomes in each of your somatic cells. In this activity meiosis is simulated using only two pairs of chromosomes. Gather four pairs of pipe cleaners to represent two pairs of double chromosomes. Each chromosome pair is unique. Each contains genes for the same traits. The two pairs of double chromosomes are different from each other in both size and the genetic traits they contain.
Figure 4.5 Two pairs of double chromosomes.
Step 2 Stack four plates on top of each other. Place your four double chromosomes in pairs on the top plate. The chromosomes are not organized in any particular way. The plate represents the parent cell. Record a colored drawing of the chromosomes on the plate on Data Sheet 1.
Step 3 Now line up the pairs of double chromosomes along a line that divides the plate into two halves. Unlike mitosis, the chromosomes do not line up in a single row. The chromosomes line up in pairs. On Data Sheet 1, record a colored drawing of the chromosomes on the plate.
Step 4 Place one member of each chromosome pair on the left side of the plate. Place the other member of each chromosome pair on the right side of the plate. Each side of the plate should have one long double chromosome and one Short double chromosome. On Data Sheet 1, record a colored drawing of the chromosomes on the plate.
Step 5 Division 1: Now it is time for the cell to divide for the first time to produce two cells. Carefully separate the stack of four plates so that you have two stacks of two plates each. This is illustrated on Data Sheet 1. Transfer one set of double chromosomes to the other stack of plates. You should have two double chromosomes, each a different size on the top plate of each stack. On Data Sheet 1, record a colored drawing of the chromosomes on the plate.
Step 6 On the top plate of each stack, separate each double chromosome by untwisting the strands. Leave them side by side on the midline through the center of the plate. Next, move one single chromosome of each pair to the left side of the plate and one to the right side of the plate. Each side of the plate should have one long and one short single chromosome. On Data Sheet 1, record a colored drawing of the chromosomes on the plate.
Step 7 Division 2: Now it is time for the cell to divide again. Position the two stacks so they are about a foot apart. Separate the two plates of each stack so that the two plates of each stack are side by side. This is illustrated on Data Sheet 1. Transfer a set of single chromosomes (one of each kind of chromosome-one short chromosome and one long chromosome) onto the empty plates so that each of the four plates has two single chromosomes. These plates represent the four daughter gamete cells.
Each newly formed daughter cell contains two single chromosomes. On Data Sheet 1, record a colored drawing of the chromosomes on the plate.
Step 8 Discuss with your partner questions 1 and 2 on the Activity Report. Then record your responses to each question.
Step 9 In this part of the activity you model what happens to the chromosomes during fertilization. Work together with another pair of students in the class. Select one of your plates and place it in front of you. Have the other team place one of their plates in front of them. Each plate should have two single chromosomes. The chromosomes on the two plates should be different colors. On Data Sheet 2, record a colored drawing of the chromosomes on the plate.
Step 10 One of the plates represents a sperm cell and the other plate represents an egg cell (ovum). Move the plate representing the sperm cell next to the plate representing the egg cell. Transfer the chromosomes from the plate representing the sperm cell onto the plate representing the egg cell. (Now, there should be four single chromosomes on one of the plates and none on the other.) This process of the union of sperm and egg is called fertilization. The cell with the chromosomes is now called the fertilized egg or zygote. On Data Sheet 2, record a colored drawing of the chromosomes on the plate.
Step 11 Discuss with your group questions 3 through 7 on the Activity Report. Then record your responses.
What is the relationship between DNA replication and the fact that chromosomes are doubled when they begin meiosis?
In the models you made of the processes of meiosis and fertilization, you used pipe cleaners in place of chromosomes. You followed only two chromosomes, but imagine how the chromosomes in humans sort and recombine when there are 46 chromosomes. In what other ways were your models of meiosis and fertilization different from the real thing?
You have learned that the genes are segments or regions of DNA and that DNA makes up chromosomes. DNA's job is to preserve a code for cells that tells the cells what to do. Interestingly, genetic codes for different genes can vary a lot. Some are short and others are long. Some have a lot of extra information in them and others do not. Sometimes, this information gets mixed up in the process of replication-resulting in a mutation. A mutation is an error in the order of one or more nucleotides of DNA. These errors can result in well-known diseases, such as those described in Section 7.
Make a list of the characteristics that make you, you. They can be both characteristics you see, and characteristics in personality or choice of activities. Now separate those characteristics into two groups-those you think you cannot control and are part of your genetic self (nature), and those characteristics you have developed and you think you can change (nurture). How much of who you are is truly genetic, and how much of who you are is a product of how you are raised?
- What is the difference between mitosis and meiosis?
- Describe the process of meiosis.
- Why is meiosis important to the continuity of a species?
- Why are models important tools for geneticists?