When We Look at Chromosomes, Identical Chromosomes Have a Common Origin

Honor’s Science

Chromosomes are microscopic strands found in the nuclei of the cells of most living things. The characteristics of an organism are located in an organism’s chromosomes. The degree of chromosome pattern similarity between two species indicates the degree of biological relationship. Therefore, to assess the evolutionary relationships between organisms of different species, one can compare chromosomes of those organisms. When we look at chromosomes, identical patterns on chromosomes have a common origin. They are inherited from the same source: a common ancestor.

Chromosomes, when treated with a particular stain, reveal characteristic banding patterns according to their molecular makeup. Where the chromosomes are tightly packed, they stain very darkly. These dark bands are NOT genes; there are many hundreds of genes in a single band.

Objective: The students will demonstrate their understanding that degrees of similarities in chromosomes correspond to degrees of evolutionary relationships

Standards: 1.4 Write in various modes in all content areas

1.5 Produce writing of high quality

1.6 Speak and listen effectively in all content areas

1.8 Conduct research in all content areas

1.3.2.10.C Apply the elements of scientific inquiry to solve problems

3.3.10.A Explain the structural & functional similarities & differences found among living things

3.3.10.D Explain the mechanisms of the theory of evolution

PROCEDURE:

Part 1: Identical Chromosomes?

1. On your data sheet are the detailed diagrams of Chromosome#3 from three different animals (orangutan, gorilla, chimpanzee) that share many characteristics with humans. In fact, even these chromosomes are very similar to the human chromosome, #3. In your envelope, you will find a copy of Chromosome #3, the human chromosome. Place it next to each of the 3 chromosomes. Is there a perfect match? If so, with which chromosome is the match? Attach the chromosome where it matches.

2. What does this suggest about the relationship between the human and this primate?

Part 2: Alike, But Different; Inversions

3. Sometimes, similar chromosomes don’t match perfectly. When this occurs, what does this suggest about the 2 organisms?

4. On your data sheet are pictures of the very similar human and chimp chromosome#4: identical in both upper and lower regions but different in the central portion. In your envelope, find the copy of the central piece of human chromosome#4. Align this piece so it matches the human chromosome. Then, flip it upside down and match it to the chimp chromosome#4. Attach it to the sheet in the position it best fits. What does this suggest about the difference between the human and chimp chromosome in this central region?

DNA analysis has shown that the chimp chromosome#4 resulted from an inversion of that section from the common ancestor, while the human chromosome#4 resulted from an exact copy of chromosome#4 from the common ancestor. In fact, researchers Yunis and Prakash concluded in 1982 that the human chromosome#4 is the ancestral chromosome since it has this simple inverted section in chimps and other apes. This type of inversion of chromosome segments can be observed in many species, suggesting that it is a common “mistake” in cell division that can result in evolutionary changes. In fact, many differences between humans and apes are a result of an apparent inversion. Inversions in the central region of the chromosome are fairly common.

Part 3: Alike, But Different; Fission or Fusion

5. Inversion is not the only process that can cause changes in related species. Sometimes, when DNA copies and divides, it can break to form 2 smaller chromosomes. This is called fission. DNA can also join to form 1 larger chromosome, called fusion.

While humans have 23 pairs of chromosomes, chimps and other apes have 24 pairs. Use fission and fusion to explain 2 different ways of how could this have happened. Let’s see if we can determine how this might have happened. Look at human chromosome #2 on your data sheet. It is similar to 2 chromosomes, 2p & 2q, found in the apes. Find the strips 2p and 2q in your envelope. These are chimpanzee chromosomes. Match them with the human chromosome #2. Attach them where they fit. How do they compare? How did the human chromosome #2 form? Chromosome 2q in orangutan and gorilla do not match the human chromosome as well as the chimp 2q does. What does this suggest about the relationship between humans and chimps as compared to humans and gorillas or orangutans?

Part 4: How do Other Apes Compare?

6. Not only do humans and chimps have similar chromosomes, but also they are very similar to those of gorillas and orangutans. In your envelope, find the paper showing 7 selected sets of chromosomes. You will notice that for each set, there is a number, the chromosome number, and there are pictures of the human, chimp, gorilla, and orangutan copy of this chromosome. In some cases, you may find that one chromosome in a set is significantly different from the other three. In other cases, all four are very similar, with none being significantly different. As you compare the chromosomes in each set, you can ignore the dark staining tip ends found on some chromosomes. They are composed of a protein and are not important differences. Complete the information on your data sheet for this section.

7. Gorilla, chimp and orangutan are classified as pongids (great apes), while humans are classified as hominoids. While these organisms are different enough to be classified in different genera, most scientists commonly believe that all four are very similar and the chimp is more like the human than the other apes. Look at the 7 sets of chromosomes and make the comparisons necessary to complete the second chart is step 4.

Conclusion:

Explain how DNA is useful in determining the course of human evolution and how the different primates are possibly related using this information.

Honors Ninth Grade Science

Hypothesis:

Part 1: Identical Chromosomes?

Part 2: Alike, But Different; Inversions

Part 3: Alike, But Different; Fission or Fusion

Part 4: How do Other Apes Compare?

9. What is the most logical assumption about the relationship between these 4 organisms?

10. How are these different organisms classified? Do you think humans and the apes should be classified in the same family? Explain your answer.

Conclusion: Explain how DNA is useful in determining the course of human evolution and how the different primates are possibly related using this information.