Teacher guide

Predict

Your body has many kinds of cells, like skin cells, bone cells and nerve cells. Some tumors grow in an ovary, the organ where eggs form. Could a tumor in an ovary grow a tooth?

Circle one. Yes / No

Watch

1. My ovary tumor grows teethyoutube.com/shorts/zf0GQxCVDcI

Word of the day

Teratoma (ter-uh-TOH-muh). A tumor made up of different types of tissue.

• Terabyte starts with tera-, from the same Greek word, téras. In units of measure, tera- means one trillion.
• Doctors use -oma in many tumor names, like melanoma and carcinoma.

Bonus word. Dermoid (DUR-moid). Derm- comes from the Greek word for skin, and -oid means like. Dermoid means skinlike. An asteroid is “starlike,” with the same -oid ending.

Questions

1. In the video, Raven calls sperm swimmers. She says sperm do two jobs. One job is adding the dad’s DNA. The other job is starting the egg dividing. A dermoid cyst starts when an egg cell, or a cell on its way to becoming an egg, starts dividing with no sperm. Which sperm job happens in a dermoid cyst, even with no sperm?

   Circle one. Adding the dad’s DNA / Starting the egg dividing

   Hint. Does a dermoid cyst have any DNA from a dad?

2. You started as one cell. That cell divided again and again, and the new cells turned into skin, bone, muscle and more. A dermoid cyst also starts as one cell, and it can grow skin, hair, fat and sometimes teeth. How can one starting cell make so many kinds of tissue?

   Hint. As one cell keeps dividing, do all the new cells stay the same? Or do some change into new kinds of cells?

3. In the video, Raven says that in some species, an egg that starts dividing on its own can grow into a baby. That doesn’t happen in mammals, like people and mice. Raven says dad’s DNA brings “the organization part.” Without dad’s DNA, the cells don’t organize into a body plan. A body plan is the layout of a body, like where the head, heart and legs go. Why does a human egg cell that starts dividing alone make a dermoid cyst instead of a baby?

   Hint. A dermoid cyst has no DNA from a dad. What does Raven say dad’s DNA brings?

4. In the video, Raven asks how close mammals are to a “virgin birth,” a baby with no dad. Scientists found a way around the need for dad’s DNA in mice. They made a live mouse named Kaguya from two eggs and no dad’s DNA. Kaguya and a dermoid cyst both have no dad’s DNA. Why did Kaguya grow into a whole mouse when a dermoid cyst grows into “body salad”?

   Hint. Kaguya came from a lab. What did the scientists find a way around?

Exit ticket

Predict

Your body has many kinds of cells, like skin cells, bone cells and nerve cells. Some tumors grow in an ovary, the organ where eggs form. Could a tumor in an ovary grow a tooth?

Circle one. Yes / No

Watch

1. My ovary tumor grows teethyoutube.com/shorts/zf0GQxCVDcI

Word of the day

Teratoma (ter-uh-TOH-muh). A tumor made up of different types of tissue.

• Terabyte starts with tera-, from the same Greek word, téras. In units of measure, tera- means one trillion.
• Doctors use -oma in many tumor names, like melanoma and carcinoma.

Bonus words. Dermoid (DUR-moid) means skinlike, from derm-, “skin,” and -oid, “like.” Homozygous (ho-mo-ZY-gus) describes a matching pair, from homo-, “same.” Heterozygous (het-er-oh-ZY-gus) describes a mixed pair, from hetero-, “different.”

Questions

1. In the video, Raven says a sperm does two separate jobs at fertilization. It delivers the father’s chromosomes, and it activates the egg to start dividing. A dermoid cyst grows from a single germ cell, a cell on its way to becoming an egg. When a dermoid cyst starts, there’s no sperm at all. Which sperm job still happens, and which one never does?

   Hint. Think about whose chromosomes end up in the dermoid cyst’s cells.

2. The cells of a dermoid cyst all come from one starting cell, and they carry the same chromosomes. Yet one cyst can hold skin, fat, cartilage and teeth. How can cells that carry the same DNA become such different tissues?

   Hint. Think about what cells do after they divide. Do skin cells and fat cells use the same genes?

3. Your body is organized in levels. Cells form tissues, tissues form organs, and organs work together in organ systems. Scientists who study dermoid cysts describe the tissues inside as a disorganized mix. Which levels of organization can a dermoid cyst reach? Which level does a dermoid cyst never reach?

   Hint. A tooth forms like a small organ. Do the parts of a dermoid cyst work together, the way your heart and blood vessels do?

4. A 2023 review of dermoid cyst studies reports teeth in 31% of cysts. Doctors at one hospital in New Delhi, India, found teeth in 10 of 223 dermoid cysts, about 4%. Why could two studies report such different numbers for teeth? Give two reasons.

   Hint. Think about how each study found and counted teeth, and which patients each study included.

Claim, evidence, reasoning

Going further: why a body plan needs both parents

1. In the video, Raven says an egg that starts dividing on its own is how some species reproduce, but not mammals. Coquita the crocodile laid an egg holding a fully formed baby with no father’s DNA. Mouse embryos made only from a mother’s chromosomes don’t survive past day 10 of pregnancy. Raven says father DNA brings “the organization part.” Scientists call the reason genomic imprinting. In mammals like mice and people, some genes are switched on only in the copy from the father, and others only in the copy from the mother, according to a review by Denise Barlow and Marisa Bartolomei. Why does a mammal body plan need DNA from both parents? Use genomic imprinting in your answer.

   Hint. A mother’s chromosomes carry the same genes as a father’s. Think about which copies are switched on.

Exit ticket

Watch

1. My ovary tumor grows teethyoutube.com/shorts/zf0GQxCVDcI

Word of the day

Teratoma (ter-uh-TOH-muh). A tumor made up of different types of tissue. A dermoid cyst is a mature cystic teratoma.

• Terabyte starts with tera-, from the same Greek word, téras. In units of measure, tera- means one trillion.
• Doctors use -oma in many tumor names, like melanoma and carcinoma.

Bonus words. Dermoid (from derm-, “skin,” and -oid, “like”) means skinlike. Homozygous (from homo-, “same”) describes a matching pair of alleles or markers. Heterozygous (from hetero-, “different”) describes a mixed pair.

Predict

1. In 1990, Urvashi Surti’s team compared chromosome markers near the centromere in dermoid cysts with the same markers in each patient’s healthy tissue. They counted only markers where the patient carried a heterozygous pair. A cyst grown from an ordinary body cell by mitosis would keep that pair heterozygous. Make a guess before you read the paper. What percent of these dermoid cysts do you think carried a homozygous pair at those markers? Give your reason in one sentence.

   Hint. In the video, Raven says a dermoid cyst starts from an egg that begins dividing on its own. Think about what meiosis does to each chromosome pair.

Read the paper

Surti, Hoffner, Chakravarti and Ferrell, “Genetics and biology of human ovarian teratomas. I. Cytogenetic analysis and mechanism of origin,” American Journal of Human Genetics, 1990. The paper is free to read as scanned pages.

The team studied 102 mature ovarian teratomas, the tumors also called dermoid cysts, from patients at Magee-Womens Hospital in Pittsburgh. Of the 102, 95 had a 46,XX karyotype. For each cyst, the team scored chromosome heteromorphisms, visible differences in size and staining near the centromeres, in the cyst and in the patient’s own normal tissue. A marker is informative when the patient’s two copies differ, and 13 cysts had no informative marker. The table gives the counts from the paper’s Results.

Counts from the paper’s Results

1. The counts table gives 58 homozygous and 31 heterozygous cysts out of 89 informative cysts. What percent of informative dermoid cysts were homozygous at the centromere markers? And why does that percent use 89 cysts instead of all 102?

   Hint. Fill in the last column first. A marker can only reveal a change when the patient’s two copies differ.

2. Mitosis, the division that copies body cells, keeps both chromosomes of each pair. Meiosis I, the first division on the way to an egg, separates the two chromosomes of each pair. What do the 58 homozygous cysts tell you about the cell each cyst started from, and how does that fit Raven’s claim in the video that the egg “starts dividing on its own”?

   Hint. Ask which kind of division could leave a cell with two matching copies where the patient carries two different ones.

3. The other 31 informative cysts stayed heterozygous at the centromere, like the patient. Surti’s team concluded that these cysts also came from germ cells, ones whose chromosome pairs never separated. Do the 31 heterozygous cysts weaken the germ cell explanation? What can centromere markers alone tell you about these cysts, and what can’t they tell you?

   Hint. Compare the centromere result you would expect from an ordinary body cell and from a germ cell whose pairs never separated.

4. Surti’s team ends the paper by describing ovarian teratomas as germ cells that begin embryonic differentiation without input from a father’s genome, giving a disorganized mix of mature tissues. In 2004, Tomohiro Kono’s team made Kaguya, a mouse with two sets of maternal chromosomes, one from an immature egg and one from a fully grown egg. The immature egg came from a mouse with a deletion in the H19 gene. Why does differentiation without a father’s genome make tissues but not an organized embryo? What does Kaguya the mouse add to that explanation?

   Hint. Think about genomic imprinting, and about which genes the H19 deletion helped express at the right levels.

Exit ticket