Friday, May 16, 2008

Are Sponge Bob's Genes Really Square?

This weekend, all of my little Geneticists will be busy completing the Bikini Bottom Genetics Take Home Test. Since the test is to be completed at home, I do expect for you to use your resources - your notebook, this blog, the internet, your textbook....whatever you need to do your best on the test. In this post, I will include reminders to several key concepts about Genetics, probability and Punnett Squares. Make sure that you read each question on the test carefully, check your work and do the best that you can!

Vocab to help you with the test (also found on your yellow vocabulary sheet):

  • Allele - an alternate form of a gene. For our study of Genetics in 7th grade, we are only working with genes that have two alleles for each trait (humans are much more complex than this, though). Represented using letters (ex: H, h, B, b, T, t)

  • Homozygous - genotype in which the two alleles in the pair are identical (ex: HH, hh). Also known as a purebred.

  • Heterozygous - genotype in which the two alleles in the pair are different from one another (ex: Hh). Also known as a hybrid.

  • Phenotype - the physical characteristic displayed by the trait (ex: brown hair, blue eyes). Can be observed.

  • Genotype - the actual genetic makeup of the organism (allele pairs). Not always able to be observed. (ex: BB, Bb, bb)

  • Dominant - form of the trait that is always expressed. Represented using uppercase letter (ex: H, B, T)

  • Recessive - form of the trait that is only expressed when paired with another recessive allele. Can be masked by a dominant allele; represented using lowercase letters (ex: h, b, t)

That's a good review to enable you to work your way through the test if you lost your vocabulary sheet (which, I hope that you didn't!).

Everyone has done such a great job working with the Punnett Squares this week - every Junior Geneticist in Team 7A has been correctly identifying the genotypes of two parents, properly placing them on the outside of the Punnett Square and accurately crossing the parents to find the possible genotypes of their offspring. However, word problems (like the ones on your take home test) are a bit are a few steps to get you through them:

1. Identify what the question is asking you to do (underline or highlight it if necessary)

2. Identify the genotypes of the parents

3. Write the genotypes of the parents along the top and side of the Punnett Square (one allele along each box)

4. Perform the cross, including two alleles in each box

5. Determine the phenotypes of the offspring's possible outcomes (refer to the question to determine which form of the trait is dominant, and which is recessive)

6. Answer the question

Don't forget that I will be collecting the tests during homeroom on Monday (I will notify your homeroom teacher to collect them and give them to one of my homeroom students as the come around) so that 4th period does not have more time to work on it than 1st period does. I will not be accepting any tests in class Monday or on Tuesday. If the school attendance documents your late arrival, I will be accomodating, however, for all other circumstances, THERE WILL BE NO LATE WORK. Absences on Monday will be handled individually.

Get the test over and done with early, so that if you have any questions you can email me immediately and so that you can enjoy the remainder of your weekend without the test hanging over your head!

Have a great weekend!!! No more rain!! : P

Tuesday, May 6, 2008

My-oh-sis! I'm half the cell I used to be.....

Meiosis I (first cell division)

.....and then.....

Meiosis II (second cell division)

Not only does cell division replace cells that needed for growth and repair, but a special type of cell division is necessary to produce gametes, or reproductive cells. In males, we call these gametes sperm cells, while they are referred to as egg cells in females. Meiosis is the special type of cell division that only produces reproductive cells.

Why do we need a different type of cell division for reproductive cells? Well, consider this: Mitosis produces daughter cells that are identical to the original parent cell, right? If our reproductive cells were produced through mitosis, we would have the same number of chromosomes in our sperm and egg cells as we would every other cell in our body, which, for humans, is 46. If a man and woman, each containing 46 chromosomes in their sperm and egg cells, conceive a child, his sperm cell would combine with her egg cell. So, his 46 + her 46 would = 92 chromosomes in their child! Not good, since each species is supposed to maintain the same number of chromosomes (humans 46, fruit flies 8, etc). Can you imagine if the 96 chromosome child grew up and reproduced on her own, conceiving a child with a 96 chromosome man?? Their child would have 192 chromosomes! It would never end!!!!

So, how do we maintain a consistent number of chromosomes in our species? The answer is simple: meiosis produces reproductive cells that contain half the number of chromosomes as our regular body cells. This means that human sperm and egg cells each have 23 chromosomes. Let's revisit our example from the previous paragraph.....If a man and woman, each containing 23 chromosomes in their sperm and egg cells, conceive a child, his sperm cell would combine with her egg cell. So, his 23 + her 23 would = 46 chromosomes in their child. Good? You bet!

How is meiotic cell division different than mitotic cell division?

1. Meiosis only produces cells that are to become gametes. All other cells divide by mitosis.

2. In meiosis, the cells divide twice. However, the chromosomes are only copied before the first division. Mitosis includes only one cell division, which always includes chromosome copying.

3. Daughter cells produced by meiosis are haploid (n), meaning that they contain half the number of chromosomes as the parent cell. In humans, this is 23. Mitosis produces diploid (2n) daughter cells, meaning that the cells are identical to the parent. These cells have a double set of chromosomes. In humans, this is 46.

4. Meiosis produces four daughter cells at its completion. Mitosis only produces two.

During Wednesday's class, we will be visiting The Biology Project's Meiosis Tutorial and completing work to learn more about meiotic cell division. Thursday's quiz will focus on identification of the phases of mitosis, as well as the major differences between mitosis and meiosis (as listed above).

Once we finish cell division, we will get into Mendelian genetics and really begin to practice the meat and potatoes of Genetics! I'm excited for some punnett squares....are you?

Monday, May 5, 2008

My-tosis or Your-tosis??

Today in class, we talked about the cell cycle, and, more specifically, cell division. The cell cycle itself is the entire process through which the cell prepares for division and then actually divides. The entire process of preparation and division takes a long time in adults - anywhere from 18 to 24 hours!

The cell does a lot in preparation for division - it has to duplicate its genetic material (DNA) in order to ensure that its daughter cells are identical to the original cell, and grow to accomodate its expanding contents. This part of the cell cycle, known as interphase, is so important that it occupies the vast majority of the cell cycle. Out of roughly 22 hours, 21 are dedicated to preparing for cell division!

The remaining hour-ish is occupied by cell division, known to us science folk as mitosis. There are four major phases of mitosis through which a cell divides:





Once the cell completes telophase, it is ready to split in two. This is the final stage of the cell cycle, also known as cytokinesis.

This type of cell division occurs in your non-reproductive body cells in order to allow you to grow (the bigger you get, the more cells you contain), develop and repair injuries that you sustain. Cell division is very important!

In an adult, it may seem as though cell division occurs rather slowly....compared to an embryo, it is slow! Remember, adult cells proceed through the cell cycle within 18-24 hours. Embryonic cells, however, divide every 30 minutes! That means that within a half an hour of your conception, you became two whole cells. An hour after you were conceived, you were nothing but 4 cells; after 90 minutes you were 8 cells; within 2 hours you were 16; then 32, then 64; then 128, and so on and so on, until you were born as a multicellular baby with specialized cells! All of that has to happen very quickly!

To revisit the interactive mitosis tutorial, visit the following website:

This week, we will also learn about how a different type of cell division, called meiosis, produces reproductive cells (sperm and egg cells) and how it compares to mitotic cell division.

This week's key terms:

  • cell cycle
  • interphase
  • mitosis
  • cytokinesis
  • asexual reproduction
  • binary fission
  • regeneration
  • gamete
  • egg
  • sperm
  • fertilization
  • meiosis

Have a great week! Sorry, but I have to split! : P