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Cytoplasmic Determinants & Signal Induction

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  • 0:00 The Chicken and the Egg
  • 0:58 Starting Gene Expression
  • 2:02 Cytoplasmic Determinants
  • 3:09 Inductive Signals
  • 4:19 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How does an entire organism develop from a single fertilized cell? In this lesson we will explore the role of cytoplasmic determinants and signal induction in the differentiation of cells.

The Chicken and the Egg

Which came first, the chicken or the egg? Who cares? What I'm interested in is how the egg became a chicken. In all creatures that reproduce sexually, the egg is just a single cell which when fertilized becomes an entire functional organism. How does this happen? Basically, through cell differentiation, the development of the distinct purpose of a cell. This cell is a brain cell, this is a feather cell, this is a beak cell, so on and so forth, until we have a complete chicken. You may notice that the brain cell in the chicken does not get a lot of activity. Chickens are dumb, that's really what I'm getting at. Anyway, how this egg becomes a chicken, or how any fertilized egg becomes a complex organism, is pretty fascinating. So where should we start? I guess with what comes first.

Starting Gene Expression

First, we have two different cells, a sperm cell and an unfertilized egg cell. When they fuse and the genetic information within each combines, together they become a zygote, a single, fertilized cell. That's how this chicken starts out, as one cell containing all of the genetic information from both parents needed to become a functional organism. From this point, the zygote cell will divide through cleavage, or cellular division, and create copies of itself.

Each cell created from the zygote, called a blastomere, therefore contains all of this chicken's genetic information. Every cell that the blastomeres turn into will also have all of this information. In fact, every cell in your body not used for reproduction contains your entire genetic sequence. But, even by the stage of turning from a zygote into blastomeres, these cells are starting to look different, despite having the same genetic information within.

Cytoplasmic Determinants

The reason that these blastomeres all seem a bit different is because within each one, different genes on the DNA are starting to be activated. Why did this happen? Because the egg cell was filled with molecules in the cytoplasm that impact gene expression called cytoplasmic determinants. Some cytoplasmic determinants are proteins, others are mRNA, or an assortment of various substances that are randomly distributed across the egg. So, this egg is covered in random cytoplasmic determinants, is then fertilized, and becomes a zygote, and the zygote divides into blastomeres

Now, when this happens, the genetic information within the blastomeres is the same, but the cytoplasmic determinants in the cytoplasm of the cell are different. This means that each nucleus is exposed to different molecules, and as those molecules interact with the genetic material inside the nucleus, they start activating different genes. So, from the first divisions of the zygote into new cells, these cells are focused on certain genes over others.

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