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Let’s explore a fundamental characteristic of multicellular organisms—cell differentiation and specialization. Before you get started, don’t forget to print out your OnTRACK Biology Journal.

TEKS Standards and Student Expectations

B(5) The student knows how an organism grows and the importance of cell differentiation. The student is expected to:

B(5)(B) Examine specialized cells, including roots, stems, and leaves of plants; and animal cells such as blood, muscle, and epithelium

Learning Objectives

Describe the process of cell differentiation and explain its importance.

Describe stem cells and their role in cell differentiation.

Explain how gene expression is related to cell differentiation and specialization.

Identify factors that can influence cell differentiation and specialization.

Describe specific examples of specialized plant and animal cells. 

Essential Questions

What is cell differentiation and why is it important?

What role do stem cells play in cell differentiation?

How is gene expression related to cell differentiation and specialization?

What type of factors can influence cell differentiation and specialization?

How are plant and animal cells specialized?

Vocabulary

  • Multicellular
  • Unicellular
  • Cell Differentiation
  • DNA
  • Stem Cells
  • Chromosomes
  • RNA
  • Metamorphosis

 

What is Cell Differentiation?

Living organisms can be made of a single cell, such as bacteria and protists, or they can be multicellular, like plants, animals, and fungi. Unicellular organisms, like bacteria, are able to perform all life functions within one single cell. They can transport molecules, metabolize nutrients, and reproduce within this one cell.

Multicellular organisms need many different types of cells to carry out the same life processes. Each of these special types of cells has a different structure that helps it perform a specific function. Humans have many different types of cells with different jobs, such as blood cells that carry oxygen and nerve cells that transmit signals to all parts of the body. Cell differentiation is the process by which cells become specialized in order to perform different functions.  

 

                                         Specialized Cells in the Human Body


           

 

Multicellular organisms begin as just one single cell—a fertilized egg. Growing from one single cell to trillions of specialized cells that perform different functions is a process that happens with the regulation of DNA and RNA

Directions: Watch Differentiation and Gene Expression for an introduction to cell differentiation and gene expression. 

Source: 
Cell Differentiation in Multicellular Organisms. Sichuan University. Retrieved from http://biology.tutorvista.com/cell/unicellular-and-multicellular-organisms.html
Differentiation and Gene Expression

1.1 Differentiation and gene expression

Source: 
Stephanie Castle. Differentiation and Gene Expression. Retrieved from https://youtu.be/qOljax2DoeE

Roles of DNA and RNA in Cell Differentiation

DNA controls the way cells function. It also determines what type of specialized cells will be made. Stem cells are cells that have the ability to become any type of specialized cell in the body. After an egg cell and sperm cell unite to begin forming a new organism, all of the DNA in each cell of that organism will be virtually identical. If every part of the DNA in each cell is the same, then how do cells become different types of cells? Let’s look more closely at DNA to find out.

                        

DNA is wound tightly into chromosomes. Different regions of the chromosome code for every different function and cell type. Not all sections of a chromosome are turned on, or expressed, at the same time. Only the regions that are needed to perform a specific function are expressed in each cell. These regions are often depicted as bands or stripes on a drawing of a chromosome. These bands are called genes, and whether or not a gene is expressed determines what type of cell will be created. For example, genes that are expressed (turned on) in a nerve cell are different from the genes that are expressed in a muscle cell. Both cells have the same DNA, but expressing different genes generates different cell types.

This process by which information from a gene is used to make the structures of a cell is called gene expression. Since RNA translates and transcribes the DNA code into proteins (the structures of a cell), it also plays a role in cell differentiation.

How to Read a Chromosome

Images of chromosomes, like the one to the left, usually look a bit like an ‘X’. That is because chromosomes come in pairs—one from each of our parents. Chromosomes in a pair contain the same genes (like genes for hair color, body height, and lipoma formation). The genes we receive from each parent might be the same (as shown for body height), or or they might be different (as shown for hair color). The alternative forms of a gene that are found at the same place on a chromosome are called alleles.
When studying gene expression, it is easier to identify alleles on chromosomes using markers. Chromosomes are sectioned into 2 portions—p and q. The p arm is the shorter arm, and the q arm is the longer arm. Then chromosomes are further divided by number. In the image below, you can see that the p section is divided into sections 1 and 2. The q section is also divided into sections 1 and 2.  Within each section, bands (which represent alleles) are also numbered.

                                                                  

Source: 
Kent Kawashama. Alleles. Retrieved from http://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/PH709_DNA-Genetics/PH709_DNA-Genetics_print.html

Now it’s your turn to see how changing the genes expressed on a chromosome can change the type of differentiated cell that is created. In this activity, you will select, or express, alleles on one X chromosome at various positions along the chromosome. Pay careful attention to the letters and numbers of each gene that is being expressed to click the right band.

Environmental Factors Influence Gene Expression

Environmental factors can also influence gene expression and cell differentiation. For example, available nutrients, salinity, and temperature are all factors that can influence gene expression in organisms. In Himalayan rabbits, genes that code for fur color are turned on and off depending on temperature. In warm parts of the rabbit's body (anything warmer than 35°C), gene expression is turned off, and the fur color is white. Cooler parts of the rabbits body (anything cooler than 35°C) turn on the gene, creating black fur. The Himalayan rabbit shown below lost black fur on her ears due to an infection. When new fur grew back in, it was white, showing that this new round of gene expression occurred at temperatures greater than 35°C.

              

Another example of environmental factors influencing gene expression is metamorphosis. Metamorphosis is regulated by external and internal factors, including temperature, available resources, and hormones. For example, a tadpole in a pond will go through many physical changes as it responds to the environment. Environmental conditions trigger hormone release that allows the tadpole to create new specialized cells so that it can survive in its environment.

Look at the pictures below. Tadpole A will spend about 16 weeks of its life undergoing metamorphosis before becoming an adult frog. Tadpole B is living in an area of drought, and in response to its environment, it will release hormones to speed up the process of metamorphosis. Tadpole B will spend about 10 weeks of its life undergoing metamorphosis before becoming an adult frog. Both tadpoles will survive to become adult frogs, but their cell differentiation is controlled at different rates.

 

Source: 
Himalayan Rabbit. Retrieved from https://www.flickr.com/photos/wilsonhui/15168552338/in/photolist-fgjuJN-fg55SK-fgjhPh-fhNXBe-p7oN9s-p6S6Lw-Vmek4-ctagdY-c7XqGb-6KGPKf-c628zd-ccQSDS-2tHADV-2owR4n

Specialized Plant Cells

Now that you know how living organisms transition from one single cell to many differentiated cells, let’s discuss some of the specialized cells that are made as a result of this process. In plants, cells are specialized for the roles they play in the plant’s survival. Three of the many specialized cell types in plants include the cells of the roots, stems, and leaves.

Directions: Click on each plus sign to learn more about these specialized cells. 

Specialized Animal Cells

Animals also require many different specialized cells in order to function. Some cells must be able to move (like sperm cells), while other cells need to contract (like muscle cells). Three of the many specialized cell types in animals include red blood cells, muscle cells, and skin cells.

Directions: Click on each plus sign to learn more about these specialized cells.

Journal Activity