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Biology

Chapter 11: Gene Expression - Differentiation

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Lecture

Specialization: Eukaryotic Cells and Differentiation

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Specialization: Eukaryotic Cells and Differentiation

The variety of controls available to the cell allow it to specialize its functions, and allow tissues to develop and perform specific tasks efficiently. Fewer mechanism would limit what the cell can do and curtail its ability to respond successfully to changes in the environment. Because there so many types of controls, cells can tailor what they do, and when and how they do it, depending on their environment, the state of the tissues, organs, and organism they are part of, the age and development of the organism, the availability of resources, and many other factors.

Eukaryotic cells

Eukaryotic organisms have special problems with DNA and RNA replication. They differ in two important ways from the prokaryotic cells of bacteria: eukaryotic DNA is much more complicated than the single loop of DNA found in bacteria, and eukaryotic DNA must code for all possibilities. This is because most eukaryotic organisms are multicellular, with different cells performaning specialized functions. Even in single-celled eukaryotic organisms, such as amoebas, the cell has many specialized organelles, each of which must have its own set of instructions so that the cell can create, replace, and control the organelles.

In order to meet these requirements, the DNA found in eukaryotic must be able to manufacture proteins efficiently and selectively. Efficiency is enhanced by using multiple copies of genes, to allow for simultaneous transcription of a protein's mRNA.

Another tactic improves on the operon organization found in bacteria. Eukaryotic genes have areas "upstream" (as the RNA polymerase moves) from the actual genes which program for enzymes. These Upstream Promotor Elements or enhancer sites, along with special activity proteins, increase the rate of replication by causing the DNA to bend in such a way that the RNA polymerase can attach more easily and quickly. There also exist repressor or silencer proteins, which prevent the DNA from forming the necessary kinks to attach RNA polymerase.

To better see how enhancers control gene expression, go to the McGraw Hill Biology text site animations on genetic expression, and click on "Transcription Complex and enhancers", which explains how transcript works with enhancers and TATA proteins.

  • What are transcription factors (TATA + basal factors)? How do they help RNA polymerase to attach to the DNA?
  • What are activators and coactivators?
  • What are enhancers? What happens when activators bind to enhancers?
  • How do repressors and silencer sequences block expression of genes?

Now work through "Control of Gene Expression in Eukaryotes". We've already seen how enhancers affect the rate at which RNA polymerase can transcribe information information in DNA to mRNA.

  • How does splicing enable the cell to produce different RNA sequences from the same DNA?
  • Why must mRNA transport across the nuclear membrane be an active process? How might exposure to active transport channels affect the mRNA?
  • How do ribonucleases affect mRNA functions and thereby control gene expression?
  • What factors affect the rate of protein synthesis?
  • What is phosphorylation? What happens in post-translational modification to affect gene expression?

Another tactic eukaryotic cells use is to attache the mRNA strands to caps and tails which prevent its premature breakdown. This means that the mRNA survives longer and can make more protein copies before it is used up.

Eukaryotic DNA contains regions called introns (unexpressed DNA) and exons (expressed DNA). The presence of introns increases the length of DNA strands, which in turn increases the possibility for crossing-over during meiosis to produce variety in the resulting chromosomes. While the intron sequences are transcribed into RNA by RNA transcriptase, before the RNA leaves the nucleus, other enzymes cut the intron-RNA sequences from the RNA strand and splice the remaining RNA together into mRNA. Differences in splicing may result in a "intron" DNA sequence becoming an "exon" DNA sequence in a subsequent transcription process. A single DNA sequence can thus produce different mRNA sequences, depending on the cell's requirements at any given time during development or maturity of the cell or the organism.

All of these mechanisms add up to a complex but very flexible system which allows a cell to fine tune its production of a specific enzyme in response to different conditions.

Differentiation and development

Differentiation is the process which results in specialized cells. Some cells (such as red blood cells) lose their DNA, but as Briggs and King's experiments (see text) with embryonic frogs show, most cells keep the entire set of DNA, even when they do not express it. The single zygote cell with its compliment of DNA from the ovum and the sperm divides by a mitotic reproduction into two, then four, then eight cells. During early divisions, the cells remain pluripotent stem cells, or undifferentiated cells able to produce many kinds of tissue cells. Stem cells can become any kind of tissue required by the organism. As the number of cells continue dividing, some stem cells turn off some exons and may turn on others. The change in gene expression is permanent, in that cells descended by mitotic division from differentiated cells inherit the parent cell characteristics. In some cases, hormones produced by one cell act as signals to cause differentiation in neighboring cells.

A key challenge to biologists is determining whether a differentiated cell can be reversed to produce stem cells. This should be theoretically possible, since with few exceptions (red blood cells in humans, for example, lose their nuclei and do not reproduce), somatic cells contain copies all of the DNA from the original parent cells. With stem cells, it is possible to grow new tissues to replace lost or damaged tissues. Plants in particular are able to grow an entire plant form a segment of the plant, not just from the seed. Such cells are called totipotent, or "able to do it all". As an organism develops, its cells need mechanisms which "turn off" the expression of some genes in favor of others so that different types of cells form.

In fruit flies, the first seven to eight divisions of the zygote results in thousands of nuclei, not one per cell. At this point, the nuclei move to the outer edges of the cells and some genes are segmented--that is--they are broken into segments by gaps or special pairings. The genes within the segment are activated or turned off, depending on the location of the nuclei containing them, so that the appropriate fly-part will develop (head, tail, legs, etc.). Each part is controlled by homeotic genes, and it is mutations in these genes which cause severe displacement of body parts in the fly, such as the appearance of legs in place of antennae!