Gene Expression And Cell Specialization

Unit : Gene expression and Regulation

Chapter: Gene expression and Cell specialization

Reference: Prokaryotic gene expression, Eukaryotic gene expression, Cell differentiation, Role of DNA in cell differentiation

Learning objectives

  • To learn about prokaryotic and eukaryotic gene expression
  • To understand cell differentiation

Prokaryotic gene expression

  • Prokaryotic organisms are single-celled organisms that lack a defined nucleus; therefore, their DNA floats freely within the cell cytoplasm.
  • To synthesize a protein, the processes of transcription (DNA to RNA) and translation (RNA to protein) occur almost simultaneously.
  •  When the resulting protein is no longer needed, transcription stops. Thus, the regulation of transcription is the primary method to control what type of protein and how much of each protein is expressed in a prokaryotic cell.
  •  All the subsequent steps occur automatically. When more protein is required, more transcription occurs.
  • Therefore, in prokaryotic cells, the control of gene expression is mostly at the transcriptional level.

Eukaryotic gene expression

  • Eukaryotic cells have intracellular organelles that add to their complexity.
  • In eukaryotic cells, the DNA is contained inside the cell’s nucleus where it is transcribed into RNA.
  • The newly-synthesized RNA is then transported out of the nucleus into the cytoplasm where ribosomes translate the RNA into protein.
  • The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus within the cytoplasm.
  • The regulation of gene expression can occur at all stages of the process. Regulation may occur when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (epigenetics), when the RNA is transcribed (transcriptional level), when the RNA is processed and exported to the cytoplasm after it is transcribed (post-transcriptional level), when the RNA is translated into protein (translational level), or after the protein has been made (post-translational level).

Cell differentiation:

  • The process by which a single cell after division becomes a distinct cell type according to its function.
  • Cell differentiation is a process by which a particular cell becomes specific or specialized.
  • In the human body stem cells can differentiate multiple cell types, as these cells can continuously divide to differentiate into different cell types.
  • During the development of the embryo, the zygote also undergoes differentiation

Role of DNA in cell differentiation

  • Deoxyribonucleic Acid, or DNA, controls the way cell’s function. It also determines what type of specialized cells will be made.
  • Stem cells are cells that can become any type of specialized cell in the body.
  • 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.
  • 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.

Solved examples

Example 1. As a fertilized egg divide, the cells differentiate because they-

a) have different sets of DNA passed on to the daughter cells

b) have different sets of genes activated in the daughter cells

c)have mutations that cause different genes to appear in the daughter cells

d)randomly assort the chromosomes into different daughter cells

Solution 1: b. As a fertilized egg divide, the cells differentiate because they have different sets of genes activated in the daughter cells

Example 2. What is the role of stem cells in cell differentiation?

a) They serve as a precursor for generalized cells

b) These cells can turn into specialized cells and back into generalized cells when needed

c) They are generalized cells that can lead to specialized cells

d) They are specialized cells that can lead to generalized cells

Solution 2: c. Stem cells are generalized cells that can lead to specialized cells.

 Summary

  • In prokaryotic cells, the control of gene expression is mostly at the transcriptional level.
  • The processes of transcription and translation are physically separated by the nuclear membrane in eukaryotes
  • Cell differentiation is a process by which a particular cell becomes specific or specialized.
  • Deoxyribonucleic Acid, or DNA, controls the way cell function. It also determines what type of specialized cells will be made.

 

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