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7.3A: The Nucleus

Skills to Develop

  1. Describe the structure and the function of the nucleus in eukaryotic cells.
  2. Define the following:
    1. nuclear envelope
    2. nucleolus
    3. nucleosome

Eukaryotic cells are generally much larger and more complex than prokaryotic. Because of their larger size, they require a variety of specialized internal membrane-bound organelles to carry out metabolism, provide energy, and transport chemicals throughout the cell. Eukaryotic cells possess what is referred to as an internal membrane system or endomembrane system that compartmentalizes the cell for various different but interrelated cellular functions. Some of these internal membrane-bound organelles, such as the nucleus and the endoplasmic reticulum, have direct connections to one another. Other organelles, such as the endoplasmic reticulum and the Golgi complex transport materials to other organelles in vesicles. A vesicle buds off of one organelle and transports materials when it fuses with another membrane.

The Nucleus

(see Fig. 31, Fig. 32A and Fig. 30)

Prokaryotic and eukaryotic cells differ a great detail in both the amount and the organization of their molecules of DNA. Eukaryotic cells contain much more DNA than do bacteria, and this DNA is organized as multiple chromosomes located within a nucleus. The nucleus in eukaryotic cells is separated from the cytoplasm by a nuclear envelope (nuclear membrane) (Figure 7.3A.1). The nucleus divides my mitosis , a process that ensures each daughter cell receives the same number of chromosomes as the original parent cell. Haploid sex cells are produced from diploid cells by meiosis.

Figure 7.3A.1: Candida albicans (Eukaryotic Cell)

The nuclear envelope consists of inner and outer membranes separated by a perinuclear space and having pores that connect with the endoplasmic reticulum (see Fig. 31, Fig. 32, and Fig. 33). The pores in the nuclear membrane allow ribosomal subunits and mRNA transcribed off genes in the DNA to leave the nucleus, enter the cytoplasm, and participate in protein synthesis.

Inside the nucleus is a fluid called nucleoplasm, a nucleolus (see Fig. 31), and linear chromosomes composed of negatively charged DNA associated with positively charged basic proteins called histones to form structures known as nucleosomes. The nucleosomes are part of what is called chromatin , the DNA and proteins that make up the chromosomes. The nucleolus is an area within the nucleus that is involved in the assembly of ribosomal subunits. An area of DNA called the nucleolar organizer directs the synthesis of ribosomal RNA (rRNA) that subsequently combines with ribosomal proteins to form immature ribosomal subunits that mature after they leave the nucleus by way of the pores in the nuclear envelope and mature in the cytoplasm. Genes located along the DNA are transcribed into RNA molecules, primarily messenger RNA (mRNA), transfer RNA (tRNA, and ribosomal RNA (rRNA). Messenger RNA is then translated into protein at the ribosomes. In general then, DNA determines what proteins and enzymes an organism can synthesize and, therefore, what chemical reactions it is able to carry out.

The DNA in eukaryotic cells is packaged in a highly organized way. It consists of a basic unit called a nucleosome , a beadlike structure 11 nm in diameter that consists of 146 base pairs of DNA wrapped around eight histone molecules. The nucleosomes are linked to one another by a segment of DNA approximately 60 base pairs long called linker DNA (see Fig. 27A). Another histone associated with the linker DNA then packages adjacent nucleotides together to form a nucleosome thread 30nm in diameter. Finally, these packaged nucleosome threads form large coiled loops that are held together by nonhistone scaffolding proteins. These coiled loops on the scaffolding proteins interact to form the condensed chromatin seen in chromosomes during mitosis.

When the cell is not replicating, the DNA and proteins appear as a threadlike mass called chromatin. During mitosis , the chromatin coils into thick rodlike bodies called chromosomes (see Fig. 31A) and a spindle apparatus guides the separation and movement of the chromosomes for cell division so each cell winds up with a full complement of chromosomes. During sexual reproduction the nuclei of sex cells divide by meiosis producing cells with half the normal number of chromosomes (one from each homologous pair).

Concept map for Eukaryotic Cell Structure

Summary

  1. Eukaryotic cells contain much more DNA than do bacteria, and this DNA is organized as multiple chromosomes located within a nucleus.
  2. The nucleus in eukaryotic cells is separated from the cytoplasm by a nuclear envelope.
  3. The nucleolus is an area within the nucleus that is involved in the assembly of ribosomal subunits.
  4. Genes located along the DNA are transcribed into RNA molecules, primarily messenger RNA (mRNA), transfer RNA (tRNA, and ribosomal RNA (rRNA). Messenger RNA is then translated into protein at the ribosomes.
  5. In general then, DNA determines what proteins and enzymes an organism can synthesize and, therefore, what chemical reactions it is able to carry out.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

  1. Match the following:

    _____ Separates the chromosomes from the cytoplasm. (ans)

    _____ An area within the nucleus that is involved in the assembly of ribosomal subunits. (ans)

    _____ A basic unit of eukaryotic DNA appearing as beadlike structures consisting of DNA wrapped around histone molecules. (ans)

    1. nuclear envelope
    2. nucleolus
    3. nucleosome
    4. chromosomes

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)