I. THE EUKARYOTIC CELL
A. COMPOSITION AND FUNCTIONS OF EUKARYOTIC CELLULAR STRUCTURES
3. The Endomembrane System
a. The Nucleus
The overall purpose of this Learning Object is to learn the chemical makeup and the functions associated with the nucleus in eukaryotic cells.
LEARNING OBJECTIVES FOR THIS SECTION
The cell is the basic unit of life. Based on the organization of their cellular structures, all living cells can be divided into two groups: prokaryotic and eukaryotic (also spelled procaryotic and eucaryotic). Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types. Only bacteria have prokaryotic cell types.
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.
We will now look at the nucleus of eukaryotic cells.
The Nucleus (def) (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.
a. Eukaryotic cells contain much more DNA than do bacteria, and this DNA is organized as multiple chromosomes located within a nucleus.
b. The nucleus (def) in eukaryotic cells is separated from the cytoplasm by a nuclear envelope (nuclear membrane) (see Fig. 32).
c. The nucleus divides my mitosis (def), a process that ensures each daughter cell receives the same number of chromosomes as the original parent cell. Haploid (def) sex cells are produced from diploid cells (def) by meiosis (def).
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 (def) (see Fig. 31), and linear chromosomes (def) composed of negatively charged DNA associated with positively charged basic proteins called histones (def) to form structures known as nucleosomes. The nucleosomes are part of what is called chromatin (def), 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. The DNA is the genetic material of the cell composed of genes that codes for protein synthesis.
The DNA in eukaryotic cells is packaged in a highly organized way. It consists of a basic unit called a nucleosome (def), 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 (def), 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 (def) producing cells with half the normal number of chromosomes (one from each homologous pair).
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