1. Nucleolemma 2. Nucleoplasm 3. Chromatin 4. Nucleolus or nuclear
membrane
a) heterochromatin b) euchromatin
Nucleus is separated from the cytoplasm by the nucleolemma. It has such morphological features:
1) It consists of two biomembranes - inner (7-8 nm) and outer (7 - 8 nm).
2) Membranes of the nucleolemma are separated by perinuclear space (20 – 60 nm).
3) There are nuclear pores in the nuclear envelope for the communication between cytoplasm and nuclear cavity. These pores are formed as a result of confluence of inner and outer membranes.
4) Nuclear pores contain fibrillar-globular molecular complexes forming the diaphragms. They permit and regulate the exchange of macromolecules and ribosomal subunits the substances between nucleus and cytoplasm. The more of nuclear pores are, the more intensively bioenergetic processes in the cell occur.
The nucleus contains:
1. Nucleoplasm. This is a fluid part of the nucleus with nuclear structure here. Nucleoplasm is like healoplasm according to its features.
2. Chromatin is the structural manifastation of chromosomes in interphase. Сhromosome is the structural and functional unit of the nucleus. It is connected with the storeage and transmission of genetic information. The molecule of DNA is molecular base of chromosome. The genetic information is in this molecule recorded.
Interphase chromosome structure.
Each chromosome contains one long DNA molecule (consisting of double helix) and DNA - connecting proteins - histones. DNA molecule is coiled on the histone proteins. Histones are situated along DNA not evenly, but in the form of blocks consisting of eight histone molecules. The protein block is called core. The site of DNA strand on the histone core is called nucleosome.
Nucleosomes form chromatin fibril. Chromatin fibril represent a beaded strand, where each bead is a nucleosome. Aditional condensation of chromatin fibril leads to the formation of loop domain. Condensed sites of chromosomes are formed, which are different from decondensed sites with less compact envelope of chromatin fibril. The presence of condensed and decondensed sites are characteristic for interphase chromosomes.
In the microscopic preparations the condensed sites represent heterochromatin (nonactive) and decondensed ones euchromatin (active), with which the information needed for the protein synthesis replicates.
Structure of nucleolus.
Nucleolus is the derivative of chromosomes. It consists of repeatedly replicated sites of DNA. Nucleolus is the place formation of ribosomatic RNA and ribosomes. It consists of fibrillar and granular components. Fibrillar component is ribonucleoproteinaceous bands, which are precursors of ribosomes and granulars are ribosome subunits, which are forming.
Cell cycle.
Cell like any biological system has the beginning and end of its existence. Life cycle of any cell begins with its formation as a results of division of precursor cell and consists of two main phases:
1. interphase (G1, S, G2 periods);
2. mitosis
Interphase is the whole life period of cell from its formation until its division. There are three periods in the interpfase: postmitotic, synthetic and premitotic. The length of each period is determined by the genetic programme.
Postmitotic (presynthetic) G1. This period is characterized by the cell growth, its differentiation, acquiring specific external and internal structure and carrying out its main functions. This is the period of intensified growth of a young cell by thanks to synthesis and accumulation of cell protein. The growth period completes in a certain period of time and the cell begins to fulfill the full volume of its functions. Under the influence of special inductors part of cells enter S phase.
Synthetic period (S). During this period of DNA molecules replication takes place, and hence, the number of chromosomes increased as well. At the end of this period the nucleus has not 2n (sets) but 4n (sets) of genetic material.
During S period centrioles dublication of cellular center occurs. In this period autosynthesis prevails over heretosynthesis. Autosynthesis is the period when synthesized protein is used by the cell itself and heretosynthesis is the period when proteins leave the cell. S-period is followed by G2 period.
Premitotic (postsynthetic) period G2. During this period heretosynthesis stops completely and synthesis of tubulin protein molecular occurs. The tubulin protein is necessary for formation of division spindle. G2 period is followed by mitosis.
In weakening of inductor action the cell can reduce biosynthetic processes considerably and turn into condition of relative rest G0. These are cells that stop dividing temporary or completely. There are several types of cells in G0 period.
The first type is the stem cells of various tissues (for example hemopoiesis). These are low differentiated cells, which are capable of dividing for a long time. They leave the cycle and enter G0 period. The cells, which lose their ability to divide during their differentiation, concern the second type. These cells, for example high specialized blood cells, perform their functions for certain period of time and then die. But there is a group of cells, which under special conditions can enter the cycle again and divide, for example liver cells.
The third type of cells during G0 period is high differentiated cells, which completely lose the ability to divide. Their life period is equal to the life period of the organism itself, for example nerve cells, cardiomyocytes.
Mitosis.
Mitosis or kariokinesis or indirect cell division. Somatic cells of the organism divide in this way. Mitosis consists of four phases: prophase, metaphase, anaphase and telophase.
- Prophase. During prophase chromosomes become coiled or condensed. Each of these chromosomes consists of two DNA molecules. The formation of the division spindle occurs as a result of divergency of centrioles cellular center to the poles of the cell. The division spindle is formed by polymerization of tubulin and is situated between centrioles. At the termination of prophase nucleolus disappears, nucleolemma destroyed and condensed chromosomes enterin the cytoplasm.
- Metaphase. All chromosomes move towards the equator of the cell. In the middle of the metaphase chromosomes are aligned along the equator and attached to the microtubules of the spindle by their centromeres (primary strangulation) and the free ends of chromosome face the cytolemma. At this stage one can see the mother aster.
- Anaphase. All sister chromatids (daughter chromosomes) in all chromosomes lose connection between themselves in the region of centromeres and begin moving toward the opposite poles of the cell.
- Telophase. When the chromosomes have diverged and stopped, telophase begins. Nucleolemma formation around every complex chromosomes and formation of new nucleoli occur. The division of cell body – cytokinesis, occurs by indrawing of cytolemma along the equatorial plate. As a result two new cell bodies of equal size are form. At the end of telophase chromosomes become decondensed, formation of membraneous and nonmembraneous structures of cell occur. These processes mean the beginning of the first life period of a new cell - G1 of interphase.
Polyploidy.
Polyploidy is the formation of cells with increased content of DNA. Appearance of polyploid somatic cells can be seen during the cytokinesis blokade. As a result two nuclei cells are formed, for example liver cells.
Endoreduplication.
Several cycles of DNA reduplication without subsequent mitosis occurs. This leads to increasing of DNA numbers in the nucleus. Such polyploid bone nuclei are in megakariocytes (cells of red marrow).
Meiosis.
Meiosis is a specialized type of cells division that produces the germ cells – ova and spermatozoa. This process has two crucial results: 1) reduction in the number of chromosomes from the diploid (2n) to the haploid (1n) number, and 2) recombination of genes, genetic variability and diversity of gene pool.
Meiosis is divided into two separate divisions.
Meiosis I (Reductional division).
Meiosis begins at the conclusion of interphase in the cell cycle. In gametogenesis, when the germ cells are in the S phase of the cell cycle preceding meiosis, the amount of DNA is doubled to 4n (sets) and the chromosome number is also doubled to 4n.
Prophase I
Prophase of meiosis I last a long time and subdivided into the five phases.
1. Leptotene. Individual chromosomes, composed of two chromatids begin to condense, forming long strands in the nucleus.
2. Zygotene. Homologous pairs of chromosomes approximate each other, lining up in register (gene locus to gene locus), and make the synaptonemal complex, forming tetrad of chromatids.
3. Pachytene. Chiasmata (crossing over sites) are formed, exchange of genetic material occurs between homologous chromosomes.
4. Diplotene. Chromosomes begin to separate, revealing chiasmata.
5. Diakinesis. Chromosomes condense maximally and nucleolus disappears.
