Viruses have the genetic material made of deoxyribonucleic acid - DNA (deoxyriboviruses) or ribonucleic acid - RNA (riboviruses). Prokaryotic (pro = before and karyon = cell nucleus) cells have the genetic material made of DNA, that is located in a cellular region called nucleoid (meaning nucleus-like). Eukaryotic (eu = good and karyon = cell nucleus) cells have the genetic material made of DNA, that formed a cell nucleus (1, 2, 3).
In this synthesis, you can read some information about the nucleoid and cell nucleus.
Nucleoid (genofor) is the genetic material of prokaryotes (bacteria and blue-green algae). The nucleoid is located central into the cell and it communicate directly with the cytoplasm because a nucleoid envelope not exist. Nucleoid consists of a single bacterial chromosome, but there are bacterial cells with 2-4 chromosomes. Bacterial chromosome made of a single circular DNA molecule. This DNA contains genes that coordinate cell metabolism, growth and cell division (amitosis). Bacteria have an independent genetic material called plasmids (4, 5). Fertility F-plasmid are capable of conjugation, transduction, transformation (6). Observing bacterial conjugation, the specialists in molecular biology have produced a multitude of vectors used for genes transfer, protein expression, etc.
Cell Nucleus
It would be useful to examine the cell cycle for understanding the structure of cell nucleus and chromosomes. Cell cycle can be defined as the time between formation of the cell by cell division, and the end of her division. Thus, the cell cycle has two periods: interphase and cell division cycle. Interphase consists of three distinct phases: G1 phase, S phase (DNA is replicated) and G2 phase. Cell division cycle has the following phases: prophase, metaphase, anaphase, telophase followed by cytokinesis. During cell division cycle, the cell chromosomes are segregated and DNA is separated between the two daughter-cells. DNA replication and separation are coordinating by several cellular mechanisms (7, 8, 9, 10).
Given the information presented, further, we describe features of interphase nucleus and chromosome.
In interphase, cell nucleus consists of a nuclear envelope (NE), nuclear lamina, nucleolus, nucleoplasm, chromosomes (chromatin) and other subnuclear body. The nuclear envelope is composed of two membranes (inner and outer) that separate the genetic material from the cytoplasm. The outer membrane is continuous of the rough endoplasmic reticulum (RER) (11). Macromolecules cannot diffuse across the double membrane. Contrary, the steroid hormones and other small lipid soluble molecules (signalling molecules) can diffuse through the nuclear double membrane. In addition, the nuclear membrane has a system of nuclear pores that allowing mediated transport (karyopherins) of most proteins, ribosomal subunits and RNA molecules (11). Nuclear lamina shows two networks located on one side and the other of nuclear membranes. Role of the two networks is to anchor the pores and chromosomes (12). Nucleoplasm (nuclaer sap) has two components - a viscous liquid and a nuclear matrix (network of fibres). This type of protoplasm surrounds the chromosomes (chromatin) and nucleoli. Free nucleotides and enzymes are dissolved in the nucleoplasm (13). Nucleolus (nucleole) is a non-membrane structure composed of proteins and nucleic acids, and it is now been determined their role in the synthesis of ribosomal subunits (14).
Chromatin consists of nucleoproteins or DNA-proteins complex (DNA, histone and non-histone proteins). DNA is a long polymer (22 to 26 Å wide) made from repeating simple units called nucleotides, that are the genes and regulating elements. The histones (H1/H5, H2A, H2B, H3 and H4) are alkaline proteins, that are involved in packaging the DNA into successive structural units called nucleosomes, ''beads-on-a-string'' structures, solenoid structure (the 30 nm fibre) and chromosome. In addition, these proteins and non-histone proteins (ex. polycomb) play a role in genes regulations. Electron microscopy has shown two types of chromatin: euchromatin (active transcription) and heterochromatin (constitutive and facultative) (15).
It's time to presents the characteristics of chromosomes. Chromosome is a chromatic structures observed under the microscope, during cell division (mitosis and meiosis). Eukaryotic species have a double (diploid) number of chromosomes in their somatic cells (2n - diploid cells) and a simple (haploid) number in germ cells (n - haploid cells). For example, the human somatic cells contain 46 chromosomes in 23 pairs of homologues chromosomes (2n = 46) and 23 chromosomes in germ cells (n = 23). Chromosomes are of two types - autosomes or non-sex chromosomes (humans have 44) and heterosomes or sex-chromosomes called X and Y. Chromosomes disorder, in number (aneuploidy) or structure (ex. deletion), determine the chromosomal and metabolic diseases (ex. Down syndrome, phenylketonuria-PKU) (16, 17).
A metaphase chromosome consists of two chromatids joined by a centromere. At the end the chromosomes have a region of repetitive DNA, called telomere (16). This region protects the end of chromosome from deterioration due to an enzyme called telomerase. This enzyme that adds DNA sequence repeats - TTAGGG to the telomere regions (17).
A chromosome consists of a single molecule DNA spooled around histone proteins. The nucleosomes are the fundamental repeating unit that are used to pack the eukaryotic DNA into the nucleus. Nucleosome consists of the core histone octamer and a DNA fragment (147 base pairs) wrapped in left-handed superhelical turns around a core histone octamer. To form chromosomes, the nucleosomes are folded in successive structures: ''beads-on-a-string'' structures and solenoid structure (the 30 nm fibre) (16, 18).
In conclusion, DNA contains genes and regulating elements and the nuclear proteins that have several roles: protection for DNA, gene regulation, enzymatic role, transmission of genetic information during cell divisions etc.
Plants and recombinant proteins1. Karyotupe
2. Human Genome Project
3. Changes in Chromosome Structure
4. Mechanism of steroid hormone
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