Eukaryotic cells have nucleus delimited by two juxtaposed membranes that continue with the membrane of the endoplasmic reticulum. The nuclear membrane, or karyotheca, presents pores through which substances pass. There are also ribosomes adhered to its external surface.
The nucleolus is a small and optically dense region in the interior of the cell nucleus. It is made of ribosomic RNA (rRNA) and proteins. One nucleus can have one or more nucleolus.
Cell Nucleus Review - Image Diversity: nucleolus
The number of chromosomes typical of each species is proximal for phylogenetically proximal species (for example, orangutan, gorilla, chimpanzee and human). But it is not impossible that evolutionary distant species, like rat and oat, bears similar karyotypes and the same total number of chromosomes.
Even presenting equal number of chromosomes evolutionary distant species have radically different characteristics since the quantity and the sequence of nucleotides that compose their respective DNA molecules are quite different.
The human haploid cell is the gamete (egg cell and sperm cell). The human gamete has 22 autosomes and 1 allosome, i.e., 23 chromosomes. The diploid cell is the somatic cell and it has 44 autosomes and 2 allosomes, i.e., 46 chromosomes.
Gametes have one sex chromosome and somatic cells have two sex chromosomes.
Sex chromosomes are also called allosomes (the other chromosomes that are not sex chromosomes are called autosomes).
Sex chromosomes get such name because they have genes that determine the sex (male or female) of an individual. Sex chromosomes also have genes related to other biological functions.
Except for clones (individuals created from nucleus transplantation, like the Dolly sheep) and monozygotic twins, it is very improbable the genomes of two individuals of the same species and generated by sexual reproduction to be identical. Nevertheless the karyotypes of two normal individuals of the same species and of the same sex are always identical. The human normal karyotype is represented by the formula 44+XX for women and 44+XY for men.
Genome is the set of DNA molecules that characterizes each living being or each species. The concept then includes the specific nucleotide sequence of the DNA molecules of each individual or species. Karyotype is the set of chromosomes of individuals of a given individual or species concerning morphology and number of each chromosome or pair of homologous.
Cell Nucleus Review - Image Diversity: karyotype
Chromosomes contain genes (genetic information in the form of nucleotide sequences) that command the protein synthesis thus regulating and controlling the activities of the cell. In the nucleus of somatic cells of diploid beings every chromosome has its correspondent homologous chromosome, both containing alleles of the same genes related to same functions. This occurs because one chromosome of one pair comes from the father and the other comes from the mother of the individual. The chromosomes that form a pair with alleles of the same genes are called homologous chromosomes. In humans, there are 22 pairs of homologous chromosomes plus the pair of sex chromosomes (the sex chromosomes are partially homologous).
The only human cells that do not have homologous chromosomes are the gametes since during meiosis the homologous chromosomes are separated.
Primary constriction is the narrower region of a condensed chromosome where the centromere, the structure that unites identical chromatids, is located. Secondary constriction is a region similar to the primary constriction, narrower than the normal thickness of the chromosome too, and in general it is related to genes that coordinate the formation of the nucleolus and control the ribosomic RNA (rRNA) synthesis. For this reason the secondary contrictions (that can be one or more in chromosome) is called nucleolus organizer region (NOR).