Cell Nucleus - Questions and Answers
1. What are cells with a delimited nucleus called ? What are the main elements of the nucleus?
Cells with delimited nucleus are called eukaryotic cells. Organisms composed of one or more eukaryotic cells are called eukaryotes.
The mains elements of the nucleus are the chromatin (made of DNA molecules), the nucleolus, the karyolymph, or nucleoplasm, and the nuclear membrane (or karyotheca).
2. Do all eukaryotic cells have nucleus and only one nucleus?
There are eukaryotic cells without a nucleus and others with more than one nucleus. Osteoclasts, the cells responsible for resorption of the osseous matrix, for example, are multinucleate cells; striated muscle fibers are multinucleate too. Red blood cells are an example of enucleated specialized cells.
Cell Nucleus Review - Image Diversity: cell nucleus miltinucleate cells enucleated cells
3. Of which substances is chromatin made?
Chromatin is made of DNA molecules associated to proteins called histones.
Cell Nucleus Review - Image Diversity: chromatin
4. What are heterochromatin and euchromatin?
Chromatin is uncondensed nuclear DNA, the typical DNA morphology in interphase (the phase of the cell cycle in which the cells is not dividing itself). In this phase of the cell cycle chromatin can be found as heterochromatin, more condensed and dark (in electronic microscopy) portions of DNA molecules, and as euchromatin, less condensed and lighter portions of DNA molecules.
Since it is uncondensed the euchromatin is the biologically active portion of the DNA, i.e., the region that has active genes to be transcripted into RNA. The heterochromatin represents the inactive portions of the DNA molecule.
Cell Nucleus Review - Image Diversity: heterochromatin euchromatin
5. What is the relation between the concepts of chromatin and chromosome? Are euchromatin and heterochromatin part of chromosomes?
Every filament of chromatin is a complete DNA molecule (a complete double helix), i.e., a complete chromosome. A DNA molecule may form euchromatin and heterochromatin portions thus both are part of chromosomes.
Cell Nucleus Review - Image Diversity: chromosome structure
6. In the phase when the cell is not dividing (interphase) is there activity within the cell nucleus?
In the interphase there is intense metabolic activity in the cell nucleus: DNA is duplicating, euchromatin is being transcripted and RNA is produced.
7. How are the concepts of chromosome, chromatin and chromatids related? In which phase of the cell cycle does DNA duplicate?
Chromatin is a set of filamentous DNA molecules dispersed in the karyoplasm forming euchromatin and heterochromatin portions. Each chromatin filament is a complete chromosome (a DNA molecule, or double helix). The chromatin of the human somatic cell is formed by 46 DNA molecules (22 homologous chromosomes and 1 pair of sex chromosomes).
In interphase the cell prepares itself for division and duplication of DNA molecules occurs. The duplication of every DNA molecule forms two identical DNA double helix bound by a structure called centromere. In this phase each identical chromosome of these pairs is called chromatid. It is also during the interphase that the chromatids begin to condensate assuming the thicker and shorter shape typical of chromosome illustrations. So the phase of the cell cycle in which DNA duplicates is the interphase.
Some Biology textbooks call the chromosome a unique filament of chromatin as well as the condensed structure made of two identical chromatids after the DNA duplication. Rigorously the pair of identical chromatids bound in the centromere are two copies of the same chromosome and therefore they are two identical chromosomes (and not only one).
Cell Nucleus Review - Image Diversity: chromatids
8. What is the structure that maintains identical chromatids bound?
The structure that maintains identical chromatids bound is the centromere.
Cell Nucleus Review - Image Diversity: centromere
9. How is the chromosome region where the centromere is located called? How are chromosomes classified in relation to the position of their centromere?
The chromosome region where the centromere is located is called primary constriction. In microscopic view this region is narrower (a stricture) than most part of the chromosome.
According to the position of the primary constriction the chromosomes are classified as telocentric, acrocentric, submetacentric or metacentric.
10. What are the primary and the secondary constrictions of a chromosome? What is the other name given to the secondary constriction?
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).
11. What are homologous chromosomes? Which are the human cells that do not have homologous chromosomes?
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.
12. What is the difference between the concepts of karyotype and genome?
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
13. Can two normal individuals of the same species with sexual reproduction have identical genomes and identical karyotypes? How is the human karyotype usually represented?
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.
14. What is the other name given to sex chromosomes? What is the function of 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.
15. How many chromosomes does a human normal haploid cell have? How many chromosomes does a human normal diploid cell have? How many are the sex chromosomes within each of them?
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.
16. Do phylogenetically proximal species have cells with proximal chromosome counts?
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.
17. What is the nucleolus?
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
18. Of which structures is the nuclear membrane composed?
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.
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