18 Questions on the Function of the Cell Nucleus

2. Do all eukaryotic cells have only one nucleus?

Some eukaryotic cells do not contain a nucleus and others contain more than one. For example, osteoclasts, the cells responsible for resorption of bone matrix, are multinucleate cells, meaning they have more than one nucleus. Striated muscle fibers are also multinucleate. Red blood cells are an example of enucleated (no nucleus) specialized cells.

• Cell Nucleus Review - Image Diversity: cell nucleus multinucleate cells enucleated cells

3. What substances is chromatin made of?

Chromatin is made of DNA molecules bound 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 during interphase (the phase of the cell cycle in which the cell does not divide). During this phase of the cell cycle, which is the chromatin can be found as heterochromatin, more condensed portion of DNA molecules and which appears darker under electron microscopy, and as euchromatin, which is less condensed and composes the lighter portions of DNA molecules.

Since it is less condensed, euchromatin is the biologically active part of the DNA, that is, the region that contains active genes to transcribe into RNA. Heterochromatin makes up the inactive portions of the DNA molecule.

• Cell Nucleus Review - Image Diversity: heterochromatin euchromatin

5. What is the relationship between the concepts of chromatin and chromosomes? Are euchromatin and heterochromatin a part of chromosomes?

Every filament of chromatin is a complete DNA molecule (a complete double helix), or rather, a complete chromosome. A DNA molecule may contain euchromatin and heterochromatin portions and, as a result, both are a part of chromosomes.

• Cell Nucleus Review - Image Diversity: chromosome structure

6. During the phase when the cell is not dividing (interphase), is there activity within the cell nucleus?

During 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 chromosomes, chromatin and chromatids related? During which phase of the cell cycle does DNA replicate?

Chromatin is a set of filamentous DNA molecules dispersed in the karyoplasm, made up of 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).

During interphase, the cell prepares itself for division and the duplication of DNA molecules occurs. The duplication of every DNA molecule forms two identical DNA double helices bound by a structure called the centromere. During this phase, each identical chromosome of these pairs is called a chromatid. It is also during interphase that chromatids begin to condense, taking on the thicker and shorter shape typical of chromosome illustrations. Therefore, the phase of the cell cycle during which DNA duplicates is interphase.

Some biology textbooks refer to a chromosome as a unique filament of chromatin as well as the condensed structure made of two identical chromatids after DNA duplication. The pair of identical chromatids bound in the centromere is always made up of two copies of the same chromosome, therefore, they are two identical chromosomes (and not only one).

• Cell Nucleus Review - Image Diversity: chromatids

8. What structure maintains the binding of identical chromatids?

The structure that maintains the binding of identical chromatids is the centromere.

• Cell Nucleus Review - Image Diversity: centromere

9. What is the name given to region of the chromosome where the centromere is located? How are chromosomes classified in relation to the position of their centromere?

The region of the chromosome where the centromere is located is called primary constriction. Under a microscopic view, this region is narrower (a stricture) than most parts of the chromosome.

Depending on the position of the primary constriction, chromosomes are classified as telocentric, acrocentric, submetacentric or metacentric.

10. What are the primary and 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 binds identical chromatids, is located. The secondary constriction is a region similar to the primary constriction, narrower than the normal thickness of the chromosome, and which is normally related to genes that coordinate the formation of the nucleolus and which control ribosomal RNA (rRNA) synthesis. For this reason, secondary constrictions (there can be one or more in a chromosome) are called nucleolus organizer regions (NOR).

11. What are homologous chromosomes? Which human cells do not have homologous chromosomes?

Chromosomes contain genes (genetic information in the form of nucleotide sequences) that control protein synthesis, thus regulating and controlling cell activities. In the nuclei of somatic cells of diploid beings, every chromosome has its corresponding homologous chromosome, both of which contain alleles of the same genes related to the same functions. This occurs because one chromosome of one pair comes from the father and the other comes from the mother of an individual. 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 one pair of sex chromosomes (sex chromosomes are partially homologous).

The only human cells that do not have homologous chromosomes are gametes, as during meiosis, the homologous chromosomes are separated.

12. What is the difference between karyotype and a genome?

A genome is the set of DNA molecules that characterizes each living being or each species. This concept includes the specific nucleotide sequence of the DNA molecules of each individual or species. A karyotype is the set of chromosomes of a given individual or species, and focuses on the number of pairs of chromosomes as well as their morphology. 

• 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 Dolly the sheep) and monozygotic twins, it is very improbable that the genomes of two individuals of the same species generated by sexual reproduction will be identical. Nevertheless, the karyotypes of two normal individuals of the same species and of the same sex are always identical. The normal human 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 (other chromosomes that are not sex chromosomes are called autosomes).

Sex chromosomes take their name from the fact that they have genes that determine the sex (male or female) of an individual. Sex chromosomes also contain genes related to other biological functions.

15. How many chromosomes does a normal human haploid cell have? How many chromosomes does a normal human diploid cell have? How many sex chromosomes do each of them contain? 

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 close species have cells with similar chromosome counts?

The number of chromosomes typical of each species is similar for phylogenetically close species (for example, orangutans, gorillas, chimpanzees and humans). However, it is not impossible for evolutionarily distant species, such as rats and oats, to have similar karyotypes and the same total number of chromosomes.

Even if they present equal number of chromosomes, evolutionarily distant species have radically different characteristics, since the quantity and the sequence of nucleotides that make up their 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. What structures make up the nuclear membrane?

Eukaryotic cells have a nucleus that is enclosed by two juxtaposed membranes that are a continuation of the membrane of the endoplasmic reticulum. The nuclear membrane, or karyotheca, contains pores through which substances pass. In addition, its external surface contains ribosomes.

• Cell Nucleus Review - Image Diversity: karyotheca