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1.4. Cell nucleus

Cell nucleus

The nucleus is the characteristic structure of the eukaryotic cell, being its largest component. Within the nucleus is DNA, with the genetic information and instructions necessary for the cell to carry out its vital functions.

Remember that the prokaryotic cell does not have its DNA wrapped by any membrane.

Although the nucleus  is characteristic of eukaryotic cells, and they usually have only one nucleus, there are some exceptions, such as red blood cells, which do not have a nucleus, and others, such as muscle cells, have many nuclei (polynucleated).

Animation: Cell nucleus.

The structure of the nucleus

The cell nucleus does not always have the same structure, but can be found in two different states, depending on the state of division in which the cell is:

This section describes the nuclear structure in the interface state . In the section " cell cycle " we will deal with the characteristics of the nucleus in a state of division.

The cell nucleus, during the interphase, is a spherical structure that is located in the central zone of animal cells. In plant cells, due to the large size of the vacuole, it moves to one side along with the other organelles.

In the interphase nucleus, when the cell is not dividing, one can distinguish:

Nuclear envelope

The nuclear envelope is made up of two membranes separated by an intermembrane space. The outer membrane carries ribosomes  attached to it and continues with the membranes of the endoplasmic reticulum. These membranes are interrupted by nuclear pores that allow the exchange of substances between the nucleus and the cytoplasm.

Nucleoplasm

The nucleoplasm is the internal aqueous environment of the nucleus, in which the nuclear components are immersed.

Nucleolus

The nucleolus is a spherical structure, without a membrane, that appears in the interphase nucleus. It has the function of synthesizing ribosomes.

Chromatin

In the interphase nucleus, the genetic material is made up of chromatin. Chromatin consists of long strands of DNA associated with proteins called histones. When the cell is about to divide, the chromatin filaments condense and form the shorter and thicker chromosomes.
In eukaryotes, each chromosome is made up of a single linear DNA molecule.

Remember that the prokaryotic cell has circular DNA and the  eukaryotic cell has linear DNA .

Interactive activity. The core.

Chromosomes

The chromosomes are filaments of genetic material (DNA and protein) displayed on the cell when in division. Chromatin is condensed to be able to divide the genetic information of the mother cell between the two daughter cells.

A chromosome has the following structure:

  • Two sister chromatids. The DNA of each chromatid is identical, which is why they are called sister chromatids.
  • Centromere. Place where the two sister chromatids meet.
  • Arms. Each chromatid is made up of two arms that can have the same or different length, depending on the chromosome. Each arm is the part of the chromatid that goes from the centromere to the telomere.
  • Telomeres. They are the ends of the arms.

Estructura del cromosoma

Alejandro Porto [CC BY-SA 3.0], via Wikimedia Commons

Number of chromosomes

The number of chromosomes that a species has is the same for all cells (except gametes) of all individuals of that species. This number is characteristic of the species and its chromosomes always have a constant size and shape. The number of chromosomes does not have a direct relationship with the complexity of the living being. For example, humans have 46 chromosomes, flies 12, dogs 78, and chimpanzees 48 chromosomes.
Most of the cells of organisms are diploid (or 2n), with two sets of chromosomes that are identical in shape and size, although possibly different genetic information, since one comes from the father and the other from the mother. These chromosomes are called homologous chromosomes.
Gametes or reproductive cells (and some other organisms) are haploid cells (or n), and they do not have homologous chromosomes. They have half the chromosomes because it will unite with another gamete to form the new individual that will be diploid. For example, human eggs and sperm have 23 chromosomes, which when fertilization occurs will give rise to a zygote (diploid) of 23 pairs of homologous chromosomes.
Rarer, but there are also polyploid organisms , with several sets of chromosomes (3n, 4n, etc.).

Chromosome types

According to the shape of the chromosomes, several types of chromosomes are distinguished according to the position of the centromere:
  • Metacentric chromosomes. The centromere is located in the middle of the chromosome, so the arms are the same length.
  • Submetacentric chromosomes  The arms are slightly unequal because the centromere is displaced to one of the extremes.
  • Acrocentric chromosomes. The arms have different lengths, one is much longer than the other, because the centromere is very displaced towards one end.
  • Telocentric chromosomes. The centromere is at one end of the chromosome, so there is only a single arm.
Tipos de cromosomas