The Cell Nucleus and Cell Cycle: Structure and Function

The Cell Nucleus

Nuclear Pores

In all nuclei, the two membranes forming the nuclear envelope fuse, giving rise to circular holes called nuclear pores.

These are aqueous channels that regulate the exchange of molecules between the nucleus and the cytosol. They allow free circulation and regulate water-soluble molecules as active transport mechanisms.

Chromatin

In the nucleus of eukaryotic cells, DNA is associated with proteins, forming a packed and compact structure called chromatin.

Features

Chromatin consists of DNA and protein binding. The binding may be of two types:

  • Histone: These are very basic proteins.
  • Nonhistone:

Ultrastructure

The observation of chromatin fibriliar reveals a constitution after a series of fibers attached to each other in a spiral k fibers called chromatin or nucleosome.

Each single fiber has the appearance of a string of beads. Each bead has a spherical shape, giving it the name of nucleosome. These nucleosomes are connected by a fibril that corresponds to the thickness of a DNA double helix.

Nucleoplasm

Also called karyoplast or nuclear matrix, the nucleoplasm is a semi-fluid matrix located inside the nucleus.

Nucleolus

A constant structure in the cell nucleus, there is usually a pro kernel.

The nucleolus is a more or less rounded, highly refractive organelle, usually near the nuclear envelope.

Functions of the Nucleolus

In the nucleolus, rRNA synthesis and processing and packaging of ribosomal subunits occur. These subunits are subsequently exported to the cytosol.

Metaphysical Structure of Chromosome

The chromosome is the most studied metaphysical structure, consisting of two chromatids parallel to each result of the duplication of genetic material, except the area of the centromere.

The centromere, or primary constriction, divides the chromosome into two arms that can be the same or different sizes. It occupies a variable but fixed position for each of them along the chromosome. The centromeres contain heterocomatina constitutive, i.e., compacted chromatin and genetically inactive in all cells.

On both sides of the centromere and on each of the two chromatids are located structures called kinetochore protein. These are the points from which microtubules are polymerized.

The Secondary Constrictions

They are the most closely identified areas in the arms and which are related.

Telomeres are protective structures located at each eukaryotic chromosome end to prevent loss of information from the ends in each replication cycle.

The bands are segments of chromatin that are colored with different intensity.

4 Types of Chromosomes

  • Metacentric: Centromere occupies the medial position; the two arms are of equal or similar length.
  • Submetacentric: Centromere occupies the submeridional position. One arm has a slightly larger size.
  • Acrocentric: Centromere position is subterminal.
  • Telocentric: Centromere occupies one of the chromosome ends.

Number of Chromosomes

The number of chromosomes is constant for all k belonging to the same body.

Most organisms are diploid, i.e., they have two sets of chromosomes. Chromosomes are k pairs of homologous chromosomes that contain genetic information for the same characters. Gametes are haploid.

There are also agencies in k where all cells are haploid, such as algae and ferns, and there are agencies in which most k has two sets of chromosomes.

The number of chromosomes is unrelated to the developmental level reached.

The set of all chromosomes of a cell is called a karyotype, and there are two types of chromosomes:

  • The autosomes: Somatic or common to both sexes of the same species and involved in developing the characteristics of soma.
  • Sex or gonosomas: Responsible for numerous presence or absence of sex determination in man are the X and Y chromosomes.

The Cell Cycle

Cell division is for the stem cell to duplicate its genetic material, which is then apportioned equally among daughter cells, and their cytoplasm divided into two.

The cell cycle is the set of changes since suffering a cell that has formed another division until it divides to give rise to two daughter cells.

In the eukaryotic cell cycle, it is divided into two phases: the interface, at which the cell grows and synthesizes various substances, and phase M, occurring in mitosis and cytokinesis.

Interface

It is the time elapsed between two successive mitoses and occupies most of the cell cycle. During interphase, there is high metabolic activity, the cell increases in size, and its genetic material doubles in preparation for cell division. There are three periods:

  • G1 phase: It synthesizes proteins needed for cell k to increase its size. The start phase ends when my hard until the beginning of DNA replication in cells that never enter into mitosis G0 – his name is stated that the cell is at rest or quiescence. It occurs in cells that have undergone an important process of differentiation.
  • S Phase: Occurs in DNA replication, and histones are synthesized. As a result of replication, each chromosome consists of two chromatids joined at the centromere.
  • G2 phase: It has a very short duration, and it is transcribed and translated genes encoding proteins necessary for the cell to divide, and the centrioles replicate.

DNA Replication

The general mechanism of replication was inferred by Watson and Crick when they established a double helical structure and complementary bases. They proposed that the DNA double helix is opened and the two chains of nucleotides are separated, removing replication models.

Possible Models for Replication

  • Conservative: A double helix retains the two original strings, and the other is formed by the two new ones.
  • Scatter: Each of the daughter strands containing fragments of the original string and pieces of the new synthesis.
  • Semiconservative: Proposed by Watson and Crick double helix, each retains a helix of both original and synthesized a new one.

Phases of Replication in Prokaryotes

It is divided into two stages: initiation and elongation. Also, during elongation, error correction is performed.

Initiation Phase

This is basically the unwinding and opening of the double helix. In the bacterial chromosome replication, there is a single origin. It begins in a region of DNA, called oriC or point of initiation.

The oriC is recognized by specific proteins that bind to it. Enzymes helicases break the hydrogen bonds between nitrogenous bases, and the double helix opens like a zipper from each of the two chains.

When you open the double helix, unwinding occurs in this area.

Ssb proteins bind to the template strand that prevents re-wrapping and frees the part of the thread that leads the bases so that these are accessible to other molecules.

On the origin of replication oriC, a bubble has formed around replication in which there are two areas known as Y-shaped replication forks, where they will synthesize new DNA strands. The replication bubble is spreading along the chromosome in both directions from there k k is said replication is bidirectional.

Elongation Stage

The stage where k is synthesized a new strand of DNA on each strand of the original double helix. Furthermore, the enzymes involved in the initiation phase of DNA elongation involve several types of DNA polymerases with dual functions:

  • Polymerase activity: Nucleotides bind to each DNA to form him. For that tour, the selected template strand and the deoxyribonucleotide whose base is complementary to the template strand and bind. The new DNA strands are synthesized by assembling deoxyribonucleotide triphosphate.
  • Exonuclease activity: Deletes nucleotides which are nitrogen bases mismatch.