Essay for Biology Students on Nucleus and Chromosomes

Nucleus:

Nucleus is the dominant organelle controlling all the activities of the eukaryotic cell. Schneider and Schwann (1838 & 1839) believed that the nucleus had a central role in the growth and differentiation of an organism.

Some cells like the sieve tubes of vascular plants and red blood cells of mammals do not possess nuclei during the major part of their existence.

They, however, possess nuclei during early stages of their development. The prokaryotes have an incomplete or incipient nucleus without a nuclear envelop while eukaryotes have prominent and well-defined nuclei. Some cells may have more than one nucleus. Rat liver cells, for example, have 2 to 3 nuclei and fungal hyphae are generally, multinucleate (coenocytic).

Structure:

The nucleus is normally a spherical body. In plant cells, the nucleus is pushed to the periphery due to the presence of a large central vacuole. It becomes lens shaped.

Irrespective of its shape, the nucleus is generally separated from the cytoplasm by a double limiting membrane called the nuclear envelope.

The two membranes are separated from each other by aperinuclear space. The envelope is absent during the stages of cell division. The outer membrane of the nucleus is often continuous with the membranes of the endoplasmic reticulum and the inner membrane surrounds the nuclear content. .

Nuclear pores interrupt the nuclear envelope. These pores serve for the passage of materials between the cytoplasm and nucleoplasm. In plant cells, these are irregularly and sparsely distributed over the surface of the nucleus. These pores are nearly 100 nm in diameter and are enclosed by circular rims called annuli. The nuclear envelope is not just a physical barrier, but it regulates the passage of ions, small molecules and RNA.

The content of the nucleus is called the nucleoplasm. It is granular in nature. It contains thread-like inter-woven structures called chromatin and one too many dense spherical bodies called nucleoli.

During the resting stage (interphase) of the cell, the chromosomes are uncoiled into a mass of loose thread-like, light staining stiuctures called chromatin the nucleolus is produced from and is associated with a specific nucleolar organising region on a chromnosome. In the nucleolus, the RNA is synthesized, which consequently associate with ribosomal proteins forming ribosomes.

Chemical composition:

In the nucleoplasm, four types of molecules such as DNA, RNA, low molecular weight basic proteins called histones and acidic proteins called nonhistones are present. DNA and histones are associated as a nucleo-protein complex with a weight ratio of approximately 1:1. The amount of RNA and acidic proteins are variable.

Function:

Nucleus is an integral part of the eukaryotic cell. It controls all the activities of the cell. It contains the bearers of hereditary characters called chromosomes. The DNA present within the chromosomes forms the molecular basis of heredity.

Chromosomes:

Chromosomes (chroma, colour; soma, body) are thread-like colour bodies, as these are known, since these are stained by characteristic basic dyes. E.Strasburger (1875) observed thread like structures during cell division. W.Fleming described the splitting of chromosomes and termed the stained material as chromatin. Waldeyer (1888) named these structures as chromosomes.

Chromosomes undergo a regular cycle of condensation and decondensation during the cell division. During interphase, the chromosomes are-elongated and fine thread-like structures.

They are present as an inter-woven mass, such that the individual chromosomes can not be identified. This mass stains lightly with basic dyes and is known as chromatin. The chromatin is heterogeneous, i.e. it consists of relatively deep and light staining regions.

The deep staining regions are known as heterochrpmatin, while the light staining regions as chromatin the maximum condensation occurs during the metaphase, such that the chromosome is visible under a low resolution microscope.

At this stage, a chromosome is seen to have two identical halves attached at a point called the centromere / kinetochore. Each half is known as a chromatid. Each chromatid has a DNA double helical fiber.

Structure:

The morphology of the chromosomes can be best studied during the metaphase, which is the period of its maximum condenstion. In a gross structure- a chromosome has two arms joined with eachother at a point called the centromere or kinetochore.

Based on the appearance, chromosomes are classified under four groups: telocentric, acrocentric, .sub-metacentric and metacentric.

This classification is based on the position of the centromere. In telocentric type, the centromere is located at one end, such that there is only one arm. Acrocentric chromosomes are small and rod shaped.

The centromere is situated at one end, such that one arm is negligibly small. Submetacentric chromosomes are ‘L’ shaped having unequal arms. Metacentric chromosomes are ‘V’ shaped having approximately equal arms. The centromere is situated in the middle. All the four types of chromosomes are depicted in the

Chromosomes have bead-like structures of chromatin material, constant in their position and extent, called nucleosomes.

According to folded fibre model, the chromosome is considered to be a single chromatin fibre that is repeatedly folded back on it to make up the body of the chromosome. These folded bead-like structures are known as nucleosomes. A nucleosome consists of 2 molecules each of 4 types of histones: H₂A, H₂B, H₃ and H₄, around which the DNA is coiled twice. Two nucleosomes are linked by a linker DNA which is associated with another histone,

The Centro mere or kinetochore is the region of the chromosome that becomes attached to the mitotic spindle. The centromere lies within a thinner segment of the chromosome, the primary constriction.

The regions flanking the centromere frequently contain highly repetitive DNA and stain intensely with basic dyes. These are heterochromatin regions. Other regions of the chromosome stain lightly and are called euchromatin.

The vast majority of chromosomes have only one centromere (monocentric chromosomes). Some species have diffuse centromere. In some chromosomal abnormalities, chromosomes may break and fuse with other segments, producing chromosomes without. Any centromere (acentric) or with two centromeres (dicentric).

The tips of the chromosomes are called telomeres. When chromosomes are broken, the free ends without teleomeres become sticky and fuse with other chromosomes. They do not fuse with a normal telomere.

Besides primary constriction, some chromosomes have secondary constrictions constant in their position and extent. These constrictions are useful in identifying particular chromosome in a set.

The secondary constriction contains DNA coding for ribosomal RNA. This DNA also organises the nucleous during the cell division. Therefore, this part of the chromosome is known as the nucleolus organising region. Certain chromosomes contain rounded structures called satellites.

The satellite is separated from rest of the chromosome by a secondary constriction. The satellite and the secondary constriction are constant in shape and size for each particular chromosome.

Chemical Composition

Normally, chromosomes consist of the following components: DNA (40%), RNA (1.5%), histone protein (50%), non-histone protein (8.5%), and trace amounts of metallic ions like magnesium, calcium etc.

Function:

Chromosomes are the carriers of the genetic (hereditary) material from generation to the generations of all living organisms. They determine the identity.of an individual organism and store and transfer the genetic information from one generation to the next. Sutton and Boveri (1902) have rightly stated that chromosomes are the physical structures that are responsible for the transmision of the hereditary characters through successive generations of all organisms.