Does the DNA really need to FOLD inside the nucleus?
A diploid human cell contains approximately 6.4 billion base pairs. These 6.4 billion base pairs are distributed in our 23 pairs (2n = 46) of chromosomes. We know that each chromosome contain a single linear segment of DNA.
According to Watson and Crick model, the distance between each base pair in a DNA double helix is 0.34 nm. Thus, the 6.4 billion base pair will constitute a total length of about 2.2 m DNA strand. The total length of DNA of a single human cell is approximately 2.2 meters long (when all 46 DNA strands are joined end to end).
The size of the nucleus in which the chromatin situated is about 10 µm in diameter. Thus, it is evident that the 2.2 m long DNA should fold several times to fit in the nucleus of 10 µm diameter. The exact nature and pattern of folding of DNA strands in the nucleus disclose the organization of genetic material in the cells.
Satellite Map of a Gamma Garden at Institute of Radiation Breeding, Hitachiohmiya, Japan
Gamma Gardens (Atomic Gardens)
What are Gamma Gardens or Atomic Gardens?
Gamma garden or Atomic garden is a concept popularized after the Word War 2 for the peaceful use of atomic energy (atoms for peace) for the crop improvement. Gamma gardens or atomic gardens are a type of induced mutation breeding where radioactive sources particularly gamma rays from cobalt -60 or Caesium-137 are used to induce desirable mutations in crop plants.
Salient features of Gamma Garden
Ø Gamma gardens are “area subjected to gamma irradiation of crop plants”.
Ø They are giant structures, enclosed by thick high wall to protect the plants and animals outside.
Ø The purpose of a gamma garden is to irradiate the whole plants during different stages of development and of varying duration.
Ø The source of radiation used is Cobalt-60.
Ø Rarely Caesium-137 is also used as the source of radiation.
Mutation Breeding (Induced Mutations for Crop Improvement)
What is mutation?
Mutation is the “Sudden heritable change in an organism”. Mutation may be the change in gene, chromosome or plasmagene (genetic material inside mitochondria and chloroplasts. The mutation produced by change in the base sequence of gene is called point mutation or gene mutation. The gene mutation may be further classified as transition, transversion, deletion, duplication or inversion. Chromosomal mutations are the change in chromosome structure. The change in the structure of chromosome can occur as a result of large deletion, inversion, duplication, translocation and change in chromosome number. Most of the mutations are lethal to the organism. A very small number of mutations are beneficial to the organism. Additionally, by the use of mutation inducing agents, a breeder can induce desirable changes in the genetic constitution of plants and thereby he can improve the performance of a cultivated variety.
Definition: “The utilization of induced mutations in crop improvement is called mutation breeding”
The term mutation breeding was first coined by Freisleben and Lein in 1944 to refer to the deliberate induction and development of mutant lines for crop improvement.
Spontaneous and Induced mutations:
(1). Spontaneous mutation:
Mutation occurs in nature are called spontaneous mutation. Spontaneous mutation occurs in the organism without any treatment at low rate in the nature. The frequency of spontaneous mutation is 10-6 (one in 10 lakhs). Different genes in and organism show different mutation rate.
(2). Induced mutation:
Mutations induced in an organism by treatment with physical or chemical mutagen are called induced mutations. The agents which are used to induce mutation are called mutagens. Certain genes in an organism promote the mutation of other genes nearby in the chromosomes. For example, the gene Dt in Chromosome number 9 of maize increases mutation rate of other genes.
Characteristics of Mutation:
Ø Mutations are generally recessive; Dominant mutations do occur in nature.
Ø Mutations are generally harmful to organism; small percentage of mutation is beneficial.
Ø Mutation occurs at random in the chromosome, may occur in any gene.
Toxins in Plant Pathogenesis Toxins in Plant Pathogen Interaction
The process of pathogenesis (the onset of diseased condition) in plants is facilitated by many factors such as virulence of the pathogen, susceptibility of the host, presence of suitable environmental conditions and various enzymes and metabolites of pathogens. Previously we have discussed the importance of different enzymes involved in plant pathogen interactions. These enzymes facilitate the entry of the pathogen into the host and help to colonize inside the host cell. The production of different hydrolytic enzymes may not always result in the establishment of pathogen inside the host tissue. Certain plant pathogen may successfully invade the host tissue area but fail to induce the onset of disease. These findings highlighted the involvement of certain toxins, microbial toxins or aggressins, in the establishment of diseased conditions in plants. The present post describes the Toxins involved in plant pathogen interaction and pathogenesis with examples.
What is meant by ‘toxin’ in Plant Pathology?
ØDefinition: Any substances produced by a pathogen which is injurious to host and it directly and immediately play a crucial role in pathogenesis.
Ø Toxins involved in plant parasite interactions are also called as aggressin.
Ø According to Koch’s postulates, a substance to be called as a ‘toxin’ should follow the following criterions:
@. The compound must be isolated from the diseased plant
@. It should NOT be present in the healthy plant
@. Compound must be chemically characterized
@. When isolated toxin is reintroduced to a healthy host, it should produce the original symptoms.
Folded Fibre Model of Chromosome (The Ultra-structural Organization of DNA and Histone Proteins in the Chromosomes)
The chromosomes of eukaryotic organisms are a complex structural organization of DNA and proteins. The exact structural organization of proteins and DNA to form the chromatin material (or chromosomes during cell division) is a curious question in the scientific community. This curiosity becomes a wonder when we realize the total length of DNA in a single cell and size of the nucleus in which this DNA is residing. For example, in a diploid human cell, there will be 46 chromosomes. The DNA in all these 46 chromosomes when joined together, it will have a distance of about 2.2 meters. Thus, the average length of DNA in a single chromosome will be 4.8 cm or 48,000 µm (2.2 X 100/46). On an average, the human chromosome at its metaphase stage is about 6 µm long. This means the 48,000 µm long DNA strand is heavily folded to from the 6 µm long chromosome with a packing ratio of about 8000 : 1. The exact folding pattern of DNA is a highly debated concept. For explaining the structural organization of DNA and proteins in the chromosome, various theories have been put forward by different scientists. DuPraw Folded Fibre Model and NucleosomeModel are the two such models trying to explain the ultra-structural organization of DNA and proteins in the chromosome. The present post describes the significance of Folded Fibre Model of Chromosomes and its merits and demerits.
Folded Fibre Model of Chromosome
Ø The Folded Fibre Model of chromosome was proposed by DuPraw in 1965.
Ø He published this model based on his studies on human chromosomes using electron microscope.