Molecular Biology Tutorials

Difference between Necrosis and Apoptosis: A Comparison Table


necrosis vs apoptosis

Apoptosis vs Necrosis
(Similarities and Differences)

Apoptosis and Necrosis are two types of cell death occur in organisms. The cells undergo death when the cell death becomes necessary as a part of developmental process or they fail to adapt to injuries. Both these types of cell deaths differ in their initial cause and progression of the cell death pathway.

Apoptosis definition (programmed cell death): a physiological process by which unwanted or useless cells are eliminated during the development and other normal biological processes. Often found during tissue homeostasis, embryogenesis, immunological reactions and development of nervous systems. During apoptotic cell death, the cells undergo some characteristic events such as chromatin condensation, nuclear and cytoplasmic aggregation and partitions of cytoplasm and nucleus into membrane bound vesicles called apoptotic bodies containing ribosomes and mitochondria. Apoptotic bodies are recognized and phagocytized by either by macrophages or adjacent cells and thus no inflammatory response are elicited during apoptotic cell death.

Necrosis definition: (accidental cell death) a pathological process occurs when the cells are exposed to serious physical or chemical insults. Occur during pathological infections such as bacterial and fungal infections, hypothermia and hypoxia conditions. The cell and cellular organelles swell and ruptures to release the entire cell content including lysosomal enzymes into the extracellular fluid.  Due to this, necrotic cell deaths are always associated with severe inflammatory response in the surrounding tissues.

The current post describes the similarities and difference between apoptotic and necrotic cell death with a comparison table.

Similarities between Apoptosis and Necrosis

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Molecular Biology Tutorials

Nucleosome Model of Chromosomes in Eukaryotes (Short Notes)


structure of nucleosome short notes

image source: scitable

Nucleosome Model of Chromosome

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.

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Molecular Biology Tutorials

Folded Fibre Model of Chromosomes


Dupraw model of chromosome

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 Nucleosome Model 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.

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Molecular Biology Tutorials

Karyotype and Idiogram: Definition and Importance of Karyotype Test (Karyotyping) in Human


what is karyotyping

(image source: wikipedia)

Karyotype, Karyotyping and Preparation of Idiogram

What is a Karyotype?

All species are characterized by a set of chromosomes to carry their genetic information. The chromosomal composition of each species has a number of characteristics. The Karyotype is a set of characteristics that identifies and describes a particular set of chromosome. These characteristics which are described by a karyotype are:-

(1).  The chromosome number

(2).  Relative size of different chromosomes

(3).  Position of centromere and length of chromosomal arms

(4).  Presence of secondary constrictions and satellites

(5).  Banding pattern of the chromosome

(6).  Features of sex chromosomes

What is Karyotyping? How to Prepare the Karyotype of Human?

Ø  The process of preparation of the karyotype of a species is called Karyotyping.

Ø  Karyotyping is now most commonly used in clinical diagnosis and clinical genetics.

Ø  Karyotype is prepared from the microphotographs of metaphase chromosomes.

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Molecular Biology Tutorials

Classification of Chromosomes based on Position of Centromere and Length of Chromosomal Arms

how chromosomes are classified

Classification of Chromosomes Based on Position of Centromere and Length of Arms

Ø  The size and shape of the chromosomes are variable in the different phases of cell cycle.

Ø  Chromosomes in the interphase of cell appear as thin, coiled, elastic and thread-like structures.

Ø  This thread-like stainable interphase chromosome is called chromatin.

Ø  During the mitotic or meiotic cell division, the chromatin materials become thicker in their width and shorter in their length.

Ø  Chromosomes in the metaphase stage of cell division show maximum condensation.

Ø  Each metaphase chromosome contains a centromere (primary constriction).

Ø  The centromere divides the chromosome into two parts called chromosomal arms.

Ø  The small arm of the chromosome is denoted as ‘p’ – arm, whereas the large arm is denoted as the ‘q’ – arm.

Ø  When chromosomes are represented as a karyotype or ideogram, each chromosome is arranged in such a way that the ‘p’ arm is positioned above the centromere and the q arm is represented below the centromere.

Ø  The position of centromere and the relative size of chromosomal arms are used as a criterion for a morphological classification of chromosomes.

Ø  This morphological classification is an important karyotypic feature of an organism.

Classification of chromosome

Ø  Based on the position of centromere and length of chromosomal arms, the chromosomes are classified into 4 groups:

(1).      Telocentric chromosomes

(2).      Acrocentric chromosomes

(3).      Sub-metacentric chromosomes

(4).      Metacentric chromosomes

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