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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Molecular Biology Tutorials

Cell Cycle Checkpoints in Regulation of Cell Division and Cancer


Cell Cycle Checkpoints and Cancer

Cell Cycle Checkpoints and Regulation of Cell Cycle

What is cell cycle checkpoint?

Every cell in our body pass through a series of different stages in a cyclic manner called cell cycle.  Cell cycle is a sequential step that taking place in a cell leading to the accurate duplication of genetic materials (DNA), precise separation of replicated genetic materials and passing them in to two daughter cells.  The process of cell cycle is very critical in each cell, thus it operate strictly under strong surveillance to prevent any mistakes. This strong surveillance system in the cell to monitor the cell cycle progression itself is called cell cycle checkpoints. Checkpoints are surveillance mechanisms that halt the progress of cell cycle if (1) any of the chromosomal DNA is damaged, or (2) critical cellular processes, such as DNA replication during S phase or chromosome alignment during M phase, have not been properly completed. Thus cell cycle checkpoints ensure that the various events in the cell cycle progression occur accurately and in correct order. In this post we will discuss the three types of cell cycle checkpoints that operate in eukaryotic cells during cell cycle progression.

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