Eukaryotic Cell in Hindi- Genetic Organization of Eukaryotes

Eukaryotic cells, which are found in plants, animals, fungi, and bacteria, have a unique genetic organization. The genetic material is organized into straight chromosomes, each of which contains a long DNA strand folded around histone proteins. These chromosomes are primarily located in the cell's nucleus. Also, plasmids, which are small circular DNA strands, may be found in the mitochondria or chloroplasts of eukaryotic cells. This structure of genetic material in eukaryotic cells as well as the genetic organization of eukaryotes (eukaryotic cell in Hindi) gives significant control over gene translation and is engaged in a variety of biological processes, including growth, development, and reproduction.

Eukaryotic Cell in Hindi

The eukaryotic genome is rather complex & enclosed by a typical nuclear membrane which forms a true nucleus. The amount of DNA in the haploid genome of eukaryotes is quite variable, it is usually expressed in terms of C-value.

The C-value is characteristic of all living species, but some variation in C-value may occur within the species.

There are two main reasons for c-value Variations:

1) The organization of eukaryotic genes.
2) Repetitive DNA.

In eukaryotes, the actual coding sequence of a gene is much smaller than the average size of the gene due to the presence of introns.

Such genes are called Split genes or interrupted genes. Since their coding or expressed sequence (exons) are divided by introns which are non-coding sequences.

The size of mature eukaryotic mRNA is about 1 / 5 of the size of the gene coding for it.

Further, the actual number of genes for the genome is much lower than the number estimated based on haploid DNA content, this is mainly due to the presence of a Variable amount of repetitive DNA. This situation is called the C-value paradox.

The eukaryotic genome is distributed into several linear chromosomes.

The chromosomes are made of DNA (nucleic acid) and proteins which together form the nucleoprotein called 'chromatin'.

Chemical Composition

Chromatin is composed of DNA (30-40%), RNA (1-10%), and proteins (50-60%).

Around six percent of total DNA in pea vegetative seedlings is involved in transcription, while 32% is involved in transcription in its developing cotyledons.

1) DNA

a) The DNA content varies among the cells of different organisms. A species' gametes (eggs and sperm) contain only half of the DNA found in its somatic tissues.

b) In humans, the DNA content of an egg or sperm is 2.8 picogram. per cell. The DNA content per cell also varies during the different storage of the cell cycle.

c) The haploid DNA content of a cell is denoted by 2c. DNA content of a somatic cell is 2c in G1, It doubles during the S-phases and becomes 4c in G2, and it remains in 4c during prophase & metaphase stages of mitosis & prophase 1 and metaphase 1 of meiosis.

d) The 4c DNA of meiotic cells is reduced to half (2c) in the two daughter cells forming the dyad (due to the first meiotic division).

e) Finally, each of the four component calls of the tetrad produced after the completion of meiosis contains 1c DNA of the gametes.

The eukaryotic genome is composed of the following two types of base sequences;

i) Unique or non-repetitive DNA
ii) Repetitive DNA

i) Unique DNA

These are the sequences that are present in a single copy, in each genome. The base sequence of unique DNA is not repeated in the genome.

The proportion of unique DNA differs among eukaryotic species. It constitutes 8% of the rye genome, 25% of a pea, 40%. of a snail, and 70% of the human genome.

A large no. of genes, for example, most of the structural genes are present in a single copy in the genome.

Bacterial DNA is considered to be composed of unique DNA. It contains only 0.3% repetitive DNA which is rDNA & Codes for ribosomes.

ii) Repetitive DNA

Repetitive DNA refers to DNA segments that appear in more than one duplicate per genome.

a) They consist of families of sequences that are not exactly similar but are related.

b) The members of each family consist of a set of base sequences that are sufficiently similar to re-associate with one another.

c) Differences among individual sequences occur due to deletion, insertion, and substitution. The size of these sequences is affected by unequal passing over.

d) Repetitive DNA sequences are classified into two main groups;

i) Moderately Repetitive Sequences

In this case, the number varies from 2 to less than 105 copies of the genome.

Drosophila melanogaster has 12% of its DNA in the shape of slightly repeated sequences, whereas humans have 13% of this type of DNA.

ii) Highly Repetitive DNA

It constitutes a smaller proportion of the genome. Generally, it consists of very short sequences which are repeated tandemly in large clusters.

The percentage of extremely repetitive DNA in mammalian chromosomes is typically less than 10%, whereas it is 17% in Drosophila melanogaster.

2) RNA

It constitutes a very small proportion of chromatin. It is 3.5% in humans, 0.3% in cows, and 10%. in peas.

The majority of RNAs are ribosomal RNA (rRNA), mRNA, and tRNA. But a special class of RNAs called "Chromosomal RNA" is associated with chromosomes.

Chromosomal RNA contributes to about 5% of overall chromosome weight. These RNAs are small molecules, containing 40-60 nucleotides.

They may be involved in the structural organization of chromatin fibers & gene regulations.

3) Proteins

It constitutes more than half of the overall number of chromosomes. They belong to two classes.

i) Histones or Basic proteins
ii) Non-Histones proteins

Another type of protein known as a "protamine" is found linked with chromosomes in the sperms in certain animal species.

Protamines are acidic proteins with molecular weights ranging from 1000 to 5000. They replace the chromatin of the sperm.

i) Histones or Basic Proteins

These are basic proteins or acid-soluble proteins and have a net positive charge.

These proteins are of 5 types, namely H1, H2A, H2B, H3, and H4. These proteins have low molecular mass.

Different histone classes of amino acids constitute pea bud histone.

The histones are divided into the classes;

i) Very rich in lysine.
ii) Moderately sich in lysine
iii) Arginine is rich in histone.

ii) Non-Histones Proteins

It occurs in a much lower proportion than histones and their proportion in the total chromosome mass varies considerably in the different organisms. These proteins make up about 4% of a few vegetative buds, 16% in growing pea cotyledon, and 25%. human He La cells.

It consists of various enzymes involved in different metabolic functions.

Examples - are DNA polymerase, RNA polymerase, nucleases, polynucleotide ligase, histone kinases, etc.

Certain non-histone proteins are found that have high electrophoretic mobility, they are called HMG (high mobility group) proteins.

Chromosome Structure

Proteins & DNA are complexed together to form nucleoprotein fibers called “chromatin". The chromatin fibers coil and fold to form the ‘chromosome’.

Very long DNA molecules are packed into chromosomes of much smaller sizes.

The amount of DNA is measured in a picogram (Pg).

1pg = 10 - 12 gm.

1pg DNA corresponds to 31 cm. of DNA double helix which contains 109 base pairs.

In man, 5.6 pg DNA is found in each cell. This DNA is equivalent to 174 cm. of DNA double helix and is distributed into 46 chromosomes which together measure only about 220 um during the metaphase of mitosis.

Organization of Chromatin Fibers

DNA of eukaryotes is complexed with proteins to form fibrils of 100 A (=10 nm) diameter which are coiled to give rise to the chromatin fiber of 300A (= 30 nm) diameter.

The chromatin fibers are dispersed during interphase but they become condensed and visible during cell division.

Two models of chromatin fiber organization have been proposed, the coiled DNA model and the nucleo-solenoid model.

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