Give the chemical nature of chromosome
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Each eukaryotic chromosome consists of one linear, unbroken, double-stranded DNA molecule running throughout its length and contains about twice as much protein by weight as DNA. Each species has a characteristic content of DNA, which is constant in all the individuals of that species and has thus been called the C-value.’
The amount of genetic material in eukaryotic cell is nearly 100 times greater than that in prokaryotic cells. However, there is no direct relationship between the C value and the structural or organisational complexity of the organism. In eukaryotic cells, the large amount of DNA remains in very compact form in its nucleus.
The complexed structure between eukaryotic DNA and protein are called chromatin. The fundamental structure of chromatin is essentially identical in all eukaryotes. There are two types of chromatin – euchromatin and heterochromatin.
In 1928, Heitz defined heterochromatin as those regions of the chromosome that remain condensed during interphase to early prophase and that stains darkly. Euchromatin, on the other hand, is chromatin that stains lightly. It is uncoiled during interphase, but becomes condensed during mitosis. Most of the genome consists of euchromatin.
Functionally, euchromatin is genetically active i.e., it contains genes that are being expressed; whereas heterochromatin is genetically inactive, either because it contains no genes or because the genes it does contain need not be expressed in that cell at that time.
Characteristically, heterochromatin replicates later in the S-phase of the cell cycle as a result of the higher degree of chromosome condensation. Heterochromatin is found in all eukaryotic species near the centromeres, at telomeres and elsewhere in a species-specific manner. Two classes of heterochromatin can be distinguished.
i. Constitutive Heterochromatin:
It is always genetically inactive and permanently condensed in all types of cell of a species. It is found at homologous sites on chromosome pairs.” This type of heterochromatin contains highly repeated DNA sequences, called satellite DNA, which might have a structural role in chromosomes. Centromeric and telomeric heterochromatin are example of constitutive heterochromatin.
ii. Facultative Heterochromatin:
It is potential to become condensed to the hetero- chromatin state. It may contain genes that are made inactive when the chromatin becomes condensed. Barr bodies i.e., inactivated X chromosomes in female mammals are examples of facultative heterochromatin.
Proteins associated with the DNA in chromatin are of two types – histones and non-histones. The DNA is wrapped around a core of histone molecule, and the non-histones are somehow associated with that complex.
Histones are small basic proteins with a high content of basic amino acid arginine and lysine and having net positive charges that facilitate their binding to the negatively charged DNA. Five main types of histones are found in chromosomes, viz., H1, H2A, H2B, H3 and H4. The four main histones, H2A, H2B, H3 and H4 are very similar in different species, being among the most conserved known proteins.
For example, only two amino acid differences exist in the H4 proteins of cow and peas, and only one amino acid difference between sea urchin and calf thymus H3. The remarkable similarity in the amino acid sequence of histones among different organisms is a strong indicator that histones play some basic role in organising the DNA in the chromosomes of all eukaryotes.
H1 is not conserved between species and has tissue-specific forms. It is present only once per 200 base pairs of DNA and is involved in the maintenance of a higher-order folding of chromatin. There are a large number of non-histone proteins that are usually acidic and are likely to bind to positively charged histones in the chromatin.
The enzymes and proteins involved with replication, transcription and the regulation of gene expression are example of chroma
The amount of genetic material in eukaryotic cell is nearly 100 times greater than that in prokaryotic cells. However, there is no direct relationship between the C value and the structural or organisational complexity of the organism. In eukaryotic cells, the large amount of DNA remains in very compact form in its nucleus.
The complexed structure between eukaryotic DNA and protein are called chromatin. The fundamental structure of chromatin is essentially identical in all eukaryotes. There are two types of chromatin – euchromatin and heterochromatin.
In 1928, Heitz defined heterochromatin as those regions of the chromosome that remain condensed during interphase to early prophase and that stains darkly. Euchromatin, on the other hand, is chromatin that stains lightly. It is uncoiled during interphase, but becomes condensed during mitosis. Most of the genome consists of euchromatin.
Functionally, euchromatin is genetically active i.e., it contains genes that are being expressed; whereas heterochromatin is genetically inactive, either because it contains no genes or because the genes it does contain need not be expressed in that cell at that time.
Characteristically, heterochromatin replicates later in the S-phase of the cell cycle as a result of the higher degree of chromosome condensation. Heterochromatin is found in all eukaryotic species near the centromeres, at telomeres and elsewhere in a species-specific manner. Two classes of heterochromatin can be distinguished.
i. Constitutive Heterochromatin:
It is always genetically inactive and permanently condensed in all types of cell of a species. It is found at homologous sites on chromosome pairs.” This type of heterochromatin contains highly repeated DNA sequences, called satellite DNA, which might have a structural role in chromosomes. Centromeric and telomeric heterochromatin are example of constitutive heterochromatin.
ii. Facultative Heterochromatin:
It is potential to become condensed to the hetero- chromatin state. It may contain genes that are made inactive when the chromatin becomes condensed. Barr bodies i.e., inactivated X chromosomes in female mammals are examples of facultative heterochromatin.
Proteins associated with the DNA in chromatin are of two types – histones and non-histones. The DNA is wrapped around a core of histone molecule, and the non-histones are somehow associated with that complex.
Histones are small basic proteins with a high content of basic amino acid arginine and lysine and having net positive charges that facilitate their binding to the negatively charged DNA. Five main types of histones are found in chromosomes, viz., H1, H2A, H2B, H3 and H4. The four main histones, H2A, H2B, H3 and H4 are very similar in different species, being among the most conserved known proteins.
For example, only two amino acid differences exist in the H4 proteins of cow and peas, and only one amino acid difference between sea urchin and calf thymus H3. The remarkable similarity in the amino acid sequence of histones among different organisms is a strong indicator that histones play some basic role in organising the DNA in the chromosomes of all eukaryotes.
H1 is not conserved between species and has tissue-specific forms. It is present only once per 200 base pairs of DNA and is involved in the maintenance of a higher-order folding of chromatin. There are a large number of non-histone proteins that are usually acidic and are likely to bind to positively charged histones in the chromatin.
The enzymes and proteins involved with replication, transcription and the regulation of gene expression are example of chroma
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