Genetic material is present as chromosomes which are complex structures consisting of DNA and proteins. These chromosomes contain genes encoding for proteins which regulate the reactions in living systems. Chromosomes are small structures. In some cases, chromosomes which are larger than the normal size and different in structure are found in plants and animals.
B chromosomes are accessory or supernumerary chromosomes present in the cell as extra chromosomes over the standard complement of diploid or polyploidy chromosomes. These chromosomes are dispensable, heterochromatinized. These are non-homologous to standard chromosomes and do not follow Mendelian laws. These unusual chromosomes are considered to be genetically inactive. B chromosomes are mostly present in plants and in a less frequency in animals. Among plants about 1000 species were found to have B chromosomes including Bryophytes and family Graminae and Liliaceae. B chromosomes are said to be formed by non-disjunction of chromosomes during inter specific and intra specific crosses. According to Peters, B chromosomes are not transferred from generation to generation and produce denova. B chromosomes are classified as interspecific autosomal; inter specific sex chromosomal, intra specific autosomal and intra specific sex chromosomal. B chromosomes play a significant role in characters. Increased number of B chromosomes reduces the vigour and fertility of pollen grains and seeds. In pollen grains, the generation cell receives all the B chromosomes and out of two sperm cells one receives all the B chromosomes and this sperm cell fuses with the egg. Whereas in animal cells, B chromosomes are confined to females. B chromosomes influence flowering in plants and in Rye, the increase in number of B chromosomes to eight increase number of chiasmata leading to more variation.
Lamp brush chromosomes were first found in oocytes of amphibians. These are found in some invertebrates and all vertebrates except in mammals. At diplotene stage, the homologous chromosomes repel from each other, they are held together at points of chiasmata. Each homologous chromosome consist of a pair of sister chromatids. They are arranged parallel to each other that often produce loops that vary in number from 1-9 pairs. The extension of each loop is about 200 microns and the length is about 700 microns. It is proved that lamp brush chromosomes forming loops produce maximum RNA and proteins sufficient for further embryogenesis. Each loop is composed of double stranded DNA that is surrounded by matrix rich in RNA and proteins. The loop is thicker at some regions which represent inactive regions and thinner at some regions which is the active part. In the loop region, transcription of proteins occurs. And these loops remain as such till fertilization. After the fertilization of the egg, the loops condense to form chromomeres and the chromosomes act like normal chromosomes entering into the M phase.
Polytene chromosomes are also known as giant chromosomes which were first identified in antipodal cells of Fritillaria. As Balbiani discovered, the cells of salivary glands of third instar larvae of Drosophila contain these chromosomes and it disintegrates when the instar pupates. These chromosomes are 1800 times larger than the normal chromosomes. Polytene chromosomes consist of five radiating arms corresponding to an X chromosome, 2L arm, 2R arm and 3R arm. The short sixth arm represents the fourth chromosome that corresponds to heterochromatin and euchromatin respectively. In euchromatin region, the chromosomes are less condensed and form the active sites for genetic regulation. Often chromonema bulges to form Balbiani ring or chromosomal puff.
This chromosome has a unique characteristic of undergoing endomitosis where these replicate during S phase, but doesn’t enter into the cell cycle. In this, there is gene amplification as the DNA keeps dividing leading to production of more strands of chromonema. The chromonema are arranged side by side corresponding to each chromonema. Endomitosis leads to gene amplification which in turn increases transcription and translation. Thereby quantity of gene products formed is increased. Chromosomes in Drosophila show somatic pairing where chromosomes paired in undivided cells as in the zygotene stage of meiosis and due to endomitosis when the diameter increases in the homologous chromosomes and refuse to join in a common arm. The centromeres form a common centromere in pairs such that one was maternal origin and the other is of paternal origin.
B chromosomes are accessory or supernumerary chromosomes present in the cell as extra chromosomes over the standard complement of diploid or polyploidy chromosomes. These chromosomes are dispensable, heterochromatinized. These are non-homologous to standard chromosomes and do not follow Mendelian laws. These unusual chromosomes are considered to be genetically inactive. B chromosomes are mostly present in plants and in a less frequency in animals. Among plants about 1000 species were found to have B chromosomes including Bryophytes and family Graminae and Liliaceae. B chromosomes are said to be formed by non-disjunction of chromosomes during inter specific and intra specific crosses. According to Peters, B chromosomes are not transferred from generation to generation and produce denova. B chromosomes are classified as interspecific autosomal; inter specific sex chromosomal, intra specific autosomal and intra specific sex chromosomal. B chromosomes play a significant role in characters. Increased number of B chromosomes reduces the vigour and fertility of pollen grains and seeds. In pollen grains, the generation cell receives all the B chromosomes and out of two sperm cells one receives all the B chromosomes and this sperm cell fuses with the egg. Whereas in animal cells, B chromosomes are confined to females. B chromosomes influence flowering in plants and in Rye, the increase in number of B chromosomes to eight increase number of chiasmata leading to more variation.
Lamp brush chromosomes were first found in oocytes of amphibians. These are found in some invertebrates and all vertebrates except in mammals. At diplotene stage, the homologous chromosomes repel from each other, they are held together at points of chiasmata. Each homologous chromosome consist of a pair of sister chromatids. They are arranged parallel to each other that often produce loops that vary in number from 1-9 pairs. The extension of each loop is about 200 microns and the length is about 700 microns. It is proved that lamp brush chromosomes forming loops produce maximum RNA and proteins sufficient for further embryogenesis. Each loop is composed of double stranded DNA that is surrounded by matrix rich in RNA and proteins. The loop is thicker at some regions which represent inactive regions and thinner at some regions which is the active part. In the loop region, transcription of proteins occurs. And these loops remain as such till fertilization. After the fertilization of the egg, the loops condense to form chromomeres and the chromosomes act like normal chromosomes entering into the M phase.
Polytene chromosomes are also known as giant chromosomes which were first identified in antipodal cells of Fritillaria. As Balbiani discovered, the cells of salivary glands of third instar larvae of Drosophila contain these chromosomes and it disintegrates when the instar pupates. These chromosomes are 1800 times larger than the normal chromosomes. Polytene chromosomes consist of five radiating arms corresponding to an X chromosome, 2L arm, 2R arm and 3R arm. The short sixth arm represents the fourth chromosome that corresponds to heterochromatin and euchromatin respectively. In euchromatin region, the chromosomes are less condensed and form the active sites for genetic regulation. Often chromonema bulges to form Balbiani ring or chromosomal puff.
This chromosome has a unique characteristic of undergoing endomitosis where these replicate during S phase, but doesn’t enter into the cell cycle. In this, there is gene amplification as the DNA keeps dividing leading to production of more strands of chromonema. The chromonema are arranged side by side corresponding to each chromonema. Endomitosis leads to gene amplification which in turn increases transcription and translation. Thereby quantity of gene products formed is increased. Chromosomes in Drosophila show somatic pairing where chromosomes paired in undivided cells as in the zygotene stage of meiosis and due to endomitosis when the diameter increases in the homologous chromosomes and refuse to join in a common arm. The centromeres form a common centromere in pairs such that one was maternal origin and the other is of paternal origin.
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