Morphology of chromosome
Nucleoplasm is the inner mass of the nue iCUS. Nucleoplasm contains chromosomes. The DNA is organized with proteins to form chromosomes. The chromosomes are thread like during interphase. So they cannot be identified as individual structures. Thus the chromosomes are appeared as mass of stained material. This stained material is collectively called as chromatin. The chromosomes condense before the division of cell. So it becomes thick. Now it can be seen as separate structures. ‘Hie chromosomes contain genes. These genes control all the character of an animal. Each eukaryotic species has a characteristic number of chromosomes. A human cell has 46 chromosomes in its nucleus. But the sex cells—eggs and sperm have only 23 chromosomes in humans.
Chromosomes widely differ in appearance. They vary in size, staining properties, the location of centromere, relative length of the two arms on either side of centromere, and the position of constricted regions along the arms. Chromosomes acquire different shapes at the time of anaphase during cell division. The usual shapes are i. j and v. A chromosome is made of telomere. chromatids, centromere, primary constriction and a secondary constriction.
- Telomere: A telomere caps each end of every chromosome. Specific proteins bind to telomere. These proteins protect chromos tiles from the attacks or exonueleases present in the same cell. Telomere permits DNA replication to continue to the very end of • chromosome. The telomere is also involved in
attachment o: enromosome ends to the nuclear membrane. It
is also involved U. pairing of homologous chromosomes during meiosis. The sti icture of telomeric DNA is very similar in all eukaryotic orgai isms. One strand of the DNA is rich in guanine. It is oriented toward the end of the chromosome. The other strand is rich in cytosine. It is oriented to \sard the centromere. In most organisms, the telomere consists or multiple copies of a very short DNA.
- Chromatids: Each chromosome contains two chromatids. Both chromatids are exact copy of each other. Chromatids are held together by centromere. These chromatids are exact copy of each other. Both chromatids are separated from each other during anaphase.
- Centromere: The chromatids of each pair of homologous chromosomes are held together at the centromere. Centromere is responsible fkm– proper segregation of each chromosome pair during cell division. The kinetochore is the attachment site Ew the mierottibules. It guides the movement of the chromosomes to the poles. Kinetochore is organized around the centromere. The molecular structures of centromere in most species are still unclear. Centromere contains repetitive DNA called heterochromatin.
- Primary and secondary constrictions: Each chromosome has special constrictions at two points. First constriction is present at sit of centromere. It is called primary constriction. One or more pairs of chromosomes in each species have also secondary constriction. It does not stain well. Nucleolus is attached with this region. This region contains multiple copies of the genes that transcribe the ribosomal RNA ( :RN A) within the nucleolus.
Types of chromosomes
The chromosomes have different types on the basis of position of centromere:
I. Telocentric: The centromere is present at the ends of the chromatids.
- Acrocentric: The centromere is present near the end of chromatids. The chromosomes have two very small arm and two very long arms.
- Sub metacentric: The centromere is present near the middle of the chromatids. Chromosome contains arms of unequal length.
- Metacentric: The centromere is present at the middle of the chromatids. Chromosome contains equal arms.
Ultrastracture of chromosome
Chromatin consists of DNA and histone proteins. This association of DNA and protein helps in the complex packing of DNA into chromosomes. So it regulates the DNA activity. There are live different histone proteins. These are HI. fbA, H2B. 113, H4
Four types of Histone protein form a core particle called nucleosome. These proteins are I I,A. 11,13. 11,, 114. The nucleosome is composed of eight molecules of histones. DNA coil around the nucleosome. Histones have a high proportion of positively charged amino acids. So they bind tightly to the negatively charged DNA and form chromatin. The unfolded chromatin shows beads on a string under
electron microscope. Chromati n organization controls the transcription of DNA. Therefore, nucleosome influences the gene expression.
- Linker protein HI
The fifth histone HI is called the linker protein. It is not needed to form the nucleosome. Rather it fixes the DNA with nueleosomes.
The beaded string undergoes higher order packing. The chromatin coils and folds further to produce the thickened. compact
chromosomes during mitosis. The beaded string can coil tightly with the help of histone HI. It makes a cylinder 30 nm in diameter. It is called solenoid.
- Looped domains
The solenoid fiber forms loops called loop domains.
- Metaphase chromosomes
The metaphase chromosome themselves coil and fold in a mitotic chromosome. It further compact all the chromatin. Thus it produces the characteristic metaphase chromosomes. Not all chromatin is equally active. Some human genes are active only after adolescence. In other cases, entire chromosomes may not function in particular cells. So there are two areas of chromosomes:
(a) Heterochromatic region: These are inactive portions of chromosomes. They show, a banding pattern with certain stains. So they are called heterochromatin. The DNA of heterochromatin is not transcribed. Euchromatic region: The active portions of chromosomes are called euchromatic regions.