Model Answer
0 min readIntroduction
Chromosomes, the carriers of genetic information, are fundamental units of heredity. Discovered by Wilhelm von Waldeim in 1888, they are typically observed during cell division. The study of chromosomes—cytogenetics—has revolutionized our understanding of inheritance, genetic disorders, and evolutionary processes. Recent advancements in genome sequencing and CRISPR technology continue to deepen our knowledge of chromosome structure and function, with implications for agriculture and medicine. This response will describe chromosomes under the requested headings, outlining their number, composition, structure, and the unique role of Y chromosomes.
(i) Chromosome Number
Chromosome number refers to the total number of chromosomes in a cell. Humans have 23 pairs (46 total), with 21 pairs of autosomes and one pair of sex chromosomes. Different species exhibit varying chromosome numbers; for example, dogs have 78 chromosomes, while a common goldfish has 100. The number is characteristic of a species and is crucial for proper cellular function. Aneuploidy, a condition where the chromosome number deviates from the normal, often leads to genetic disorders like Down syndrome (trisomy 21).
(ii) Chemical Constituents of Chromosomes
Chromosomes are primarily composed of DNA (deoxyribonucleic acid) and proteins. DNA carries the genetic code, organized into genes. Proteins, particularly histones, are crucial for packaging and organizing DNA. Histones form structures called nucleosomes, which are the basic units of chromatin. Approximately 50% of a chromosome’s mass is DNA, and the other 50% is protein. RNA (ribonucleic acid) is also present in smaller quantities, playing a role in gene expression. The ratio of DNA to protein can vary depending on the organism and the stage of cell cycle.
(iii) Chromomeres
Chromomeres are localized condensations of chromatin visible along chromosomes during prophase I of meiosis. They appear as distinct, bead-like structures. Each chromomere typically represents a region of actively transcribed genes. The number and arrangement of chromomeres are species-specific and can be used for chromosome identification. The formation of chromomeres is linked to the folding and compaction of chromatin around histone cores. They provide a visual representation of the underlying genetic organization within a chromosome.
(iv) Y Chromosomes
The Y chromosome is one of the sex chromosomes in mammals, responsible for male sex determination. It is smaller than the X chromosome and carries relatively few genes, approximately 70-80 genes. The SRY gene, located on the Y chromosome, is the primary determinant of male sex. Its presence triggers the development of testes, leading to the production of testosterone. The Y chromosome also contains genes involved in spermatogenesis (sperm production) and male fertility. Its size and gene content have reduced over evolutionary time, leading to it being termed a "degenerated" chromosome, although it still plays vital roles.
Conclusion
In essence, chromosomes are the repositories of genetic information, meticulously organized and regulated. From their varying numbers across species to their intricate chemical composition and specialized roles like the Y chromosome's contribution to sex determination, they represent a cornerstone of biological inheritance. Ongoing research continues to unveil the complexities of chromosome structure and function, promising further advancements in our understanding of life itself.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.