Chromosomes

Introduction 

Chromosomes are aptly referred to as hereditary vehicles because they maintain morphological and physiological characteristics and properties through successive generations. They can cause self-reproduction and seamless transmission of the genetic material through generations.

Viral chromosomes represent a single DNA or RNA molecule in viruses and macrophages. In a bacterial chromosome, a double-stranded DNA contains the hereditary matter. Eukaryotic chromosomes consist of DNA and proteins and are specific bodies with individual existence.

Chromosomes: Key Aspects to Know

A chromosome resembles a small thread and with a coil-like structure containing proteins. Chromosomes store the crucial genetic material DNA and are present in the nuclei of all cells. The DNA in chromosomes passes the genetic information to successive generations. Humans have 23 pairs of chromosomes. There are specific numbers of chromosomes in a given species.

By considering the location of the centromere, we can divide chromosomes into four different types as follows:

• Metacentric chromosomes: The location of the centromere is precisely at the centre, meaning that both arms are of equal length. The metacentric chromosome looks like the English alphabet V
• Sub-metacentric chromosome: The centromere is away from the centre of the chromosome, making one arm shorter than that of the other
• The acrocentric chromosome: The centromere is at the far end of an arm in these chromosomes. The acrocentric chromosome appears like the letter J
• Telocentric chromosome: The centromere lies at the end of the chromosome

Sex chromosomes and autosomes are the two types of chromosomes as per characteristics . One is a sex chromosome from the 23 pairs of chromosomes in humans. The remaining 22 pairs are autosomes.

Homologous Chromosomes

Homologous chromosomes or homologs consist of two chromosomes. One chromosome on the homologous chromosome is from the mother, and another is from the father. These chromosomes undergo pairing during the process of fertilization. These have their centromere in a similar location to facilitate proper pairing of the centromere. Both chromosomes have identical lengths for alignment during pairing. These properties prevent separation before meiosis.  

The dissimilarity between alleles is a vital difference between the two chromosomes. It ensures that the phenotypes of the offspring are different. Both parents supply a complete genome as 23 chromosomes from encoding the same genes from each parent, this is regarding humans. Homologous chromosomes provide multiple versions of the genes to boost variety.

Homologous Chromosomes Facilitate Variety and Stability

Humans are diploid organisms because they have two copies of the genome in every cell. These are homologous chromosomes that enable different versions of genes or alleles. Various permutations and combinations of good and bad proteins result in different phenotypes to boost variety.

Complex, dominant or recessive relationships of alleles help produce significantly different versions of the organisms. It helps populations of organisms survive the challenges of environmental changes.  

During the formation of gametes, homologous chromosomes participate in homologous recombination. Crossing over is the process that results in an exchange of some parts of homologous chromosomes after coming in close contact. It is possible because the chromosomes are of identical size and length. The resultant genetic material exchange is vital for maintaining variety within the given population. Homologous chromosomes ensure the random mixing of traits from parents.

Chromosomes: Constituents and Functions 

Chromosomes are made up of protein and a molecule of Deoxyribonucleic Acid (DNA). These thread-like structures are present in animal and plant cells. The special structure of chromosomes keeps DNA in a tightly wrapped condition, surrounded by proteins. The spool-like proteins are also known as histones. A chromosome has a dumbbell-like shape with a constriction in the middle.

Chromosomes are made up of the following:

• Centromere: The chromosome divides into two parts because of the constriction or centromere. It is at the junction of spindle fibres and enables chromosomes to move during cell division
• Chromatid: A centromere connects two chromatids on both ends. Chromatid contains DNA and forms separate chromosomes during the anaphase
• Chromatin: Consisting of DNA, RNA, and protein, chromatin forms chromosomes within the eukaryotic cell
• Secondary constriction: It enables a nucleolar organisation
• Telomere: Each chromosome has two telomeres at both terminals
• Chromonema: It bears the genes and has a coil-like structure 
• Chromomeres: The thread-like chromonema bears the chromomeres that resemble tiny beads. Chromomeres transmit the genes to successive generations 
• Matrix: The jelly-like matrix inside the pellicle, acting as a surrounding membrane for each chromosome. The matrix contains non-genetic material

Chromosomes are made up of DNA to provide genetic information to execute several cellular functions that are vital for survival, growth and reproduction. Proteins in chromosomes offer protection to the DNA material against physical shocks and chemicals. It is crucial during the cell division process. The histone and non-histone proteins in chromosomes control the action of genes. Activation and deactivation of these proteins cause the expansion and contraction of chromosomes.

XX chromosome

A person’s biological sex relates to the presence of X or Y sex chromosomes. These are X chromosomes and Y chromosomes. The XX is specific for females, and XY denotes males. Unlike homologous chromosomes, the X and Y chromosomes carry different genes( heterologous).

XX chromosome represents a biologically female individual. Females inherit X chromosomes from both parents. The X chromosome is three times larger than the Y chromosome as it contains 900 genes against just 55 genes in the Y chromosome. Females in the mammal population carry XX chromosomes in each cell. There is an inactivation of one of the X chromosomes to prevent double-dosing of X-linked genes.

Following are the two conditions that may result from the structural changes in X chromosomes or change in their numbers:

• Klinefelter’s Syndrome: is caused due to the presence of an additional copy of X-chromosome resulting in a karyotype of 47, XXY. Such an individual has overall masculine development, however, the feminine-development (development of breast, i.e. Gynaecomastia) is also expressed. Such individuals are sterile
• Turner syndrome: A woman inherits a single copy of the X chromosome(44+XO).It is also called gonadal dysgenesis

Conclusion

The credit of coining the term chromosomes goes to the German anatomist Heinrich Waldeyer. Chromosomes are made up of DNA and proteins and have thread-like structures. There are 46 chromosomes and 23 pairs with complex functions in humans. Chromosomes carry vital information about personal traits and characteristics from one generation to another. One pair of chromosomes is a sex chromosome. Homologous chromosomes maintain variety in a given population of species. X chromosomes and Y chromosomes are responsible for determining the gender of the offspring. XX chromosome is specific to females, and XY is a male sex chromosome.