Chromosome Structure and Function

What are Chromosomes?

Chromosomes are the threadlike structures found in the nucleus of almost all organisms. Chromosomes are made up of protein and DNA. They are passed from parents to offspring and contain all the instructions required to produce a complete individual. Every species has a specific number of chromosomes in their nucleus. For example, humans have 46 chromosomes in the form of 23 pairs in their somatic cell nuclei. However, they have only 23 chromosomes in their sperms and eggs, which fuse to form a complete organism haveing 46 chromosomes. Chromosomes are invisible under the microscope if a cell is in the non-dividing state. However, during cell division, they are tightly coiled so become visible under the microscope. In the following paragraphs, we will discuss chromosome structure and function.

Structure of Chromosomes

The structure of chromosomes is variable between prokaryotes and eukaryotes. Eukaryotes have larger linear chromosomes while prokaryotes have smaller and coiled DNA. Chromosomes of the eukaryotes are condensed and packed by proteins in the form of  chromatin. Condensation helps the longer DNA molecules to fit into the cell’s nucleus. As the chromosomes contain the genetic material, they are replicated and passed on to the next generations.

In eukaryotes, chromosomes are made up of 50% DNA and 50 % proteins. DNA is tightly coiled around histone proteins. Histones are the proteins which help package the DNA by wrapping them around it. There are five types of histones present in DNA molecules. These are positively charged molecules so they bind tightly to the negatively charged phosphate molecules in the DNA.

Chromosomes are duplicated during S phase of the cell cycle. There is a constriction point in each chromosome called centromere which divides the chromosomes into two arms. Centromeres help chromosomes to be properly aligned during cell division. On both sides of the chromosomes, the regions are called arms of the chromosomes. Each chromosome has a short arm called “p arm” and a long arm called “q arm”. There are repetitive stretches of DNA located at the ends of chromosomes called telomeres.

How are Chromosomes Inherited?

Almost all organisms have a diploid number of chromosomes in their somatic cells. The chromosomes are found in the form of pairs in a somatic cell’s nuclei. Of these pairs, one chromosome is inherited from the mother and one from the father. This is why we inherit some traits from our father and some from our mother.

Types of Chromosomes

There are two types of chromosomes: Autosomes and allosomes (Sex chromosomes). Autosomes are same in males and females. However, there is a slight difference in sex chromosomes present in males and females. For example humans, out of 46 chromosomes, 44 are autosomes while only two are sex chromosomes. In females, there are two X chromosomes, while in males there is one X chromosome and one Y chromosome.

Function of Chromosomes

Chromosomes contain the genetic material called the DNA which is needed by an organism to develop and grow. As the chromosomes are duplicated and passed on to to the next  generations, they help in  the storage and genetic code.

They also help in sex determination. In humans, there are 23 pairs of chromosomes, out of which one pair is of sex chromosomes. Sex is determined by the chromosome given by male. If an X chromosome of the male fuses with X of female the child is a female and if Y chromosome fuses with the X the child is a male.

Chromosomes also control successful cell division as well they form proteins and store them.

Next: Difference between DNA, Genes, and Chromosomes

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