Chromosome definition
Chromosome consists of a large, linear DNA molecule that is associated with proteins responsible for its packaging, forming chromatin (this structure is similar in eukaryotes, including fungi, plants and animals). The main function of chromosome is transportation of genes from parental cell to daughter cells.
Chromosome constitution
Due to its large size, the DNA present in cells is packaged and organized in a specific way, which is crucial for the architecture of nucleus and cell itself. In bacteria, DNA is organized into large circular chromosomes, unlike eukaryotic cell’s DNA which is arranged in a linear double-helix fashion on several chromosomes.
Chromosomes consist of:
- Centromere, which is a specialized region that plays a key role in the distribution of chromosomes by daughter cells and is located in the middle of the chromosome;
- Long and short arm, which are determined by the position of the centromere;
- Telomeres, which play a key role during chromosome duplication and maintenance and which are located at the edges.
Chromosome structure
During cell cycle chromosomes exhibit variations in their packaging state which is essential to minimize the loss of genetic information during cell division and to facilitate replication and transcription at interphase.
During interphase, cell cycle stage where there is no cell division, chromosomes are in a more decondensed/relaxed state in order to facilitate the access of replication machinery (proteins and other molecules), responsible for DNA duplication, and machinery of transcription, necessary to transcribe the DNA into RNA. After duplication of chromosomes, there are two identical copies of the same chromosome in the cell, which are called ‘chromatids’.
During mitosis, cell cycle stage where the cell division occurs, chromosomes are highly condensed and are easily observed under optical microscope, being called ‘mitotic chromosomes’. In this step, chromosomes (which are duplicated and held together by a centromere) are separated by connecting the mitotic spindle to the centromere and equally distributes to each daughter cells (each chromatide migrates to a different cell), allowing the passage of exactly the same information for each daughter cell.
Human karyotype
Human cell contains two copies of each chromosome, one inherited from paternal progenitor and other inherited from maternal progenitor (called homologous chromosomes), on a total of 46 chromosomes (22 pairs of somatic chromosomes and 1 pair of sex chromosomes). There are two distinct sex chromosomes: X chromosome and Y chromosome, the pair of sex chromosomes being XX in women and XY in men.
Chromosome study
To the study and visualization of chromosome’s structure several techniques are used. For example: G and R banding (with the CG-rich zones being colored by a different color from the C-G-poor zones, which gives a chiselled appearance to chromosomes) and hybridization (each chromosomes is stained with a specific color). Through studies of chromosomes it is possible to ascertain about its structure and if they are in correct number, making possible the detection of several chromosomal anomalies.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
