The DNA of an organism are packed in their cells to protect, they also regulate the accessibility of the DNA. The packaging of DNA helps conserve space in the cells. Approximately, two meters of the human DNA can fit into a cell that is only a few micrometres wide. Chromosomes are made up of DNA segments. Chromosomes carry all the information that helps a cell grows, survive and reproduce. DNA segments with specific patterns are called genes. The chromosomes are found in the nucleus of the cell. In prokaryotic organisms, the DNA is not present in the nucleus; the DNA floats in the cytoplasm in the area called the nucleoid.
The chromosomes vary widely between different organisms. Eukaryotic cells have a large number of linear chromosomes and cells of prokaryotes have smaller and circular DNA. Cells may contain more than one type of chromosome, like in most eukaryotic cells, the mitochondria and the chloroplasts in plant cells possess their own set of chromosomes.
In the nucleus of the eukaryotic organism, the chromosomes are packed with proteins to form a compact structure called chromatin. This condensation allows long molecules of DNA to fit into the cell nucleus. Chromosomes are more condensed than the chromatin and they are essential for cell division. The chromosomes are replicated, divided and passed on to the daughter cells, to ensure genetic diversity and survival of the progeny.
Duplicated chromosomes contain two identical copies known as chromatids or sister chromatids, they are joined by a centromere. Compaction of the chromosomes during the cell division process results in the four-arm structure.
Recombination of chromosome plays a vital role in genetic diversity. Incorrect multiplication of the chromosomes may lead to mitotic failure or death of the cell, it may lead to apoptosis and sometimes may be cancerous.
What are Chromosomes?
- In eukaryotic cells, chromosomes are composed of the single molecule of DNA with many copies of five types of histones.
- Histones are proteins molecules and are rich in lysine and arginine residues, they are positively charged. Hence they bind tightly to the negatively charged phosphates in the DNA sequence.
- A small number of non-histone proteins are also present, these are mostly transcription factors. Transcription factors regulate which parts of DNA to be transcribed into RNA.
- During most of the cell’s life cycle, chromosomes are elongated and cannot be observed under the microscope.
- During the S phase of the mitotic cell cycle, the chromosomes are duplicated.
- At the beginning of mitosis, the chromosomes are duplicated and they begin to condense into short structures which can be stained and observed easily under the light microscope.
- These duplicated condensed chromosomes are known as dyads.
- The duplicated chromosomes are held together at the region of centromeres.
- The centromeres in humans are made of about 1-10 million base pairs of DNA.
- The DNA of the centromere are mostly repetitive short sequences of DNA, the sequences are repeated over and over in tandem arrays.
- The attached, duplicated chromosomes are commonly called sister chromatids.
- Kinetochores are the attachment point for spindle fibers which helps to pull apart the sister chromatids as the mitosis process proceeds to anaphase stage. The kinetochores are a complex of about 80 different proteins.
- The shorter arm of the two arms of the chromosome extending from the centromere is called the p arm and the longer arm is known the q arm.
Function of Chromosomes
- Genetic Code Storage: Chromosome contains the genetic material that is required by the organism to develop and grow. DNA molecules are made of the chain of units called genes. Genes are those sections of the DNA which code for specific proteins required by the cell for its proper functioning.
- Sex Determination: Humans have 23 pairs of chromosomes out of which one pair is the sex chromosome. Females have two X chromosomes and males have one X and one Y chromosome. The sex of the child is determined by the chromosome passed down by the male. If X chromosome is passed out of XY chromosome, the child will be a female and if a Y chromosome is passed, a male child develops.
- Control of Cell Division: Chromosomes check successful division of cells during the process of mitosis. The chromosomes of the parent cells ensure that the correct information is passed on to the daughter cells required by the cell to grow and develop correctly.
- Formation of Proteins and Storage: Proteins are essential for the activity of a cell. The chromosomes direct the sequences of proteins formed in our body and also maintain the order of DNA. The proteins are also stored in the coiled structure of the chromosomes. These proteins bound to the DNA help in proper packaging of the DNA.