Index
An organism’s DNA is packaged in its cells to protect it; these also regulate access to DNA. DNA packaging helps conserve space in cells.
Approximately two meters of human DNA can fit in a cell only a few micrometers wide. Chromosomes are made up of segments of DNA. Chromosomes contain all the information that helps a cell grow, survive, and reproduce.
DNA segments with specific patterns are called genes. Chromosomes are found in the nucleus of the cell. In prokaryotic organisms, DNA is not present in the middle; DNA floats in the cytoplasm in the nucleoid area.
Chromosomes are an organized structure of DNA and proteins found in cells. They are thread-like structures located within the nucleus of animal and plant cells. Chromosomes are made up of proteins and a deoxyribonucleic acid (DNA) molecule.
Chromosomes vary widely between different organisms. Eukaryotic cells have many linear chromosomes, and prokaryotic cells have smaller, circular DNA.
Cells can contain more than one type of chromosome; as in most eukaryotic cells, the mitochondria and chloroplasts in plant cells have their own set of chromosomes.
In the nucleus of the eukaryotic organism, chromosomes are packed with proteins to form a compact structure called chromatin. This condensation allows long DNA molecules to enter the middle of the cell.
Chromosomes are more condensed than chromatin and are essential for cell division.
Cellular division
Chromosomes replicate, divide, and are passed on to daughter cells, to ensure genetic diversity and survival of the progeny. Duplicated chromosomes contain two identical copies known as chromatids or sister chromatids, to which a centromere is attached.
The compaction of chromosomes during cell division results in the four-armed structure.
In eukaryotic cells, chromosomes are made up of a single DNA molecule with many copies of five types of histones.
Histones are protein molecules and are rich in lysine and arginine residues; they have a positive charge. Therefore, they bind tightly to negatively charged phosphates in the DNA sequence.
Many proteins are not histones; these are mainly transcription factors. Transcription factors regulate which parts of DNA are transcribed into RNA.
During most of the cell’s life cycle, chromosomes are elongated and cannot be seen under a microscope. During the S phase of the mitotic cell cycle, chromosomes duplicate.
At the beginning of mitosis, chromosomes duplicate and condense into temporary structures that can be easily stained and seen under a light microscope.
These duplicated condensed chromosomes are known as dyads. The replicated chromosomes are held together in the region of the centromeres.
Centromeres
Centromeres in humans are approximately 1-10 million base pairs of DNA.
Centromere DNA is short, primarily repetitive DNA sequences; the sequences repeat over and over again in tandem arrays.
The attached duplicated chromosomes are commonly called sister chromatids.
Kinetochores are the point of attachment of the spindle fibers that help separate sister chromatids as the mitosis process moves to the anaphase stage. Kinetochores are a complex of approximately 80 different proteins.
The shorter arm of the two arms of the chromosome that extends from the centromere is called the C-arm, and the more extended arm is known as the q-arm.
The structure of the X and Y chromosomes
While the chromosomes for other body parts are the same size and shape, forming an identical pairing, the X and Y chromosomes have different structures.
