Eukaryotic chromosomes are organized structures found within the nucleus of plant and animal cells. These structures house the organism’s entire genetic information, encoded in long strands of deoxyribonucleic acid (DNA). Chromosomes act like organized shelves, storing genetic “books” of information compactly within the cell’s nucleus, making them accessible when needed.
Structural Organization and Composition
Chromosomes are constructed from DNA molecules tightly wound around specific proteins called histones. Histones are positively charged and act like spools. DNA, being negatively charged, strongly binds to these histones, allowing for effective coiling.
The fundamental unit of this packaging is a nucleosome. A nucleosome consists of approximately 147 base pairs of DNA wrapped around an octamer of eight histone proteins. Multiple nucleosomes, connected by short segments of linker DNA, resemble “beads on a string”.
This “beads on a string” structure further coils and folds into more condensed forms, initially forming a 30-nanometer fiber through a process called supercoiling. These fibers then loop and fold again into even more compact structures. This hierarchical packaging allows the enormously long DNA molecule, which can be billions of nucleotides long, to fit efficiently within the small nucleus of a eukaryotic cell. The entire complex of DNA and its associated proteins is collectively referred to as chromatin, which condenses significantly to form visible chromosomes during cell division.
Key Anatomical Features
A replicated chromosome, visible during cell division, has distinct anatomical features. It consists of two identical copies, called sister chromatids, joined lengthwise. These sister chromatids contain the same genetic information, ensuring accurate distribution to daughter cells.
The centromere is a constricted region on the chromosome that connects sister chromatids. This specific DNA sequence is where proteins attach, aiding in chromosome separation during cell division. Its location can vary, influencing the appearance of the chromosome arms.
At the ends of each chromosome arm are specialized protective caps known as telomeres. These regions consist of repetitive DNA sequences and associated proteins that shield genetic information from damage or degradation during DNA replication. They prevent the “fraying” of genetic material.
Eukaryotic organisms that reproduce sexually have two sets of chromosomes, one inherited from each parent. These are homologous chromosomes, pairs that carry genes for the same traits at corresponding positions. While they carry genes for the same traits, the specific versions (alleles) may differ between them.
Function During Cell Division
Chromosomes play an important role during cell division, ensuring accurate genetic information transmission to new cells. During mitosis, a process for growth and repair, each chromosome duplicates to form two sister chromatids. These replicated chromosomes then align in the cell’s center before being pulled apart to opposite ends. This segregation ensures each new daughter cell receives a complete and identical set of 46 chromosomes in humans.
Meiosis, a different form of cell division, produces gametes (sperm and egg cells) for sexual reproduction. In this process, homologous chromosomes pair up and exchange genetic material through crossing over. This genetic recombination introduces new gene combinations, contributing to genetic diversity among offspring.
Following recombination, the chromosome number is halved, resulting in daughter cells with one set of chromosomes. For humans, gametes contain 23 chromosomes, half the number found in somatic (body) cells. This reduction maintains the correct chromosome number in offspring when two gametes fuse during fertilization.
Chromosomal Abnormalities
Errors can occur during cell division, leading to chromosomal abnormalities, which involve changes in chromosome number or structure. Numerical abnormalities, known as aneuploidy, happen when an individual has an extra or missing chromosome. An example is Trisomy 21 (Down syndrome), where an individual has three copies of chromosome 21 instead of two. Another instance is Turner syndrome, where females have only one X chromosome (XO) instead of two (XX).
Structural abnormalities involve alterations within the chromosome itself. These include deletions, where a segment of a chromosome is lost, or duplications, where a segment is repeated. Translocations occur when a segment of one chromosome breaks off and attaches to a different, non-homologous chromosome.
These errors often arise from mistakes during meiosis, particularly a failure of chromosomes to separate properly, a phenomenon called nondisjunction. Such abnormalities can lead to developmental disorders, genetic syndromes, or, in some cases, may not have noticeable effects, depending on the specific change and the genes involved.