Chromosomes

Chromosomes are long, thread-like structures composed of DNA and proteins that carry genetic information. Found in the nuclei of cells, chromosomes are the physical structures that organize and store the vast amount of DNA required for an organism’s development, function, and reproduction.

Structure of Chromosomes

1. DNA Molecule:
   • Chromosomes are made of tightly coiled DNA, which contains genes, the functional units of heredity.
   • In eukaryotic cells, DNA wraps around proteins called histones, forming a complex called chromatin.
2. Chromatin Organization:
   • Chromatin exists in two forms:
     • Euchromatin: Loosely packed, transcriptionally active (genes are expressed).
     • Heterochromatin: Densely packed, transcriptionally inactive.
3. Condensation:
   • During cell division, chromatin condenses further to form the characteristic “X-shaped” structure of chromosomes.
4. Centromere:
   • A constricted region that divides the chromosome into two arms:
     • Short arm (p arm).
     • Long arm (q arm).
   • The centromere is essential for proper chromosome segregation during cell division.
5. Telomeres:
   • Repeated sequences of DNA at the ends of chromosomes.
   • Protect chromosomes from degradation and prevent them from fusing with one another.

Types of Chromosomes

1. Autosomes:
   • Non-sex chromosomes that carry genes determining general traits.
   • Humans have 22 pairs of autosomes.
2. Sex Chromosomes:
   • Determine the biological sex of an organism:
     • XX: Female.
     • XY: Male.
   • Humans have one pair of sex chromosomes.
3. Circular Chromosomes (Prokaryotes):
   • In prokaryotic cells (e.g., bacteria), DNA exists as a single, circular chromosome located in the nucleoid region.

Functions of Chromosomes

1. Storage of Genetic Information:
   • Chromosomes contain all the genes required for the organism’s development, survival, and reproduction.
2. DNA Protection:
   • Chromosomes protect DNA from damage and help maintain its structural integrity.
3. Gene Regulation:
   • Chromosomes control gene expression by regulating which parts of the DNA are accessible for transcription.
4. Inheritance:
   • Chromosomes ensure the precise transmission of genetic information from parent to offspring during reproduction.

Number of Chromosomes

• The number of chromosomes varies between species:
  • Humans: 46 chromosomes (23 pairs).
  • Fruit Fly: 8 chromosomes.
  • Dogs: 78 chromosomes.
• The complete set of chromosomes in a cell is called the karyotype.

Chromosomes During Cell Division

1. Replication:
   • Before cell division, each chromosome is replicated to form two identical sister chromatids joined at the centromere.
2. Mitosis:
   • Chromosomes are equally divided between two daughter cells to ensure genetic consistency.
3. Meiosis:
   • In reproductive cells, chromosomes undergo a specialized division to produce gametes (sperm and egg) with half the number of chromosomes.

Abnormalities in Chromosomes

1. Numerical Abnormalities:
   • Aneuploidy: Missing or extra chromosomes.
     • Example: Down syndrome (trisomy 21).
2. Structural Abnormalities:
   • Deletions, duplications, inversions, or translocations of chromosome segments.

Importance of Chromosomes

1. Basis of Heredity:
   • Chromosomes carry genes that determine an organism’s traits and enable genetic inheritance.
2. Role in Evolution:
   • Variations in chromosome structure and number contribute to species evolution.
3. Medical Relevance:
   • Chromosomal analysis helps diagnose genetic disorders and study diseases like cancer.

In summary, chromosomes are essential structures that store, protect, and transmit genetic information. They play a crucial role in heredity, cellular function, and evolution, making them central to the study of biology and medicine.