Understanding DNA Mutations and Chromosomal Anomalies

DNA Mutations

Mutations are alterations to a DNA sequence that occur when a gene is damaged or changed, altering the genetic message. These mutations can have varying effects on health, depending on their location and whether they alter essential protein functions.

Mutations can occur at three levels:

1. Molecular (Gene or Point)
   • These mutations affect the chemical constitution of genes, specifically the DNA bases.
2. Chromosomal
   • These mutations affect a segment of a chromosome, altering its structure through deletions, duplications, or rearrangements.
3. Genomic
   • These mutations affect the entire genome, altering the number of chromosome sets (polyploidy or haploidy) or individual chromosomes (e.g., trisomy 21).

Mutations can be of different types:

1. Gene Mutations: Alterations in the nucleotide sequence of a gene.
   • Substitution of Bases: Incorrect bases are paired.
   • Insertion: Extra bases are added.
   • Deletion: Bases are removed.

   Insertions and deletions can cause frameshift mutations, altering the reading frame of the genetic code.

2. Chromosomal Mutations: Alterations of the gene sequence on a chromosome.
   • Deletion: Genetic material breaks off.
   • Duplication: Extra gene copies are generated.
   • Inversion: A chromosome segment is reversed.
   • Translocation: A fragment from one chromosome attaches to another.

Chromosomal Anomalies

Chromosomal abnormalities can be numerical or structural.

• Numerical Anomalies: An extra or missing chromosome.

Structural Anomalies: Part of a chromosome is missing, duplicated, moved, or inverted.

Conditions Related to Chromosomes 21 and 23

Down Syndrome (Trisomy 21)

Down syndrome occurs when an individual has an extra copy of chromosome 21. This leads to intellectual disability, low muscle tone, distinct physical features, and other health issues.

Trisomy 21

Caused by nondisjunction during cell division, resulting in three copies of chromosome 21.

Translocation Down Syndrome

The extra chromosome 21 is attached to another chromosome and can be inherited.

Mosaicism

A mixture of cells with 46 and 47 chromosomes, leading to fewer characteristics of Down syndrome.