Molecular Genetics

Concept: In 1953, James Watson and Francis Crick proposed the double-helix model of DNA, which explains the molecular structure and mechanism of genetic inheritance. Their model was based on X-ray diffraction data (notably by Rosalind Franklin) and Chargaff’s base pairing rules.
Explanation: According to the Watson--Crick model, the important features of the DNA double helix are:

• DNA consists of two long polynucleotide strands coiled around each other to form a right-handed double helix.
• Each strand is made of repeating units called nucleotides, consisting of:
  • A deoxyribose sugar
  • A phosphate group
  • A nitrogenous base
• The two strands run in opposite directions (antiparallel), i.e., one 5' 3' and the other 3' 5'.
• The sugar-phosphate backbone forms the outer framework, while nitrogenous bases project inward.
• Complementary base pairing occurs:
  • Adenine pairs with thymine via two hydrogen bonds
  • Guanine pairs with cytosine via three hydrogen bonds
• The helix has a uniform diameter of about 2 nm and one complete turn every 3.4 nm (approximately 10 base pairs per turn).

This model explained how genetic information is stored and replicated, making it a cornerstone of molecular biology.

Concept: DNA consists of two polynucleotide strands held together by hydrogen bonds between complementary nitrogenous bases. The pairing follows Chargaff’s base pairing rule:

• Adenine (A) pairs with Thymine (T)
• Guanine (G) pairs with Cytosine (C)

Each base pair is stabilized by a specific number of hydrogen bonds, which contribute to the stability of the DNA double helix.
Explanation: Adenine and thymine form two hydrogen bonds between them. In contrast, guanine and cytosine form three hydrogen bonds, making the G--C pair more stable than the A--T pair. Thus, the number of hydrogen bonds between adenine and thymine is 2.

Concept: Genetic disorders in a developing fetus can be detected using prenatal diagnostic techniques. These methods involve analyzing fetal cells or tissues to identify chromosomal abnormalities or inherited diseases. One of the most widely used procedures is amniocentesis. Amniocentesis is a medical procedure in which a small amount of amniotic fluid is withdrawn from the amniotic sac surrounding the fetus. This fluid contains fetal cells that can be analyzed for:

• Chromosomal abnormalities (e.g., Down syndrome)
• Genetic disorders (e.g., thalassemia, sickle cell anemia)
• Certain metabolic disorders

Explanation: During pregnancy, usually between 15--20 weeks, a thin needle is inserted into the uterus under ultrasound guidance to collect amniotic fluid. The fetal cells present in this fluid are cultured and examined for genetic abnormalities. Thus, the method used for detecting genetic disorders during pregnancy is amniocentesis.