Molecular Basis of Inheritance: DNA Structure, Replication, and Chromosomal Organization

L10: Molecular Basis of Inheritance

Introduction

This study guide covers the molecular basis of inheritance, focusing on the structure of DNA, its replication, and chromosomal organization. These topics are central to understanding how genetic information is stored, transmitted, and maintained in living organisms.

Genetic Inheritance and Carrier Probability

Hemophilia and X-linked Inheritance

Hemophilia is a genetic disorder often inherited in an X-linked recessive manner. Understanding carrier probability requires knowledge of pedigree analysis and sex-linked inheritance.

• X-linked recessive inheritance: The gene causing hemophilia is located on the X chromosome. Males (XY) with the mutant allele will express the disease; females (XX) must inherit two mutant alleles to express the disease but can be carriers with one mutant allele.

• Carrier probability: If a woman's paternal or maternal grandfather had hemophilia, the probability she is a carrier depends on the transmission of the X chromosome through her parents. For example, if her paternal grandfather had hemophilia, her father must have inherited the affected X chromosome and would express the disease (since males have only one X). If her maternal grandfather had hemophilia, her mother could be a carrier, and the woman has a 50% chance of being a carrier if her mother is heterozygous.

• Example: If a woman's maternal grandfather had hemophilia, and her mother is a carrier, the woman has a 50% chance of being a carrier.

• Additional info: Pedigree analysis and Punnett squares are useful tools for calculating carrier probabilities in X-linked traits.

Telomeres: Definition and Etymology

What are Telomeres?

Telomeres are specialized structures at the ends of linear chromosomes that protect genetic material during cell division.

• Etymology: The term telomere comes from Greek: telos meaning "end" and meros meaning "part".

• Function: Telomeres prevent the loss of important DNA sequences during replication and play a role in cellular aging and stability.

Structure of DNA

The DNA Strand

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms.

• Components: Each DNA strand consists of a sugar-phosphate backbone and nitrogenous bases (adenine, thymine, guanine, cytosine).

• Base pairing: Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

• Antiparallel orientation: The two strands run in opposite directions (5' to 3' and 3' to 5').

• Example: The structure is often depicted as a ladder, with the sugar-phosphate backbone as the sides and base pairs as the rungs.

Building a Structural Model of DNA

Scientific Inquiry and Discovery

The discovery of DNA's structure was a major milestone in biology, involving several key scientists and techniques.

• X-ray crystallography: Maurice Wilkins and Rosalind Franklin used this technique to study DNA's molecular structure.

• Rosalind Franklin's contribution: Franklin produced X-ray diffraction images that revealed DNA's helical structure.

• Watson and Crick: Used Franklin's data to deduce the double helix structure and built models to fit the chemical and physical data.

The Double Helix

Structure and Base Pairing

DNA's double helix consists of two strands twisted around each other, stabilized by specific base pairing.

• Helical structure: DNA is a right-handed double helix with a diameter of 2 nm and 10 base pairs per turn (3.4 nm per turn).

• Base pairing specificity: Purines (A, G) pair with pyrimidines (T, C) to maintain a uniform width. Pairing like with like (A with A, etc.) does not fit the observed structure.

• Chargaff's rules: In any organism, the amount of A equals T, and the amount of G equals C.

• Antiparallel backbones: The two sugar-phosphate backbones run in opposite directions.

• Example: The double helix model explains how genetic information is stored and replicated.

Base Pairing in DNA

Purines and Pyrimidines

Base pairing in DNA is governed by the chemical structure of the bases.

• Purines: Adenine (A) and Guanine (G) are double-ringed structures.

• Pyrimidines: Thymine (T) and Cytosine (C) are single-ringed structures.

• Pairing: A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds.

• Example: The stability of the DNA molecule is enhanced by the specific hydrogen bonding between base pairs.

Summary Table: DNA Base Pairing

Base	Type	Pairs With	# of Hydrogen Bonds
Adenine (A)	Purine	Thymine (T)	2
Thymine (T)	Pyrimidine	Adenine (A)	2
Guanine (G)	Purine	Cytosine (C)	3
Cytosine (C)	Pyrimidine	Guanine (G)	3

Key Equations

• Chargaff's Rule:

$[A] = [T]$ $[G] = [C]$

Additional Info

• DNA's structure is essential for its function in heredity, replication, and mutation.

• Understanding the molecular basis of inheritance is foundational for genetics, molecular biology, and biotechnology.