The Molecular Basis of Inheritance

The Molecular Basis of Inheritance

Overview

  • Life’s Operating Instructions: In 1953, James Watson and Francis Crick introduced a groundbreaking model for the structure of deoxyribonucleic acid (DNA), resembling a double helix.

  • This structural framework of DNA directs the development of various traits including biochemical, anatomical, physiological, and to some extent, behavioral characteristics.

DNA as Genetic Material

  • DNA is confirmed as the genetic material.

  • Early 20th Century: Identifying the molecular basis of inheritance was a significant challenge for biologists.

  • Frederick Griffith conducted pivotal studies on bacteria, specifically using Streptococcus pneumoniae, in his quest to develop a vaccine, which led to insights into DNA’s role in heredity.

Griffith’s Experiment

  • Griffith discovered that mixing heat-killed pathogenic strain with live non-pathogenic strain resulted in some living cells acquiring pathogenic traits, indicating a transfer of genetic material.

Additional Evidence for DNA as Genetic Material

  • Studies, such as the Hershey-Chase Experiment, provided more evidence that DNA acts as genetic material, particularly through the analysis of the T2 virus that infects E. coli.

Knowledge Before 1950s

  • Before the 1950s, it was established that DNA is a polymer of nucleotides consisting of:

    • A nitrogenous base

    • A sugar

    • A phosphate group

  • In 1947, E. Chargaff noted variability in the DNA base composition across different species, which added to its credibility as genetic material.

  • Example DNA Base Composition for Humans:

    • A = 30.3%

    • T = 30.3%

    • G = 19.5%

    • C = 19.5%

Structural Model of DNA

  • After establishing DNA as the genetic material, determining its structure was crucial to understand its role in inheritance.

  • Maurice Wilkins and Rosalind Franklin employed X-ray crystallography to analyze DNA’s molecular structure.

  • Franklyn’s most notable contribution was a picture of the DNA molecule that revealed:

    • DNA’s helical structure

    • The double-helical model composed of two antiparallel sugar-phosphate backbones

    • Pairing of nitrogenous bases within the helical interior.

    • Important measurements:

    • The helix makes a turn every 3.4 nm.

    • Bases are spaced 0.34 nm apart.

Base Pairing Specificity

  • Watson and Crick interpreted Franklyn’s images and further reasoned that base pairing followed specific rules, with each base pair forming a different number of hydrogen bonds:

    • Adenine (A) and Thymine (T) are joined by two hydrogen bonds.

    • Cytosine (C) and Guanine (G) are joined by three hydrogen bonds.

  • DNA's structure is antiparallel and right-handed.

DNA Replication Basics

  • Base Pairing to a Template Strand: Each DNA strand serves as a template for synthesizing new strands during replication. The process involves:

    • The unwinding of the parent molecule and construction of new daughter strands based on base-pairing rules where:

    • A pairs with T

    • G pairs with C.

  • Replication Steps:

    1. The parent molecule uncoils into two complementary strands.

    2. Each strand is a template to determine the order of nucleotides in new strands.

    3. Nucleotides connect to form sugar-phosphate backbones of the daughter strands, creating a structure that includes one parental and one new strand in each daughter molecule.

Semiconservative Nature of DNA Replication

  • DNA replication is semiconservative, which means that each new DNA molecule includes one old strand from the parent and one newly synthesized strand.

Detailed Mechanism of DNA Replication

  • The replication process is fast and precise, requiring various enzymes:

    • Helicase: unwinds DNA strands.

    • Topoisomerase: alleviates strain and rejoins parental strands.

    • Primase: synthesizes short RNA primers to start new DNA strands.

    • DNA Polymerase III: adds DNA nucleotides to the 3' end of the new strand.

    • DNA Polymerase I: removes RNA primers and replaces them with DNA.

    • DNA Ligase: connects Okazaki fragments to create continuous DNA strands.

Origins of Replication

  • The replication process begins at origins of replication where parental strands separate, creating replication bubbles. Eukaryotic chromosomes can have hundreds or thousands of these origins. Bubbles grow laterally as replication occurs bidirectionally, eventually merging to synthesize complete daughter strands.

Leading vs. Lagging Strands in DNA Synthesis

  • The leading strand synthesizes continuously in the 5' to 3' direction.

  • The lagging strand is synthesized in fragments (Okazaki fragments), also in the 5' to 3' direction.

  • DNA ligase then joins these fragments to form a continuous strand.

Proofreading and Repair Mechanisms

  • DNA polymerases possess proof-reading capability to rectify any incorrect nucleotides, with errors occurring in approximately 1 in 100,000 nucleotides.

  • Mismatch repair mechanisms identify and correct base pairing errors, reducing final error rates to around 1 in 10 billion nucleotides.

Nucleotide Excision Repair

  • Repair Process:

    1. A nuclease enzyme removes damaged sections of DNA.

    2. DNA polymerase synthesizes to fill in missing nucleotides.

    3. DNA ligase seals the repaired strand.

Telomeres and Chromosomal DNA Replication

  • With every replication, the ends of eukaryotic DNA become shorter. This occurs in lagging strands due to the inability of DNA polymerase to replace RNA primers at the ends.

  • Telomeres are repetitive nucleotide sequences that prevent the loss of essential genes from being replicated.

  • Telomerase is an enzyme present in germ cells that can extend telomeres, replenishing lost sequences during replication.

Telomerase Activity

  • Mechanism when telomerase binds to the telomere:

    1. Telomerase attaches to the 3' overhang of the telomere, pairing with the complementary RNA.

    2. Bases are added using the telomerase RNA as a template.

    3. Telomerase shifts forward, repeating the process to extend the telomere using a sequence of bases that protect genetic information from erosion.

  • This activity highlights the critical interplay between DNA replication and cellular longevity, particularly seen in germ cells where telomerase is active.