Molecular Biology Chapter

Chapter 9: Molecular Biology Overview

Dolly the sheep was the first cloned mammal, representing a significant advancement in genetic research. Applications of DNA analysis are vast, including:

  • Crime solving

  • Paternity testing

  • Human identification

  • Tracing genealogy

  • Identifying pathogens

  • Clinical applications in diagnostics and cancer therapy.

Molecular genetics enables profound insights into evolution and relationships among species by sequencing genomes.

9.1 The Structure of DNA

Learning Objectives

  • Understand the structure of DNA and its arrangement in prokaryotic and eukaryotic cells.

Discovery of DNA Structure

In the 1950s, Francis Crick and James Watson proposed the double helix structure of DNA, which was complemented by data from Rosalind Franklin and Maurice Wilkins. Chargaff's rules highlight that two types of nucleotides are always found in equal amounts: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). This discovery earned them the Nobel Prize in 1962.

Components of DNA

  • Nucleotides: The building blocks of DNA, consisting of:

    • Deoxyribose (sugar)

    • Phosphate group

    • Nitrogenous bases:

      • Purines: Adenine (A), Guanine (G)

      • Pyrimidines: Cytosine (C), Thymine (T)

  • Backbone Formation: Covalent bonds between phosphate groups and sugars of adjacent nucleotides create a polynucleotide strand.

DNA Double Helix Structure

DNA exists as two strands forming a right-handed double helix. Key features include:

  • Base-pairing: A pairs with T (2 hydrogen bonds), and C pairs with G (3 hydrogen bonds).

  • Antiparallel Strands: One strand runs 5' to 3'; the complementary strand runs in the opposite direction.

RNA Structure

Ribonucleic Acid (RNA) shares similarities with DNA but has distinct features:

  • Contains ribose instead of deoxyribose.

  • Nitrogenous bases include Adenine, Cytosine, Guanine, and Uracil (U instead of Thymine).

  • RNA is typically single-stranded.

Types of RNA

  • mRNA (messenger RNA): Carries genetic information from DNA to the ribosome for protein synthesis.

  • tRNA (transfer RNA): Brings amino acids to ribosomes during translation.

  • rRNA (ribosomal RNA): A component of ribosomes that facilitates protein synthesis.

DNA Arrangement in Cells

Prokaryotic Cells

  • Contain a single circular chromosome located in the nucleoid region.

  • Compact structure due to supercoiling.

Eukaryotic Cells

  • Multiple linear chromosomes located within a nucleus.

  • DNA is wrapped around histones to form nucleosomes, resulting in a compact structure ("beads on a string" model). At metaphase, chromosomes are maximally condensed (~700 nm wide).

9.2 DNA Replication

Learning Objectives

  • Explain the DNA replication process and its significance.

Importance of Replication

DNA must replicate before cell division to ensure that each daughter cell receives an identical copy of genetic information.

Mechanism

  • Semi-conservative Replication: Each original strand serves as a template for a new strand, maintaining both parental and new strands.

  • Key Steps:

    • DNA unwinds at origins of replication.

    • DNA polymerase synthesizes new complementary strands, utilizing RNA primers.

    • Leading strand is synthesized continuously while the lagging strand is synthesized in short Okazaki fragments.

    • RNA primers are replaced by DNA, and fragments are joined by DNA ligase.

Eukaryotic Replication vs. Prokaryotic Replication

Eukaryotes have multiple origins of replication, facilitating simultaneous replication, while prokaryotic cells replicate efficiently using a singular origin of replication.

Telomere Replication

  • Telomeres: Repetitive sequences that protect chromosome ends from degradation and are shortened during cell division.

  • Telomerase: An enzyme that extends telomeres, allowing replication at the ends of linear chromosomes. It is active in germ cells and stem cells and is associated with aging reduction and potential applications in regenerative medicine.

DNA Repair Mechanisms

  • Proofreading: DNA polymerase corrects mismatches during replication.

  • Mismatch Repair: Enzymes identify and correct errors post-replication.

  • Nucleotide Excision Repair: Repairs DNA damage, such as thymine dimers caused by UV light.

9.3 Transcription

Learning Objectives

  • Describe the process of transcription and the handling of eukaryotic mRNA.

Central Dogma of Molecular Biology

The central dogma illustrates the flow of genetic information: DNA → RNA → Protein.

Transcription Steps

Transcription occurs in three stages:

  1. Initiation: Unwinding DNA and binding RNA polymerase to the promoter region.

  2. Elongation: RNA polymerase synthesizes mRNA from the DNA template.

  3. Termination: Signals instruct RNA polymerase to release mRNA, completing the transcript.

Post-Transcriptional Modifications in Eukaryotes

Eukaryotic mRNA undergoes several modifications before translation:

  • Addition of a 5' cap for stability and recognition.

  • Polyadenylation at the 3' end (addition of a poly-A tail).

  • Splicing to remove introns (non-coding sequences) and join exons (coding sequences).

These modifications are crucial for the translation of mRNA into protein and protect mRNA from degradation.