Ch. 6: Molecular Genetics Handout

Introduction to Molecular Genetics

• Central Dogma: Biological theory describing the direction of genetic information flow ( DNA → RNA → Protein )

• Genetic Building Blocks: Nucleotides as basic units

Nucleotides vs Nucleosides

• Nucleoside: Consists of 1 ribose or deoxyribose sugar and 1 nitrogenous base

• Nucleotide: Comprises 1 sugar, 1 nitrogenous base, and 1 or more phosphate groups

  • E.g. Nucleotide with 3 phosphates = Nucleoside triphosphate

RNA vs DNA

• RNA (Ribonucleic acid):

  • Single-stranded, contains ribose sugar; hydroxyl groups on both 2' and 3' carbons

• DNA (Deoxyribonucleic acid):

  • Double helix, contains deoxyribose sugar; hydroxyl group on only 3' carbon

Nitrogenous Bases

• Purines: Two ring structures

  • Adenine (A) and Guanine (G)

  • Mnemonic: PURe As Gold

• Pyrimidines: One ring structure

  • Cytosine (C), Thymine (T), and Uracil (U)

  • Mnemonic: CUT the PYe

Base Pairing

• In DNA:

  • A pairs with T

  • G pairs with C

• In RNA:

  • A pairs with U

  • G pairs with C

Strength of Base Pairs

• Strength depends on hydrogen bonds:

  • A-T or A-U has 2 hydrogen bonds

  • G-C has 3 hydrogen bonds

  • Regions with G-C pairs are harder to separate due to more bonds which require more tempature to break the bond.

DNA Organization

Nucleosomes

• Nucleosomes: DNA wrapped around histone proteins

Chromatin

• Chromatin Structure:

  • Euchromatin: Loosely packed, easily accessible for transcription

  • Heterochromatin: Tightly packed, mostly inaccessible for transcription

Histone Modifications

• Acetylation: Addition of acetyl group which loosens nucleosome packing

• Deacetylation: Removal of acetyl group which tightens nucleosome packing

• Methylation: Can increase or decrease transcription based on gene needs

DNA Replication: Overview and Initiation

• Semiconservative Replication: Each new DNA helix consists of one old and one new strand

• Antiparallel Strands: DNA has 5' (phosphate) and 3' (hydroxyl) ends

Steps of DNA Replication

1. Initiation

• Creation of an origin of replication; initiates with specific sequences recognized by enzymes

2. Elongation

• Involves nucleotides being added to the growing strand

  • Helicase: Unzips DNA

  • Topoisomerase: Relieves supercoiling

3. Termination

• Occurs when DNA polymerase reaches the end of chromosomes, leading to incomplete replication at telomeres

G1/S Checkpoint

• Quality control before DNA replication

• Favorable conditions required for cells to enter S phase; otherwise, cells enter G0 phase

Transcription Overview

• Gene: Basic unit of heritable information, encodes proteins/RNA

• Initiates with a promoter, ends with a terminator

Eukaryotic vs Prokaryotic Transcription

• Eukaryotic: Transcription in the nucleus, translation in cytoplasm
  - Prokaryotic: Both processes occur simultaneously in the cytoplasm

Post-Transcriptional Modifications in Eukaryotes

• Includes 5' capping, 3' polyadenylation, and splicing of introns

Ribosome Structure and Function

• Ribosomes aid in translation, composed of rRNA and proteins

  • Eukaryotic: 80S (60S + 40S); Prokaryotic: 70S (50S + 30S)

Translation Steps

1. Initiation: Start codon is recognized (AUG)

2. Elongation: tRNAs deliver amino acids; peptide bonds form

3. Termination: Stop codons signal for halting synthesis

Types of DNA Mutations

1. Point Mutations: Base substitutions - can be silent, missense, or nonsense

2. Frameshift Mutations: Insertions or deletions that alter gene reading frame

3. Chromosomal Mutations: Affect whole chromosome structure

   • Includes deletions, duplications, inversions, and translocations

Bacterial Genetic Diversity

• Bacteria utilize binary fission, depend on horizontal gene transfer for genetic diversity

• Mechanisms of horizontal gene transfer: Conjugation, Transformation, Transduction