Molecular Biology of the Gene: DNA Structure, Replication, Transcription, and Translation

Molecular Biology of the Gene

Introduction

This study guide covers the molecular basis of genetic information, focusing on the structure and function of DNA and RNA, the processes of replication, transcription, and translation, and the regulation and expression of genes. It also addresses viral genetics, mutations, and the mechanisms of genetic transfer in bacteria.

DNA Structure and Function

Key Features of DNA

• DNA (Deoxyribonucleic Acid) is the hereditary material in almost all living organisms.

• Composed of nucleotides, each containing a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, guanine).

• Double helix structure with complementary base pairing (A-T, C-G).

• Antiparallel strands: one runs 5' to 3', the other 3' to 5'.

DNA vs. RNA

• RNA (Ribonucleic Acid) contains ribose sugar and uracil instead of thymine.

• RNA is usually single-stranded.

Nucleotides and Nucleic Acids

• Nucleotides are the building blocks of nucleic acids.

• Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

• DNA nucleotides: deoxyribose sugar; RNA nucleotides: ribose sugar.

DNA Replication

Mechanism of Replication

• Replication is semiconservative: each new DNA molecule consists of one old strand and one new strand.

• DNA polymerase adds nucleotides to the 3' end of the growing strand.

• Replication proceeds in the 5' to 3' direction.

• Leading strand is synthesized continuously; lagging strand is synthesized in Okazaki fragments.

Equation:

$\text{DNA}_{\text{parent}} \rightarrow \text{DNA}_{\text{daughter}} + \text{DNA}_{\text{daughter}}$

Key Enzymes

• Helicase: Unwinds the DNA double helix.

• DNA polymerase: Synthesizes new DNA strands.

• Ligase: Joins Okazaki fragments on the lagging strand.

Transcription: DNA to RNA

Process Overview

• Transcription is the synthesis of RNA from a DNA template.

• Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• RNA polymerase binds to the promoter region to initiate transcription.

• mRNA is synthesized in the 5' to 3' direction.

mRNA Processing in Eukaryotes

• Introns are removed, and exons are spliced together.

• A 5' cap and a 3' poly-A tail are added for stability and export from the nucleus.

Translation: RNA to Protein

Genetic Code

• The genetic code is a set of three-nucleotide sequences (codons) on mRNA that specify amino acids.

• It is universal, redundant, and unambiguous.

Translation Process

• Occurs in the cytoplasm on ribosomes.

• tRNA molecules bring amino acids to the ribosome, matching their anticodon to the mRNA codon.

• Ribosomes have three sites: A (aminoacyl), P (peptidyl), and E (exit).

• Translation proceeds through initiation, elongation, and termination.

Sequence of Events in Translation

• Codon recognition

• Peptide bond formation

• Translocation

• Termination

Example: Using the Genetic Code Table

Given mRNA sequence: 5' CGGUUACAGGUCUGUCA 3'

To determine the amino acid sequence, use the genetic code table to translate each codon.

Sample Genetic Code Table

Codon	Amino Acid
UUU, UUC	Phe
UUA, UUG, CUU, CUC, CUA, CUG	Leu
AUG	Met (Start)
UAA, UAG, UGA	Stop

Mutations

Types of Mutations

• Silent mutation: No change in amino acid sequence.

• Missense mutation: Changes one amino acid to another.

• Nonsense mutation: Changes an amino acid codon to a stop codon.

• Frameshift mutation: Insertion or deletion alters the reading frame.

Example

The sentence "The dog did not eat" can be altered by a frameshift mutation to "The dgd idn ote at."

Viruses and Gene Transfer

Viral Life Cycles

• Lytic cycle: Virus replicates rapidly and lyses the host cell.

• Lysogenic cycle: Viral DNA integrates into host genome and replicates with it.

Gene Transfer in Bacteria

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between bacterial cells via a pilus.

Gene Regulation

Control of Gene Expression

• Genes can be regulated at the level of transcription, RNA processing, translation, and post-translational modification.

• Promoters and enhancers are DNA sequences that regulate gene expression.

Applications and Implications

Antibiotic Resistance

• Human activities, such as the use of antibiotics in agriculture, can increase the number of bacteria with resistance genes.

Genetic Engineering

• Plasmids are used as vectors in genetic engineering to introduce new genes into bacteria.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded
Function	Genetic information storage	Protein synthesis, gene regulation

Additional info: This guide expands on the exam questions by providing definitions, explanations, and examples for each concept, ensuring a comprehensive review of molecular genetics as covered in a General Biology course.