Molecular Biology of the Gene: Key Terms and Concepts

Ch. 10 Molecular Biology of the Gene: Vocabulary and Concepts

Introduction

This section covers essential vocabulary and concepts related to the molecular biology of the gene, focusing on the structure and function of DNA and RNA, the processes of transcription and translation, and the genetic code. Understanding these terms is fundamental for grasping how genetic information is stored, expressed, and transmitted in living organisms.

DNA Structure and Components

Deoxyribonucleic Acid (DNA)

• Definition: DNA is a double-stranded molecule that carries genetic information in all living organisms and many viruses.

• Double Helix: The structure of DNA, consisting of two polynucleotide strands wound around each other.

• Sugar-Phosphate Backbone: The repeating chain of sugar and phosphate groups that forms the structural framework of DNA and RNA molecules.

• Nucleotide: The basic building block of nucleic acids, consisting of a sugar, a phosphate group, and a nitrogenous base.

• Polynucleotide: A long chain of nucleotides joined together.

Nitrogenous Bases

• Adenine (A): A purine base that pairs with thymine in DNA and uracil in RNA.

• Thymine (T): A pyrimidine base found only in DNA; pairs with adenine.

• Cytosine (C): A pyrimidine base that pairs with guanine.

• Guanine (G): A purine base that pairs with cytosine.

• Uracil (U): A pyrimidine base found only in RNA; replaces thymine and pairs with adenine.

Semiconservative Model

• Definition: The model of DNA replication in which each new DNA molecule consists of one old strand and one newly synthesized strand.

• Example: During cell division, DNA replicates so that each daughter cell receives one parental strand and one new strand.

Gene Expression: Transcription and Translation

Transcription

• Definition: The process by which a DNA sequence is copied into messenger RNA (mRNA).

• RNA Polymerase: The enzyme that synthesizes RNA from a DNA template.

• Promoter: A DNA sequence where RNA polymerase binds to initiate transcription.

• Terminator: A DNA sequence that signals the end of transcription.

• RNA Splicing: The process of removing introns and joining exons in a pre-mRNA transcript.

• Exons: Coding regions of a gene that remain in the mature mRNA.

• Introns: Non-coding regions of a gene that are removed during RNA splicing.

Translation

• Definition: The process by which the sequence of an mRNA molecule is used to direct the synthesis of a polypeptide (protein).

• Ribosome: The molecular machine that facilitates the translation of mRNA into protein.

• Ribosomal RNA (rRNA): RNA molecules that form the core of the ribosome's structure and catalyze protein synthesis.

• Messenger RNA (mRNA): The RNA copy of a gene that carries genetic information from the DNA to the ribosome.

• Transfer RNA (tRNA): RNA molecules that bring amino acids to the ribosome during translation.

• Anticodon: A sequence of three bases in tRNA that pairs with the complementary codon in mRNA.

• A Site: The region of the ribosome that holds the tRNA carrying the next amino acid to be added.

• P Site: The region of the ribosome that holds the tRNA carrying the growing polypeptide chain.

Genetic Code

• Triplet Code: The genetic instructions for a polypeptide chain are written in DNA as a series of non-overlapping three-nucleotide words (codons).

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal.

• Start Codon: The codon (AUG) that signals the start of translation and codes for methionine.

• Stop Codon: Codons (UAA, UAG, UGA) that signal the end of translation.

Mutations and Genetic Variation

Mutation

• Definition: Any change in the nucleotide sequence of DNA.

• Mutagen: An agent, such as radiation or a chemical substance, that causes mutations.

Types of Mutations

• Silent Mutation: A mutation that does not change the amino acid sequence of a protein.

• Missense Mutation: A mutation that changes one amino acid in a protein, potentially altering its function.

• Nonsense Mutation: A mutation that changes a codon to a stop codon, resulting in a truncated protein.

• Frameshift Mutation: A mutation caused by insertions or deletions that change the reading frame of the genetic message.

Other Key Terms

• Bacteriophage: A virus that infects bacteria.

• DNA Ligase: An enzyme that joins DNA fragments together during DNA replication and repair.

• DNA Polymerase: An enzyme that synthesizes new DNA strands using a DNA template.

• Transformation: The uptake of foreign DNA by a cell, often used in genetic engineering.

Summary Table: Key Terms and Functions

Term	Definition/Function
DNA Polymerase	Enzyme that synthesizes new DNA strands
RNA Polymerase	Enzyme that synthesizes RNA from DNA template
Ligase	Enzyme that joins DNA fragments
Promoter	DNA sequence where transcription begins
Terminator	DNA sequence where transcription ends
Exon	Coding region of a gene
Intron	Non-coding region of a gene
Codon	Three-nucleotide sequence in mRNA
Anticodon	Three-nucleotide sequence in tRNA
Start Codon	Initiates translation (AUG)
Stop Codon	Terminates translation (UAA, UAG, UGA)

Key Equations

• Base Pairing Rule:

$\mathrm{A} = \mathrm{T}$ $\mathrm{G} = \mathrm{C}$

• Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Additional info:

• Some terms (e.g., A site, P site) refer to specific locations within the ribosome that are critical for the translation process.

• Mutations can have varying effects on protein function, from no effect (silent) to complete loss of function (nonsense or frameshift).