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 exploring how genetic information is stored, expressed, and transmitted in living organisms.

DNA Structure and Components

• Deoxyribonucleic acid (DNA): The molecule that stores genetic information in all living organisms. DNA is composed of two strands forming a double helix structure.

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

• Polynucleotide: A long chain of nucleotides joined together; DNA and RNA are both polynucleotides.

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

• Double helix: The spiral structure formed by two complementary strands of DNA.

• Adenine (A), Thymine (T), Guanine (G), Cytosine (C): The four nitrogenous bases in DNA. Adenine pairs with thymine, and guanine pairs with cytosine.

• Uracil (U): A nitrogenous base found in RNA, replacing thymine.

Genetic Code and Codons

• Triplet code: The genetic code is read in sets of three nucleotides (codons), each specifying a particular amino acid.

• Codon: A sequence of three nucleotides in mRNA that codes for a specific amino acid or a stop signal during translation.

• 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.

• Genetic code: The set of rules by which information encoded in genetic material is translated into proteins by living cells.

Gene Expression: Transcription and Translation

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

• RNA polymerase: The enzyme that synthesizes RNA from a DNA template during transcription.

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

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

• Messenger RNA (mRNA): The RNA molecule that carries genetic information from DNA to the ribosome for protein synthesis.

• RNA splicing: The process of removing introns and joining exons in a pre-mRNA transcript to produce mature mRNA.

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

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

Translation and the Role of RNA

• Translation: The process by which the sequence of an mRNA is decoded to produce a specific polypeptide (protein).

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

• Ribosomal RNA (rRNA): A type of RNA that, together with proteins, makes up the ribosome.

• Transfer RNA (tRNA): RNA molecules that carry amino acids to the ribosome and match them to the coded mRNA message using their anticodon.

• Anticodon: A set of three nucleotides in tRNA that is complementary to an mRNA codon.

• A site and P site: Sites on the ribosome where tRNA molecules bind during translation. The A site holds the incoming tRNA, and the P site holds the tRNA with the growing polypeptide chain.

DNA Replication

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

• DNA polymerase: The enzyme that synthesizes new DNA strands by adding nucleotides to a pre-existing chain.

• DNA ligase: The enzyme that joins fragments of DNA together during replication and repair.

Mutations and Their Effects

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

• Mutagen: An agent, such as radiation or a chemical, that increases the mutation rate.

• 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.

• Nonsense mutation: A mutation that changes a codon to a stop codon, resulting in a shortened 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.

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

Summary Table: Types of Mutations

Mutation Type	Description	Effect on Protein
Silent Mutation	Change in DNA that does not alter amino acid sequence	No effect
Missense Mutation	Change in DNA that results in a different amino acid	May alter protein function
Nonsense Mutation	Change in DNA that creates a stop codon	Produces a truncated, usually nonfunctional protein
Frameshift Mutation	Insertion or deletion that shifts the reading frame	Alters all downstream amino acids; usually nonfunctional protein

Key Equations and Models

• Base Pairing Rule:

$\text{A pairs with T (or U in RNA), G pairs with C}$

• Semiconservative Replication:

$\text{Each new DNA molecule: one parental strand + one new strand}$

Example: Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: These terms are foundational for understanding advanced topics such as gene regulation, biotechnology, and genomics.