How Genes Work: From DNA to Protein and Mutation

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How Genes Work

Introduction to How Genes Work

Understanding how genes function at the molecular level is central to modern biology. Genes are segments of DNA that contain instructions for building molecules essential for cellular function. The process by which information in DNA is converted into functional molecules is called gene expression.

DNA is often referred to as the blueprint of life.
Gene expression involves converting DNA information into functioning molecules within the cell.

What Do Genes Do?

Beadle and Tatum's Experiments

Beadle and Tatum used bread mold (Neurospora crassa) to investigate gene function by inducing mutations and observing their effects on phenotype.

Damaging a gene and observing the resulting phenotype reveals gene function.
Nonfunctioning alleles are called null or loss-of-function alleles.

The One-Gene, One-Enzyme Hypothesis

This hypothesis states that each gene contains the information needed to make one enzyme. Srb and Horowitz tested this by studying the metabolic pathway for arginine synthesis in N. crassa.

They performed a genetic screen by growing mold on medium lacking arginine.
Mutants unable to grow were missing an enzyme in the pathway.

Figure: Each step in the arginine pathway is catalyzed by a different enzyme.

Experimental Test and Modern View

Srb and Horowitz isolated mutants, each deficient in a different enzyme of the pathway.
Results supported the one-gene, one-enzyme hypothesis, now refined as the one-gene, one-polypeptide hypothesis.

The Genetic Code Hypothesis

DNA as a Code

Francis Crick proposed that the sequence of DNA bases acts as a code, with different combinations specifying the 20 amino acids.

DNA is an information storage molecule.
A gene specifies the amino acid sequence of a protein.
Information in DNA is not directly translated into protein sequence.

RNA as the Intermediary

Jacob and Monod suggested that RNA links genes in the nucleus to protein synthesis in the cytoplasm.
Messenger RNA (mRNA) carries information from DNA to the site of protein synthesis.
RNA polymerase synthesizes RNA using DNA as a template.

The Central Dogma of Molecular Biology

The central dogma summarizes the flow of genetic information:

DNA → RNA → Protein
Genes (DNA) code for RNA, which codes for proteins.

Transcription and Translation

Transcription: Using a DNA template to make complementary RNA (copying information).
Translation: Using information in mRNA to synthesize proteins (interpreting nucleotide language to amino acids).

Linking Genotypes to Phenotypes

Genotype: Determined by the sequence of bases in DNA.
Phenotype: Observable traits produced by proteins.
Alleles of the same gene differ in DNA sequence, leading to different protein products and phenotypes.

Modifications to the Central Dogma

Some genes code for RNAs that are not translated into proteins but have important functions (e.g., rRNA, tRNA).
In some viruses, information can flow from RNA back to DNA via reverse transcriptase:

The Genetic Code

Structure of the Genetic Code

The genetic code specifies how nucleotide sequences code for amino acid sequences.
Each "word" in the code is a codon—a group of three bases specifying a particular amino acid.
There are 64 possible codons (), more than enough to code for 20 amino acids.

Cracking the Code

Nirenberg and Leder determined which codons code for each amino acid.
There is one start codon (AUG) for methionine and three stop codons (UGA, UAA, UAG) that signal the end of translation.

Codon Type	Function
Start codon (AUG)	Initiates protein synthesis; codes for methionine
Stop codons (UGA, UAA, UAG)	Signal end of protein-coding sequence
Other codons	Code for amino acids

Properties of the Genetic Code

Redundant: Most amino acids are encoded by more than one codon.
Unambiguous: Each codon codes for only one amino acid.
Non-overlapping: Codons are read one at a time.
Nearly universal: The code is shared by almost all organisms.
Conservative: Codons for the same amino acid often share the first two bases.

Applications of the Genetic Code

Predicting amino acid sequences from DNA sequences.
Determining possible DNA or mRNA sequences for a given amino acid sequence (due to redundancy).

Types and Consequences of Mutation

Definition and Types of Mutation

A mutation is any permanent change in an organism's DNA, resulting in a modification of its genetic information and potentially creating new alleles.

Point mutations: Changes in one or a few bases.
Chromosome-level mutations: Larger-scale changes affecting chromosome structure or number.

Types of Point Mutations

Missense mutations: Change an amino acid in the protein.
Silent mutations: Do not change the amino acid sequence (due to code redundancy).
Frameshift mutations: Shift the reading frame, altering all subsequent codons.
Nonsense mutations: Change a codon to a stop codon, terminating translation early.

Name	Definition	Example	Consequence
Silent	Change in nucleotide sequence that does not change amino acid	UCU → UCC (both code for Ser)	No change in phenotype
Missense	Change in nucleotide sequence that changes amino acid	UCU → UGU (Ser → Cys)	Change in protein structure/function
Nonsense	Change in nucleotide sequence that results in a stop codon	UCU → UAA (Ser → Stop)	Leads to mRNA breakdown or shortened protein
Frameshift	Addition or deletion of a nucleotide	UCU GAA → UCG AA...	Reading frame is shifted, altering all downstream amino acids

Effects of Mutations on Fitness

Beneficial mutations: Increase fitness (survival and reproduction).
Neutral mutations: No effect on fitness.
Deleterious mutations: Decrease fitness.

Some mutations outside coding regions can affect gene expression and phenotype.

Chromosome Mutations

Inversion: Segment of chromosome breaks off, flips, and rejoins.
Translocation: Segment breaks off and attaches to another chromosome.
Deletion: Segment is lost.
Duplication: Segment is present in multiple copies.

Chromosome mutations can be beneficial, neutral, or deleterious. They are often visualized using a karyotype (complete set of chromosomes in a cell).

Type	Description
Inversion	Segment flips and rejoins
Translocation	Segment attaches to another chromosome
Deletion	Segment is lost
Duplication	Segment is duplicated

Example: Chromosomes of cancer cells often exhibit deleterious chromosome mutations, including aneuploidy, inversions, translocations, deletions, and duplications.

Additional info: Chromosome-level mutations can be detected by analyzing karyotypes, which display the number and structure of chromosomes in a cell.