09 Molecular Genetics and Gene Expression

09 Molecular Genetics and Gene Expression: Information in DNA

Molecular genetics follows information from its storage in DNA to its use in cells. DNA is a polymer of nucleotides, and each nucleotide contains a deoxyribose sugar, a phosphate group, and one base: adenine, thymine, cytosine, or guanine. The two DNA strands are antiparallel and held together by complementary hydrogen bonding: $\mathrm{A-T}$ and $\mathrm{C-G}$. Because each base has a specific partner, the sequence of one strand determines the sequence of the other.

A gene is a DNA sequence that contributes to a functional product. The complete DNA content of an organism is its genome. In eukaryotes, DNA is organized into chromosomes and wrapped around histone proteins to form nucleosomes and chromatin. Chromatin structure can make a gene more or less accessible to the machinery required for transcription.

Takeaway: DNA stores information in the order of its bases, and complementary pairing makes that information readable and copyable.

Copying Genetic Information Accurately

Before a cell divides, its DNA must be copied. DNA replication is semiconservative: each new DNA molecule contains one original strand and one new strand.

The main sequence of events is:

• Helicase unwinds the double helix at a replication fork.

• Single-strand-binding proteins stabilize the separated strands.

• Primase makes a short RNA primer because DNA polymerase needs a pre-existing $3'$ hydroxyl group.

• DNA polymerase adds complementary nucleotides to the new strand in the $5'$ to $3'$ direction.

• RNA primers are removed and replaced with DNA.

• DNA ligase seals breaks between adjacent DNA fragments.

Because the template strands are antiparallel, the leading strand is synthesized continuously toward the replication fork, while the lagging strand is synthesized discontinuously as Okazaki fragments. Proofreading and repair systems improve accuracy, although some changes remain and become mutations.

For example, a strand written as $3'\text{–TAC GGA CTT–}5'$ has the complementary strand $5'\text{–ATG CCT GAA–}3'$. Each original strand can then serve as a template for a new complementary strand.

Takeaway: DNA replication preserves genetic information by combining semiconservative copying, complementary pairing, and error correction.

From DNA to RNA

Gene expression uses DNA information to make functional RNA or protein. For protein-coding genes, the central pathway is $\mathrm{DNA} \rightarrow \mathrm{RNA} \rightarrow \mathrm{protein}$, with transcription connecting DNA to RNA and translation connecting mRNA to a polypeptide.

During transcription, RNA polymerase binds to a promoter, opens a local section of DNA, and builds an RNA strand in the $5'$ to $3'$ direction. RNA contains ribose and uracil instead of deoxyribose and thymine, and it is usually single-stranded. The RNA sequence is complementary to the DNA template strand and resembles the coding strand except that $\mathrm{U}$ replaces $\mathrm{T}$.

In eukaryotes, the initial pre-mRNA is modified before it leaves the nucleus. A $5'$ cap and $3'$ poly-A tail are added, introns are removed, and exons are joined. Alternative splicing can combine exons in different ways, allowing one gene to produce multiple related proteins. In prokaryotes, which lack a nucleus, transcription and translation can occur nearly simultaneously.

Takeaway: Transcription produces an RNA copy of gene information, while RNA processing prepares many eukaryotic transcripts for translation.

Reading Codons and Building Proteins

Translation converts an mRNA nucleotide sequence into a polypeptide. The ribosome reads mRNA in groups of three nucleotides called codons. Messenger RNA carries the coding sequence, transfer RNA brings specific amino acids and uses an anticodon to pair with each codon, and ribosomal RNA contributes to ribosome structure and peptide-bond formation.

Translation proceeds in stages:

1. During initiation, a ribosomal subunit binds mRNA and an initiator tRNA pairs with the start codon.

2. During elongation, tRNAs pair with successive codons, peptide bonds form, and the ribosome moves along the mRNA.

3. During termination, a stop codon recruits release factors and the completed polypeptide is released.

4. The polypeptide folds and may be chemically modified or transported to a particular cellular location.

The genetic code is degenerate, meaning that more than one codon can specify the same amino acid. For the mRNA sequence $5'\text{–AUG GCU UUU UGA–}3'$, the ribosome reads methionine, alanine, phenylalanine, and then a stop signal. The polypeptide therefore begins with methionine–alanine–phenylalanine.

Takeaway: Ribosomes translate codons into amino acids, but the final protein also depends on folding, modification, and cellular location.

Controlling Gene Expression

Cells do not express every gene continuously. Regulation saves energy and allows cells with the same genome to develop different structures and functions.

In many bacteria, genes for a shared pathway are arranged in an operon. An operon can contain a promoter, an operator, and several structural genes transcribed into one mRNA. A repressor reduces transcription by binding regulatory DNA, an activator increases transcription, and an inducer changes the activity of a regulatory protein.

The lac operon is inducible: when lactose is available and glucose is scarce, transcription of lactose-use genes increases. The trp operon is repressible: when tryptophan is abundant, it helps activate a repressor that decreases transcription of tryptophan-synthesis genes.

Eukaryotic regulation can occur at many levels, including chromatin accessibility, DNA methylation, transcription-factor binding, RNA processing, mRNA stability, translation, and protein modification or degradation. Epigenetic regulation changes gene activity without changing the DNA base sequence. Signals such as nutrients, hormones, temperature, stress, and communication from neighboring cells can be integrated into regulatory networks.

Takeaway: Regulation controls when, where, and how much gene product a cell makes.

Mutations and Their Consequences

Mutations are changes in DNA sequence. They can arise from replication errors, DNA damage, environmental mutagens, or some viral processes. Mutations in germline cells may be inherited, whereas mutations in somatic cells usually affect the individual cell and its descendants.

Common small-scale changes include:

• A substitution replaces one base pair with another.

• An insertion adds one or more base pairs.

• A deletion removes one or more base pairs.

A substitution may be silent if the altered codon still specifies the same amino acid, missense if it specifies a different amino acid, or nonsense if it becomes a stop codon. An insertion or deletion can cause a frameshift when its length is not a multiple of $3$, changing the grouping of downstream codons.

The effect of a mutation depends on its location and type. A mutation may have no detectable effect, alter a protein's structure or activity, change the amount or timing of gene expression, cause disease, or provide a beneficial variation in a particular environment. Mutations introduce variation; natural selection changes the frequency of variants that affect survival or reproduction. Selection does not create mutations because organisms need them.

Takeaway: A DNA change can influence a protein, gene regulation, phenotype, or evolutionary variation, but its consequence must be evaluated in context.

Biotechnology: Analyzing and Modifying DNA

Molecular biotechnology applies the principles of DNA structure, replication, expression, and regulation to detection, analysis, production, and modification.

Polymerase chain reaction (PCR) amplifies a selected DNA region. A reaction contains template DNA, two primers, free nucleotides, and a heat-stable DNA polymerase. Each cycle includes denaturation, annealing, and extension. Repeated cycles produce many copies of the target sequence.

Gel electrophoresis separates DNA fragments by movement through a porous gel in an electric field. Because DNA has negatively charged phosphate groups, it moves toward the positive electrode. Smaller fragments usually travel farther than larger fragments, and a DNA ladder provides reference sizes.

DNA sequencing determines the order of bases in a DNA molecule. It can identify genes, reveal variants, compare organisms, identify pathogens, and help evaluate whether a mutation may affect disease risk. Recombinant DNA combines DNA molecules from different sources; a gene can be joined to a vector, introduced into a host cell, and used to produce or study a product such as human insulin or a research protein.

Genome editing changes DNA at a selected location. In CRISPR-based systems, a guide RNA directs a nuclease to a complementary sequence. Repair may disrupt a gene or, when a template is provided, introduce a planned change. Important considerations include off-target changes, delivery, incomplete editing, immune responses, effects on future generations, informed consent, privacy, ecological risk, and equitable access.

Takeaway: Biotechnology turns molecular mechanisms into practical tools, but reliable evidence, safety review, ethical consideration, and fair access are essential.

Investigating Molecular Genetics

Molecular genetics can be investigated by connecting a testable question to observable evidence. A strong investigation identifies variables, includes controls and replicates, collects quantitative measurements when possible, and considers uncertainty and alternative explanations.

Useful investigation designs include:

• Model semiconservative replication by separating parental strands and adding complementary nucleotides.

• Transcribe a DNA template, divide the mRNA into codons, and use a codon chart to predict a polypeptide.

• Change one nucleotide and classify the result as silent, missense, nonsense, or frameshift.

• Simulate operon regulation under different combinations of lactose, glucose, and tryptophan.

• Use a DNA ladder and appropriate controls to interpret gel bands.

• Select forward and reverse primers that flank a target region and predict the amplicon length.

• Model how substitutions, insertions, deletions, or premature stop codons affect protein folding or active-site shape.

For example, a mutation in an enzyme gene can be amplified by PCR, identified by sequencing, modeled through transcription and translation, and investigated by producing normal and mutant proteins with recombinant DNA. Genome editing might then be examined as a possible correction strategy in cultured cells, subject to safety and ethical review.

The complete chain is not always linear. A regulatory mutation may change the amount or timing of a protein without changing its amino acid sequence, and environmental factors or multiple genes may contribute to the final phenotype.

Takeaway: Molecular evidence becomes meaningful when experimental design connects a DNA change to RNA, protein behavior, cell function, and phenotype.