Unit 6: Genetic material and molecular mechanisms of gene expression - Subjective Questions

Genetic material is the biological molecule that stores, replicates, expresses, and transmits hereditary information from one generation to the next.

A molecule must possess the following properties to serve as genetic material:

• Information storage: It must contain sufficient biological information to determine the structure and functions of an organism.
• Faithful replication: It must produce accurate copies of itself before cell division.
• Chemical and structural stability: It should remain sufficiently stable so that hereditary information is preserved.
• Capacity for mutation: It must undergo occasional heritable changes that generate genetic variation and permit evolution.
• Expression of information: Its stored information must be capable of producing functional products, such as RNA and proteins.
• Transmission: It must be transferred from parent cells to daughter cells and from parents to offspring.

DNA fulfills all these requirements in most organisms. RNA acts as the genetic material in some viruses, such as tobacco mosaic virus and many animal viruses.

DNA can adopt different helical conformations depending on nucleotide sequence, hydration, ionic conditions, and interaction with proteins.

Comparison of major DNA forms:

• B-DNA:

  • It is the predominant physiological form described by Watson and Crick.
  • It is a right-handed double helix with about 10.5 base pairs per turn.
  • Its diameter is approximately 2 nm, and its helical pitch is approximately 3.4 nm.
  • It has a wide major groove and a narrow minor groove.

• A-DNA:

  • It is a right-handed, shorter, and wider helix.
  • It contains about 11 base pairs per turn.
  • It forms under dehydrating conditions and is also seen in DNA-RNA hybrids and double-stranded RNA.
  • Its base pairs are tilted relative to the helix axis.

• Z-DNA:

  • It is a left-handed helix with a zigzag sugar-phosphate backbone.
  • It contains about 12 base pairs per turn.
  • It is favored by alternating purine-pyrimidine sequences, particularly GC repeats.
  • It may form transiently during transcription and may participate in gene regulation.

Thus, B-DNA is the common cellular form, whereas A-DNA and Z-DNA occur under particular structural or physiological conditions.

Cellular RNA molecules can be classified into coding and non-coding types.

• Messenger RNA (mRNA): Carries genetic information copied from DNA to ribosomes. Its codons specify the amino acid sequence of a protein.
• Transfer RNA (tRNA): Functions as an adaptor during translation. Each tRNA carries a specific amino acid and contains an anticodon complementary to an mRNA codon.
• Ribosomal RNA (rRNA): Combines with proteins to form ribosomes. It has both structural and catalytic roles; the large-subunit rRNA catalyzes peptide-bond formation.
• Heterogeneous nuclear RNA (hnRNA): Represents primary RNA transcripts in eukaryotes before processing into mature mRNA.
• Small nuclear RNA (snRNA): Participates in pre-mRNA splicing as a component of the spliceosome.
• Small nucleolar RNA (snoRNA): Guides processing and chemical modification of rRNA in the nucleolus.
• MicroRNA (miRNA): Regulates gene expression by inhibiting translation or promoting degradation of target mRNAs.
• Small interfering RNA (siRNA): Directs sequence-specific degradation of complementary RNA and contributes to RNA interference.
• Long non-coding RNA (lncRNA): Regulates chromatin organization, transcription, RNA processing, and other cellular processes.

These RNA classes collectively connect genetic information with protein synthesis and gene regulation.

According to the Watson-Crick model, DNA consists of two polynucleotide chains wound around a common axis to form a double helix.

Structural features:

• Each nucleotide contains deoxyribose sugar, phosphate, and a nitrogenous base.
• Adjacent nucleotides in a strand are joined by - phosphodiester bonds.
• The two strands are antiparallel: one runs from to and the other from to .
• Sugar-phosphate backbones lie on the outside, while bases face inward.
• Adenine pairs with thymine through two hydrogen bonds, whereas guanine pairs with cytosine through three hydrogen bonds.
• Complementary pairing maintains a nearly uniform helix diameter.
• Base stacking and hydrogen bonding stabilize the double helix.
• The helix possesses major and minor grooves that provide binding sites for proteins.

Relation to replication:

Because the two strands have complementary sequences, each parental strand can act as a template for the synthesis of a new complementary strand. This provides a simple molecular mechanism for accurate, semiconservative DNA replication.

Several classical experiments established DNA as the genetic material.

Griffith's transformation experiment:

• Virulent smooth bacteria killed mice, whereas non-virulent rough bacteria did not.
• A mixture of heat-killed smooth bacteria and living rough bacteria killed mice.
• Living smooth bacteria were recovered, indicating that a heritable transforming principle had passed to the rough cells.

Avery, MacLeod, and McCarty experiment:

• Extracts from heat-killed smooth bacteria transformed rough bacteria.
• Protease and RNase did not eliminate transformation.
• DNase destroyed transforming activity.
• Therefore, DNA was identified as the transforming principle.

Hershey-Chase experiment:

• Bacteriophage DNA was labeled with radioactive phosphorus, , and phage protein was labeled with radioactive sulfur, .
• During infection, most entered bacterial cells, while most remained outside.
• Radioactive phosphorus was also associated with progeny phages.

Together, these experiments showed that DNA enters cells, directs the production of progeny, and transmits hereditary information.

Semiconservative replication means that each daughter DNA molecule contains one parental strand and one newly synthesized strand.

Meselson-Stahl experiment:

• Escherichia coli cells were grown for several generations in a medium containing heavy nitrogen, , so that their DNA became heavy.
• The cells were transferred to a medium containing ordinary nitrogen, .
• DNA samples were collected after successive generations and separated by cesium chloride density-gradient centrifugation.
• Before transfer, DNA formed a single heavy band.
• After one generation in , DNA formed one intermediate-density band. This result excluded conservative replication.
• After two generations, two bands appeared: one intermediate and one light. This result excluded dispersive replication.
• In later generations, the proportion of light DNA increased, while the intermediate band persisted in decreasing proportion.

The observations matched the prediction that each first-generation molecule contained one parental strand and one newly synthesized strand, proving semiconservative replication.

DNA replication begins at an origin and proceeds through replication forks.

Major events at a replication fork:

• Initiation proteins recognize the origin and locally open the double helix.
• Helicase unwinds DNA by disrupting hydrogen bonds between complementary bases.
• Single-strand-binding proteins prevent the separated strands from reannealing.
• Topoisomerase relieves torsional strain and supercoiling ahead of the fork.
• Primase synthesizes short RNA primers that provide free -OH groups.
• DNA polymerase adds deoxyribonucleotides only in the to direction.
• The leading strand is synthesized continuously toward the replication fork.
• The lagging strand is synthesized discontinuously away from the fork as Okazaki fragments.
• RNA primers are removed, the resulting gaps are filled with DNA, and DNA ligase seals remaining nicks.
• Proofreading by DNA polymerases removes incorrectly incorporated nucleotides and increases fidelity.

Replication is therefore bidirectional, semiconservative, semidiscontinuous, and highly accurate.

The two parental DNA strands are antiparallel, but DNA polymerases can synthesize DNA only in the to direction by adding nucleotides to a free -OH group.

• On the leading-strand template, polymerase moves in the same overall direction as fork opening. The new strand is therefore synthesized continuously from a single primer.
• On the lagging-strand template, the required direction of synthesis is opposite to fork movement. DNA must therefore be synthesized in short segments.
• Each segment, called an Okazaki fragment, begins with a separate RNA primer.
• Primase repeatedly produces primers as additional template DNA becomes exposed.
• DNA polymerase extends each primer until it reaches the preceding fragment.
• The RNA primers are removed and replaced with DNA.
• DNA ligase catalyzes formation of the final phosphodiester bonds between adjacent fragments, producing a continuous lagging strand.

This unequal pattern is called semidiscontinuous replication.

Linear eukaryotic chromosomes face an end-replication problem because conventional DNA polymerases cannot completely replicate the ends of the lagging strand.

• Removal of the terminal RNA primer leaves a gap at the end of the newly synthesized strand.
• No upstream -OH group is available for DNA polymerase to fill this gap.
• Without a compensating mechanism, chromosomes would become shorter after every cell division.

Role of telomeres and telomerase:

• Telomeres are repetitive, non-coding DNA sequences at chromosome ends that protect genes from progressive loss.
• Telomerase is a ribonucleoprotein reverse transcriptase containing an internal RNA template.
• It binds to the overhang and extends the parental strand by adding telomeric repeats.
• Primase and DNA polymerase then synthesize the complementary strand.
• Telomerase is active in germ cells, many stem cells, and most cancer cells but has low activity in many somatic cells.

Telomeres also prevent chromosome ends from being mistaken for broken DNA. Progressive telomere shortening is associated with cellular senescence, while abnormal telomerase activation can support unlimited proliferation of cancer cells.

Transcription is the synthesis of an RNA molecule complementary to the template strand of DNA.

Initiation:

• The RNA polymerase holoenzyme contains a core enzyme and a sigma factor.
• The sigma factor recognizes promoter elements, commonly the and regions.
• RNA polymerase binds the promoter to form a closed complex and then locally unwinds DNA to form an open complex.
• The first phosphodiester bonds are formed without requiring a primer.
• After promoter clearance, the sigma factor usually dissociates.

Elongation:

• The core RNA polymerase reads the DNA template in the to direction.
• RNA is synthesized in the to direction.
• A short transcription bubble moves along DNA while the DNA behind the polymerase reanneals.

Termination:

• In intrinsic termination, a GC-rich RNA hairpin followed by uracil residues destabilizes the RNA-DNA hybrid.
• In rho-dependent termination, rho protein moves along RNA and disrupts the transcription complex.

The completed RNA is released, and RNA polymerase dissociates from DNA.

Prokaryotic and eukaryotic transcription follow the same basic principle but differ in organization and regulation.

• Location: Prokaryotic transcription occurs in the cytoplasm, whereas eukaryotic transcription occurs mainly in the nucleus.
• RNA polymerases: Prokaryotes generally use one major RNA polymerase. Eukaryotes use RNA polymerases I, II, and III for different RNA classes.
• Promoter recognition: Prokaryotes use sigma factors. Eukaryotes use general transcription factors and other regulatory proteins.
• Promoters: Prokaryotic promoters commonly contain and elements. Many eukaryotic promoters contain elements such as the TATA box, initiator, or downstream promoter element.
• Chromatin: Prokaryotic DNA is relatively accessible. Eukaryotic transcription requires regulation of nucleosomes and chromatin structure.
• RNA processing: Prokaryotic mRNA generally undergoes limited processing. Eukaryotic pre-mRNA commonly receives a cap, a poly(A) tail, and undergoes splicing.
• Coupling: Transcription and translation can be coupled in prokaryotes. They are spatially and temporally separated in eukaryotes.
• mRNA organization: Prokaryotic mRNAs are often polycistronic, whereas most eukaryotic mRNAs are monocistronic.

These differences allow eukaryotes to exercise additional levels of gene regulation.

A eukaryotic protein-coding transcript is initially produced as pre-mRNA and must be processed before translation.

• capping: A modified guanine nucleotide, 7-methylguanosine, is added through an unusual - linkage. The cap protects RNA from degradation and assists in nuclear export, splicing, and translation initiation.
• Splicing: Introns are removed and exons are joined by the spliceosome, which contains snRNAs and proteins. Splicing generally recognizes the splice site, branch-point adenine, and splice site.
• cleavage and polyadenylation: The transcript is cleaved downstream of a polyadenylation signal, and poly(A) polymerase adds a poly(A) tail. The tail increases mRNA stability and supports export and translation.
• Alternative splicing: Different combinations of exons can be joined to generate multiple mRNA and protein products from one gene.
• RNA editing: In some transcripts, nucleotides are inserted, deleted, or chemically altered, changing the RNA sequence.

Processing converts an unstable primary transcript into a mature mRNA and greatly expands the diversity and regulation of gene products.

The genetic code is the set of rules by which nucleotide triplets in mRNA specify amino acids or translation signals.

Major characteristics:

• It is a triplet code: each codon contains three nucleotides.
• It is unambiguous: a particular codon normally specifies only one amino acid.
• It is degenerate: most amino acids are specified by more than one codon.
• It is non-overlapping: each nucleotide normally belongs to only one codon in a reading frame.
• It is comma-less: codons are read continuously from the initiation site.
• It is nearly universal, although mitochondria and a few organisms show exceptions.
• AUG generally acts as the initiation codon and codes for methionine.
• UAA, UAG, and UGA are termination codons.
• The code is read on mRNA in the to direction.

Wobble pairing refers to flexible base pairing between the third base of an mRNA codon and the first base of a tRNA anticodon. This flexibility allows one tRNA to recognize multiple synonymous codons, reducing the number of distinct tRNAs required by a cell.

tRNA acts as the adaptor that connects an mRNA codon with its corresponding amino acid.

Structure of tRNA:

• The secondary structure resembles a cloverleaf.
• The acceptor stem terminates in the sequence CCA at the end, where the amino acid is attached.
• The anticodon loop contains a triplet complementary to an mRNA codon.
• The D loop and TC loop assist in folding and molecular recognition.
• The variable loop differs in size among tRNAs.
• The tertiary structure is compact and L-shaped.

Role of aminoacyl-tRNA synthetases:

• Each synthetase recognizes a particular amino acid and the identity elements of its corresponding tRNA or tRNAs.
• The amino acid is first activated with ATP to form aminoacyl-AMP.
• It is then transferred to the end of the tRNA.
• The overall reaction can be represented as:

• Many synthetases possess editing sites that remove incorrectly activated amino acids.

The ribosome checks codon-anticodon pairing, but it does not directly verify the attached amino acid. Therefore, accurate charging by aminoacyl-tRNA synthetases is crucial for translational fidelity.

Translation converts the codon sequence of mRNA into the amino acid sequence of a polypeptide.

Initiation:

• The small ribosomal subunit binds the mRNA with the help of initiation factors.
• The initiation codon AUG is positioned correctly in the ribosome.
• The initiator tRNA pairs with AUG in the P site.
• The large ribosomal subunit joins to form the complete initiation complex.

Elongation:

• An aminoacyl-tRNA complementary to the next codon enters the A site with an elongation factor.
• The ribosome verifies codon-anticodon pairing.
• Peptidyl transferase activity of large-subunit rRNA forms a peptide bond, transferring the growing chain from the P-site tRNA to the A-site tRNA.
• The ribosome translocates by one codon in the to direction.
• The empty tRNA moves to the E site and exits, while the peptidyl-tRNA moves to the P site.

Termination:

• When a stop codon enters the A site, no corresponding tRNA binds.
• A release factor recognizes the stop codon and promotes hydrolysis of the completed polypeptide from tRNA.
• The polypeptide, mRNA, and ribosomal subunits are released.

Protein synthesis requires energy from ATP during tRNA charging and GTP during initiation, elongation, and termination.

Transcription and translation differ as follows:

• Meaning: Transcription copies genetic information from DNA into RNA, whereas translation decodes mRNA to synthesize a polypeptide.
• Template: Transcription uses one DNA strand as its template. Translation uses mRNA as its template.
• Major machinery: Transcription is catalyzed by RNA polymerase and supported by transcription factors. Translation is performed by ribosomes, tRNAs, aminoacyl-tRNA synthetases, and translation factors.
• Building units: Transcription joins ribonucleotides. Translation joins amino acids.
• Direction: RNA is synthesized in the to direction. The ribosome also reads mRNA in the to direction, while the polypeptide grows from its amino terminus to its carboxyl terminus.
• Product: Transcription produces RNA. Translation produces a polypeptide.
• Signals: Promoters and terminators control transcription. Start and stop codons define the translated region.
• Location: In prokaryotes, both occur in the cytoplasm and may be coupled. In eukaryotes, transcription occurs mainly in the nucleus, while translation occurs on cytoplasmic or rough-endoplasmic-reticulum-associated ribosomes.

Both processes are central components of gene expression.

The meaning of the term gene has changed with advances in genetics and molecular biology.

• Mendel proposed discrete hereditary factors that control characters, although he did not use the term gene.
• Johannsen introduced the term gene for a unit of inheritance.
• Morgan and coworkers placed genes in a linear order on chromosomes and treated the gene as a unit of recombination, mutation, and function.
• Beadle and Tatum proposed the one gene-one enzyme concept from studies of metabolic mutants.
• The concept was revised to one gene-one polypeptide because many proteins are not enzymes and some contain multiple polypeptide chains.
• Benzer's fine-structure studies divided the classical gene into the cistron, recon, and muton, corresponding broadly to units of function, recombination, and mutation.
• Discovery of introns, alternative splicing, overlapping genes, RNA genes, and regulatory sequences showed that a gene is not always one continuous DNA segment producing one protein.

A modern gene may be defined as a DNA sequence, together with the regulatory information required for its expression, that produces a functional RNA or one or more related polypeptide products. The exact boundaries of a gene can depend on the biological context.

A typical eukaryotic protein-coding gene contains regulatory and transcribed regions arranged in a functional organization.

• Core promoter: Located near the transcription start site and provides a platform for assembly of general transcription factors and RNA polymerase II. Some promoters contain a TATA box.
• Proximal promoter elements: Bind regulatory transcription factors and influence the frequency of initiation.
• Enhancers and silencers: May be located upstream, downstream, or within introns. They bind activators or repressors and can act over long distances.
• Transcription start site: The nucleotide at which RNA synthesis begins.
• untranslated region: Transcribed but not translated; it influences mRNA stability and translation.
• Exons: Sequences retained in mature RNA. Coding portions of exons specify the amino acid sequence, while some exon regions may be untranslated.
• Introns: Intervening sequences removed from pre-mRNA by splicing.
• Start codon: Usually AUG in mRNA and defines the beginning of the coding sequence.
• Stop codon: Ends translation but does not itself encode an amino acid.
• untranslated region: Contains elements controlling localization, stability, and translation of mRNA.
• Polyadenylation signal: Directs cleavage of the transcript and addition of the poly(A) tail.

The final expression pattern results from interaction among these regions rather than from the coding sequence alone.

An operon is a group of prokaryotic structural genes controlled together by common regulatory sequences and transcribed into a polycistronic mRNA.

The lac operon of E. coli contains:

• lacZ, which encodes beta-galactosidase.
• lacY, which encodes lactose permease.
• lacA, which encodes thiogalactoside transacetylase.
• A promoter where RNA polymerase binds.
• An operator where the lac repressor binds.
• A regulatory gene, lacI, which encodes the repressor.

Negative regulation:

• In the absence of lactose, the repressor binds the operator and blocks transcription.
• When lactose is present, its derivative allolactose binds the repressor and changes its conformation.
• The repressor leaves the operator, allowing transcription.

Positive regulation:

• When glucose is scarce, cellular cAMP increases.
• cAMP binds CAP, and the CAP-cAMP complex binds near the promoter, increasing RNA polymerase recruitment.
• When glucose is abundant, cAMP is low, so lac operon transcription remains weak even if lactose is present.

Maximum expression therefore occurs when lactose is available and glucose is scarce. The system integrates substrate availability with the cell's preferred energy source.

Eukaryotic gene expression is controlled at multiple levels.

• Chromatin and epigenetic regulation: DNA methylation, histone modifications, nucleosome positioning, and chromatin-remodeling complexes alter DNA accessibility. Open euchromatin is generally more transcriptionally active than compact heterochromatin.
• Transcriptional regulation: Activators, repressors, enhancers, silencers, promoters, mediator, and general transcription factors control transcription initiation.
• RNA processing: Alternative promoter usage, alternative splicing, RNA editing, and alternative polyadenylation produce different mature RNAs from related transcripts.
• RNA export and localization: Only properly processed RNAs are efficiently exported, and localization signals direct some mRNAs to particular regions of the cell.
• mRNA stability: RNA-binding proteins, poly(A) tail length, miRNAs, and degradation pathways determine mRNA half-life.
• Translational regulation: Initiation factors, regulatory proteins, upstream reading frames, and non-coding RNAs control ribosome recruitment and translation efficiency.
• Post-translational regulation: Protein folding, cleavage, phosphorylation, glycosylation, localization, complex formation, and other modifications regulate protein activity.
• Protein degradation: Ubiquitin-mediated proteasomal degradation and lysosomal pathways control protein abundance.

Regulation at several stages allows genes to respond precisely to developmental signals, environmental conditions, and cellular requirements.