Chapter 7

MLO 7.1: Distinguishing between the Structure of DNA and RNA

DNA (Deoxyribonucleic Acid)

DNA is the fundamental genetic material found in all cellular organisms and many viruses. It is typically a double-stranded molecule, structured as a double helix, resembling a twisted ladder. Each strand is a polymer of deoxyribonucleotides, linked by phosphodiester bonds.

  • Sugar: Contains deoxyribose, a five-carbon sugar that lacks a hydroxyl (ext−OHext{-OH}) group at its 2' carbon position, distinguishing it from ribose.

  • Bases: Composed of four nitrogenous bases:

    • Adenine (A)

    • Guanine (G)

    • Cytosine (C)

    • Thymine (T)
      Specific base pairing occurs, where Adenine always pairs with Thymine (A-T) via two hydrogen bonds, and Guanine always pairs with Cytosine (G-C) via three hydrogen bonds. This complementary pairing is crucial for DNA's structure and function.

  • Structure: The double helix comprises two antiparallel strands, meaning they run in opposite 5' to 3' directions. The sugar-phosphate backbone forms the outer 'rails' of the ladder, and the paired bases form the internal 'rungs'.

  • Function: Primarily serves as the long-term, stable repository of genetic information, storing the instructions for building and maintaining an organism. It is responsible for heredity, ensuring genetic traits are passed to successive generations.

RNA (Ribonucleic Acid)

RNA is a diverse nucleic acid molecule, typically single-stranded, involved in various aspects of gene expression. While primarily single-stranded, it can fold into complex three-dimensional structures through intramolecular base pairing.

  • Sugar: Contains ribose, a five-carbon sugar that possesses a hydroxyl (ext−OHext{-OH}) group at its 2' carbon position.

  • Bases: Contains four nitrogenous bases:

    • Adenine (A)

    • Guanine (G)

    • Cytosine (C)

    • Uracil (U)
      In RNA, Adenine pairs with Uracil (A-U), and Guanine pairs with Cytosine (G-C).

  • Structure: Usually single-stranded, but internal base pairing can lead to secondary structures like hairpins and loops, essential for its function (e.g., tRNA has a 'cloverleaf' structure). These structures can then fold into complex tertiary structures.

  • Function: Plays multiple transient roles in converting the genetic information from DNA into proteins. Key types include messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).

MLO 7.2: Steps of DNA Replication

DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. It is a semiconservative process, meaning each new DNA double helix consists of one original (parental) strand and one newly synthesized (daughter) strand.

  1. Initiation: The process begins at specific sequences on the DNA called origins of replication. In prokaryotes, there is typically one origin of replication, while eukaryotes have multiple.

    • The enzyme DNA helicase unwinds and separates the two DNA strands by breaking the hydrogen bonds between the complementary base pairs, creating a Y-shaped structure known as a replication fork.

    • Single-strand binding proteins (SSBs) promptly bind to the separated single strands to prevent them from reannealing (rejoining) and protect them from degradation.

    • Topoisomerases (like DNA gyrase in bacteria) relieve the torsional stress (supercoiling) that builds up ahead of the replication fork due to the unwinding process.

  2. Elongation: New DNA strands are synthesized by DNA polymerase enzymes, which can only add nucleotides in the 5' to 3' direction.

    • Primase (an RNA polymerase) synthesizes short RNA primers. DNA polymerase cannot initiate synthesis on its own; it requires a pre-existing 3'-OH group to add nucleotides. The RNA primer provides this starting point.

    • DNA polymerase III (in prokaryotes) then binds to the primer and adds complementary DNA nucleotides to the 3' end of the primer, following the base-pairing rules (A-T, G-C). It moves along the template strand, synthesizing the new strand.

  3. Termination: This stage ensures the complete duplication of the genome.

    • In prokaryotes, termination occurs when two replication forks meet or when replication reaches specific termination sequences. In eukaryotes, it involves the completion of many replication forks all along the chromosomes.

    • DNA polymerase I (in prokaryotes) removes the RNA primers and replaces them with appropriate DNA nucleotides.

    • DNA ligase then forms phosphodiester bonds to seal the nicks (gaps) in the sugar-phosphate backbone between the newly synthesized DNA fragments and the DNA that replaced the primers, creating a continuous strand.

MLO 7.3: Distinguishing Between Leading and Lagging Strand Replication

Since DNA polymerase can only synthesize DNA in the 5' to 3' direction and the two template strands are antiparallel, DNA replication proceeds differently on each strand at the replication fork.

  • Leading Strand Replication:

    • This daughter strand is synthesized continuously in the 5' to 3' direction, moving towards the replication fork as the DNA unwinds.

    • It requires only one RNA primer at the origin of replication.

    • DNA polymerase can add nucleotides seamlessly and without interruption as the template strand becomes available.

  • Lagging Strand Replication:

    • This daughter strand is synthesized discontinuously in the 5' to 3' direction, moving away from the replication fork.

    • Because DNA synthesis must proceed 5' to 3', and the fork is opening in the opposite direction, DNA polymerase must repeatedly detach and reattach.

    • This leads to the synthesis of short fragments of DNA called Okazaki fragments, each requiring a separate RNA primer.

    • After synthesis, the RNA primers are removed by DNA polymerase I (in prokaryotes) and replaced with DNA nucleotides. Finally, DNA ligase joins the Okazaki fragments together by forming phosphodiester bonds, creating a complete, continuous strand.

MLO 7.4: Steps of Gene Expression

Gene expression is the multi-step process by which the genetic information encoded in DNA is used to synthesize a functional gene product, typically a protein or a functional RNA molecule. This central dogma of molecular biology involves two main stages:

  1. Transcription (DNA to RNA):

    • This is the process where the genetic information from a specific gene on a DNA template is copied into a complementary RNA molecule.

    • Initiation: RNA polymerase binds to a specific DNA sequence called the promoter, located upstream of the gene. This binding positions the polymerase to begin transcription and unwinds a small section of the DNA double helix.

    • Elongation: RNA polymerase moves along the template DNA strand (the antisense strand) in the 3' to 5' direction. It synthesizes a new RNA strand in the 5' to 3' direction, adding complementary RNA nucleotides (A with T/U, G with C).

    • Termination: Transcription ends when RNA polymerase encounters a specific termination sequence in the DNA. This causes the release of the newly synthesized RNA molecule and the detachment of RNA polymerase from the DNA.

    • In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes RNA processing (e.g., splicing to remove introns, addition of a 5' cap, and a poly-A tail) to become mature messenger RNA (mRNA) before it leaves the nucleus.

  2. Translation (RNA to Protein):

    • This is the process where the genetic information carried by mRNA is decoded to synthesize a specific sequence of amino acids, forming a polypeptide chain (protein).

    • Initiation: The mRNA molecule binds to a ribosome (the cellular machinery for protein synthesis). The ribosome scans the mRNA until it finds the start codon (usually AUG), which codes for methionine. A tRNA carrying methionine then binds to the start codon.

    • Elongation: The ribosome moves along the mRNA, reading codons (three-nucleotide sequences) one by one. For each codon, a complementary tRNA molecule carrying its specific amino acid arrives at the ribosome. Peptide bonds are formed between adjacent amino acids, elongating the polypeptide chain.

    • Termination: Elongation continues until the ribosome encounters one of the three stop codons (UAA, UAG, UGA) on the mRNA. Release factors then bind to the stop codon, causing the polypeptide chain to be released from the ribosome, and the ribosomal subunits dissociate.

MLO 7.5: Comparing Transcription to DNA Replication

While both processes involve nucleic acids and are fundamental to genetic information flow, they have distinct purposes and mechanisms.

Similarities:

  • Both use a DNA template strand to guide synthesis.

  • Both involve the unwinding of the DNA double helix (locally in transcription; extensively in replication).

  • Both synthesize a new polynucleotide strand using polymerase enzymes (DNA polymerase in replication, RNA polymerase in transcription).

  • Both follow base-pairing rules (A-T/U, G-C) for adding complementary nucleotides.

  • New strand synthesis in both processes occurs in the 5' to 3' direction.

Differences:

  • Product: DNA replication produces two identical DNA molecules (a complete copy of the genome). Transcription produces one RNA molecule (mRNA, tRNA, or rRNA) from a specific gene.

  • Purpose: DNA replication's primary purpose is to duplicate the entire genome so that genetic information can be passed accurately to daughter cells during cell division. Transcription's purpose is to synthesize specific RNA molecules that carry out gene expression.

  • Template Used: DNA replication uses both strands of the DNA double helix as templates to create two new DNA strands. Transcription typically uses only one DNA strand (the template or antisense strand) of a specific gene as its template.

  • Enzyme: DNA replication is catalyzed by DNA polymerase, which requires an RNA primer to start synthesis. Transcription is catalyzed by RNA polymerase, which can initiate RNA synthesis directly without a primer.

  • Specificity/Scope: DNA replication typically replicates the entire chromosome or genome. Transcription targets and transcribes specific genes or sets of genes.

  • Duration/Frequency: DNA replication occurs once per cell cycle (e.g., before mitosis or meiosis). Transcription occurs many times for various genes throughout a cell's life, depending on cellular needs.

MLO 7.6: Roles of rRNA, mRNA, and tRNA in Translation

Translation is the cellular process where the genetic code carried by mRNA is used to synthesize proteins. Each type of RNA molecule plays a distinct and critical role in orchestrating this complex process.

  • mRNA (messenger RNA):

    • Role: mRNA molecules carry the translated genetic code from the DNA (in the nucleus of eukaryotes, or nucleoid region of prokaryotes) to the ribosomes in the cytoplasm.

    • Function: The sequence of three-nucleotide units on the mRNA, called codons, directly dictates the specific order in which amino acids are to be assembled into the polypeptide chain. mRNA essentially serves as the blueprint for protein synthesis.

  • tRNA (transfer RNA):

    • Role: tRNA molecules act as adapter molecules, bridging the gap between specific mRNA codons and their corresponding amino acids.

    • Structure: Each tRNA molecule has a distinct L-shaped three-dimensional structure. It features an anticodon loop, which contains a three-nucleotide sequence (the anticodon) that can base-pair complementarily with a specific mRNA codon. At its 3' end, it has an amino acid attachment site where the specific amino acid corresponding to its anticodon is covalently bound.

    • Function: tRNAs are responsible for accurately delivering the correct amino acid to the ribosome during protein synthesis, ensuring that the genetic code is translated faithfully into the amino acid sequence.

  • rRNA (ribosomal RNA):

    • Role: rRNA is a fundamental structural and catalytic component of ribosomes, the large macromolecular complexes that serve as the cellular machinery for protein synthesis.

    • Structure: rRNA molecules combine with numerous ribosomal proteins to form the two ribosomal subunits (large and small). These subunits come together on the mRNA to form a functional ribosome.

    • Function: rRNA contributes to the ribosome's overall structure and plays a key catalytic role. Specifically, the rRNA within the large ribosomal subunit possesses peptidyl transferase activity, which catalyzes the formation of peptide bonds between amino acids, linking them together to form the growing polypeptide chain. rRNA also helps in orienting the mRNA and tRNAs for efficient translation.

MLO 7.7: Determining the Polypeptide Produced from a Given Nucleotide Sequence

To translate a nucleotide sequence into an amino acid sequence (polypeptide), one must use the genetic code, which defines the correspondence between mRNA codons and specific amino acids. The process is as follows:

  1. Identify the mRNA sequence: If you are given a DNA template strand, you must first transcribe it into its complementary mRNA sequence. Remember that during transcription, adenine (A) in DNA pairs with uracil (U) in RNA, and guanine (G) pairs with cytosine (C). If you are given a coding DNA strand, the mRNA sequence will be nearly identical, but thymine (T) will be replaced by uracil (U).

    • Example: DNA template: 3′−extTAC−extGAG−extTTG−extACT−5′3' - ext{TAC} - ext{GAG} - ext{TTG} - ext{ACT} - 5'

    • mRNA sequence: 5′−extAUG−extCUC−extAAC−extUGA−3′5' - ext{AUG} - ext{CUC} - ext{AAC} - ext{UGA} - 3' (Note: this is transcribed from the template strand; the coding strand would be 5′−extATG−extGAC−extAAC−extTGA−3′5' - ext{ATG} - ext{GAC} - ext{AAC} - ext{TGA} - 3')

  2. Find the Start Codon: Translation typically initiates at the first AUG codon encountered from the 5' end. This codon sets the reading frame for the entire sequence and codes for the amino acid methionine (Met) in eukaryotes and formylmethionine (fMet) in prokaryotes.

  3. Read in Codons: Once the start codon is identified, the mRNA sequence is read in non-overlapping groups of three nucleotides, called codons, moving in the 5' to 3' direction.

  4. Use the Genetic Code Table: Match each subsequent codon to its corresponding amino acid using a standard genetic code table (which can be found in microbiology or genetics textbooks).

  5. Stop at a Stop Codon: Continue translating until one of the three stop codons is reached: UAA, UAG, or UGA. These codons do not specify an amino acid; instead, they signal the termination of translation, leading to the release of the completed polypeptide chain.

Example: If an mRNA sequence fragment is 5′−extAUG−extCUC−extAAU−extUGA−3′5' - ext{AUG} - ext{CUC} - ext{AAU} - ext{UGA} - 3',

  • AUG codes for Methionine (Met)

  • CUC codes for Leucine (Leu)

  • AAU codes for Asparagine (Asn)

  • UGA is a Stop codon

The resulting polypeptide sequence would be Met-Leu-Asn.

MLO 7.8: Comparing Regulation of Inducible and Repressible Operons in Prokaryotes

Operons are coordinated units of gene expression in prokaryotes, consisting of a promoter, an operator, and a set of structural genes that encode enzymes for a specific pathway. They allow bacteria to quickly adapt to changes in their environment by regulating gene expression. The two major types of operons are inducible and repressible.

Inducible Operons (e.g., lac operon)

  • Function: These operons typically control genes that code for enzymes involved in the catabolism (breakdown) of complex substrates. They are usually off but can be turned on (induced) when the specific substrate for the enzymes is present in the environment.

  • Mechanism: A repressor protein is constitutively synthesized and is normally active, meaning it readily binds to the operator region (a DNA sequence located between the promoter and the structural genes). When the repressor is bound to the operator, it physically blocks RNA polymerase from binding to the promoter and transcribing the structural genes, thus keeping the genes off.

  • Induction: When an inducer molecule (the substrate itself, or a derivative of it, like allolactose in the lac operon) is present, it binds to the repressor protein. This binding causes a conformational change in the repressor, inactivating it. The inactive repressor can no longer bind to the operator, allowing RNA polymerase to bind to the promoter and proceed with transcription. This leads to the synthesis of the enzymes needed to metabolize the substrate, effectively turning the genes on.

  • Example: The lac operon in E. coli is induced by lactose. When lactose and its isomer allolactose are present, allolactose binds to the lac repressor, preventing it from blocking transcription of the genes required for lactose uptake and breakdown (e.g., β\beta-galactosidase).

Repressible Operons (e.g., trp operon)

  • Function: These operons typically control genes that code for enzymes involved in the anabolism (synthesis) of essential molecules, such as amino acids or nucleotides. They are usually on, allowing constitutive production of these molecules, but can be turned off (repressed) when the end product is abundant.

  • Mechanism: A repressor protein is constitutively synthesized but is normally inactive; it cannot bind to the operator on its own. Therefore, RNA polymerase can bind to the promoter and transcribe the structural genes, keeping the genes on, resulting in the continuous production of the pathway enzymes.

  • Repression: When a corepressor molecule (the end product of the pathway, like tryptophan in the trp operon) is present in high concentrations, it binds to the inactive repressor protein. This binding causes a conformational change that activates the repressor. The now active repressor-corepressor complex binds to the operator region, blocking RNA polymerase and preventing transcription of the structural genes, effectively turning the genes off.

  • Example: The trp operon in E. coli is repressed by tryptophan. When tryptophan levels are high, tryptophan acts as a corepressor, binding to the trp repressor and enabling it to block the transcription of genes necessary for tryptophan synthesis, thus conserving cellular resources.

MLO 7.9: Distinguishing Between Different Types of Mutations

Mutations are heritable changes in the nucleotide sequence of a genome. They can occur spontaneously or be induced by mutagens. Mutations vary significantly in their scale and impact on gene function or the organism. Here are different types:

I. Point Mutations (Base Substitutions): These involve a change in a single nucleotide base pair within the DNA sequence.

  • Silent Mutation: A change in a single nucleotide that does not alter the amino acid sequence of the protein. This often happens because of the degeneracy (redundancy) of the genetic code, where multiple codons can code for the same amino acid (e.g., GGU, GGC, GGA, GGG all code for Glycine).

  • Missense Mutation: A change in a single nucleotide that results in a codon coding for a different amino acid. The effect can vary from negligible (if the new amino acid is chemically similar or in a non-critical region) to severe (if the new amino acid significantly alters protein structure or function, e.g., sickle cell anemia caused by a single missense mutation in the β\beta-globin gene).

  • Nonsense Mutation: A change in a single nucleotide that results in a premature stop codon (UAA, UAG, or UGA). This leads to the truncation (shortening) of the polypeptide chain, which is often non-functional because it lacks critical parts needed for its structure or activity.

II. Frameshift Mutations: These are caused by the insertion or deletion of nucleotides that are not a multiple of three within the coding sequence.

  • This type of mutation drastically alters the reading frame of the genetic code downstream from the mutation. Since codons are read in groups of three, adding or removing one or two bases shifts all subsequent codons. This generally leads to a completely different amino acid sequence from that point onward, often generating a premature stop codon, and typically results in a severely altered or non-functional protein.

    • Deletion: Removal of one or more nucleotides from the DNA sequence.

    • Insertion: Addition of one or more nucleotides into the DNA sequence.

III. Large-Scale Mutations (Chromosomal Mutations): These involve larger segments of DNA, often affecting multiple genes or the overall structure of chromosomes. While more common in eukaryotes, similar large-scale changes can occur in prokaryotes.

  • Deletions: The loss of a segment of a chromosome, which can remove several genes. The impact depends on the size and importance of the deleted region.

  • Duplications: A segment of a chromosome is repeated, leading to extra copies of particular genes. This can sometimes provide raw material for evolution by allowing one copy to mutate and acquire new functions.

  • Inversions: A segment of a chromosome breaks off, flips 180 degrees, and reattaches in a reversed orientation. This can disrupt gene function if the breakpoints occur within a gene or affect gene regulation.

  • Translocations: A segment of one chromosome breaks off and attaches to a different, non-homologous chromosome. This can lead to gene dosage problems or disrupt genes at the breakpoints.

MLO 7.10: Distinguishing Between Conjugation, Transformation, and Transduction

These are the three primary mechanisms of horizontal gene transfer (HGT) in prokaryotes, particularly bacteria. HGT allows bacteria to acquire new genetic material from other organisms, contributing significantly to genetic diversity and adaptation (e.g., spread of antibiotic resistance). Unlike vertical gene transfer (parent to offspring), HGT transfers genes between unrelated organisms or within the same generation.

1. Conjugation:

  • Mechanism: This is the direct transfer of genetic material (usually a plasmid or a portion of the bacterial chromosome) from one bacterial cell (donor) to another (recipient) through direct, temporary physical contact.

  • Process: A donor cell, which typically contains a conjugative plasmid (like the F plasmid or fertility factor), forms a specialized pilus called a conjugation pilus (or F pilus). This pilus reaches out and attaches to a recipient cell that lacks the plasmid. The pilus then retracts, bringing the two cells into close contact. A protein channel forms between the cells, and a single strand of the plasmid DNA is transferred from the donor to the recipient. Both cells then replicate the single strand to become double-stranded, resulting in two cells, both containing the plasmid.

    • If the F plasmid integrates into the donor's chromosome, the donor becomes an Hfr cell (High-frequency recombination). Hfr cells can transfer a portion of their chromosome along with part of the F plasmid to a recipient.

  • Outcome: The recipient cell acquires new genetic traits encoded on the plasmid (e.g., antibiotic resistance, virulence factors, metabolic capabilities) and often becomes a donor itself. It requires cell-to-cell contact.

2. Transformation:

  • Mechanism: This process involves the uptake of naked DNA (DNA released from dead or lysed cells) from the external environment by a recipient bacterial cell.

  • Process: Some bacterial species are naturally competent, meaning they have the physiological ability to take up extracellular DNA. When a donor bacterial cell dies and undergoes lysis, its cellular contents, including fragmented DNA, are released into the surrounding environment. A competent recipient cell can then bind these free DNA fragments to its cell surface and transport them across its cell membrane into the cytoplasm. Once inside, if the foreign DNA is sufficiently homologous (similar) to a segment of the recipient's chromosome, it can be integrated into the host genome via homologous recombination. Alternatively, if the DNA is a plasmid, it may remain as an extrachromosomal element.

  • Outcome: The recipient cell acquires new genetic characteristics encoded by the taken-up DNA. Transformation does not require cell-to-cell contact; it's a passive acquisition of environmental DNA. This mechanism was famously demonstrated in Griffith's experiment with Streptococcus pneumoniae.

3. Transduction:

  • Mechanism: The transfer of bacterial DNA from one bacterial cell to another via a bacteriophage (a virus that infects bacteria).

  • Process: During the lytic cycle of a bacteriophage, the phage infects a donor bacterium and replicates. Sometimes, during the assembly of new phage particles, pieces of the host bacterial DNA are mistakenly packaged into the phage head instead of or in addition to the viral DNA. The resulting phage particle containing bacterial DNA is called a transducing particle.

    • This transducing particle then infects a new recipient bacterial cell. When it injects its genetic material, it injects the bacterial DNA it acquired from the previous host. This injected bacterial DNA can then integrate into the recipient cell's chromosome via homologous recombination.

  • Outcome: The recipient bacterium acquires new genes from the donor bacterium, mediated by the bacteriophage. This transfer is not direct cell-to-cell contact but relies on a viral vector.