Unit 2.1 Notes

Central Dogma of Molecular Biology

  • The Central Dogma is a fundamental framework that explains the flow of genetic information within a biological system, dictating how genetic instructions are converted into functional products. It consists of the following interconnected processes:

    • DNA Replication:

    • This is the process by which a cell makes an identical copy of its DNA (DNA \rightarrow DNA). It is a semi-conservative process, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.

    • Enzyme involved: DNA polymerase, which synthesizes new DNA strands by adding nucleotides complementary to the template.

    • Transcription:

    • The synthesis of a messenger RNA (mRNA) molecule from a DNA template (DNA \rightarrow RNA). This process occurs in the nucleus for eukaryotes and in the cytoplasm for prokaryotes.

    • Enzyme involved: RNA polymerase, which binds to specific promoter regions on the DNA to initiate RNA synthesis.

    • Translation:

    • The synthesis of proteins from an mRNA template (RNA \rightarrow Protein). During this process, the genetic code carried by mRNA is decoded to produce a specific amino acid sequence.

    • Site of translation: Ribosome, which facilitates the binding of transfer RNA (tRNA) molecules carrying specific amino acids to the mRNA codons, forming a polypeptide chain.

Objectives and Key Questions

  1. Draw and contrast the structure of an RNA nucleotide vs a DNA nucleotide.

  2. Contrast the structure of an RNA polymer vs a DNA polymer.

  3. Identify the bonds between nucleotides and the 5′5' & 3′3' ends of DNA & RNA polymers.

  4. Draw and describe the structure of DNA. Identify the bonds holding the two strands together and define antiparallel strands.

  5. Draw and describe how DNA is packaged into chromosomes in the nucleus (including the terms chromatin, histones, & nucleosomes) and explain the importance of DNA packaging.

  6. Contrast purines and pyrimidines, categorize nitrogenous bases (A, T, G, C, U) into these categories, and list the base pairing rules.

  7. Predict the proportion of nitrogenous bases in a molecule of DNA using the base pairing rules.

  8. Trace the flow of information from DNA to a functional protein (Central Dogma).

Major Concepts

  • Central Dogma: The fundamental process by which genetic information flows from DNA to RNA to protein, ultimately transforming genotype into phenotype.

  • Deoxyribonucleic Acid (DNA): - The primary genetic material found in the chromosomes of all living organisms and many viruses, responsible for heredity.

    • DNA is a double-stranded polymer made of repeating nucleotide units. These two strands wind around each other to form a characteristic right-handed double helix structure.

What Is a Gene?

  • Gene: - Definition: A specific sequence of deoxyribonucleotides or ribonucleotides that constitutes a region of DNA (or RNA, in some viruses) that carries information for a discrete hereditary characteristic.

    • Corresponds to: A gene can contain the instructions for synthesizing:

      • A single protein via an mRNA intermediate.

      • A single functional RNA molecule, such as transfer RNA (tRNA) or ribosomal RNA (rRNA), which are not translated into proteins but play crucial roles in protein synthesis or gene regulation.

    • Genes often include regulatory sequences (like promoters and enhancers) that control their expression.

Key Definitions

  • Genotype: The complete set of genes and alleles in an organism's DNA, representing its genetic makeup.

  • Phenotype: The observable physical or biochemical characteristics of an organism, resulting from the expression of its genotype and its interaction with the environment. It is how the collection of genes creates proteins and gene products, leading to a unique individual.

Central Dogma Flow

  • The genetic information stored in DNA is first transcribed into messenger RNA (mRNA) in the nucleus, a process facilitated by RNA polymerase.

  • The mRNA molecule then migrates to the cytoplasm where it is translated into a protein via ribosomes.

  • Catalysts: - Definition: Biological substances (enzymes, in this context) that increase the rate of specific biochemical reactions without being consumed in the process. They lower the activation energy required for the reaction to occur, thus speeding it up.

Locations of Macromolecules

  • DNA: Primarily located in the nucleus of eukaryotic cells, organized into chromosomes. In prokaryotes, it is found in the nucleoid region of the cytoplasm.

  • RNA: Synthesized in the nucleus (e.g., mRNA, tRNA, rRNA) from a DNA template. mRNA then exits the nucleus and is utilized in the cytoplasm on ribosomes for protein synthesis. Other RNAs also perform various functions in the cytoplasm and nucleus.

Structure of Nucleic Acids

  • Nucleotide: The fundamental building block (monomer) of nucleic acids (DNA and RNA).

    • Consists of three chemically distinct components:

      • Phosphate group: Typically attached to the 5′5' carbon of the sugar.

      • 5-carbon sugar (pentose): Deoxyribose in DNA or ribose in RNA.

      • Nitrogenous base: Attached to the 1′1' carbon of the sugar.

    • Nucleotides are linked together by phosphodiester bonds, which form between the phosphate group of one nucleotide and the 3′3' hydroxyl group of the sugar of the next nucleotide, creating a sugar-phosphate backbone.

Types of Sugars

  • Deoxyribose (in DNA): A pentose sugar that characteristically lacks an oxygen atom on its 2′2' carbon, contributing to DNA's greater stability compared to RNA.

  • Ribose (in RNA): A pentose sugar that contains a hydroxyl (-OH) group bonded to the 2′2' carbon, making RNA more reactive and less stable than DNA.

Nitrogenous Bases

  • Purines: Larger, double-ring structures (a six-membered ring fused to a five-membered ring), consisting of Adenine (A) and Guanine (G).

  • Pyrimidines: Smaller, single-ring structures (a six-membered ring), consisting of Cytosine (C), Uracil (U, exclusively found in RNA), and Thymine (T, exclusively found in DNA).

  • Base Pairing Rules (Chargaff's Rules):

    • Adenine (A) always pairs with Thymine (T) in DNA, forming two hydrogen bonds (A=T). In RNA, Adenine (A) pairs with Uracil (U).

    • Guanine (G) always pairs with Cytosine (C) in both DNA and RNA, forming three hydrogen bonds (G≡\equivC). These specific pairings ensure the consistent width of the DNA double helix and accurate replication and transcription.

Stability of DNA

  • GC content: The proportion of guanine-cytosine pairs in a DNA molecule has a significant impact on its thermal stability. Higher guanine-cytosine content increases DNA stability due to the presence of three hydrogen bonds between G and C, compared to two between A and T.

  • High GC content requires more energy (higher temperature) to denature (separate) the DNA strands, which is a critical consideration in molecular biology techniques like Polymerase Chain Reaction (PCR) and DNA melting temperature assays.

Eukaryotic DNA Packaging

  • Chromatin: The complex formed by DNA tightly wrapped around specialized basic proteins called histones. This fundamental unit of DNA packaging is termed a nucleosome, which resembles 'beads on a string.' Each nucleosome consists of approximately 146146 base pairs of DNA wrapped around an octamer of histone proteins (two copies each of H2A, H2B, H3, and H4).

  • Importance of DNA Packaging: DNA packaging is crucial for several reasons:

    • Compaction: Allows the very long DNA molecules to fit within the small confines of the nucleus.

    • Protection: Protects DNA from damage.

    • Regulation: Regulates gene expression by controlling the accessibility of DNA to transcription machinery.

  • Chromatin exists in two main forms, influencing gene accessibility:

    • Euchromatin (active, unpacked): Loosely packed chromatin that is rich in genes and is generally under active transcription. Its open structure allows access for RNA polymerase and regulatory proteins.

    • Heterochromatin (inactive, packed): Densely packed chromatin that is transcriptionally inactive or silenced. It typically contains repetitive DNA sequences and plays a role in centromere and telomere function.

DNA Compaction During Cell Division

  • During cell division (mitosis and meiosis), nucleosomes can further condense and coil into thicker 3030 nm fibers, which then form highly compact looped domains. This hierarchical compaction eventually leads to the formation of visible, rod-like structures termed mitotic chromosomes, ensuring accurate segregation of genetic material to daughter cells.

Flow of Genetic Information

  • Genes are precisely transcribed into complementary RNA strands by RNA polymerase. This RNA can be mRNA, tRNA, or rRNA.

  • Messenger RNA (mRNA) acts as an intermediary, transporting the genetic information encoded in the DNA from the nucleus to the ribosomes in the cytoplasm, where it serves as a template for protein synthesis.

  • Not all genes encode proteins; a significant portion of the genome generates functional RNAs, such as ribosomal RNA (rRNA) which forms the structural and catalytic core of ribosomes, and transfer RNA (tRNA) which carries specific amino acids to the ribosome during translation.

Upcoming Topics

  • A deeper discussion of RNA synthesis (transcription), gene transcription regulation, eukaryotic RNA processing, and the complex nature of eukaryotic gene interactions will continue. Specific aspects like alternative splicing, the role of RNA polymerase in transcription initiation and elongation, and various regulatory elements will be explored in detail.

Transcription Learning Objectives

  1. Structure of a eukaryotic gene, including relevant regions (promoter, terminator, transcribed region, introns, and exons).

  2. Identify proteins required to initiate eukaryotic gene transcription, including where they bind.

  3. Identify components of pre-mRNA removed in RNA splicing, labeling a mature mRNA, and discussing the significance of the 5′5' cap and poly A tail.

  4. Explain alternative splicing and its impact on protein diversity.

  5. Predict mRNA transcribed from a given gene sequence.

  6. Predict the impact of chromatin remodeling and DNA methylation on transcription.

  7. Contrast the roles of transcriptional activators and repressors, identifying their DNA-binding sites.

Transcription Mechanism

  • A functional gene in eukaryotes consists of several key regions that orchestrate its expression:

    • Promoter Region: A DNA sequence located upstream (before) the transcribed region that acts as the binding site for RNA polymerase and various transcription factors. It signals the precise starting point for transcription and determines the rate of gene expression. Promoters often contain specific consensus sequences, like the TATA box, which helps position RNA polymerase.

    • Transcribed Region (Coding Region): The section of DNA that is copied into an RNA molecule. This region includes both exons (coding sequences) and introns (non-coding sequences) in eukaryotic genes.

    • Terminator Region: A DNA sequence downstream (after) the transcribed region that signals the endpoint for transcription, leading to the dissociation of RNA polymerase from the DNA template and the release of the newly synthesized RNA.

DNA Template and RNA Synthesis

  • RNA polymerase synthesizes RNA by moving along the template strand (also known as the antisense or non-coding strand) of the DNA double helix in the 3′−to−5′3'-to-5' direction. It adds ribonucleotides that are complementary to the template strand, forming the new RNA strand in the 5′−to−3′5'-to-3' direction.

  • Eukaryotic promoters typically contain well-defined sequences such as the TATA box (consensus sequence TATAAA, located about 25−3525-35 base pairs upstream of the transcription start site). These promoters require the assembly of a complex of general transcription factors (GTFs), which recruit RNA polymerase I, II, or III to the promoter to initiate transcription effectively.

Differences Between Prokaryotic and Eukaryotic Transcription

  • Compartmentalization: In prokaryotes, transcription and translation occur simultaneously in the cytoplasm. In eukaryotes, transcription occurs in the nucleus, and translation in the cytoplasm due to the presence of a nuclear envelope.

  • Gene Structure: Eukaryotic genes typically feature non-coding regions (introns) that interrupt coding sequences (exons). Prokaryotic genes are generally continuous coding sequences.

  • RNA Processing: Eukaryotic pre-mRNA undergoes extensive post-transcriptional processing in the nucleus, which is not usually seen in prokaryotes. This includes the addition of a 5′5' cap and a 3′3' poly-A tail, as well as the removal of introns (splicing). These modifications enhance mRNA stability, aid in nuclear export, and increase translation efficiency and regulation.

RNA Processing (Eukaryotes)

  • Once pre-mRNA is transcribed, it undergoes several processing steps before becoming mature mRNA:

    • 5′5' Capping: Capping factors add a modified guanine nucleotide (a 77-methylguanosine cap, m7^7G) to the 5′5' end of the pre-mRNA shortly after transcription begins. This cap protects the mRNA from degradation by exonucleases, plays a crucial role in the export of mRNA from the nucleus, and is essential for initial ribosomal binding during translation.

    • 3′3' Polyadenylation: An enzyme adds a tail of approximately 50−25050-250 adenine nucleotides (poly-A tail) to the 3′3' end of the pre-mRNA. This tail is important for mRNA stability, nuclear export, and efficient translation.

    • RNA Splicing: Splicing factors, primarily components of the spliceosome (a complex of small nuclear ribonucleoproteins, snRNPs), recognize specific sequences at the exon-intron boundaries in the pre-mRNA. The spliceosome then precisely removes introns while joining exons together to form a continuous coding sequence.

Alternative Splicing

  • The incredibly versatile process by which a single pre-mRNA transcript can produce multiple distinct mRNA variants (isoforms) by selectively including or excluding various exons. This allows for the generation of a diverse range of protein products (a phenomenon called proteome diversity) from a limited number of genes, critically influencing protein expression, function, and cell-specific roles.

Structure of Mature mRNA

  • A mature mRNA molecule contains:

    • A 5′5' cap that stabilizes the mRNA and enhances its translation by ribosomes.

    • A 3′3' poly-A tail for nuclear export, stability, and protection from enzymatic degradation.

    • Untranslated Regions (UTRs): Non-coding sequences located at both the 5′5' and 3′3' ends (5' UTR and 3' UTR). These regions do not encode amino acids but are vitally important for regulating translation initiation, mRNA stability, localization, and overall gene expression.

Conclusion

  • The regulation of protein expression is a tightly controlled and multi-layered process, including mechanisms such as mRNA degradation (which controls the lifespan of mRNA molecules in the cytoplasm, affecting how much protein can be produced) and DNA methylation (an epigenetic modification where a methyl group is added to cytosine bases, typically in CpG islands, often leading to gene silencing and impacting the availability and production of proteins over time). These regulatory mechanisms ensure that genes are expressed at the appropriate time, in the correct cell type, and at the required levels for proper cellular function and development.