👨‍👩‍👦‍👦General Genetics Unit 6 – Molecular Structure of DNA and RNA

Key Concepts

• DNA and RNA are nucleic acids that store and transmit genetic information
• DNA is a double-stranded molecule, while RNA is typically single-stranded
• Nucleotides are the building blocks of DNA and RNA, consisting of a sugar, phosphate group, and nitrogenous base
• DNA uses the bases adenine (A), thymine (T), guanine (G), and cytosine (C), while RNA uses A, G, C, and uracil (U) instead of thymine
• Base pairing occurs between complementary bases: A with T (or U in RNA) and G with C
• The double helix structure of DNA is stabilized by hydrogen bonds between base pairs and the sugar-phosphate backbone
• Replication is the process of making a copy of DNA, while transcription involves synthesizing RNA from a DNA template
• Understanding the molecular structure of DNA and RNA is crucial for fields such as genetics, molecular biology, and biotechnology

DNA Structure

• DNA (deoxyribonucleic acid) is a double-stranded molecule composed of nucleotides
• The sugar in DNA nucleotides is deoxyribose, which lacks an oxygen atom compared to the ribose sugar found in RNA
• DNA strands are antiparallel, meaning they run in opposite directions (5' to 3' and 3' to 5')
• The two strands of DNA are held together by hydrogen bonds between complementary base pairs
• The sugar-phosphate backbone is on the outside of the DNA double helix, while the nitrogenous bases are on the inside
• The diameter of a DNA double helix is approximately 2 nanometers (nm)
• The distance between consecutive base pairs in a DNA double helix is about 0.34 nm
• The double helix structure of DNA was first proposed by James Watson and Francis Crick in 1953

RNA Structure

• RNA (ribonucleic acid) is typically a single-stranded molecule composed of nucleotides
• The sugar in RNA nucleotides is ribose, which has an additional oxygen atom compared to the deoxyribose sugar found in DNA
• RNA strands are directional, running from 5' to 3'
• RNA can form secondary structures, such as hairpin loops and stem-loops, through base pairing within the same strand
• There are three main types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)
  • mRNA carries genetic information from DNA to ribosomes for protein synthesis
  • tRNA transfers specific amino acids to the ribosome during protein synthesis
  • rRNA is a component of ribosomes and plays a role in catalyzing peptide bond formation
• Some viruses, such as retroviruses (HIV), have genomes made of RNA instead of DNA

Nucleotides and Base Pairing

• Nucleotides are the monomers that make up DNA and RNA
• Each nucleotide consists of three components: a sugar (deoxyribose in DNA or ribose in RNA), a phosphate group, and a nitrogenous base
• There are five nitrogenous bases: adenine (A), thymine (T), guanine (G), cytosine (C), and uracil (U)
  • A and G are purines, which have a double-ring structure
  • T, C, and U are pyrimidines, which have a single-ring structure
• Base pairing occurs through hydrogen bonding between complementary bases
  • A pairs with T (in DNA) or U (in RNA)
  • G pairs with C
• The base pairing rules (A-T/U and G-C) ensure the specificity and stability of the DNA double helix and RNA secondary structures
• Proper base pairing is essential for accurate replication, transcription, and translation of genetic information

Double Helix vs. Single Strand

• DNA typically exists as a double helix, while RNA is usually single-stranded
• The DNA double helix is stabilized by hydrogen bonds between complementary base pairs and the sugar-phosphate backbone
  • There are two hydrogen bonds between A and T, and three hydrogen bonds between G and C
• The antiparallel nature of the DNA strands allows for the formation of the double helix structure
• RNA, being single-stranded, can form secondary structures through base pairing within the same strand
  • Examples of RNA secondary structures include hairpin loops, stem-loops, and pseudoknots
• The flexibility of single-stranded RNA allows it to perform various functions, such as catalyzing reactions (ribozymes) and regulating gene expression (microRNAs and siRNAs)
• In some cases, DNA can also exist in single-stranded form, such as during replication or in certain viral genomes (ssDNA viruses)

Replication and Transcription Basics

• Replication is the process of making an identical copy of DNA, which occurs during cell division
  • DNA replication ensures that each daughter cell receives a complete set of genetic instructions
• Replication begins at specific sites called origins of replication and proceeds bidirectionally
• The enzyme DNA helicase unwinds the double helix, separating the two strands
• DNA polymerase synthesizes new strands of DNA using the original strands as templates
  • DNA polymerase can only add nucleotides to the 3' end of a growing strand
• Transcription is the process of synthesizing RNA from a DNA template
  • Transcription is the first step in gene expression, leading to the production of functional molecules such as proteins
• RNA polymerase is the enzyme responsible for transcription
  • RNA polymerase binds to specific sequences called promoters to initiate transcription
• During transcription, RNA polymerase reads the DNA template strand in the 3' to 5' direction and synthesizes a complementary RNA strand in the 5' to 3' direction
• The newly synthesized RNA strand is called the primary transcript and may undergo further processing (splicing, capping, polyadenylation) to become a mature RNA molecule

Comparing DNA and RNA

• DNA and RNA are both nucleic acids that store and transmit genetic information, but they have several key differences
• DNA is typically double-stranded, while RNA is usually single-stranded
• DNA contains the sugar deoxyribose, while RNA contains the sugar ribose
  • The presence of an additional oxygen atom in ribose makes RNA more susceptible to hydrolysis compared to DNA
• DNA uses the nitrogenous bases A, T, G, and C, while RNA uses A, U, G, and C
  • Uracil (U) in RNA is structurally similar to thymine (T) in DNA and base pairs with adenine (A)
• DNA is more stable than RNA due to its double-stranded structure and the absence of the reactive 2' hydroxyl group on the sugar
• DNA is primarily found in the nucleus of eukaryotic cells, while RNA can be found in both the nucleus and cytoplasm
• DNA serves as the long-term storage of genetic information, while RNA acts as a temporary messenger and performs various functions in the cell
  • Examples of RNA functions include protein synthesis (mRNA), amino acid transfer (tRNA), and catalysis (ribozymes)

Real-World Applications

• Understanding the molecular structure of DNA and RNA has led to numerous advancements in various fields
• In medicine, knowledge of DNA and RNA has enabled the development of targeted therapies and personalized medicine approaches
  • For example, antisense oligonucleotides can be designed to bind to specific mRNA sequences and inhibit the expression of disease-causing genes
• Genetic engineering techniques, such as recombinant DNA technology and CRISPR-Cas9 gene editing, rely on the manipulation of DNA and RNA
  • These techniques have applications in agriculture (genetically modified crops), industry (production of biopharmaceuticals), and research (studying gene function)
• DNA profiling, also known as genetic fingerprinting, is used in forensic science to identify individuals based on their unique DNA sequences
  • Short Tandem Repeats (STRs) are commonly used genetic markers in DNA profiling
• RNA-based technologies, such as RNA interference (RNAi) and mRNA vaccines, have emerged as promising tools for treating diseases and preventing infections
  • RNAi can be used to silence specific genes by targeting their mRNA for degradation
  • mRNA vaccines (COVID-19 vaccines) deliver genetic instructions for the production of antigens, triggering an immune response