General Genetics

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

DNA and RNA are the building blocks of life, storing and transmitting genetic information. DNA's double-stranded structure contrasts with RNA's single-stranded form, while both use nucleotides as their basic units. These molecules play crucial roles in replication and transcription processes. Understanding the molecular structure of DNA and RNA is essential for genetics and molecular biology. Their unique properties enable various functions, from long-term genetic storage to temporary messaging. This knowledge has led to groundbreaking applications in medicine, biotechnology, and forensic science.

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


© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.