Chapter10

DNA and RNA: The Molecular Basis of Heredity

Overview of DNA and Proteins

• Role of Proteins: Crucial for cell function, providing structural integrity and catalyzing reactions.

• Location of Genetic Information: Recipes for proteins are encoded in DNA.

• Genes: Organized segments of DNA, each specifying a different protein.

Nucleic Acid Structure and Function

• Functions of DNA:

  • Passes genetic information to the next generation.

  • Controls the synthesis of proteins.

• Unique Structure of DNA: Essential for its functions.

Structure of DNA

• Composition: Nucleic acids are large polymers consisting of nucleotides:

  • Sugar: Deoxyribose for DNA and ribose for RNA.

  • Phosphate Group: Forms the backbone of the DNA structure.

  • Nitrogenous Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T).

DNA Strand Description

• Double-Stranded Structure:

  • Strands are connected by hydrogen bonds between bases: A-T and G-C.

DNA Replication Process

• Definition: DNA replication is the process of copying DNA before cell division.

• Importance: Ensures that daughter cells receive an identical genetic copy.

• Base-Pairing Rules: Key for accurate replication.

Enzymes Involved in Replication

• DNA Polymerase:

  • Binds to DNA and forms replication bubbles.

  • Builds new strands by adding complementary nucleotides based on the existing strands.

• Helicases: Separate the two strands of DNA for replication to occur.

Repairing Genetic Information

• Error Correction: If mutations occur during replication, the old strand can be used to correct errors.

The DNA Code

• Genetic Information: Order of bases (A, T, G, C) forms codons that specify amino acids, creating a recipe for proteins.

• Triplet Codons: Each set of three nucleotides translates to a specific amino acid.

RNA Structure and Function

• Differences from DNA:

  • RNA contains ribose sugar, Uracil (U) instead of Thymine (T), and is typically single-stranded.

• RNA Synthesis: Begins in the nucleus, with RNA synthesized following base-pairing rules (A-U; G-C).

Transcription and Translation

• Transcription: Process by which DNA is used to create RNA; occurs in the nucleus with RNA polymerase serving as the main enzyme. It involves multiple steps:

  • RNA polymerase binds to the promoter region to start transcription.

  • Transcription terminates at specific signals.

• Translation: The mRNA is translated into a protein by ribosomes, where the sequence of mRNA is read in codons. This involves two primary RNAs:

  • mRNA (messenger RNA): Carries the genetic code.

  • tRNA (transfer RNA): Brings the correct amino acids during protein formation.

Genetic Code Universality

• Commonality Across Organisms: Nearly all organisms share a similar genetic code, allowing for the use of bacterial systems for protein production (e.g., insulin).

Gene Expression and Control

• Definition: The process by which cells produce proteins from genes, unique to cell types based on gene expression.

• Regulation of Protein Production:

  • Cells control how much protein is synthesized through mechanisms affecting mRNA availability and transcription rate.

  • Regulatory sequences on DNA (enhancers and silencers) impact the rate of transcription.

Protein Quality Control

• Eukaryotic Gene Management: Introns are non-coding regions that must be spliced out during post-transcription processing, allowing for the synthesis of multiple proteins from a single gene.

Mutations and Their Impact on Synthesis

• Mutations: Can occur due to replication errors or external influences, with the potential to alter protein structures and functions.

• Types of Mutations:

  • Point Mutations: Single nucleotide changes, which can be silent, nonsense, or missense.

  • Insertions/Deletions: These can cause frameshifts, dramatically altering the translation outcome.

Chromosomal Aberrations**: Alterations at the chromosome level can have widespread effects on many genes and proteins, including inversions, translocations, duplications, and deletions.