Biol 20: Chapter 10

  • Chapter 10: Biochemistry of the Genome, Part One

  • Discovery of DNA:

    • DNA was discovered well before its role in heredity was understood.

    • Microbiologists played a crucial role in establishing DNA as the hereditary material in cells.

  • Timeline of Key Events:

    • Gregor Mendel (1850s-1860s): Conducted experiments with garden peas to illustrate the heritability of traits (phenotype and genotype concepts).

    • Sutton and Bawry (1902): Proposed the chromosomal theory of inheritance, linking chromosomes to genetic inheritance.

    • Beadle and Tatum: Showed that the production of proteins is controlled by single genes, leading to the concept of "one gene - one enzyme" which is part of the central dogma of molecular biology.

    • Frederick Griffith (1928): Conducted experiments demonstrating bacterial transformation.

    • Avery and McCarthy: Identified DNA as the transforming principle responsible for heredity.

    • Hershey and Chase (1952): Used radioactive labeling to prove DNA is the hereditary material.

  • Griffith's Transformation Experiments:

    • Showed that hereditary information could be transferred from one bacterium to another, also termed horizontal gene transfer.

    • Streptococcus pneumoniae strains:

      • Rough strain (R strain): non-pathogenic, lacks a capsule.

      • Smooth strain (S strain): pathogenic, has a capsule which aids in evading the host's immune response.

    • Experimental Design:

      1. Infecting mice with the live R strain: Mice lived.

      2. Infecting mice with heat-killed S strain: Mice lived.

      3. Mixing heat-killed S strain with live R strain: Mice died.

        • Conclusion: Something from the heat-killed S strain transformed the R strain into a pathogenic form, termed the "transforming principle".

      4. Pathogenic S strain extracted from dead mice killed when reintroduced to healthy mice.

  • Avery, MacLeod, and McCarthy's Experiments:

    • Investigated Griffith's transforming principle by isolating components and deducing their roles.

    • Found that only when DNA was degraded did the mixture fail to transform R strain, indicating DNA as the transforming agent.

    • Despite results, some scientists disputed the findings, believing proteins contaminated the extracts and were responsible for transformation.

  • Hershey and Chase Experiments:

    • Labeled DNA and protein of bacteriophages to determine which component was the genetic material.

    • Demonstrated that DNA, not protein, is responsible for the production of new phage particles.

  • Conclusion:

    • Set the stage for further discussions on the structure of DNA and related concepts in upcoming sections.

video 2:

  • Nucleic Acids

    • One of the four major biological molecules, along with carbohydrates, lipids, and proteins.

    • Composed of nucleotides.

  • Nucleotide Structure

    • Consists of three components:

      • Sugar: A pentose sugar (five-carbon sugar).

      • Phosphate Group

      • Nitrogenous Base

  • Types of Nucleotides in DNA

    • Deoxyribonucleotides (in DNA) contain:

      • Deoxyribose as the sugar.

    • Nitrogenous Bases:

      • Purines: Adenine (A) and Guanine (G)

      • Pyrimidines: Cytosine (C) and Thymine (T)

  • Structure of Deoxyribose

    • Identified by the designation of five carbon atoms:

      • 1', 2', 3', 4', and 5' positions identified.

  • Phosphodiester Bonds

    • Connect nucleotides:

      • Bond between the 5' phosphate group of one nucleotide and the 3' hydroxyl group of another.

  • Base Pairing Rules

    • Chargaff's rules:

      • Amount of Adenine (A) = Amount of Thymine (T)

      • Amount of Guanine (G) = Amount of Cytosine (C)

    • Pairs:

      • A with T (2 hydrogen bonds)

      • G with C (3 hydrogen bonds)

  • Double Helix Model

    • Proposed by Watson and Crick based on:

      • Chargaff’s research

      • Rosalind Franklin’s X-ray diffraction studies

    • Structure described:

      • Two complementary strands oriented antiparallel to each other.

      • Sugar-phosphate backbones on the exterior, with bases in the interior.

  • Denaturation and Renaturation of DNA

    • Can be denatured through heat or chemicals, breaking hydrogen bonds.

    • Renaturation occurs upon cooling, allowing hydrogen bonds to reform.

  • Transposons

    • Proposed by Barbara McLintock:

      • Also known as “jumping genes,” segments of DNA that can move within the genome.

    • Discovery significant after its initial proposal in the 1950s.

  • Conclusion for Chapter 10, Part Two

    • Understand the basic structure of DNA, including nucleotide composition, bonding, and the significance of base pairing.

    • Next topic: RNA and protein synthesis.

video 3

Structure of RNA

  • RNA (Ribonucleic Acid)

    • Typically single-stranded

    • Contains ribose sugar (pentose)

    • Utilizes uracil (U) instead of thymine (T), which is found in DNA

Differences between RNA and DNA

  • Sugar type:

    • RNA: Ribose

    • DNA: Deoxyribose

  • Base pairing differences:

    • RNA: Adenine (A) pairs with Uracil (U)

    • DNA: Adenine (A) pairs with Thymine (T)

  • Structural differences:

    • RNA can undergo significant base pairing to form three-dimensional structures

    • DNA is typically double-stranded

Base Pairing in RNA

  • Adenine pairs with Uracil (A-U)

  • Guanine pairs with Cytosine (G-C)

Types of RNA Involved in Protein Synthesis

  1. Messenger RNA (mRNA)

    • Intermediary between DNA and proteins

    • Central dogma: One gene = one protein

    • Transcribes the DNA message into RNA

  2. Transfer RNA (tRNA)

    • Carries amino acids to the ribosomes

    • Has an anticodon that pairs with codons in mRNA

  3. Ribosomal RNA (rRNA)

    • Major constituent of ribosomes

    • Ensures proper alignment of mRNA and ribosomes during protein synthesis

Protein Synthesis Process

  • Transcription:

    • Process of converting DNA information into RNA

    • mRNA is formed as the result of transcription

  • Translation:

    • mRNA read in codons (three-letter sequences)

    • tRNA brings the corresponding amino acids to the ribosomes

    • Formation of polypeptide chain until a stop codon is reached

Ribosomes and Their Role

  • Ribosomes are the site of protein synthesis

  • Composed of rRNA

    • Large and small subunits

  • Important for correctly reading mRNA codons and forming proteins

Gene Expression

  • Genes code for proteins and can be influenced by environmental conditions

  • Example: Serratia marcescens produces pigment only under certain temperatures

  • Phenotype: Observable characteristics influenced by gene expression

    • Same genotype may express different phenotypes under different conditions

DNA Structure

  • Coding Regions: Parts of DNA that code for proteins

  • Noncoding Regions: Include introns, which are spliced out during RNA processing

Differences Between Prokaryotes and Eukaryotes

  • Prokaryotes:

    • Typically haploid with singular, circular chromosomes

    • DNA is in nucleoid region, no true nucleus

  • Eukaryotes:

    • Diploid with linear chromosomes found in the nucleus

    • DNA tightly packed with histones into chromatin

Extrachromosomal DNA and Plasmids

  • Plasmids provide additional genetic functions such as antibiotic resistance

  • Example:

    • Bacillus anthracis has plasmids that encode for toxins, unlike Bacillus cereus

Review Notes

  • Focus on the importance of mRNA, tRNA, and rRNA in protein synthesis processes

  • Remember the details about gene expression influenced by environmental factors, and differences between prokaryotic and eukaryotic cells.

  • Prepare for calculations related to nucleotide composition (A, G, C, T) in DNA.

  • Review the start and stop codons, and how they relate to mRNA and protein synthesis.