Chapter 10: Genetic Material and Information Flow

What is the Genetic Material?

• Griffith, 1928:
  • Mixed nonpathogenic bacteria with killed pathogenic bacteria.
  • Result: Nonpathogenic bacteria became pathogenic.
  • Pathogenicity trait was inherited by future generations.
• Hershey and Chase “blender” experiments, 1952:
  • Radioactive phage DNA, but not protein, found in bacteria, indicating DNA is the genetic material.

DNA and RNA are Polymers of Nucleotides

• Nucleic acids are polynucleotides made of long chains of nucleotide monomers.
• Nitrogenous bases:
  • Single-ring pyrimidines: thymine (T), cytosine (C).
  • Double-ring purines: adenine (A), guanine (G).
• Sugar-phosphate backbone is present.

Differences Between DNA and RNA

• Nitrogenous bases:
  • DNA: A, C, G, T
  • RNA: A, G, C, U
• Sugars:
  • DNA: deoxyribose
  • RNA: ribose

DNA Structure

• Watson and Crick:
  • James Watson and Francis Crick worked out the three-dimensional structure of DNA based on X-ray crystallography by Rosalind Franklin.
  • DNA consists of two polynucleotide strands wrapped around each other in a double helix.
  • Sugar-phosphate backbones are on the outside, and nitrogenous bases are on the inside.
• Base pairing:
  • Each base pairs with a complementary partner – A with T, and G with C.
  • Hydrogen bonds between the bases hold the strands together.
• The Watson-Crick model suggested a molecular explanation for genetic inheritance, with the specific pairing suggesting a copying mechanism.

DNA Replication

• Mechanism:
  • DNA strands separate.
  • Enzymes use each strand as a template to assemble new nucleotides into complementary strands.
• Semiconservative replication:
  • Each new double helix consists of one old and one new strand.
  • Occurs during the S phase of interphase.
  • Parent strand untwists and unzips via helicase.
  • DNA polymerase attaches unattached nucleotides to complimentary nucleotides in the old strand.
• A DNA molecule is “semi-conserved” following replication due to each new helix having one old and one new strand.

DNA Replication: A Closer Look

• Origins of replication:
  • DNA replication begins at specific sites on the double helix.
  • Proteins attach and separate the strands.
  • Replication proceeds in both directions, creating replication bubbles.
• Parent strands open, daughter strands elongate at many sites simultaneously.
• DNA strands have opposite orientations.
• The enzyme DNA polymerase adds nucleotides only at the 3' end.
  • One daughter strand is synthesized continuously.
  • The other strand is synthesized in a series of short pieces.
  • The two strands are connected by the enzyme DNA ligase.

Enzymes Involved in Replication

• DNA polymerases.
• DNA ligase.

Flow of Genetic Information: From DNA to RNA to Protein

• The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits.
• A gene—a linear sequence of many nucleotides—specifies a particular polypeptide.
• The flow of genetic information:
  1. Transcription of the genetic information in DNA into RNA.
  2. Translation of RNA into the polypeptide.
• Beadle-Tatum one gene-one enzyme hypothesis:
  • Studies of inherited metabolic disorders in mold suggested that phenotype is expressed through proteins.
  • A gene dictates the production of a specific enzyme; the hypothesis has been restated to one gene-one polypeptide.

Genetic Code

• Genetic information flows from DNA to RNA to protein.
• Nucleotide monomers represent letters in an alphabet that can form words in a language.
• Triplet code:
  • Three-letter words (codons).
  • Each word codes for one amino acid in a polypeptide.
• The genetic code specifies the correspondence between RNA codons and amino acids in proteins.
• Includes start and stop codons.
• Redundant: More than one codon can code for an amino acid, but not ambiguous: a codon can code for only one amino acid (e.g., UCA will only code for Serine and nothing else).
• Nearly all organisms use exactly the same genetic code.

Transcription

• Transcription produces genetic messages in the form of RNA.
• One DNA strand serves as a template for the new RNA strand.
• RNA polymerase constructs the RNA strand in a multistep process.
  • Initiation: RNA polymerase attaches to the promoter, and synthesis starts.
  • Elongation: RNA synthesis continues, the RNA strand peels away from the DNA template, and DNA strands come back together in the transcribed region.
  • Termination: RNA polymerase reaches a terminator sequence at the end of the gene and detaches.

Eukaryotic RNA Processing

• The RNA that encodes an amino acid sequence is messenger RNA (mRNA).
• In prokaryotes, transcription and translation both occur in the cytoplasm.
• In eukaryotes, RNA transcribed in the nucleus is processed before moving to the cytoplasm for translation.
• RNA Splicing:
  • Noncoding segments called introns are cut out.
  • Remaining exons are joined to form a continuous coding sequence.
• A cap and a tail are added to the ends.

Translation

• Transfer RNA (tRNA) molecules match the right amino acid to the correct codon.
• tRNA is a twisted and folded single strand of RNA.
  • An anticodon loop at one end recognizes a particular mRNA codon by base pairing.
  • The amino acid attachment site is at the other end.
• Each amino acid is joined to the correct tRNA by a specific enzyme.

Ribosomes

• A ribosome consists of two subunits.
  • Each is made up of proteins and ribosomal RNA (rRNA).
• The subunits of a ribosome:
  • Hold the tRNA and mRNA close together in binding sites during translation.
  • Allow amino acids to be connected into a polypeptide chain.

Initiation of Translation

• The initiation phase of translation:
  • Brings together mRNA, a specific tRNA, and the two subunits of a ribosome.
  • Establishes exactly where translation will begin.
  • Ensures that mRNA codes are translated in the correct sequence.
• Initiation is a two-step process:
  1. mRNA binds to a small ribosomal subunit; initiator tRNA, carrying the amino acid Met, binds to the start codon.
  2. A large ribosomal subunit binds to the small one, forming a functional ribosome; initiator tRNA fits into one binding site; the other is vacant for the next tRNA.

Elongation and Termination of Translation

• Once the initiation is complete, amino acids are added one by one in a three-step elongation process:
  1. Codon recognition.
  2. Peptide bond formation.
  3. Translocation.
• Elongation continues until a stop codon reaches the ribosome’s A site, terminating translation.

Review: Flow of Genetic Information

• The sequence of codons in DNA, via the sequence of codons in RNA, spells out the primary structure of a polypeptide.
  1. Transcription of mRNA from a DNA template.
  2. Attachment of amino acid to tRNA.
  3. Initiation of polypeptide synthesis.
  4. Elongation.
  5. Termination.

Mutations and Disease

• Gene mutation:
  • Change sequence, structure, interactions, and function.
• Disease aspects:
  • Metabolic disorders.
  • Cancer.
  • Developmental disorders.
• Infection by pathogenic organism (bacteria, fungus, virus):
  • Pathogens have specialized factors that help them invade cells, evade the immune response, and take over cells replication machinery; they rely on interactions with host molecules.
• Normal gene to Oncogene conversion:
  • Mutation within the gene.
  • Multiple copies of the gene.
  • Gene moved to a new DNA locus, under new controls.
• Mutation types:
  • Base substitution.
  • Nucleotide deletion.

Tumor Suppressor Genes

• Normal growth-inhibiting protein becomes defective or non-functioning, leading to uncontrolled cell division.

Cancer Development

• Proto-oncogene (normal) can be converted to an oncogene via mutation or virus.
• Accumulation of mutations in the development of a cancer cell.
• Stepwise development of a typical colon cancer as an example.