Biology Chapter: DNA and Genetics

DNA Discovery - Humble Beginnings

• Nuclei were first purified from pus cells in the 1860s.
• Friedrich Miescher was the scientist who first purified the nuclei.
• Chemical analysis of the purified nuclei revealed the presence of hydrogen, carbon, oxygen, nitrogen, and phosphorus
• This same substance was found in all cells examined. It was called nuclein.

Chromosomes Carry Genetic Information

• Chromosomes carry genetic information.
• Chromosomes contain an acidic portion (DNA) and a basic portion (protein).
• DNA is composed of four different nucleotides.
• Proteins are composed of twenty different amino acids.
• The question arose: which component, DNA or protein, carried the genetic information?

Pneumonia Experiments

• Griffith's experiments used two strains of Streptococcus pneumoniae bacteria.

• Strain S (smooth) had a capsule, enabling infection.

• Strain R (rough) lacked a capsule and was non-infectious.

• Mice injected with live S strain died.

• Mice injected with live R strain lived.

• Mice injected with heat-killed S strain lived.

• Mice injected with heat-killed S strain plus live R strain died, and live S strain bacteria were found in their blood.

• This indicated a transfer of genetic material (transformation) from the dead S strain to the live R strain.

Nucleic Acid Carries Genetic Material

• Avery, MacLeod, and McCarty's experiments in 1944 proved that DNA, not protein, was the genetic component of cells.
• Purified nucleic acid from the S strain bacteria.
• RNAse treatment, leaving DNA, with live R cells produced S cells
• DNAse treatment, eliminating the DNA, with live R cells did not transform the cells into S cells

Viruses to the Rescue

• Bacteriophages, viruses that infect bacteria, further confirmed DNA's role in carrying genetic information.
• Hershey and Chase experiments:
  • Radioactive labeling of protein and DNA components of a bacteriophage
  • Determination of which component entered a bacterial cell post-infection. Only DNA entered, establishing DNA as the genetic material

Hershey and Chase Experiments Summary

• DNA, not proteins, goes into bacterial cells and directs how new bacteriophages are made.

Nucleic Acid Structure

• DNA and RNA are both made of nucleotide subunits.
• Nucleotides:
  • Phosphate group
  • Sugar (deoxyribose in DNA; ribose in RNA)
  • Nitrogen-containing base (purines or pyrimidines)

Nucleotide Components - Bases

• Purines: Adenine (A) and Guanine (G)
• Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U)

Chargaff's Rules of DNA Composition

• The amount of adenine equals thymine, and the amount of cytosine equals guanine.
• The amount of purines equals the amount of pyrimidines.

Rosalind Franklin

• Used X-ray diffraction to study DNA structure.

Watson and Crick

• Built a model of DNA's double helix structure.
• Their model used known information from other scientists' earlier work

DNA is a Double-Stranded Helix

• DNA is a double helix structure.
• Held together by hydrogen bonds between the bases (AT and GC).
• Strands run antiparallel (5' to 3' and 3' to 5').

DNA Replication - Semi-conservative

• One old strand and one new strand are present in each of the two new DNA molecules formed.
• Proves DNA is semi-conservative

DNA Replication - 4 Major Enzymes

• Helicase: Unwinds DNA
• DNA Polymerase III: Adds new bases; proofreading
• Primase: Adds RNA primer for DNA polymerase
• DNA Ligase: Joins Okazaki fragments

Okazaki Fragments

• Short stretches of DNA on the lagging strand.
• Joined together by DNA ligase.

DNA Replication - Leading and Lagging

• Leading strand: synthesizes continuously
• Lagging strand: synthesizes in fragments (Okazaki fragments)

DNA Polymerase III Quality Control

• Proofreading mechanism that corrects errors during replication.

Bi-directional DNA Replication

• DNA replication proceeds in both directions from a single origin of replication.

The Organisation of DNA into Chromosomes

• DNA compaction: DNA winds around proteins called histones, forming nucleosomes, then further compacted.
• Nucleosomes condense DNA. Further compaction results in chromosomes.
• DNA is further packaged

Centromeres and Telomeres

• Centromere: The central region of a chromosome where sister chromatids attach.
• Telomere: The protective caps at the ends of each chromosome.

Role of Centromeres

• Crucial for cell division
• Important to ensure proper chromosome segregation

Role of Telomeres

• Prevent chromosomal degradation during replication & cell division
• Provide stability by capping the ends of chromosomes.

Nuclear Organisation

• Each chromosome occupies a specific territory within the nucleus.
• Organisation is linked with the functions of the chromosomes.

RNA - Structure and Function

• RNA is a single-stranded nucleic acid
• Different from DNA via a different sugar and different bases (A,C,G and U)
• Three important types of RNA.
• mRNA: Carries instructions for protein synthesis from the DNA.
• tRNA: Brings amino acids to the ribosomes, enabling protein synthesis.
• rRNA: Part of the ribosomes.