Complete notes for Chapter 20 Chromosomes and DNA. Covers chromosome structure, DNA replication, Central Dogma, Mutations, and key experiments (Morgan, Griffith, Hershey-Chase).
Definition: Thread like structures which are present in the nucleus. They appear at the time of cell division.
Discovery: German embryologist Walther Fleming in 1882 first observed the chromosomes. He discovered them in the rapidly dividing cells of salamander larva.
Number of Chromosomes: Different species have different number of chromosomes. Humans (46), Drosophila (8), Corn (20), Sugarcane (80), Frog (26), Mouse (40).
Importance: Chromosomes play an important role in the inheritance of a species. Each of them has genes which determines how the person's body develops and functions.
Based on the position of centromere:
Chemically chromosomes are composed of two basic portions: DNA (40%) and Histone Proteins (60%).
DNA and histone proteins combine to form nucleosome. In one nucleosome there are nearly 200 nucleotides and this DNA duplex is coiled around a core of eight histone proteins.
Histones: Have positive charge due to basic amino acids arginine and lysine, which attracts them to negatively charged phosphate groups of DNA.
Sutton's Work: In 1902 Sutton proposed that similar chromosomes paired with one another during meiosis describe the role of chromosomes in heredity.
Morgan's Experiment: In 1910 Thomas Hunt Morgan studied Drosophila melanogaster. Detected a white-eyed mutant male. Crossed with normal red-eyed female. F1 were all red-eyed. F2 showed 3:1 ratio but all white-eyed flies were males. This confirmed sex-linkage and that genes reside on chromosomes.
Studied Streptococcus pneumoniae (Virulent S-strain and Harmless R-strain). Injected heat-killed S-strain and live R-strain into mice. Mice died. Found live S-strain in blood. Concluded something transformed R to S.
Purified transforming principle. Found it was DNA, not protein.
Used Bacteriophages. Labeled DNA with P32 and Protein with S35. Found that only P32 (DNA) entered bacteria, proving DNA is hereditary material.
Discovery: Friedrich Miescher in 1869 discovered "Nuclein".
Components: Nucleotides composed of Deoxyribose sugar, Phosphate group, Nitrogenous bases (Adenine, Guanine, Cytosine, Thymine).
Chargaff's Rule: Amount of Adenine equals Thymine (A=T) and Guanine equals Cytosine (G=C).
Used N15 (heavy) and N14 (light) isotopes. Grew bacteria in N15, then transferred to N14. 1st generation density was intermediate (hybrid). 2nd generation showed intermediate and light. Confirmed Semi-conservative model.
Flow of genetic information: DNA → RNA → Protein.
Synthesis of RNA from DNA. Enzyme: RNA Polymerase. Steps: Initiation (Sigma factor binds Promoter), Elongation, Termination.
Synthesis of protein from mRNA. Occurs on ribosomes. tRNAs bring specific amino acids matching mRNA codons. Steps: Initiation (AUG codon), Elongation (Peptide bond formation), Termination (UAA, UAG, UGA stop codons).
Garrod: Studied Alkaptonuria and Albinism. Proposed enzyme defects cause genetic disorders.
Beadle and Tatum: Experimented on Neurospora crassa. Induced mutations with X-rays. Found mutants that could not synthesize specific amino acids. Concluded one gene specifies one enzyme (now One Gene One Polypeptide).
Change in structure of DNA.
Single base change.
Change in chromosome structure or number. Examples: Inversion, Deletion, Insertion (Translocation).