Lecture 1 - Early Chromosome research

Discovery of Chromosomes?

Chromosomes = Colored bodies

• Staining of cells showed colored areas (nucleus/chromosomes)

Carl Rabl

• What was the observation

• What was his experiment?

• What was his discovery?

Observation: Identifiable chromosomes became a blob during interphase

Discovery: Chromosomes are not destroyed during interphase

• Followed cell through replication + found that the colored chromosomes did not disappear during interphase but rather CHANGED APPEARANCE

What was Discovered to be the FUNCTION OF CHRM?

Chromosome theory of Heredity

• Proposed by Boveri + Sutton

• Proved/confirmed by stevens + Morgans

(names are not testable)

Theodor Boveri

• What did he Study

• What was Discovered?

Only Sea Urchin Embryos with the CORRECT # of Chromosomes developed normally

• Implication: Wrong # chrm correlates to wrong number of genes (which is the unit of heredity)

  • (chromosome unit of heredity)

Walter Sutton

• What did he Study?

• What did he discover?

Gametes get ONE of each pair of chromosomes

• Studied both Adult + Gamete Grasshopper chromosomes during prophase of spermatogenesis

Discovery:

• Adults = 2 of each gene + 11 pairs of chromosomes

• Gamete = 1 of each gene + 11 single chromosomes

Proposal:

• 2 genes = correlates to 2 sets of chromosomes in adults and to 1 in gametes

Nettie Stevens

• What did She discover?

Sex chromosomes X + Y

What is the answer + WHY?

B. 1 of Spermatogenesis

• Why Meiosis 1?

  • It shows the separation of Homologous chromosomes and not sister chromatids (because the size of each are diff + not identical)

• Why Spermatogenesis?

  • X + Y are both present which is only present in males

  • Females would have identical XX chromosomes

Thomas Morgan

• What did He discover?

Gene for Drosophila eye color is on the X chromosome

What is the Summary/ Conclusion for Stevens + Morgan’s Experiments?

White eye gene is on the X chromosome found by stevens

= Genes are on chromosomes (Chromosome function = heredity as genes are the unit of heredity)

SUMMARIZE + Describe the 4 experiments that Determined The Function of Chromosomes (Chromosome theory of heredity)

Proposed:

• Boveri = Sea urchin + Wrong # of chromosomes = Sad

• Sutton = Grasshoppers (Adults + Gametes); Gametes only get one of each pair of chromosomes)

Confirmed:

• Stevens = Sex chromosomes (X + Y)

• Morgan = Fruit fly + White eye color gene on X chromosome

Genome size: What is a Genome?

ALL the DNA in a organisms cells

• eg. humans have 46 Chromosomes + 1 mitochondria

Genome size: What is C value?

The amount of DNA an organism has in a Haploid cell in G1

• The smallest amount of DNA that can be ever found in an organism

Genome size: What is C value paradox?

The lack of correlation between the C value of a species + it’s complexity

• Mice + Humans have relatively similar values

• Some Plants can have ~50x’s the C value of humans (Japanese canopy plant)

DNA reassociation Experiments

• Procedure

1. Isolate DNA

2. Shear into fragments

3. Denature with Heat

4. Anneal by cooling

*****Results of DNA reassociation Experiments?

1. Prokaryotes: Genomes reanneals Slowly = Low amounts of highly repetitive DNA (Each piece of ssDNA only had a single matching piece)

2. EUKARYOTES: Found 3 distinct stages of reannealing

   1. Fast = Highly repetitive

   2. Interm. = Moderately Repetitive

   3. Slow = Unique/ Not repetitive DNA

3. Proportions of each of the 3 stages vary between species

What does the Prokaryote reannealing graph look like compared to the Eukaryotes?

What is the DNA reannealing experiment also called + WHY?

COT CURVE EXPERIMENT

• Cot ½ = Time at which half of the DNA is reannealed

Describe the DNA centrifugation experiments

• what were the steps

• What 2 chemicals were used + What are their functions?

Steps

• Isolate DNA

• Shear DNA

• Centrifuge @ high speeds for a long time

CsCl = Cesium Chloride = salt that creates a GRADIENT based on concentration

EtBr = Ethidium bromide = Intercalates in DNA to make it glow orange

What were the Results of the Centrifugation experiment?

2 Bands were visualized: Main band + Satellite band

• BASE COMPOSITION DIFFERED between the 2 bands

  • Satellite = Lower GC content = lighter

  • Main = Higher GC content = Heavier

What is the Answer?

A