Chromosome Organization and Function in Higher Organisms
Series of lectures given by McClintock at Columbia University. The notes are partially written in McClintock's hand.
Number of Image Pages:
5 (469,289 Bytes)
1964-04-08 (April 8, 1964)
Original Repository: American Philosophical Society. Library. Barbara McClintock Papers
Reproduced with permission of the Barbara McClintock Papers, American Philosophical Society.
Medical Subject Headings (MeSH):
Gene Expression Regulation
Principles Derived from Study of Bacterial and Phage Genetic Systems (April 6, 1964)
Gene-Control Mechanisms in Higher Organisms. Part II (April 15, 1964)
Gene Control Mechanisms in Higher Organisms: Two Common Types and Several Special Types (April 13, 1964)
Box Number: 2
Folder Number: 2
Lecture 2 Columbia University. April 8, 1964
Chromosome Organization and Function in Higher Organisms.
I. A view of differences in the organization of chromosomes in higher organisms: Appearances of chromosomes at the pachytene
stage of meiosis:
Slide 1. The 10 bivalents in maize: Morgan photo. Nucleolus; nucleolus organizing chromosome (small knob). Chr.7 with knob.
chr. 5. knob; chr. 9 knob. Chromomere near end of short arm of chr. 4. Chr. 3 chromomere. Chr. 6 chromomere. Centromeres.
Heavily staining regions near centromeres: Contain genes.
Slide 2. The nucleolus chromosome maize: Knob (b) position; The fused knobs.
Slide 3. The 10 chromosomes of Sorghum -- relative of maize: The deep staining regions near centromeres; The ghost parts towards
Slide 4. Bermuda grass: The chromomeres; the deep staining regions (heavy chromomeres); the terminal knobs.
Slide 5. Tomato chromosomes: Deep chromomeres near centromeres; the ghost regions. Nucleolus chromosome: position of centromere.
Slide 6. Antirrhinum (snapdragon). Heavy chromomeres; terminal knobs.
Slide 17: Somatic prophase of Rye chromosomes. The terminal knobs.
II. The position of the chromosomes in the somatic and meiotic nuclei: Ends of chromosomes; centromeres; knobs: At the nuclear
III. The knobs: Present in a large number of organisms: those with a general chromomere pattern with no distinct "heterochromatin"
type organization near centromeres.
1. Knobs examined in detail only in maize; other organisms with knobs not so examined.
2. Knobs in maize: can be very varied: depends on the race of maize.
3. The knob-forming regions in each of the ten chromosomes of maize:
(a). The variation of knobs at any one position: Slide 19 (Morgan photo)
Knobs - 9; 7, 5; small knob 6-a.
(1) No knobs - 10, 8, 4, 3, 2, 1.
(2). Medium knob, homozygous; chr. 5. Slides 20, 21
Medium-large knob; chr. 5. Slide 22 Large knob, chr. 2.
(3). Chromosome 9: No knob, small knob, medium knob; large knob: Slides 23, 24, 25.
(4). Heterozygous knobs: Slides 26 and 27
4. The mode of action of the knob-forming regions: Controls of size and shape:
Example: Slide 28 - chr. 4
5. The proof of the knob-forming regions: X-rays: cut into two parts: Knob, short arm of chromosome 9: Rearranged chromo.
Constancy in generation after generation.
6. The races of maize and their chromosome differences. The Americas.
1). Differences among ears of the races: Slides 29, 30, 31. Small Ear
2). Knob constitutions: Central America; West Coast of Mexico: Very large knobs at nearly all knob forming regions.
3). The Abnormal chromosome 10: In Western Mexico: Slide 32
7. The constitution of knobs in different types of cells: Feugen. Large knob race. Relation to nuclear membrane.
8. Conclusions on knobs: Not a constant of genotype; regions show specific genetic differences among races; inherited differences;
redundancy of parts; changes in constitution in different types of cells.
Need for further cytochemical studies.
General prevalence of knobs in organisms with particular type of chromosome organization.
No knob vs. many large knobs: no differences in the plants; No known effects in the hybrids.
IV. Nucleolus organizers: The organizers in maize: different sizes. Different positions where nucleolus is organized at most
1. The redundancy in the nucleolus organizer:
(a). The maize case -- already examined. Diagram break and action.
(b). Chirmomous: similar to maize - breaks
(c) Drosophila melanogaster and buskii - same as maize.
2. The need for the nucleolus: Assembly plant for ribosomes (RNA from genes in chromosomes). Methylation of transper RNA.
3. Absence of nucleolus organizer - no nucleolus - many small nucleolus-like bodies; The genetic effect in Chironomous and
V. The centromeres: Redundancy. Divisions into two parts - several organisms. Same result as in maize; different parts function.
VI. The significance of redundancy:
1. Shown in the knobs of maize
2. Shown in the nucleolus organizers
3. Shown in the ever present polyploidy in many organisms - especially recognized in plants. Some organisms with very great
degree of redundancy of chromosome sets.
4. Two major aspects of this: Safety measure - when something happens to major active part, other parts take over;
Required for effective gene action - as will be clear shortly.
Part II. Regulation of gene action. Structural level.
I. Regulation of level of gene action through redundancy of genes: within different cells of the same organisms.
1. Increase in whole sets of chromosomes in different nuclei of same organism: Methods
(a). Polyploid nuclei - increase in sets; chromosomes separated.
(c) Combination of this: Some cells of the cecidomyids.
2. Increases in single chromosomes of the set:
(a). X chromosome in Drosophila males: XX females vx. XY males.
(b) Single chromosome in Salivaries - Cecidomyid: Slide 33
3. Increases in parts of chromosomes: Drosophila X chromosome: Ganglia cells: ratio DNA: Euchromatin 2 : 1 heterochromatin.
Salivaries: ration DNA: Euchromatin 50 : 1 heterochromatin
4. Increase in single bands (genes) in salivaries just before gene becomes active: Rhyncosciara; Drosophila. Somatic cells
II. Regulation of level of gene action through reduction in gene potentials.
1. Control of gene action through loss of whole sets -- males. Many insect forms.
2. Control of gene action through loss of individua1 chromosomes - Examples given in X chromosome of Sciara.
3. Control of gene action through loss of heterochromatic parts of chromosomes in the soma but not the germ line: Many examples
of this in insects.
III. Regulation in plants and animals: Animals with their soma which dies has experimented with gene control in many different
ways, some described above.
Plants: regeneration of parts occurs from somatic cells; regulation is more conservative. Cannot lose parts, increase of genes
by reduplication of gene itself; Must have other method of control of gene action.
IV. The regulation of gene action through structural modifications within the nucleus.
1. Level of the gene: Removal of the histon - protein associated with DNA in order that gene be transcribed.
2. Control of gene action through condensation of the chromosome parts:
Levels: Complete condensation - metaphase chromosome type: No RNA produced by metaphase condensed chromosome.
Partial condensations - smaller parts of chromosomes.
3. Complete condensations:
(a) Whole set of chromosomes: Coccid: Slide 34. Male set.
(b) X chromosomes in mammals: Reviewed previously.
(c) Position of condensed chromosomes: at the nuclear membrane.
4. Partial condensations: Differences in different cells of plants and animals. Same type always found in same type of cell.
(a). Very active cells: Large nucleolus; chromosomes not condensed; finely granular within the nucleus.
(b) Nuclei with low gene action: small nucleoli; highly granular chromatin, much of this at nuclear membrane. Small nuclei,
5. Examples of differential condensation:
(a). Retina cells in 10 day old kitten. Slides 35 and 36.
(b). Lymphocytes: Slides 37 and 38. Position of condensed chromatin.
V. Summary. [Remainder of document handwritten]
RNA production: 14% in [ . . . ] path = 70% g
[ . . . ]ation after new RMA product - 80% of DNA in condensed [ . . . ] = contains 17% RNA and only 14% [ . . . ] genes in
condensed portion of DNA = [ . . . ]
Slides - lecture 2
1. 10 bivalents in maize - Morgan -
2. Cles-6. In maize
3. Sorghum - 10 Chr.
4. Bermuda grass.
5 Tomato chromosomes:
9. Salvia glutinosa
10. Salvia horminum
11 to 14 neurospora
15. Cyclops - copepod - germ line cell
16 " diplotene
17 Rye-somatic chromosomes - Terminal knobs
18 - R[ . . . ] - Knob forming regions.
19 - [ . . . ] in maize - (Morgan photo) - knob
20-21 Chr 5 - medium knob chr. 5 Large knob chr. 2
23-25. Knob, every chr 9 short arm.
26-27 Heterozygous knobs.
28. Chr. 4 knob - maize.
29-31 Ears of maize differences.
32. Abnormal chr. 10
33. Cecidomyid salivaries
34. [ . . . ]: Whole set of chr - heterochromate[?]