1. Chromosome Structure, Nucleosome Model
Chromosome Structure, Nucleosome Model & Variation in chromosome number, Chromosome Alterations
Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany Deptt.
P.G.G.C.G. -11,Chandigarh

2. Chromosomes and Genetics
Basic review:
Chromosomes are long pieces of DNA, with supporting proteins.
Genes are short regions of this DNA that hold the information needed to build and maintain the body
Genes have fixed locations: each gene is in a particular place on a particular chromosome
Diploids have 2 copies of each chromosome, one from each parent. This means 2 copies of each gene.
The interactions between the 2 copies of each gene give rise to the various forms of dominance.

3. Chromosomes
The essential part of a chromosome is a single very long strand of DNA. This DNA contains all the genetic information for creating and running the organism.
Each chromosome has a central constricted region called a centromere that serves as an attachment point for the machinery of mitosis.

4. Chromosomes
Chromosomes exist in 2 different states, before and after they replicate their DNA. Before replication, chromosomes have one chromatid. After replication, chromosomes have 2 sister chromatids, held together at the centromere. Each chromatid is one piece of DNA with its supporting proteins.
In mitosis, the two chromatids of each chromosome separate, with each chromatid going into a daughter cell.

5. Chromosomes = DNA

6. Eukaryotic chromosomal organization
2 main groups of proteins involved in folding/packaging eukaryotic chromosomes
Histones = positively charged proteins filled with amino acids lysine and arginine that bond
Nonhistones = less positive

7. Model for Chromatin Structure
Chromatin is linked together every 200 bps (nuclease digestion)
Chromatin arranged like
“String onBeads” (electron microscope)
8 histones in each nucleosome
147 bps per nucleosome core particle with 53 bps for linker DNA (H1)
Left-handed superhelix

8. Eukaryotic chromosomal organization
Histone proteins
Abundant
Histone protein sequence is highly conserved among eukaryotes
Provide the first level of packaging for the chromosome
DNA is wound around histone proteins to produce nucleosomes; stretch of unwound DNA between each nucleosome

9. Eukaryotic chromosomal organization
Nonhistone proteins
Other proteins that are associated with the chromosomes
Many different types in a cell; highly variable in cell types, organisms, and at different times in the same cell type
Amount of nonhistone protein varies
May have role in compaction or be involved in other functions requiring interaction with the DNA
Many are acidic and negatively charged; bind to the histones; binding may be transient

10. Eukaryotic chromosomal organization
Histone proteins
5 main types
H1—attached to the nucleosome and involved in further compaction of the DNA (conversion of 10 nm chromatin to 30 nm chromatin)
H2A
H2B H3 H4
This structure produces 10nm chromatin
Two copies in each nucleosome ‘histone octomer’; DNA wraps around this structure1.75 times

11. Nucleosome structure

12. 10 nm chromatin is produced in the first level of packaging.
Nucleosomes connected together by linker DNA and H1 histone to produce the “beads-on-a-string” extended form of chromatin
10 nm chromatin is produced in the first level of packaging.
Linker DNA H1
Histone octomer

13. - Core DNA = 146 bp
- Linker DNA = bp (usually 55bp)
- DNA turns 1 and ¾ times around histone octamer.

15. Types of Genetic variation
Allelic variations
mutations in particular genes (loci)
Chromosomal aberrations
Changes in chromosome Number ( Numerical)
Changes in chromosome Structure (Structural)

16. VARIATIONS IN CHROMOSOME Number

17. Variation In Chromosome Number
Euploidy
Normal variations of the number of complete sets of chromosomes
Haploid, Diploid, Triploid, Tetraploid, etc…
Aneuploidy
Variation in the number of particular chromosomes within a set
Monosomy, trisomy, polysomy

18. VARIATIONS IN CHROMOSOME STRUCTURE (CHROMOSOMAL REARRANGEMENTS)
Deletions
Loss of a region of a chromosome
Duplications
Inversions
Pericentric – inversion about the centromere
Paracentric – inversion not involving the centromere
Translocations
Exchange or joining of regions of two non-homologous chromosomes

19. Polyploidy v Aneuploidy

20. 8-51

21. Euploidy Variations Plants commonly exhibit polyploidy
30-35% of ferns and flowering plants are polyploid
Many of the fruits & grain are polyploid plants
Polyploid strains often display desirable agricultural characteristics
wheat
cotton
strawberries
bananas
large blossom flowers

22. and two copies of other chromosomes one copy of some chromosomes
Polyploidy
Polyploids with odd chromosome sets are usually sterile
produce mostly aneuploid gametes
rare a diploid & haploid gamete are produced
and two copies of other chromosomes
Each cell receives
one copy of some chromosomes

23. Benefit of Odd Ploidy-Induced Sterility
Seedless fruit
watermelons and bananas
asexually propagated by human via cuttings
Seedless flowers
Marigold flowering plants
Prevention of cross pollination of transgenic plants

24. Generation of Polyploids
Autopolyploidy
Complete nondisjunction of both gametes can produce an individual with one or more sets of chromosomes

25. Interspecies Crosses can Generate Alloploids
Alloploidy
Offspring generally sterile

26. Interspecies Crosses Result in Alloploids
Allodiploid
one set of chromosomes from two different species
Allopolyploid
combination of both autopolyploidy and alloploidy
An allotetraploid: Contains two complete sets of chromosomes from two different species

27. Experimental Treatments Can Promote Polyploidy
Polyploid and allopolyploid plants often exhibit desirable traits
Colchicine is used to promote polyploidy
Colchicine binds to tubulin, disrupting microtubule formation and blocks chromosome segregation

28. Variation In Chromosome Structure
Amount of genetic information in the chromosome can change
Deficiencies/Deletions
Duplications
The genetic material remains the same, but is rearranged
Inversions
Translocations

29. Deficiencies (Deletions)
A chromosomal deficiency occurs when a chromosome breaks and a fragment is lost

30. Deficiencies Phenotypic consequences of deficiency depends on
Size of the deletion
Functions of the genes deleted
Phenotypic effect of deletions usually detrimental

31. Cri-du-chat Syndrome

32. Duplications
A chromosomal duplication is usually caused by abnormal events during recombination

33. Duplications
Phenotypic consequences of duplications correlated to size & genes involved
Duplications tend to be less detrimental

34. Bar-Eye Phenotype in Drosophila
Ultra-bar (or double-bar) is a trait in which flies have even fewer facets than the bar homozygote
Trait is X-linked and show intermediate dominance

35. Bar-eye Phenotype due to Duplication
Figure: 07-12a
Caption:
(a) Genotypes & phenotypes.

36. Inversions
A segment of chromosome that is flipped relative to that in the homologue
Centromere lies within inverted region
Centromere lies outside inverted region

37. Inversions No loss of genetic information Break point effect
Many inversions have no phenotypic consequences
Break point effect
Inversion break point is within regulatory or structural portion of a gene
Position effect
Gene is repositioned in a way that alters its gene expression
separated from regulatory sequences, placed next to constitutive heterochromatin
~ 2% of the human population carries karyotypically detectable inversions

38. Inversion Heterozygotes
Individuals with one copy of a normal chromosome and one copy of an inverted chromosome
Usually phenotypically normal
Have a high probability of producing gametes that are abnormal in genetic content
Abnormality due to crossing-over within the inversion interval
During meiosis I, homologous chromosomes synapse with each other
For the normal and inversion chromosome to synapse properly, an inversion loop must form
If a cross-over occurs within the inversion loop, highly abnormal chromosomes are produced

39. Crossing Over Within Inversion Interval Generates Unequal Sets of Chromatids

40. Crossing Over Within Inversion Interval Generates Unequal Sets of Chromatids

41. Inversions Prevent Generation of Recombinant Offspring Genotypes
Only parental chromosomes (non-recombinants) will produce normal progeny after fertilization

42. Translocations
When a segment of one chromosome becomes attached to another
In reciprocal translocations two non-homologous chromosomes exchange genetic material
Usually generate so-called balanced translocations
Usually without phenotypic consequences
Although can result in position effect

43. Fig. 8.13b(TE Art) Nonhomologous chromosomes 1 1 7 7 Crossover between
Reciprocal
translocation
Nonhomologous crossover

44. Fig. 8.13a(TE Art) 22 22 Environmental agent 2 2 causes 2 chromosomes
to break. 2 2
Reactive ends
DNA repair enzymes
recognize broken ends
and connect them.
Chromosomal breakage and DNA repair

45. Example: Familial Down Syndrome
In simple translocations the transfer of genetic material occurs in only one direction
These are also called unbalanced translocations
Unbalanced translocations are associated with phenotypic abnormalities or even lethality
Example: Familial Down Syndrome
In this condition, the majority of chromosome 21 is attached to chromosome 14

46. Balanced Translocations and Gamete Production
Individuals carrying balanced translocations have a greater risk of producing gametes with unbalanced combinations of chromosomes
This depends on the segregation pattern during meiosis I
During meiosis I, homologous chromosomes synapse with each other
For the translocated chromosome to synapse properly, a translocation cross must form

48. Meiotic segregation can occur in one of three ways
1. Alternate segregation
Chromosomes on opposite sides of the translocation cross segregate into the same cell
Leads to balanced gametes
Both contain a complete set of genes and are thus viable
2. Adjacent-1 segregation
Adjacent non-homologous chromosomes segregate into the same cell
Leads to unbalanced gametes
Both have duplications and deletions and are thus inviable
3. Adjacent-2 segregation
Adjacent homologous chromosomes segregate into the same cell