Lec 2: Structure of the chromosome Flashcards
What are the names of the 3 positions of the centromere?
Telocentric - at the end of the chromosome
Metacentric - in the middle of the chromosome
Acrocentric - between the end and the middle of the chromosome
Polytene chromosomes are found where and how are they identified?
- In the salivary glands of many flies
- They show transverse banding in staining, allowing chromosomes to be distinguished
Polytene chromosomes are … DNA?
Euchromatic
With a large haploid number…
More variation is generated by independent assortment in gamete formation
Constitutive heterochromatin is…
- A permanent feature of specific chromosome location
- Poor in genes
- Can inactivate genes
Facultative heterochromatin is…
- Sometimes found in particular locations
- Normal gene content
- Can be switched on/off
- Characteristic of sex chromosomes
The number of chromosome pairs is not related to …
- Genome size
- Complexity
Fungi and Algae are usually…
Haploid
Plants and animals are usually…
Diploid
The karyotype is the morphology of all the chromosomes at which stage of the cell cycle?
Mitotic metaphase
How can telomeres be seen?
By hybridising fluorescent labels to them, as they are generally not visible with staining techniques
What ia the nuclear organiser region and where is it found?
- Depends on the species
- Its genes encode for rRNA
- Composed of tandem repeats
- Sometimes not visible, and is on AT LEAST one chromosome
What do nuclear organiser regions look like?
Constrictions, called the secondary constriction
Which is the smallest chromosome in human DNA?
chromosome 21
What are nucleoli?
They are organelles in the nucleus that contain rRNA and ribosome components
What is Euchromatin?
Loosely packed form of chromatin
Rich in genes
High recombination frequencies
Stains weakly at interphase
What is Heterochromatin?
Tightly packed/Coiled form of chromatin
Fewer genes
Lower recombination frequencies
Stains densely at interphase
What is a common DNA stain?
Giemsa
- used after mild proteolytic digestions of the chromosomes
- produces G-light and G-dark regions
- difference originally thought to be GC content of DNA, howveer seems to depend on chromatin packing density
What are polytene chromosomes?
- large chromosomes
- arise from repeated rounds of DNA replication without cell division
- formed of only the euchromatic portion of DNA
Why is chromosome packing necessary?
- Because of the large physical size of the genome
- So it can be moved around during cell division
What are nucleosomes?
DNA wrapped twice around histone protein cores made of:
2 of each H2A, H2B, H3, H4
- Positively charged amino acids in histones interact with negative charges on phosphates to cause tight association
What is a solenoid?
- 6 Histone octamers with 6 H1 histones (stabilising) per turn
- Organised coils
- 30nm
- Formed at high salt concentrations
