Lecture 2 Flashcards
What is the difference between haploid and diploid numbers?
The haploid number is how many chromosomes are in a single set, however as most organisms have two sets the haploid number is doubled to give the diploid number or the full complement of chromosomes in an organism
What groups of organisms are typically haploid as opposed to the more common diploid?
Fungi and Algae
What is significant about the variation of chromosome numbers between species?
It is highly variable between species however it does not reflect evolutionary complexity
What is the significance of the number of chromosomes a species has?
Proportional to the number of gametes that are able to be generated due to reassortment (although an upper limit is seen as if the chromosome number is too large viable gametes can not be produced)
Not related to the total genome size of the organism
What is the range of chromosome size in humans?
A three to four fold range with chromosome 1 being he largest and 21 being the smallest
What is a karyotype?
Morphology of all the chromosomes at mitotic metaphase
What are the different morphologies a chromosome can take?
Metacentric (1:1 arm length ratio, V shaped)
Telocentric (1:0 arm length ratio, I shaped)
Acrocentric (1:3 arm length ratio, J shaped)
What are the key structural features of a eukaryotic chromosome
A telomere at both ends of the chromosome, a centromere which may connect to a sister chromosome if on a duplicated chromosome, kinetochores which connect the spindle microtubules to centromere
A nucleolar organiser maybe present
What are telomeres?
Specialised regions at the end of chromosomes which contain tandemly repeated arrays of simple DNA sequences which are not distinctly visible through microscopy unless probes have been used
What is the nucleolar Organiser?
It is a secondary constriction in a chromosome seen in all functional sets of chromosomes but not in all individual chromosomes
Is an organelle which codes for ribosomal RNA
Located in different positions in chromosomes but this position is conserved within species
What is chromatin and what are its different forms?
A complex of DNA and protein, can be present as loosely packed euchromatin which is transcriptionally active, or densely packed heterochromatin which may be constitutive (permanent and typically gene poor) or facultative (which can be turned on and off and is a common feature of sex chromosomes)
What are chromosome banding patterns?
Patterns that are formed due to different chromosome staining techniques, the position and size of the stripes formed are consistent between specific chromosomes and species
Giemsa is a commonly used stain which causes more densely packed regions to stain dark
What are polytene chromosomes?
Giant Chromosomes found only in some organisms such as drosophila
Formed due to repeated rounds of DNA replication without cell division
Only the euchromatin portion of the genome becomes polytene with the heterochromatin forms a densely packed chromocentre
Has a distinct advantage in that it will form visible puffs when transcription is occuring
What are the ‘orders’ of chromosome packing?
DNA is wrapped around a histone to form a nucleosome
This nucleosome is then placed into a series of coils to form a solenoid
The solenoids are then mounted on a proteinaceous scaffold to form a chromosome
What is the structure of the histone?
Octomer of symmetrically arranged H2A,H2B,H3,H4 which results in a helical groove with positive charge which attracts the negatively charged DNA which is then secured with an H1 linker protein
