18/ chromosomes structure

Question 1

define karyotype

Answer 1

organised representation of all chromosomes in a eukaryotic cell at metaphase

Question 2

how are individual chromosomes arranged even in interphase nuclei

Answer 2

distinct subnuclear territories

Question 3

under electron microscope, interphase chromatin resembles what

Answer 3

• beads on string
• beads are nucleosomes

Question 4

structure of nucleosome diagram

Answer 4

• core histones - proteins around which dna is wound, like cotton on a bobbin
• 8 core histone subunits (H2A, H2A, H2B, H2B, H3, H3, H4, H4)
• n terminal tails of core histones project out and are free to interact w other proteins
• facilitates reg of chromatin structure and function

Question 5

what do linker histones (example) do

Answer 5

• strap dna onto histone octamers and limit movement of dna relative to the histone octamer
• stabalises formation of 30nm fibre
• facilitates establishment of transcriptionally silent heterochromatin
• positive, can bind to negative dna. binds off charge not sequence

Question 6

structure of 30nm chromatin fibre

Answer 6

• supercoiled array of nucleosomes
• felxible scaffold
• can be re modelled to accommodate protein (sequence spec) complexes involved in gene transcription and dna replication
• mediated by histones

Question 7

interphase chromatin movement

Answer 7

• dynamic fractal globules (globules within globules)
• can reversibly condense and decondense w/o getting knotted

Question 8

the nuclear periphery vs middle of interphase cells are composed of what

Answer 8

• periphery - transcriptionally inactive dna (protect genes inside)
• rna transcripts in the middle

Question 9

what are telomeres

Answer 9

• specialised repetitive dna sequences at chromosome ends
• replicated by a dna polymerase called telomerase
• define chromosome ends and maintain chromosome integrity
• single stranded 3’ overhanging TTAGGG repeat arrays. can be several hundred nucleotides long

Question 10

function of alpha satellite dna repeats in centromeres

Answer 10

• readily form condensed chromatin w histone octamers containing unusual subunits
• this interacts w spec histones and the kinetochore

Question 11

kinetochore binding

Answer 11

• inner plate proteins bind to chromatin containing alpha-satellite dna
• kinetochore outer plate proteins bind to protein components of mitotic spindle like microtubule

Question 12

kinetochore in yeeast

Answer 12

• kinetochore is a basket that links a single nucleosome of centromeric chromatin to a single microtubule

Question 13

what % of our genome is accounted for by genes

Answer 13

20

Question 14

what % of our genome encodes info for making cellular proteins

Answer 14

1.5

Question 15

how do you inc biological complexity

Answer 15

• inc number of protein coding genes
• inc number of regukatory genes - when and where proteins are expressed

Question 16

what is cis-regulatory dna

Answer 16

• non protein coding
• determines when and where in the body adjacent protein coding genes are transcribed

Question 17

50% of the human genome is made up of what, what is most of this?

Answer 17

• repeated dna sequence elements
• copies of retrotransposons - ‘parasitic dna’. integrated from viruses
• can jump around genome
• good for genetic diversity but inc risk of mutation. no particular function

Question 18

3 types of transposons, function

Answer 18

• dna transponsons
• retroviral retrotransponsons
• non-retroviral polyA retrotransponsons
• not really! defective generally. relics of formally functional elements

Question 19

how do dna transponsons move

Answer 19

• cut and pase mechanism
• without self duplication
• enzyme transposase required, recognises short inverted repeat sequences

Question 20

example of dna transponson

Answer 20

• p element in fly
• activator-dissociator in maize
• Tn3/Tn10 in e coli

Question 21

how do retrovial retrotransponsons work

Answer 21

• behave like retroviruses like hiv
• replicate via rna intermediates
• produce new dna copies that integrate at new genomic locations
• using self encoded reverse transcriptase

Question 22

how do non retroviral poly A retrotransposons work

Answer 22

• abundant in vertebrate genomes
• replicates via rna intermediate
• using own retrotransponson - encoded reverse transcriptase which binds to poly a tail (?)
• copy and paste

Question 23

examples of non retroviral polyA retrotransponsons

Answer 23

• human LINE1 elements. some of which cause human diseases like haemophilia. these products are able to be directly integrated into new location w/o being packaged into virus like particle
• human Alu elements. SINE
• mouse B1 elements. SINE

Question 24

LINEs and SINEs, nonretroviral polyA retrotransponsons

Answer 24

• LINEs - long interspersed elements. self sufficient, encode proteins for their own movement
• SINEs - short …. , need LINEs

Question 25

Common ancestor of human Alu and mouse B1

Answer 25

7SL RNA gene in common ancestor