Genomes

Question 1

what is a genome

Answer 1

complete haploid set of genetic material of an organism

Question 2

what is the size of the human genome

Answer 2

3.2 x 10⁹ base pairs

Question 3

chromosome consist of?

Answer 3

• single, enormously long linear DNA molecule
• proteins involved in DNA packaging e.g. histones

Question 4

What is chromatin?

Answer 4

a complex of DNA + tightly bound proteins

Question 5

Define a gene

Answer 5

• Most genes: segment of DNA that codes for a particular protein (or a family of related proteins)
• some genes (RNA genes): codes for functionally significant RNA molecules as final product

Question 6

What are introns and exons?

Answer 6

• sections within genes
• introns are long streches of non-coding DNA that interuptes relatively short exons (coding DNA)
• introns spliced out after transcription in mRNA processing

Question 7

What is C-value

Answer 7

C-value is the amount, in picograms, of DNA contained within a haploid nucleus

Question 8

What is the C-value paradox?

Answer 8

• lack of relationship between the complexity of organism and genome size
• closely related species, with similar number of genes can differ hundredfold in genome size
• especially prominent in eukaryotes
• prokaryotes have concise genome

Question 9

What caused the C-value paradox?

Answer 9

• Large amount of “junk DNA” (non-coding DNA)
• Some non-coding DNA has regulatory functions
  
  • important for multicellular organisms, they need to turn genes on and off throughout development

Question 10

How does junk DNA persist in the eukaryotic genome?

Answer 10

• most junk DNA has no clear function
  
  • not under strong selection pressure
  • not easily lost
• junk DNA enhances their own transmission by e.g. over-replication
• have histones to turn genes off

Question 11

Why do prokaryotes have concise genome?

Answer 11

• need for constant DNA replication => selcection pressure for efficiency/speed in replication process
• don’t have histones to switch genes off => more prone to deleterious effects of parasitic sequences => selection pressure against junk DNA

Question 12

What are LINEs and SINEs?

Question 13

Why does DNA need packaging

Answer 13

• 2m of DNA need to fit in 5-6µm nucleus

Question 14

Why is DNA packaging dynamic

Answer 14

• need to be unpackaged for the access of enzymes to carry out:
  
  • gene expression
  • DNA replication
  • DNA repair
• require specialised proteins to package DNA

Question 15

How is DNA packaged?

Answer 15

• DNA is wrapped around histones to form nucleosomes
  
  • converts a DNA molecule into a chromatin thread about one-third of its initial length
  • 11nm in diameter
• nucleosomes further packaged, forming 30nm chromatin fibre
  
  • fifth histone (H1) pulls the nucleosomes into a regular repeating array
• 30nm fibre undergoes more packaging by unknown processes
• end product is approx. condensed 10000 times

Question 16

structure of nucleosomes

Answer 16

• complex of 8 histone proteins assemble to form a protein core which DNA is wrapped around about 2 times
  
  • histone proteins: two molecules each of histones H2A, H2B, H3, and H4

Question 17

How does the packing state of interphase chromosome differ between different regions?

Answer 17

• genes that are being expressed => more extended
• quiescent genes => more compact

Question 18

in what way is access to DNA allowed?

Answer 18

• typically localised:
  
  • only regions where the cell needs to access are exposed

Question 19

How does cell unpackage DNA?

Answer 19

• chromatin remodelling complexes
  
  • energy: ATP hydrolysis
  • changes structure of neucleosomes
  • make DNA accessible to proteins (especially ones involved in gene expression, replication and repair)