The nucleus

Question 1

Why do we have a nucleus in cells?

Answer 1

-separated transcription + translation
-can modify mRNA before translation
-can translate mRNA immediately, later or not at all
-enables alternative splicing (remove introns)

Question 2

What is the first hypothesis as to where the nucleus comes from?

Answer 2

-ancient prokaryotic cell
-membrane moves in around DNA
-results in encapsulation of DNA - formation of primitive nucleus

Question 3

What is the second hypothesis as to where the nucleus comes from?

Answer 3

-different kinds of cell were in the same environment
-one cell engulfs the other - engulfed cell became nucleus

Question 4

What evidence is there to support the second hypothesis as to where the nucleus comes from?

Answer 4

-compare all yeast genes with genes from archaea + bacteria
-use homology hit analysis - compare genes to see if they’re related
-yeast genes more similar to bacterial mitochondrial genes than archaea + more similar to archaea nuclear genes than bacterial
-suggests bacterial cell was engulfed + became mitochondria
-suggests archaea cell was engulfed + became nucleus

Question 5

What is found within the nucleus?

Answer 5

-DNA in nucleus packaged into chromosomes
-DNA wrapped around histones for effective packaging

Question 6

What is packing essential for?

Answer 6

Ensure the entire genome is within a single nucleus

Question 7

In non-dividing cells, how is DNA packed?

Answer 7

Loosely packed forming a tangle of strands - chromatin

Question 8

What technique reveals additional chromatin organisation?

Answer 8

EM staining

Question 9

What is heterochromatin?

Answer 9

Dense staining of interphase DNA

Question 10

What is euchromatin?

Answer 10

Less dense staining of interphase DNA

Question 11

What is the nucleolus?

Answer 11

Highly dense staining of RNA

Question 12

What are territories and how can they be identified?

Answer 12

-chromosomes occupy specific territories within nucleus
-identified by chromosomal painting

Question 13

Are territories inherited and can they be changed?

Answer 13

-they’re inherited
-can change following differentiation or disease

Question 14

What does the location of a gene within the nucleus change depending on?

Answer 14

Its transcriptional status

Question 15

What happens in the nucleolus?

Answer 15

-processes ribosomal RNA to produce ribosome
-processes mRNA tRNA

Question 16

What happens in speckles?

Answer 16

pre-mRNA processing

Question 17

What happens in cajal bodies and PML bodies?

Answer 17

-cajal = splicing
-PML = storage depot

Question 18

Describe the structure of the nucleolus

Answer 18

-a collection of macromolecules
-not membrane bound

Question 19

Describe the structure of the nuclear envelope

Answer 19

-double unit membrane perforated with pores
-supported by a fibrous meshwork called the lamina
-lamina partly responsible for ensuring the asymmetric nature of double unit membrane

Question 20

What do the nuclear pores do?

Answer 20

Control nuclear access in a size-dependent manner

Question 21

What molecules can’t enter the nucleus via nuclear pores and how do they enter instead?

Answer 21

-particles with MW>50,000 can’t enter by diffusion
-can by active signal-dependent transport