Lecture 4 - Nuclear Structure and Function

Question 1

What are the 5 main functions of the nucleus?

Answer 1

1. Protect chromosomes
2. Repair chromosomes
3. Facilitate expression of genes
4. Construct ribosomes

Question 2

Describe the basic structure of the nucleus…

Answer 2

Nuclear envelope (double membrane) punctuated by nuclear pores surrounded by intermediate filaments. Chromosomes spread inside of nucleus with nucleolus (non membrane bound sub-compartment) present.

Question 3

Describe the nuclear envelope

Answer 3

Double membrane structure that is continuous with the ER

Question 4

Describe the function of the nuclear lamina

Answer 4

Support the nuclear envelope, organize chromosomes, and regulate nuclear breakdown at mitosis.

Question 5

What are nuclear pores?

Answer 5

Semi-selective gates through which proteins and ribonucleoprotein complexes are moved into and out of the nucleus.

Question 6

Describe the general structure of the nuclear pores

Answer 6

Question 7

How is nuclear transport facilitated?

Answer 7

A intranuclear Ran GTP vs cytosol Ran GDP gradient exists which provides the energy for transport.

Question 8

What is a nuclear localization signal?

Answer 8

Large proteins being imported into the nucleus requires a NLS - contiguous sequence of 4-8 positively charge amino acids (lysine and/or arginine).

Question 9

How does nuclear import work?

Answer 9

The protein’s NLS combines with an importin carrier protein and directs it through the nuclear pore. Ran GTP then binds to the importin, which releases the cargo, and returns the importin to the cytosol. Through GRP hydrolysis, the importin is free to bind more cargo.

Question 10

Describe how RNA export works.

Answer 10

The substrate is recognized by the carrier protein (exportin) and Ran-GTP, and released outside of the nucleus when Ran-GTP undergoes hydrolysis.

Question 11

Describe the main structure and function of the nucleolus.

Answer 11

Non membrane bound, sub compartment of the nucleus whose primary function is ribosome biogenesis. Formed on rDNA repeats that are on 5 different chromosomes.

Question 12

Recognize and describe the 3 main regions of the nucleolus.

Answer 12

1. Fibrillar center - contains DNA that is not being transcribed.
2. Dense Fibrillar Component - this region contains rRNA molecules actively being synthesized (many from the same DNA strand at once).
3. Granular component - contains maturing ribosomal precursor particles.

Question 13

Describe ribosome biogenesis.

Answer 13

45S rRNA genes are transcribed in the nucleolus. Ribosomal proteins and processing factors assemble along with 5S rRNA. 45S rRNA then processed by nucleases to create 18S rRNA of the small ribosomal subunit, and the 5.8S rRNA and 28S rRNA of the large subunit. They are then transferred into the cytoplasm for final assembly.

Question 14

Replication origins are what….

Answer 14

Required for, and are the sites of, the initiation of DNA replication - there are many spread over the length of chromosomes

Question 15

What is a centromere?

Answer 15

Serves to attach the chromosome to the mitotic apparatus during cell division via kinectechores.

Question 16

Describe telomeres!

Answer 16

Required to stabilize the chromosome ends against the shortening of DNA replication. Keep actual genetic info from being cut during replication. Telomerase functions to extend the length of the telomeres.

Question 17

What percent of the human genome codes is expressed?

Answer 17

95%

Question 18

What are 3 examples of repetitive junk DNA?

Answer 18

1. Alu sequences - 5% of the genome, contain recognition sequence for restriction enzyme AluI
2. Transposons - mobile DNA elements
3. Pseudogenes - copies of true genes, became non-functional

Question 19

Describe the process from transcription to translation…

Answer 19

Question 20

What is chromatin?

Answer 20

The sum of chromosomal DNA and its associated proteins

Question 21

What subunits comprise the nucleosome?

Answer 21

Histones H2A, H2B, H3, and H4

Question 22

How many bp wrap around a nucleosome? (how many with linker DNA?)

Answer 22

146 bp, (200 bp)

Question 23

How is the 30-nm fiber constructed?

Answer 23

Histone H1 subunits connect to coil the DNA wrapped nucleosomes in a 6 member coil.

Question 24

Describe changes to histone proteins that might affect DNA transcription.

Answer 24

Methylation and acetylation of the histone N-terminal domains can affect expression of genes.

Question 25

What is the function of the nuclear matrix?

Answer 25

Mostly made of protein, it is believed to provide structure and organization to the nucleus - gives distinct territories to the nucleus. Maxtrix-associated regions are where the chromosomes attach to the nuclear matrix.

Question 26

What happens in the nucleus at mitosis?

Answer 26

All transcriptional activity stops, chromosomes condense (nucleolus goes away), lamin proteins phosphorylated and break apart.

Question 27

What are examples of diseases arising from defects in the nucleus?

Answer 27

Systemic lupus erythematosus - caused by production of antibodies to self-antigens in the nucleus

Acute promyelocytic leukemia - caused by mutation of nuclear protein PML (creates subcompartment)

Spinal muscular atrophy - mutations in protein SMN, involved in RNA splicing.