Week 9 - Nucleus and Nucleolus

Question 1

What is a histone?

Answer 1

A protein that binds to DNA to form eukaryotic chromosomes

Question 2

What is a nucelosome?

Answer 2

A protein - DNA complex containing 8 histone proteins (it is the most basic level of chromosome packing)

Question 3

What is a Chromatin?

Answer 3

Repeating nucleosome structure, like beads on a necklace, also bound to none-histone proteins

Question 4

What are chromosomes?

Answer 4

Defined region of chromatin that carries information relating to specific set of genes ( 1/3 DNA, 2/3 protein)

Question 5

What are sister chromatids?

Answer 5

identical copies of the same chromosome, metaphase of mitosis

Question 6

What is the process called when you go from DNA to MRNA?

Answer 6

Transcription - occurs in the nucleus, requires specific enzymes, called RNA polymerase

Question 7

What is the process called when you go from mRNA to proteins?

Answer 7

Translation - occurs on the ribosomes in the cytosol, specific amino acids are matxhed to the mRNA sequence to form a polypeptide

Question 8

What is the nucleus and its characteristics?

Answer 8

The nucleus is the biggest single organelle within eukaryotic cells, containing the genetic material (DNA), packages as a chromatin, and is the site of RNA synthesis ( transcription) , also contains an area termed the nucleolus

Question 9

What is the nuclear envelope?

Answer 9

A double membrane that surrounds the nucleus. Its outer membrane is continuous with the endoplasmic reticulum of the cell. It controls the entry and exits of materials in and out of the nucleus and contains the reactions taking place within it.

Question 10

What are nuclear pores?

Answer 10

Allow the passage of large molecules in and out of the cell

Question 11

What is the structure of the nuclear pore complex?

Answer 11

• large quaternary protein structure
• aqueous channel that connects the nuceloplasm to the cytosol
• very ordered 8 fold symmetry
• made up of nucleophorins

Question 12

What is the nucleoplasm?

Answer 12

The jelly-like material that makes up the bulk of the nucleus

Question 13

What is the definition of genetic material?

Answer 13

Consists of protein-bound, linear DNA (deoxyribonucleic acid)

Question 14

What is the nucleolus?

Answer 14

A small region within the cytoplasm which maufactures ribosomal RNA and assembles the ribosomes (process called ribosomal biogenesis). The nucleolus is NOT surrounded by a membrane.

Question 15

What is the nuclear lamina?

Answer 15

A network of laminate intermeidate filaments which form a cage-like structure around the genetic material (protection)

Question 16

What protein sizes can freely diffuse through the NPC?

Answer 16

< 5000 DA
proteins up to 60 KDa but takes longer

Question 17

What size proteins cannot diffuse through NPC?

Answer 17

> 60 KDa CANNOT diffuse
require a specialised mechanism and ENERGY

Question 18

What is GTP?

Answer 18

Guanosine Triphosphate

Question 18

What are Nuclear Import Receptors?

Answer 18

• They recognise specific Nuclear Locialisation Signals
• Bind to FG repeats on nucleoporins in the cytosolic domain of NPC
• Proteins move through the NPC binding to FG repeats

Question 19

Explain GTP binding protein

Answer 19

Protein that binds GTP and also acts as a GTPase (hydrolyses GTP to GDP)

Question 20

Explain GAP

Answer 20

GTPase activating protein. Binds to GTP binding protein and induces GTP hydrolysis

Question 21

Explain GEF

Answer 21

Guanine nucleotide exchange factor. Binds to GTP-binding protein and causes it to release GDP

Question 22

What are the 2 types of Chromatin?

Answer 22

1 - Heterochromatin = transcriptionally inactive
2 - Euchromatin = transcriptionally active

Question 23

What are ribosomes the site of?

Answer 23

Protein synthesis - mRNA translation

Question 24

What does the unit S stand for?

Answer 24

Svedberg unit - non linear, influenced by mass, density and shape
It is how quickly an organelle sinks to the bottom of a tube if centrifuged

Question 25

What subunits make up a ribosome?

Answer 25

• large subunit - 60S (5S + 5.8S + 28S rRNA + 49 proteins)
• small subunit - 40S (18S rRNA + 33 proteins)
• all together = 80S as it is non-linear

Question 26

What are the characteristics of rRNA genes?

Answer 26

• 4 rRNA’s are encoded by more then 200 rRNA genes split across 6 chromosomes
• rRNA genes are clustered together on a DNA sequence called TANDEM REPEATS (this speeds up transcription - as it is more efficient)

Question 27

What are 18S, 5.8S and 28S rRNA’s transcribed by?

Answer 27

RNA polymeriase 1 in the nucleolus

Question 28

What is 5S rRNA transcribed by?

Answer 28

RNA polymerase 3 in the nucleoplasm and then transported to the nucleolus

Question 29

What do snoRNP’s do?

Answer 29

• Small nucleolar ribonucleoproteins are small nucleolar RNAs complexed with proteins
• complexed with structural proteins and enzymes which modify the 90S ‘pre-ribosome’
• recognise specific areas of rRNA that require modification

Question 30

What are the 3 modifications snoRNPs are responsible for?

Answer 30

1 - Methylation
2 - Isomerisation
3 - Cleavage - 45S pre-ribosomal RNA modification

Question 31

What are the Cleavage enzymes called and their function?

Answer 31

1 - Exoribonucleases - remove externally transcribed spacers
2 - Endoribonucease - removed internally transcribed spacers