Cell Structure

Question 1

What are the two major intracellular compartments?

Answer 1

The nucleus with its subdomain nucleolus.

Cytoplasm with two subdomains

Cytsol: cytoplasm outside the organells containing enzymes, cytoskeletal elements and FREE POLYSOMES.

Membrane bound organelles: RER, Golgi, mitochondria, and lysosomes.

Question 2

Describe the four major structural features of the nucleus.

Answer 2

Nuclear envelope, nuclear pore, nucleolus and nuclear lamina.

Nuclear envelope. Double membrane, is continuous with the rough ER and outer membrane has ribosomes attached to it.

Nuclear pore: allows the bidirectional transport.
mRNA, ribosome subunits, tRNA LEAVE the nucleus
proteins used in gene regulation (histones, polymerases, transcription factors) ENTER the nucleus, made in cytoplasm.

Nucleolus - site of rRNA synthesis and assembly of ribosome subunits.

Nuclear lamina: intermediate filament under the nuclear membrane which provides structural support. Phosphorylation destablizes the lamins, breaking down the nuclear envelope in mitosis.

Question 3

What is the composition of the cytoplasm?

Answer 3

Several types of proteins: actin-containing filaments called microfilaments

tubulin containing structures called microtubules

intermediate filaments similar to lamina.

Metabolic products like lipid droplets and glycogen (polymer of glucose) are not membrane bound and exist as inclusions.

Many ribosomes are found associated with other ribosomes called free polysomes.

Question 4

Secretory pathway vs non

Answer 4

Ribosomes attached to ER (RER) will produce proteins that enter the secretory pathway.

Free polysomes synthesize proteins used in the cell and are not secreted. They can enter the nucleus, peroxisomes, mitochondria. And proteins that regulate replication (polymerases, TF and ribosome) are made on free ribosomes.

Question 5

Describe the pathway for a ribosomal proteins made on free ribosomes?

Answer 5

1. mRNA for ribosomal proteins are transcribed from DNA in nucleus then through nuclear pores across nuclear envelop to cytoplasm.
2. Ribosomal proteins are synthesized on free ribosomes then go through the pore and enter the nucleus.
3. rRNA is transcribed in nucleolus and prcessed and rRNA sequences associate with ribosomal proteins to create the 60S and 40S ribosomal subunits.
4. These subunits leave the nucleus through the pore and assemble into mature ribosomes in the presence of mRNA for translation.

Question 6

IF nuclear transport was completely blocked, what would happen? (3 things)

Answer 6

mRNA exit from nucleus

assembly of ribosomal subunits (needs ribosomal proteins)

Appearance of new ribosomal subunits in the cytoplasm