3.1 Cell Nucleus and Gene Expression

Question 1

2 Forms of Control of gene expression in prokaryotes

Answer 1

Inducible

Repressible

Question 2

Inducible Control

Answer 2

Example: Lactose

• addition of lactose= increases β-galactosidase
• Minimal conditions- no β-galactosidase expressed

Question 3

Repressible Control

Answer 3

Example: Tryptophan

• Typ is a corepressor.
• if Typ is present, it binds to the repressor and turns it on stopping synthesis of Typ

Question 4

Parts of a Bacterial Operon

Answer 4

• Structural Genes- code for enzymes-genes transcribed as one mRNA
• Promoter-site where RNA Polymerase binds
• Operator- Binding site for repressor protein
• Regulatory gene-encodes repressor protein
• if If repressor is bound it blocks polymerase from binding so no transcription

Question 5

Lac-Operon

Answer 5

1) Inducer (lactose) binds to repressor protein
2) Prevents attachment to operator
3) RNA Polymerase transcribes genes
4) Genes transcribed and translated
5) Lactose degraded by enzymes
6) Fall in lactose concentration permits repressor to bind (operon repressed)

Lac-operon encodes three genes
z-gene encodes b-galactosidase (breaks down lactose)
y-gene encoes galactoside permease-enables entry of lactose into cell
a-gene- encodes thiogalactoside acetyltransferase

Question 6

Trp-Operon

Answer 6

1) Corepressor (tryptophan) binds to repressor protein
2) Repressor binds to operator
3) RNA Polymerase unable to bind
4) Repressor inactive when
tryptophan levels fall
5) Genes are transcribed
6) Enzymes translated
7) Tryptophan synthesized

Typ repressor normally inactive

Question 7

Positive Control by cAMP on Lac Operon

Answer 7

• If Gluc present then bacteria break down glue instead
• Bacteria preferentially catabolize glucose over lactose
• Glucose suppresses production of enzymes for other substrates
• cAMP high when glucose levels are low
• cAMP binds to cAMP receptor protein (CRP)
• cAMP-CRP complex binds to site on lac operon (positive regulator)
• Lactose only catabolized when glucose levels are low
• Binding of CAP to the promoter is needed for lac operon induction

Question 8

What is the nucleolus for?

Answer 8

Ribosome Assembly

Question 9

Movement in the nuclear envelope, what’s going in, what going out?

Answer 9

IN:

• Proteins imported from cytoplasm
• snRNPs imported

OUT:

• mRNAs, tRNAs, ribosome subunits exported
• snRNAs exported

snRNAs move out to associate with proteins and snRNPs are transported back into nucleus

Question 10

Nuclear Lamina

Answer 10

• Mechanical support and site of chromatin attachment

- Assembly/disassembly regulated by phosphorylation levels (cyclin dependent kinases)

Question 11

Hutchinson-Gilford Syndrome

Answer 11

Disease associated with Lamina A gene associated with rapid aging rapidly. syonomous changes, same a.a. Single nucleotide change results in a splicing defect results in protein defect.
lamina has irregular structure to it. causes a truncated protein

Question 12

How are proteins target to be brought into the nucleus?

Answer 12

Nuclear targeted proteins have nuclear localization signals (NLS)

Question 13

Nuclear Pore Complex

Answer 13

• Composed of nucleoporin proteins
• Low molecular weight substances readily pass through pores
• Nonnuclear proteins do not pass through
• based structure extends into nuclear side, filaments into cytoplasm side
• FG repeats are phenoalanone and glycine repeats (they’re little filters in the pore)
• NLS has short stretches of positively charged amino acids

Question 14

Importing Proteins to Nucleus (5 Steps)

Answer 14

Step 1: NLS containing cargo protein binds to a heterodimeric soluble NLS receptor called importin å/ß

Step 2: NlS transport receptor escorts the protein cargo to the outer surface of the nucleus where it docks with the cytoplasmic filaments that extend from the outer ring of the NPC

Step 3: Filaments bend toward the nucleus delivering the receptor-cargo complex to specific binding sites on the NPC. The protein is bound by the fillaments. The filaments weave a round and the protein is lodged into the nuclear pore

4) Ran-GTP binds to Importin/NLS and causes disassembly-Imported cargo released
5) Part of NLS receptor (importin β) shuttled back to cytoplasm with Ran-GTP . Ran-GTP hydrolyzed and released (Ran-GDP) and released

Question 15

Chromosome Structure

Answer 15

• Chromosomes composed of DNA and protein (chromatin)
• Histones-small basic proteins (positivly charged proteins that bind the negatively charged DNA backbone, rich in lysine and arginine)
• Non-histone proteins-diverse size and function
• Proteins aid in packing of DNA

Question 16

Hierarchy of Packing

Answer 16

• Beads on a String
• 30 nm fiber-interaction
• Chromatin Loops
• Mitotic chromo

Question 17

1st Level of Packing:

Answer 17

Beads on a String:

• nucleosome core
• has 8 total protein that makes the core, then also H1 which is a linker protein since it links it together.
• 8 proteins with DNA wrapped around it (about 2 wraps around it)
• Octamer comprised of 2 H2A, 2 H2B, 2 H3, 2 H4 proteins
• 2 dimers H3/H4 dimerize together, H2A/H2B dimerize together so 2 of each
• H2A you can see the C terminal and the N tail, everyone else you just see the N tail

Question 18

Characteristics of Histone Proteins

Answer 18

Histones are highly concerved. Change over time is very slow.
Histome don’t aquire mutations at a fast rate: extremly slow evolving bc thery’re really conserved since they’re really important for packing so can’t tolerate any changes to their structure
-Each core histone consists of a globular “histone fold” region composed of three alpha helices and a flexible extended N-terminal tail