Nucleus

Question 1

Nucleus

Answer 1

Question 2

Nuclear Staining Varies Greatly Between Cell Types:

Some Nuclei Stain Pale and Have Uncondensed DNA -

Some Nuclei Stain Dark and Have Condensed DNA-

Answer 2

Euchromatin (dispersed and transcriptionally active DNA)

Heterochromatin (condensed and transcriptionally inactive DNA)

Cells with abundant heterochromatin are transcribing few genes

Question 3

Why Do Nuclei Vary So Much?

Answer 3

• Variations in nuclear function and framework
• Variations in gene transcription

Question 4

• Complex of proteins + rRNAs that forms around the genes for ribosomal RNA
• Site of assembly of ribosomal subunits

Answer 4

Nucleolus

Cells with prominent or multiple nucleoli produce larger amounts or ribosomes

Question 5

DNA and Associated Proteins

Answer 5

Chromatin

Question 6

DNA in Chromatin percentages

Answer 6

• About 1% of DNA in chromatin actually codes for proteins (20,000 genes)
• About 3% of DNA is “satellite” DNA (14-500 base sequences repeated 1000’s of times)
• About 45% of DNA is formed from “moderately repeated” sequences of several thousand bases (transposons)

Question 7

Allows Specific Labeling of All Chromosomes

Answer 7

In Situ Hybridization of Chromosome-Specific Repeated Sequences

Question 8

• Performed on squashed cells that are halted in metaphase by microtubule-destabilizing drugs
• Used to assess chromosomal number, banding patterns, and possible chromosomal breakages

Answer 8

Karyotype Analysis

Question 9

Occupy discrete territories within the nucleus

Answer 9

Interphase Chromosomes

Question 10

In the Cell Cycle, 2 things can happen to chromosomal DNA:

Answer 10

Duplication and Separation

Question 11

DNA Synthesis is Initiated When

Answer 11

a Kinase (Cyclin Dependent Kinase) Phosphorylates Rb

Question 12

Cyclins and Cyclin-dependent Kinases

Answer 12

• 11 Cd kinases are known, but only 4 seem important for the cell cycle
• All 4 Cdk’s (Cdk1,2,4 & 6) can inhibit Rb and initiate DNA synthesis
• Cdk2 phosphorylates Lamins and causes the disappearance of the nuclear envelope during mitosis
• All Cdk’s must bind a protein, cyclin, to be active

Question 13

Mitosis

Answer 13

• Prophase (condensation)
• Prometaphase (nuclear envelope breaks down)
• Metaphase (chromosomes line up & attach to spindle)
• Anaphase (chromosomes are pulled apart)
• Telophase

Question 14

Prophase

Answer 14

• All chromosomes become greatly condensed
• Chromosome 22 shrinks in length from 1.5 centimeters to about 2 micrometers (10,000 fold decrease in length)
• Centrioles are duplicated and the mitotic spindle of microtubules is assembled

Question 15

Metaphase

Answer 15

• Nuclear envelope breaks down, permitting attachment of microtubules to the kinetochore (a complex of proteins attached to each chromosome at the centromere)
• Chromosomes line up in the middle of the mitotic spindle

Question 16

During Metaphase, ________ is released from sister chromatids except at the ________

Answer 16

cohesin; kinetochore

Question 17

Anatomy of Kinetochore

Answer 17

Question 18

Anaphase

Answer 18

• The so-called Anaphase Promoting Complex (APC), attached to the kinetochore, degrades cohesin
• Chromatids move apart
• APC also degrades cyclins, so that cyclin-dependent kinases stop functioning and mitosis completes itself

Question 19

As chromosomes move apart along kinetochore microtubules, so called _______ repel each other and push the two spindles apart

Answer 19

polar microtubules

Question 20

Mitosis vs. Meiosis

Answer 20

Unique features of 1st Meiotic Division:

• Pairing of homologous chromosomes
• Crossing over during 1st divison
• 2nd Division
• Haploid cells

Question 21

How do homologous chromosomes pair so precisely?

Answer 21

Chromosome tips attach to nuclear envelope during prophase 1 so that homologous pairs are aligned perfectly

Question 22

How do homologous chromosomes “recognize” each other?

Answer 22

• Uncharacterized proteins prevent improper pairing?
• Or Epigenetic Marks?

Question 23

What holds homologous chromosomes together?

Answer 23

Formation of Synaptonemal Complex (SCP1 protein)

Question 24

Proteins of Synaptonemal Complex

Answer 24

• Synaptonemal Complex Protein 1 Binds adjacent DNA strands to each other
• RAD51 recombinase protein causes cutting and splicing of DNA during Crossing Over
  
  • (Also found in lymphoid organs)

Question 25

Protein that is localized between paired chromosomes

Answer 25

SCP1

Question 26

What drives the switch from mitosis to meiosis?

Answer 26

Retinoic acid appears to be a key player in this process

Question 27

Gene Transcription Varies Between Cell Types

Answer 27

Question 28

Control of Gene Transcription

Answer 28

* Each gene is marked by a specific sequence called the TATA box * The TATA box binds proteins needed for DNA transcription * All of these TATA binding proteins are only weakly active * To increase their activity, a Transcription Regulating Factor is required * 2000 different TRF’s are known; each cell produces only its own subset, leading to a unique pattern of DNA transcription for each cell

Question 29

Control **Nanog** gene to maintain pluripotency of stem cells

Answer 29

OCT4 & SOX2 Transcription Regulating Factors

Question 30

Transcription Factors Drive Cell Differentiation

Answer 30

Question 31

X-Chromosome Inactivation

Answer 31

* In each female cell, one X-chromosome is randomly inactivated and condensed * In many cells, the X-chromosome becomes attached to the nucleolus In some cells (white blood cells) the X-chromosome appears as a dark-staining Barr Body within the nucleus

Question 32

Lamin Proteins

Answer 32

Question 33

Lamin Proteins

Answer 33

* Intranuclear varieties of intermediate filaments * Form the so-called nuclear lamina * Lamin A, B, and C subtypes are known * Interact with DNA, Transcription Regulating Factors (Rb protein), and the inner nuclear envelope to regulate nuclear shape and function

Question 34

Inherited abnormalities in Lamin A protein cause a syndrome, \_\_\_\_\_\_\_, with symptoms of \_\_\_\_\_\_\_\_

Answer 34

Progeria; accelerated aging Symptoms include: weak bones, facial abnormalities, and distorted nuclear shapes; Disease is very rare

Question 35

Abnormal Lamin A Protein Becomes Inapproperiately \_\_\_\_\_\_\_\_

Answer 35

Farnesylated

Question 36

Abnormally farneslated Lamin-A interacts abnormally with a major depressor of transcription called the \_\_\_\_\_\_\_\_\_\_\_

Answer 36

retinoblastoma (Rb) Protein Drugs that block the farnesylation of lamin-A relieve many of the symptoms of progeria in mice

Question 37

A Typical Cell Nucleus Possesses \_\_\_\_\_\_\_\_\_Nuclear Pores

Answer 37

3000-4000

Question 38

Pores are composed of

Answer 38

Nucleoporin proteins

Question 39

Nucleoporin Proteins

Answer 39

* 456 individual protein molecules are found in each pore; there are about 30 varieties of nucleoporins * Function to bind and import proteins into the nucleus. Examples: * DNA, RNA polymerases * Transcription factors, histones * Nuclear proteins contain a sequence of basic amino acids termed a Nuclear Localization Sequence

Question 40

Nucleoporin Proteins Function:

Answer 40

* Function to transport out of the nucleus proteins that bind to mRNA molecules * Function to export ribosomal subunits, which are assembled within the nucleus