Nucleus and cell division

Question 1

Which animals have anucleate RBCs?

Answer 1

Mammalian RBCs are anucleate. Avian and amphibian are not

Question 2

Which cells do not have a nucleus?

Answer 2

RBCS and the top layer of
the epidermis (stratum corneum- dead cells)

Question 3

Lens fibers don’t have nuclei. Why is this important?

Answer 3

The lens degrades its organelles to create an organelle-free zone. It contributes to making the lens transparent so as not to obstruct vision. Has an epithelial layer (single layer of cells), the cells divide in the lens at the equator

Question 4

Enucleate vs anucleate

Answer 4

Enucleate is a verb- removing the nucleus. Anucleate means there is no nucleus

Question 5

Nuclear pores

Answer 5

Multiple nuclear pores perforate the nuclear envelope in all eukaryotic cells. Each nuclear pore is formed from a large protein structure called the nuclear pore complex. Ions, small metabolites, and globular proteins can passively diffuse through an aqueous region in the nuclear pore complex. However, large proteins and other macromolecules can’t diffuse into the nucleus. They are actively transported through the NPC using soluble transport proteins

Question 6

Nucleoporins

Answer 6

Make up the nuclear pore complex, interact with the soluble transport proteins that are responsible for active transport

Question 7

Nucleoplasmin

Answer 7

A molecular chaperone. Npl functions in the proper assembly of nucleosomes and chromatin structures. It participates in histone storage, sperm chromatin decondensation, and nucleosome assembly, genome stability, ribosome biogenesis, DNA duplication and transcriptional regulation.

Question 8

Chromatin

Answer 8

DNA in eukaryotic cells isn’t free. It’s associated with a mass of proteins called chromatin. The nucleosome is a structural unit of chromatin, which is wrapped around a cone of histone proteins

Question 9

Nuclear dissolution

Answer 9

The nuclear envelope has to be broken down as cells go from prophase to prometaphase during mitosis, so the nuclear lamina has be disassembled. Protein kinases (mitotic CDKs) drive cells into mitosis, and they use lamins as one of their substrates. When lamin A, B, and C are phosphorylated, they disassemble the intermediate filament lattice into lamin dimers.

Question 10

Nuclear lamina

Answer 10

A protein meshwork lining the inner surface of the nuclear envelope and forms part of the nuclear skeletal network (karyoskeleton) anchoring the chromatin.

Question 11

2 major phases of the cell cycle

Answer 11

1. Interphase
2. Mitotic (M) phase

Question 12

Interphase

Answer 12

The cell grows and makes a copy of its DNA. Includes G1, S, and G2. Interphase occurs between mitotic phases.

Question 13

M phase

Answer 13

The cell separates its DNA into two sets and divides its cytoplasm, forming two new cells. Includes mitosis and cytokinesis

Question 14

G1 phase

Answer 14

During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps

Question 15

Do all cells grow before they divide?

Answer 15

In the great majority of cases, cells do indeed grow before division. However, in certain situations during development, cells may intentionally split themselves up into smaller and smaller pieces over successive rounds of cell division. For instance, this happens in very early development of an African clawed frog (Xenopus laevis) embryo. See the end of the article for a video of cell divisions in early frog embryos.

Question 16

S phase

Answer 16

In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase.

Question 17

G2 phase

Answer 17

During the second gap phase, or G2 phase, the cell verifies that all of the DNA has been correctly duplicated and all DNA errors have been corrected. Chromosome condensation is initiated, there is early organization of the cell cytoskeleton, and mitotic cyclin dependent kinases initiate
activity. This phase ends when mitosis begins.

Question 18

Mitosis

Answer 18

The nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis takes place in four stages: prophase, metaphase, anaphase, and telophase. This phase is relatively short and occurs after S and G2.

Question 19

Cytokinesis

Answer 19

The cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells

Question 20

G0 phase

Answer 20

Depending on what type of cell they are, new daughter cells might divide rapidly, or they might exit G1 and enter a resting phase (G0). In G0, a cell is not actively preparing to divide, it’s just doing its job. For instance, it might conduct signals as a neuron (like the one in the drawing below) or store carbohydrates as a liver cell. G0 is a permanent state for some cells, while others may re-start division if they get the right signals.

Question 21

Cell fusion experiments

Answer 21

Revealed some basic rules for the cell cycle. The experiment involved fusing cells at different stages of the cell cycle together and seeing what happens to the chromosomes. If one fuses a cell in mitosis with a cell in any other stage of the cell cycle, the chromosomes from the “non-mitosis” cell replicate and enter mitosis. This indicates that mitosis dominates

Question 22

Cell fusion of M and G1 cell

Answer 22

Some entity causes nuclear dissolution via lamin phosphorylation

Question 23

Maturation promoting factor (MPF)

Answer 23

The primary function of MPF is to promote spindle assembly, chromatin condensation and the breakdown of the nuclear envelope. It is involved in nuclear dissolution and reassembly, which indicates lamin involvement. Lamin B Phosphorylation by MPF is required for nuclear dissolution

Question 24

Progeria

Answer 24

Patients with progeria experience premature aging. It is caused by a mutation in a gene that codes for lamin A, which is a protein that forms a scaffolding on the inside of the cell nucleus. In normal cells, a sticky molecule called farnesyl is added to lamin A so that it can get to the nuclear lamina, and then farnesyl is removed. With progeria, farnesyl cannot be removed from lamin A, so lamin A accumulates at the nuclear envelope.

Question 25

Lonafarnib

Answer 25

A farnesyltransferase
inhibitor that is used to treat Progeria.

Question 26

3T3 Cells

Answer 26

Commonly used for cell cycle and oncogene studies because they are easy to convert from normal to cancer cells. Isolated from mouse embryo tissue, made famous by Arthur Pardee

Question 27

Checkpoints

Answer 27

Surveillance mechanisms at the border between different cell cycle phases. Cells have to pass through the G1-S checkpoint to pass through to the S phase, and the G2-M checkpoint before they can enter mitosis. During G1, a cell evaluates the cell’s size, nutrient status, substrate attachment, density of neighboring cells, and the presence of extracellular growth factors to determine if it’s appropriate to commit to doubling and cell division

Question 28

Mitogens

Answer 28

Growth factors and chemicals that promote cells to transit through the cell cycle and divide

Question 29

Restriction point

Answer 29

A point in the G1 phase that cells have to cross to commit to cell division (and enter the S phase).

Question 30

Sea urchin experiment (Hunt)

Answer 30

Researchers added radioactive methionine to fertilized sea urchin eggs and noticed the gradual accumulation of a radiolabeled protein that was quickly degraded in mitosis. This protein has been identified as cyclin B.

Question 31

Cyclin B

Answer 31

Cyclin B binds to a cyclin dependent kinase (CDK1) to form an active protein kinase complex that drives the entry of G2 cells into the M phases, and they help the cell cycle to progress. Surveillance mechanisms at the G2-M checkpoint ensure that cells won’t progress to the M phase if they have cell stress or damage

Question 32

Phases of mitosis (4)

Answer 32

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 33

Prophase

Answer 33

In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes. The chromosomes start to condense and the mitotic spindle begins to form. In late prophase, the mitotic spindle begins to capture and organize the chromosomes. The chromosomes become even more condensed, so they are very compact, and the nuclear envelope breaks down, releasing the chromosomes.

Question 34

Mitotic spindle

Answer 34

The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart.

Question 35

Metaphase

Answer 35

The spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. All the of chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up).
At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles.

Question 36

Anaphase

Answer 36

The sister chromatids separate from each other and are pulled towards opposite ends of the cell.
The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell.
Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer. These processes are driven by motor proteins

Question 37

Telophase

Answer 37

The cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. The mitotic spindle is broken down into its building blocks, and two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear, and the chromosomes begin to decondense and return to their “stringy” form.

Question 38

How is CDK activity regulated during the cell cycle?

Answer 38

By binding to cyclins. The activity of CDKs varies depending on the phase of the cell cycle, and the levels of different cyclins rise and fall in a cell cycle specific manner.

Question 39

Cyclin D

Answer 39

Researchers noticed that cells would still progress into the S phase, and believed that some factor was accumulating during G1, and the cell would be destined for DNA replication once enough of the factor had accumulated. This factor was identified as cyclin D. Cyclin D is a regulatory factor that cycles in abundance as cells go through different stages of cell duplication, and it is required for cell cycle progression through G1

Question 40

Brdu cells

Answer 40

Brdu is used as thymidine substitute which get incorporated into newly synthesized DNA replication in the S phase of the cycle. Fluorescent antibody specific to BrdU can be used to detect actively replicating DNA. This compound was used in a cyclin D experiment. It was found that cells with cyclin D bound to antibodies were Brdu negative, while the cells without the antibody were Brdu positive- cyclin D blocks incorporation of Brdu

Question 41

Replicons

Answer 41

The S phase continues until replication from each of the multiple origins (regions) the lie along each chromosome has resulted in complete replication of the entire chromosome. The DNA that is copied from a single region is called a replicon. The process of creating and fusing the replicons ensures that each DNA strand is copied only once, maintaining the correct gene copy number each time a cell replicates.

Question 42

Fiber autoradiography

Answer 42

DNA fiber autoradiography is a technique that allows analysis of replication events on mammalian chromosomal DNA. 3H-thymidine is used to pulse-label DNA, which is subsequently released from lysed cells and extended linearly over a glass slide. The effects of inhibition of protein synthesis on DNA replication were examined using this technique. Analysis of the autoradiographic patterns has shown that inhibition of protein synthesis slows the rate of replication fork progression.

Question 43

Replication origins

Answer 43

For DNA to function as a template during replication, the two strands have to be unwound. Unwinding begins at segments in a DNA molecule called replication origins

Question 44

Helicase

Answer 44

Enzyme that unwinds parent DNA strands so one strand can be used as a template for DNA replication

Question 45

Reassembly of the nuclear envelope

Answer 45

Occurs during telophase. Extensions of the endoplasmic reticulum associate with each decondensing chromosome and then fuse with each other to form a double membrane around the chromosome. The de-phosphorylated nuclear pore subcomplexes reassemble into nuclear pores to form mini nuclei called karyomeres. The nuclear envelopes of the karyomeres fuse to form a single nucleus containing a full set of chromosomes

Question 46

Tumor suppressor genes

Answer 46

Genes for which the oncogenic forms arise because of a loss of function mutation. p53 is one example.

Question 47

Ruth Sager

Answer 47

Ruth was among the first to study the role of mutations in suppressor genes that neutralized their restraint on cell reproduction. She further hypothesized that genes of normal cells that contributed to controlled growth and replication could lose that function by regulatory changes that reduced their expression. Ruth championed the notion that tumorigenicity was more likely to be multigenic. A series of experiments with cell hybrids or cybrids that resulted in the suppression of the tumor cell phenotype strongly supported her suspicion that suppressor genes were important and probably several such genes are involved.

Question 48

p53

Answer 48

A tumor suppressor gene. It is responsible for monitor cells and either induce apoptosis for fix their DNA damage before they can continue in the cell cycle/

Question 49

Budding yeast

Answer 49

This research established that mitotic entry required the activity of a specific protein kinase (cdc28). This characterized some aspects of how the protein kinase is regulated

Question 50

Fission yeast

Answer 50

This research revealed that the yeast CDK, which was a regulator of Cdc2, was another cell cycle division gene (Cdc13). Cdc13 is a fission yeast homolog of cyclin B