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Flashcards in Cell Division Deck (43):

What is Cell Division? Karyokinesis ? Chromosomes? Chromatids?

What does it function in?

What is it in diploid cells

Cell division is nuclear division(karyokinesis) followed by cytokinesis.

It functions in growth, repair, reproduction.

In diploid cells, there are two copies of every chromosome, forming a pair(homologous chromosome)

Humans have 46 chromosomes, 23 homologous pair, a total of 92 chromatids.



Describe what it is and name 2 functions

Microtubule organizing centers aka centrosomes. Pair of these lay outside nucleus.

2 main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spindle apparatus, which separate the chromosomes during cell division.

In animal cells, each MTOC contains a pair of centrioles.




What breaks down? What forms?

Prophase: nucleus disassembles: nucleolus disappears, chromatin condenses into chromosomes, and nuclear envelope breaks down.

Mitotic spindle is formed.




What is each chromosome complete with?
What forms at the opposite side?

Chromosomes line up in single file at the center

Centrosomes at opposite ends of cell.

Spindle fibers run from the centrosomes to the kinetochores in the centromeres.





Microtubules shorten, each chromosome is pulled apart into two chromatids(once separated, chromosomes double) Pulls chromosomes to opposite poles(disjunction); at the end of this phase, each pole has a complete set of chromosome, same as original cell before replication.




Nuclear division, nuclear envelop develops, chromosomes ----> chromatin, nucleoli reappear.


Mitosis: Telophase---cytokinesis

Actually begins in later stages of mitosis(end of anaphase). Division of cytoplasm to form 2 cells.


Mitosis: Cyokinesis----cleavage furrow

In plant cells, it Forms down the middle of a cell. Actin and myosin microfilaments shorten, pull plasma membrane into center(animal)


Mitosis: Cytokinesis - Cell Plate

In plant cells, a cell plate forms during telophase as vesicles from the Golgi go straight down the middle of the cell.



Begins after mitosis and cytokinesis are complete

Cell cycle= M,G1, S, G2 phases


Interphase: What is S phase?

Where is more time spent in? Mitosis/Interphase

During S phase, second molecule of DNA replicated from the first, provides sister chromatids.

More time spent in interphase than mitosis(>90%) Growth occurs in all 3 interphases, not just G's.

During G2, material for next mitotic division are prepared.


When are the checkpoints in Interphase?

Near the end of G1 - cell growth assessed and favorable conditions checked. If fails, cells enters G0.

End of G2- checks for sufficient Mitosis Promoting Factor (MPF) levels to proceed.

M checkpoint during mitosis that triggers start of G1.


Meiosis I:

Also called the reduction division. In this process. homologous chromosomes separate.



Prophase I

Nucleus disassembles: nucleolus disappears and nuclear envelop breaks down, chromatin condenses, spindle develops. MT's begin attaching to kinetochores. Crossing over means genetic recomb.



Prophase I -- Synapsis, Chiasmata, Synaptonemal complex

Synapsis: aka Tetrad. The pairing of homologues occurs.

Chiasmata: Visiable manifestations of cross-over events occur.

Synaptonemal complex: protein structure that temporarily forms between homologous chromosomesL gives rise to the tetrad w/ chiasmata and crossing over.


Describe 5 steps in Prophase I

4. Diplotene
5. Diakinesis

1.Leptotene- chromosomes start condensing
2.Zygotene- Synapsis begins; synaptonemal complex forming)
3.Pachytene - Synapsis complete, crossing over
4. Diplotene - synatopnemal complex disappears, chiasma still present.
5.Diakinesis - nuclear envelop fragments, chromosomes complete condensing, tetrads for ready for metaphase.


Meiosis - Metaphase I:

homologous pairs of chromosomes line up in double file along the plate.


Meiosis - Anaphase I

Homologoues chromosomes are pulled by spindle fibers to opposite sides (disjunction)


Meiosis - Telophase I

Nuclear membrane develops. Each pole forms a new nucleus that has half number of chromosomes( from homologous pair to each chromosome = 2 sister chromatids)

Chromosomes reduction phase to haploid.
- Interphase may occur in between here, depending on the species.


Meiosis II

The same are Mitosis. Each parent cell contains four chromosomes.


Meiosis - Prophase II

Nuclear envelop disappears and spindle develops etc., no chiasmata and no crossing over.


Meiosis- Metaphase II

Chromosomes pair up and align on plate like in mitosis but now with half number of chromosomes (no extra copy)


Meiosis - Anaphase II

Each chromosome is pulled into 2 separate chromatids by microtubles of the spindle apparatus.


Meiosis - Telophase II

Nuclear envelop reappears and cytokinesis occurs ----> habloid cells( each chromosome = 1 chromatid)


Mitosis is for ____ cells

Meiosis is for ____ cells

Mitosis is in somatic cells

Meiosis in gametes(egg, sperm, pollen)


Genetic Variation: Genetic recombination during meiosis and sexual reproduction originates from 3 events:

1. Crossing over during prophase I
2. Independent assortment of homologous during metaphase I ( which chromosomes goes into which cell)
3. Random joining of gametes aka germ cells(which sperm fertilizes which egg - genetic composition of gamete affects this)


Regulation of Cell Cycle:

1. Surface-to-volume ratio (S/V)

Volume gets much larger when cells grow. When S/V is large, exchange becomes much easier. When S/V is small, exchange is hard, leads to cell death or cell division to increase SA.


Regulation of Cell Cycle:

2. Genome-to-volume ratio (G/V)

Genome size remains constant throughout life; as cell grows, only volume increases.

G/V will be small and thus exceed the ability of its genome to produce sufficient amounts of regulator of activities. Some large cells(paramecium, human skeletal muscle) are multi-nucleated to deal with this.


Checkpoints: cell specific regulations

1.G1 checkpoint

aka restriction point, the most important one. At the end of G1 phase, if cell is not ready to divide it may act here (G0 phase - nerve and muscle cells remain here, rarely divide after maturing) and never proceed or wait until it is ready.


Checkpoints: cell specific regulations

2. G2 checkpoint:

End of G2 phase, evaluates accuracy of DNA replication and signal whether to begin mitosis.


Checkpoints: cell specific regulations

3. M checkpoint

During metaphase, ensures microtubules are properly attached to all kinetochores.


Cyclin-dependent kinases (Cdk's)

Cdk enzyme activates proteins that regulate cell cycle by phosphorylation; Cdk's are activated by protein cyclin.


Growth factor:

Plasma membrane has receptors for growth factors that stimulate cell for division(such as damaged cell)


Density-dependent inhibition:

Cells stop dividing when surrounding cells density reaches maximum.


Anchorage dependence:

Most cells only divide when attached to an external surface such as neighboring cells or side of culture dish.

-Cancer cells defied all of the 5 conditions above( such cells are called TRANSFORMED cells)


At anaphase of mitosis, there would be a total of _____ chromosome/chromatids if a cell has ____ chromosomes at beginning. Even when pulled apart of sister chromatids, each one is now a complete chromosome.

At anaphase of mitosis, there would be a total of 92 chromosome/chromatids if a cell has 46 chromosomes at beginning. Even when pulled apart of sister chromatids, each one is now a complete chromosome.


At anaphase I, there would be a total of ___ chromosomes if a cell has ___ chromosomes at beginning because ___ chromosomes are pulled to each pole by independent assortment and no chromatids are separated at anaphase I.

At anaphase I, there would be a total of 46 chromosomes if a cell has 46 chromosomes at beginning because 23 chromosomes are pulled to each pole by independent assortment and no chromatids are separated at anaphase I.


Do plants have centrioles?(formation of cell plate)

Plants do not have centrioles.


Is mitosis a genetic variation?

Mitosis = no genetic variations.


Where does meiosis occur in?

In sexually reproducing organisms and the results are cells that are haploids.


In meiosis, a replicated chromosome consists of _____

two sister chromatids, where one is an exact copy of the other.


A kinetochore is______

a disc-shaped protein on the centromere that attaches the chromatid to the mitotic spindle during cell division.


Alternation of Generations:

Describe them

Gametophyte(n) ------>Mitosis-----> Makes Gametes(n) ---->Fertilization---->Sporophytes(2n) ----->Meiosis------>Spores(n)