Cell Division,Mitosis,Meiosis,Amitosis

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Definition of Terms

gene - basic unit of heredity; codes for a specific trait

locus - the specific location of a gene on a chromosome(locus - plural loci)

genome - the total hereditary endowment of DNA of a

cell or organism

somatic cell - all body cells except reproductive cells

gamete - reproductive cells (i.e. sperm & eggs)

chromosome - elongate cellular structure composed of DNA and protein - they are the vehicles which carryDNA in cells



diploid (2n) - cellular condition where eachchromosome type is represented by two homologous

chromosomes

haploid (n) - cellular condition where eachchromosome type is represented by only onechromosome

homologous chromosome - chromosome of thesame size and shape which carry the same type of genes

chromatid - one of two duplicated chromosomesconnected at the centromere

centromere - region of chromosome wheremicrotubules attach during mitosis and meiosis



- sequence of phases in the life cycle of the cell

- the period from the beginning of one cell division to

the beginning of the next cell divisionTwo main parts

1. growth and preparation (interphase)

2. cell divisionA. mitosis (nuclear division)

B. cytokinesis (cytoplasm division).

Cell Cycle



Cell Division

- the distribution of identical geneticmaterial, DNA, to two daughters cells

- functions in Reproduction, Growth and Repair



State Phase Abbreviation Description

quiescent/

senescent Gap 0 G0 Resting phase

Interphase

Gap 1 G1

period of activity

characterized be

growth and

development

Synthesis S

DNA

replication occur

s during this

phase.

Gap 2 G2

cell synthesizes

organelles and

other materials

Cell division Mitosis M

Cell growth stops

at this stage and

cellular energy is

focused on the

orderly division

into twodaughter cells.



C

hromosome structure



nterphase -T

he cell is engaged in metabolic activity andperforming its prepare for mitosis. Chromosomes are not clearlydiscerned in the nucleus, although a dark spot called thenucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of

which are organizational sites for microtubules

PHASESOF MITOSIS



Prophase: The chromatin, diffused in interphase,condenses into chromosomes. Each chromosome hasduplicated and now consists of two sister chromatids. Atthe end of prophase, the nuclear envelope breaks down intovesicles.



Metaphase: The chromosomes align at the equatorial plateand are held in place by microtubules attached to themitotic spindle and to part of the centromere.



Anaphase: The centromeres divide. Sister chromatidsseparate and move toward the corresponding poles.



Telophase: Daughter chromosomes arrive at the poles andthe microtubules disappear. The condensed chromatinexpands and the nuclear envelope reappears. The cytoplasmdivides, the cell membrane pinches inward ultimately

producing two daughter cells (phase:

Cytokinesis).



-type of cell division by which germ cells (eggsand sperm) are produced

- process that reduces the number of chromosomes by half, so that the daughtercells are haploid (n)

- Comprise of two successive nuclear divisions

with only one round of DNA replication

MEIOSIS



STAGESOF MEIOSIS

I.Interphase: Before meiosis begins, genetic material is

duplicated.

II. Meiosis 1

Prophase 1:

Duplicated chromatin condenses.Each chromosome consists of

two, closely associated sister

chromatids. Crossing-over can

occur during the latter part of

this stage.

(Micrograph showing condensed chromosomes in blue and the mitotic spindle in green

during prometaphase of mitosis)



Metaphase 1:

Homologous chromosomes

align at the equatorial plate.

Anaphase 1: Homologous

pairs separate withsister chromatids

remaining together.



Telophase 1:

Two daughter cells

are formed with each

daughter containing

only one chromosome

of the homologous pair.



III. Meiosis 2: Gamete formation

Prophase 2: DNA does not replicate.

Metaphase 2: Chromosomes align at the equatorialplate.

Anaphase 2: Centromeres divide and sister

chromatids migrate separately to each pole.



Telophase 2: Cell division is complete. Four haploiddaughter cells are obtained



OOGENESIS



CHECKPOINTSTO ENSURE FIDELITY OF CELL DIVISION

1. G1/S monitors the size of the cell and whether theDNA have been damaged

2.G2/M checkpoint controls the physiological

conditions in the cell prior entering to mitosis,especially DNA replication

3. M checkpoint monitors the formation of spindlefibers to the kinetochores associated with the

centromere

If the regulation of the cell cycle was not met in somecases this may lead to tumor formation.



Amitosis

-the direct method of cell division characterized bysimple division of the nucleus without formation of chromosomes

Cell Division,Mitosis,Meiosis,Amitosis

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