Mitosis, Meiosis I , Meiosis II

Cell Cycle

Two Major Phases:

-interphase

-M (mitotic) phase which includes mitosis (period of active cell division) and cytokinesis (cytoplasmic division)

Cell Cycle

-cycle of the phases of a somatic cell. a somatic cell is a cells that does not produce gametes. in other words, it is a normal body cell.

mitosis

the cell division process that results in two daughter cells that are identical to the original cell

pay attention to:

how many chromosomes are present, how many chromatids are present on each centromere, and the location of the chromosomes in each stage

G0 phase

stable, nondividing period of variable length

interphase

= G1+S+G2

G1 = gap 1

S = synthesis

G2 = gap 2

interphase

the nuclear membrane is present and chromosomes are relaxed. location: nucleus.

G1 phase

growth and development of the cell

G1/S checkpoint

S phase

synthesis of DNA

G2 phase

preparation for division; G2/M checkpoint

m phase

prophase

prometaphase

metaphase

anaphase

telophase

cytokinesis

prophase

chromosomes condense. each chromosome possesses two chromatids. the mitotic spindle forms.

prometaphase

nuclear envelope disintegrates, and spindle microtubules anchor to kinetochores

metaphase

chromosomes align on the metaphase plate; spindle-assembly checkpoint

anaphase

sister chromatids separate, becoming individual chromosomes that migrate toward spindle poles

telophase

chromosomes arrive at spindle poles, the nuclear envelope re-forms, and the condensed chromosomes relax

cytokinesis

cytoplasm divides; cell wall forms in plant cells

number of chromosomes per cell

G1 = 4

S = 4

G2 = 4

prophase = 4

prometaphase = 4

metaphase = 4

anaphase = 8

telophase = 4

cytokinesis = 4

number of DNA molecules per cell

G1 = 4

S = 4 ---> 8

G2 = 8

prophase = 8

prometaphase = 8

metaphase = 8

anaphase = 8

telophase = 4

cytokinesis = 4

meiosis

process by which gamete-producing cell divides to produce meiotic products each with half of the original chromosome number.

meiosis I

reduction division (I)

first phase of meiosis. in meiosis I, chromosome number is reduced by half.

number of chromosomes in the daughter cells is half the number in the original cell (one of each homologous pair in each daughter cell)

meiosis II

equational division (II)

second phase of meiosis. events in meiosis II are similar to those in mitosis.

number of chromosomes at the beginning and end of the division cycle is the same, but the daughter cells only have one chromatid.

prophase I

leptotene - chromosomes appear

ZYGOTENE - homologous chromosomes synapse together. synaptonemal complex forms.

PACHYTENE - crossing over occurs.

diplotene - centromeres repel; chiasmata visible

diakinesis - terminalization of chiasmata

chiasmata

singular chiasma

plural chiasmata

point of attachment between homologous chromosomes at which crossing over took place. result of crossing over

metaphase I

homologous pairs of chromosomes line up on the metaphase plate

anaphase I

the two chromosomes (each with two sis chromatids) of each homologous pair separate and move toward opposite poles

telophase I

chromosomes arrive at the spindle poles

cytokinesis

the cytoplasm divides to produce two cells, each having half the original number of chromosomes

interkinesis

in some types of cells, the spindle breaks down, chromosome relax, and a nuclear envelope re-forms, but no DNA synthesis takes place.

prophase II

chromosomes condense, the spindle forms, and the nuclear envelope disintegrates

metaphase II

individual chromosomes line up on the metaphase plate

anaphase II

sister chromatids separate and move as individual chromosomes toward the spindle poles

telophase II

chromosomes arrive at the spindle poles; the spindle breaks down and a nuclear envelope re-forms.

cytokinesis

the cytoplasm divides

significance of meiosis

1. along with fertilization, maintains constant number of chromosomes from generation to generation

2. provides variability from generation to generation

-shuffling of maternal and paternal chromosomes so that a gamete usually contains a combination of maternally derived and paternally derived chromosomes. (independent assortment)

- crossing over allows each chromosome to contain some maternal and some paternally derived alleles.

how do chromatids stick together and what makes them come apart?

cohesin and shugoshin (guardian spirit)

cohesin

holds sister chromatids together in mitosis and meiosis. it also acts at chiasmata in meiosis to hold homologs together. different forms of this protein in mitosis and meiosis

shugoshin

guardian spirit.

PROTECTS cohesin at the centromere in ANAPHASE I of meiosis, but is DEGRADED by ANAPHASE II. This allows homologs to separate in ANAPHASE I, but keeps the sister chromatids together.

nondisjunction

mistakes in separation of chromosomes in meiosis

can occur in either the first or second division of meiosis

results in gametes with an abnormal number of chromosomes

trisomic (2n+1)

monosomic (2n-1)

mitosis:

one division results in 2 daughter cells

chromosome number per nucleus maintained in mitotic products

1 pre-mitotic S phase per cell division

normally, no pairing of homologous chromosomes

meiosis:

two divisions result in 4 products of meiosis

chromosome number halved in meiotic products

1 pre-meiotic S phase per meiotic process (2 divisions)

homologous chromosomes synapse

mitosis:

normally no crossing over

centromeres divide in anaphase

conservative process: daughter cell genotypes are exactly like parent cell

cell undergoing mitosis can be haploid or diploid

meiosis:

usually, at least one cross over per homologous pair

centromeres do not divide in anaphase I but do divide in anaphse II

promotes variation among products of meiosis

works best with diploid cells