Cell Division

Cell Xu Zhaoyang Department of cell biology Basic Medical college Zhengzhou university

Division

Preface Countless divisions of a single-cell zygote produce an organism of astonishing cellular complexity and organization. Cell division does not stop with the formation of the mature organism but continues in certain tissues( i.e. bone marrow ,intestinal tract) throughout life. This enormous output of cells is needed to replace cells that have aged or died.

Division fashion of eukaryotic cell Amitosis

just take place in a small amount of cells

Mitosis

serve as the basis for producing almost all new cells

Meiosis

serve as the basis for producing sexually reproducing organisms.

Amitosis 1.First found in the RBC of chick embryo by Remark. 2.Spindle and chromosome are not formed. 3.Genetic material was separated to two cells unevenly.

Mitosis Cells divide to produce two identical daughter cells. While all the other organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis.

Interphase The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus 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

The picture of interphase

Whitefish cell

The onion root tip cells

Stage

of

prophase prometaphase metaphase anaphase telophase

mitosis

Prophase 1. The chromosomes condense. The proteins attached to the DNA cause the chromosomes to go from long thin structures to short fat one, which makes them easier to pull apart. 2. The nuclear envelope disappears. The double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division 3. The centrosomes move to opposite poles. During interphase, the pair of centrosomes were together just outside the nucleus. In prophase they separate and move to opposite ends of the cell. 4. The spindle starts to form, growing out of the centrosomes towards the chromosomes.

The picture of prophase

White fish cell

The onion root tip cell

Metaphase Metaphase is a short resting period where the chromosomes are lined up on the equator of the cell, with the centrosomes at opposite ends and the spindle fibers attached to the centromeres. Everything is aligned for the rest of the division process to occur.

The mitotic spindle of an animal cell

Three classes of microtubules make up the mitotic spindle Subunits are rapidly lost and added from the plus end of the chromosomal microtubules, with more subunits are added to the plus end than are lost, there is a net addition of subunits at the kinetochore. Meanwhile, the minus ends of the microtubules experience a net loss, and thus subunits are thought to move along the chromosomal microtubules from the kinetochore toward the pole.

Astral microtubules centriole centromere

chromosome microtubules centrosome

Polar microtubules

The structure of aster and spindle

The picture of metaphase

Whitefish cells

The onion root tip cells

Anaphase In anaphase, the centromeres divide. At this point, each individual chromosome goes from: 1 chromosome with 2 chromatids to: 2 chromosomes with one chromatid each. Then the spindle fibers contract, and the chromosomes are pulled to opposite poles.

The picture of anaphase

Whitefish cells

The onion root tip cells

Telophase In telophase the cell actually divides. The chromosomes are at the poles of the spindle, disperse and no longer visible under the light microscope. . The spindle disintegrates The nuclear envelope re-forms around the two sets of chromosomes. The cytoplasm is divided into 2 separate cells, the process of cytokinesis.

The picture of telophase

Whitefish cells The onion root tip cells

Cytokinesis The organelles get divided up into the two daughter cells passively: they go with whichever cell they find themselves in.

Different cytokinesis ways in plant and animal cells Plant and animal cells divide the cytoplasm in different ways. In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei, about where the chromosomes lined up in metaphase. Cell membranes form along the surfaces of this wall. When the new wall joins with the existing side wall, the 2 cells have become separate. In animal cells, a ring of actin fibers (microfilaments are composed of actin) forms around the cell equator and contacts, pinching the cell in half.

Summary of Mitosis Prophase: Chromosomes condense Nuclear envelope disappears centrosomes move to opposite sides of the cell Spindle forms and attaches to centromeres on the chromosomes Metaphase Chromosomes lined up on equator of spindle centrosomes at opposite ends of cell Anaphase Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes Chromosomes pulled to opposite poles by the spindle Telophase Chromosomes de-condense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells

The process of mitosis Central plate

Animal cell mitosis

Nuclear envelope nucleus chromatin centriole interphas e

prophase

metaphase

Contractile ring

telophase

anaphase

Meiosis Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I.

Meiosis a cell division forming gametes

 Goal: reduce genetic material by half  Why? from mom

from dad

child too much!

meiosis reduces genetic content

The first meiotic division ProphaseⅠ Metaphase Ⅰ Anaphase Ⅰ Telophase Ⅰ

ProphaseⅠ Leptotene Zygotene Pachytene Diplotene diakinesis

Leptotene The chromosome condense and become gradually visible in the light microscope ( there is no indication that each chromosome is composed of identical chromatids). The telomeres cluster at the inner surface of the nuclear envelope to facilitate the alignment of homologues in preparation for synapsis

Zygotene and pachytene Axial elements approach one another, becoming lateral elements of synatonemal complex

early zygotene

Mature synaptonemal complexes consist of a pair of paralell lateral elements flanking a central element. The elements are connected by transverse fibers.

Late zygotene

pachytene

Diplotene Chromosome continues shortening and thickening The 2 chromosomes in each bivalent begin to repel each other and a split occurs between the chromosomes, but still stick together at chiasmata.

Fewer nodules are present after desynapsis.

Diakinesis Meiotic spindle is assembled; The chromosomes , condense and become more compact, highly dispersed ,are prepared to separation.

Metaphase Ⅰ Metaphase I is when tetrads line-up along the equator of the spindle. Spindle fibers attach to the centromere region of each homologous chromosome pair. Other metaphase events as in mitosis.

Anaphase Ⅰ Anaphase I is when the tetrads separate, and are drawn to opposite poles by the spindle fibers. The centromeres in Anaphase I remain intact and Sister chromatids remain attached at their centromeres.

Telophase Ⅰ Telophase I is similar to Telophase of mitosis, except that only one set of (replicated) chromosomes is in each "cell". Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Prophase II During Prophase II, nuclear envelopes dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II is very similar to mitosis.

Metaphase II Metaphase II is similar to mitosis, with spindles moving chromosomes into equatorial area and attaching to the opposite sides of the centromeres in the kinetochore region

Anaphase II During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.

Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

interphase

leptotene

metaphase I

prophase II

zygotene

pachytene

anaphase I

metaphaseII

diplotene

telophaseI

anaphase II

telophase II

diakinesis

interphase

How can we produce genetically different offsprings random (independent) assortment crossing-over recombination from different individuals during fertilization

Independent assortment The homolog of one chromosome can be inherited with either homolog of a second chromosome.

Independent assortment Since the combination of maternal and parental chromosomes received by a gamete is random. And we have 23 pairs of chromosomes The possible combinations in an egg or a sperm are 223 = 8,388,608 combinations in an offspring 223 X 223 = 70,368,744,177,664 Result: Generates new combinations of genes (alleles) when the genes are located on different chromosomes.

Crossing-over Crossing-over - the physical exchange of chromosomal material between chromatids of homologous chromosomes. - Result: Generation of new combinations of genes (alleles) if the genes are located on the same chromosome.

The difference between mitosis and meiosis Mitosis

Meiosis

Number of divisions

1

2

Number of daughter cells

2

4

Yes

No

Same as parent

Half of parent

Where

Somatic cells

Germline cells

When

Throughout life

At sexual maturity

Role

Growth and repair

Sexual reproduction

Genetically identical? Chromosome #