Development Including Mitosis and Meiosis

Biology 155

Krilowicz

Spring 2010

I. The Cell Cycle

Regular sequence of events in which a cell grows and carries out regular activities, prepares to divide and then divides to produce two new cells

G = gap

M = mitosis or meiosis, DNA is divided between two new cells

Division of cytoplasm and organelles between new cells

Fig. 8.5

II. Comparison of Mitosis and Meiosis (Fig. 8.15)

Mitosis

A. Results in 2 new cells with the same # of chromosomes as the original cell

Meiosis

A. Results in 4 new cells each with ½ the # of chromosomes as the original cell

2n = diploid cell

2 X 2n cells

2n cell 4 X 1n = haploid cells

II. Comparison of Mitosis and Meiosis - continued

MitosisB. Cytokinesis is usually

approximately equalC. Used for –

growth repairdevelopment (embryogenesis)asexual reproduction

Meiosis

B. Cytokinesis can be unequal (oogenesis)

C. Used for –

gametogenesis

Definitions: Chromosome = DNA and its associated proteins, 46 in humans

Homologous chromosomes = chromosomes that contain information to control the same genetic trait, one member of each pair came from mom and one came from dad

Fig. 20B

Definitions – continued:

Copies of the same chromosome that result from S phase

Site where sister chromatids are held together, microtubules of spindle apparatus attach here during cell division

Fig. 8.4B

Definitions – continued:

Equator = center plane of the cell

Spindle apparatus = portion of the cytoskeleton that orients and separates sister chromatids or homologous chromosomes during cell division; contains poles and microtubules

Steps in MitosisInterphase nucleus with 2 homologous pairs of chromosomes

S phase

Interphase nucleus with 2 homologous sets of chromosomes, each replicated so they contain sister chromatids attached at centromere

mitosis

Spindle apparatus forms and microtubules from each pole attach to centromeres of individual chromosomes lined up on equator of the cell

Spindle “contracts” and separates sister chromatids, moving them to the poles of the cell

Cytokinesis

2 new cells with same genetic information as the original cell

2n 2 X 2n

Fig. 8.6

Steps in MeiosisNucleus at the end of interphase with 4 replicated chromosomes

Meiosis division 1

Synapsis = physical joining of homologous chromosomes as they move to the equator of the cell, microtubules from one pole of the spindle apparatus attach to the centromere of one member of each homologous pair

Spindle “contracts” and separates homologous chromosomes

cytokinesis

2n 2 X 1n, but with sister chromatids

4 X 1n

division 2 cytokinesis

Individual chromosomes move to equator

Spindle “contracts” and separates sister chromatids

4 new cells with ½ the genetic information of the original cell

Fig. 8.14

Steps in Development

A. Gametogenesis = production of gametes through the process of meiosis

1. Spermatogenesis = sperm cell production

2n1n 1n 1n 1n

2 meiotic divisions

2n (diploid) testis cell

4 X 1n (haploid) sperm cellsFig. 27.5A

A. Gametogenesis - continued

B. Oogenesis = egg cell production

2n 1n1n

1n 1n

2 meiotic divisions

2n (diploid) follicle cell 1 haploid (1n) egg + 3 haploid

polar bodies

Gets all the cytoplasm and organelles at cytokinesis

essentially 3 haploid nuclei that are discarded

Fig. 27.5B

B. Fertilization The union of an egg cell and a sperm cell that

produces a zygote (fertile egg) and re-establishes the diploid condition

1n

1n

2nfusion

egg

sperm

zygote

Fig. 27.9A

C. Embryogenesis

embryo formation – consists of cleavage + gastrulation + organ formation

All three steps will make use of mitosis to produce new cells

1. Cleavage = division of the single celled zygote into a many celled structure called a blastula (hollow ball of cells); uses mitosis so that all cells in the developing animal will have a complete set of genes

Requires multiple mitotic divisions2n

2n

2n

2n

2n

Fluid-filled

X.S.Fig. 27.10

2. Gastrulation = transformation using complex cell movements of a blastula into a multi-layered animal (gastrula); mitotic cell division continues as developing animal continues to increase in size

All cells are 2n

l.s.

x.s.

becomes mouth or anus

Breaks through to form mouth or anus

gut

gut

Fig. 27.11

3. Organ Formation

Differentiation and specialization of embryonic tissues (endoderm, ectoderm and mesoderm) into adult tissues (epithelial, connective, muscle and nervous tissues) and organs

Final Steps in Development

D. Growth = increase in size; differentiation usually continues

E. Maturation = attainment of adult body form capable of reproduction

metamorphosis as an example of a dramatic maturation process