Mitosis & Meiosis

The Cell Life Cycle

INTERPHASE

THE CELL

CYCLE

MITOSIS AND CYTOKINESIS (See Figure 2.21)

Indefinite period G0

Specialized cell functions

G1 Normal cell functions plus cell growth, duplication of organelles, protein synthesis

G2 Protein synthesis

S DNA

replication, synthesis

of histones

M

Mitosis

• A cell divides to produce two identical cells

– To heal an injury

– To grow (increase cell number)

• Cells are not always dividing- most of their “life”

is spent between divisions (interphase G1)-

carrying out the organelle jobs

• Division costs lots in terms of energy!

• Uncontrolled cell division produces tumors

(cancers).

Stages of cell’s life cycle:

(PMAT) Interphase- (between divisions, can’t see chromosomes)

• G1- cell is not ready to divide, carries out normal functions

• S- cell commits to divide, and copies all the DNA

• G2- cell prepares for division, and generates more lipids and Proteins

Mitosis- Division of genetic information (division of the nucleus): think P.M.A.T. (“Passed My Anatomy Test”)

– Prophase- preparations (preliminary steps)- package chromosomes

– Metaphase- chromosomes line up in the middle of the cell

– Anaphase- chromosomes separate and the two halves move apart to opposite sides of the cell

– Telophase – chromosomes are surrounded by membranes to form two nuclei

Cytokinesis- Division of the cell into two identical cells

Interphase and Mitosis

INTERPHASE

MITOSIS BEGINS

EARLY PROPHASE LATE PROPHASE

Nucleus

Centrioles

(two pairs)

Astral rays Spindle

fibers

Centriole Chromosome

with two sister

chromatids

No chromosomes visible

Regular functions Chromosomes condense; Centrioles attach to them;

Microtubules form the “spindle”

METAPHASE ANAPHASE TELOPHASE INTERPHASE

CYTOKINESIS Metaphase

plate

Chromosomal

microtubule

Daughter

chromosomes

Cleavage

furrow

Daughter

cells

Chromosomes line

up in the middle

(metaphase plate)

Chromosomes

separate at the

centromere; each

identical chromatid

(piece of DNA) goes

to the opposite side

2 nuclei form 2 cells form

Interphase and Mitosis

Development of Gametes:

Meiosis • Mitosis is the production of two identical

daughter cells through DNA replication

and division of a cell.

• Meiosis: production of four unique haploid

cells from one diploid cell.

• Meiosis is the mechanism for producing

haploid sex cells (spermatozoa and

oocytes).

Spermatogenesis

• Stem cells (spermatagonia) constantly undergo mitosis, to produce more spermatagonia (2n)

• Spermatagonia can differentiate (mature) to form primary spermatocytes (still 2n)

• Primary spermatocytes enter meiosis.

• At the end of the first meiotic division, each primary spermatocyte forms two secondary spermatocytes (two identical copies of 23 chromosomes)

• Each secondary spermatocyte undergoes second meiotic division to form two spermatids (1 n).

Spermiogenesis

• Spermatids are round cells.

• To be functional, spermatids must mature into

spermatazoa, by the process of spermiogenesis.

• During spermiogenesis, sperm develop distinct

head and tail regions.

• Head contains acrosomes (enzymes required to

fertilize egg)

• Tail required form motility

Spermatogenesis

Oogenesis • Oogenesis: development of oocytes

• Oocytes are found in ovary, enclosed within follicles.

• During development, all stem cells for oocytes enter first part of meiosis, and then stops (at prophase I, “primary oocyte”).

• Follicles remain “suspended” for years (or decades)

• During each ovulatory cycle, some follicles begin growth.

• Each month, one follicle reaches full maturation (preovulatory or Graafian follicle).

• The ova is released from the follicle (ovulated) due to the midcycle surge of LH.

Follicle Development & Ovulation

Oogenesis

• Just before ovulation, in response to the LH surge, the

oocyte completes meiosis I (now “secondary oocyte”).

• End product of meiosis I: secondary oocyte and 1st polar

body.

• If the ovulated secondary oocyte is fertilized, it finishes

meiosis II, becoming a mature ova (egg)

• End product of meiosis II: mature ova and second polar

body.

Summary of

Oogenesis

End Products of Spermatogenesis and

Oogenesis

• Spermatogenesis: One primary

spermatocytes results in four spermatids,

which result in four spermatazoa.

• Oogenesis: One primary oocyte gives rise

to one mature ova and three polar bodies.