Chapter 8: Cellular Reproduction Chapter 8.pdf · Stages of the Cell Cycle G 1: • preparation for DNA replication • non-dividing cells are arrested at this stage of the cell cycle

Chapter 8: Cellular Reproduction

2. Mitosis

1. The Cell Cycle

3. Meiosis

2 Types of Cell Division

Mitosis:• occurs in somatic cells (almost all cells of the body)• generates cells identical to original

2n1n

Meiosis:• occurs in gamete production (sperm & egg)• ½ the normal chromosome content (haploid or “1n”)

ChromosomeContent

• somatic cellsare diploid (2n)• 2 of eachchromosome

• i.e., 1 from eachparent

• gametes arehaploid (1n)• 1 of eachchromosome

23 pairs of human chromosomes

1. The Cell Cycle

The Cell Division

Cycle

• all eukaryoticcells progressthrough the 4phases of the cell cycle witheach division

G1G2S

M

Stages of the Cell CycleG1:

• preparation for DNA replication• non-dividing cells are arrested at this stage of

the cell cycle (referred to as G0)

• replication of genetic material (DNA Synthesis)S phase:

• preparation for cell divisionG2:

• cell division (Mitosis or Meiosis)M phase:

***G1, S & G2 collectively make up “Interphase”***

2. Mitosis

4 Stages of Mitosis1) Prophase

2) Metaphase

3) Anaphase

4) Telophase & Cytokinesis

Interphase

G1, S phase & G2

• all events in preparationfor cell division• DNA replication

• duplication of organelles

• preparation of “mitotic apparatus”

Prophase1) duplicated

chromosomes condense, remain attached at thecentromere

2) break down ofnuclear envelope

3) centrosomes moveto opposite poles

4) spindle fibers form

Duplicated Chromosomes• in prophase, condensed duplicated chromosomesremain attached at the centromere

• each “half” of duplicated chromosome referred toas sister chromatids

Still considered as1 chromosome!

Metaphase

• chromosomesalign along themetaphase plateof the cell

• mediated by spindlefibers connecting each centromere toa centrosome at ea“pole” of the cell

Anaphase• sister chromatidsare pulled toopposite poles ofthe cell

• separate at thecentromere, eachchromatid still attached to aspindle fiber

• spindle fibersshorten, pullingeach chromatid toopposite poles

Telophase

• nuclear envelope reforms aroundchromosomes ateach pole

• spindle fibers break down

• chromosomes decondense

Cytokinesis begins…

Opposite of prophase:

CytokinesisDivision of the cytoplasmcompletes the process:

• the 2 daughter cells are genetically identical to the original parent cell

• each daughter cell is now in G1 (thebeginning of interphase)

3. Meiosis

Meiosis Produces 1n GametesMeiosis requires 2 rounds of cell division:

Meiosis I• differs from mitosis in prophase, metaphase, anaphase

Meiosis II• basically the same process as mitosis

(DNA content)

(chromosome #)

4n 2n

2n 1n

1n

Meiosis IDiffers from mitosis in several key ways:

Prophase I: homologous chromosomes pair, exchangegenetic material by “crossing over”

Metaphase I: homologous chromosomes align as pairs

Anaphase I: homologous chromosome pairs separate(sister chromatids are NOT separated)

prophase I metaphase I anaphase I

Coat-colorgenes

Eye-colorgenes

Tetrad (homologous pair of chromosomes in synapsis)

Breakage of homologous chromatids

C E

c e

1

C E

c e

Joining of homologous chromatids2

C E

c e

Chiasma

3Separation of homologous chromosomes at anaphase I

C E

C e

c E

c e

4Separation of chromatids at anaphase II and completion of meiosis

C E

C e

c E

c e

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

“Crossing Over” in Prophase I

• an important source of new genetic combinations (genetic variability)

Combination 1 Combination 2 Combination 3 Combination 4

Gametes

Metaphase II

Two equally probablearrangements of chromosomes at

metaphase I

Possibility 1 Possibility 2

Independent Assortment of Chromosomesdue to alignment in Metaphase I

• an additional source of genetic variability

2n

possibilities(n = # of

homologous pairs, e.g. 23)

Meiosis II

• essentially the same as mitosis • results in four haploid (1n) gametes

• each is genetically unique due to 1) “crossing over” and 2) independent assortment of homologous chromosomes

prophase II metaphase II anaphase II telophase II/cytokinesis

Key Terms for Chapter 8

• sister chromatids vs non-sister chromatids

• centromere, centrosome, spindle fibers

• cell cycle, mitosis, meiosis

• interphase, prophase, metaphase, anaphasetelophase, cytokinesis

• haploid, diploid, somatic cell, gamete

• homologous chromosomes

• crossing over, independent assortment

Relevant Review Questions: 1-6, 8, 9, 13, 17, 20