Unit 9: Meiosis/Mitosis

Growth in

multicellular organisms

Repair & regeneration of

tissues/organs

Asexual repro in multicellular

organisms

• e.g. spores, veg. propagation

Asexual repro of unicellular

organisms

Asexual reproduction is energetically

cheap

However, asexual repro has a lack of

genetic variation

Mitosis steps

• Prophase

• Metaphase

• Anaphase

• Telophase

• (Cytokinesis)

Nuclear membrane is visible during

interphase

• no chromosomes present

Nuclear membrane disappears during

prophase

• chromosomes appear

What begins to appear during prophase

mitotic spindle

during prophase, centrioles on animals

appear & migrate towards poles of cell

Chromosomes in single line along

equator of cell during metaphase

Metaphase

mitotic spindle formed

Sister chromatids separate during

anaphase and migrate to poles of cell

Sister chromatids (individual chromosomes) clustered at poles of cell

Telophase

During telophase, nuclear membrane begins to

reform

• mitotic spindle disappears

Cleavage furrow for

animals

Cell plate for

plants

Cytokinesis

• cleavage furrow/cell plate visible

• chromosomes disappear/invisible

• nuclear membrane reformed (end)

1 diploid (2N) somatic cell

becomes 2 diploid (2N) somatic cells

Both cells should be identical except for

point mutations

Meiosis used by multicellular organisms only for

production of gametes

Only occurs in

gonadal tissues

Ovaries

ova (egg)

Testes

sperm

Sexual reproduction has

genetic variation

However, sexual reproduction is energetically

expensive

Nuclear membrane disappears

in Prophase I

• chromosomes appear

• centrioles appear and begin to migrate to poles

“Tetrads” form

Prophase I

“Crossing over” occurs during

synapsis - still in Prophase I

Metaphase I - homologous pairs line up as

partners at equator

Mitotic spindle fully formed

Metaphase I

• maternal/paternal chromo. randomly situated

Homologous pairs separate during

Anaphase I

• whole chromo. migrate to poles

• random distribution of maternal/paternal chromo. to poles

Random distribution of chromo. ensures

“independent assortment” of genetic info.

Chromosomes collected at poles during

Telophase I

• mitotic spindle disappear

• nuclear membrane reappears

Cleavage furrow/cell plate reappears during

Cytokinesis I

• cell divided into 2

Brief Interphase

“Interkinesis” occurs

• chromo. disappear

Nuclear membrane disappears again

during Prophase II

• chromo. appear

• centrioles appear and migrate to poles of cell

Mitotic spindle fully formed again during

Metaphase II

• single chromo line up at equator

• centrioles at poles

Sister chromatids separate

Anaphase II

• migrate to opposite poles

Sister chromatids (individual chromo.) clustered at poles

Telophase II

• nuclear membrane reappears

• mitotic spindle disappears

Cleavage furrow/cell plate appears, but

cell membrane/cytoplasm splits (Cytokinesis II)

1 diploid (2N x 2) cell becomes

4 haploid (1N) gametes (all 4 are unique)

3 ways meiosis creates diversity in offspring

1. point mutations

2. independent assortment

3. crossing over

Independent assortment

• random distribution of maternal & paternal chromo. to gametes

• homo. pairs in tetrads line up randomly

Crossing over

exchange of genetic info. between homo. chromo.

Spermatogenesis

• occurs in male gonads (testes)

• produces 4 haploid sperm of = size

Oogenesis

• occurs in female gonads (ovaries)

• has 1 large haploid cell (ootid) - only this may be fertilized

• has 3 small polar bodies

Fertilization

union of egg & sperm

• result: zygote (full set of genetic info.)