MEIOSISMaking Haploid Gametes

Both chromosomal problems (monosomy/trisomy & breakage) are result of improper haploid cell formation/division in process known as meiosisTwo phases: meiosis I &

meiosis IIGoal: 2n (diploid) n

(haploid)

Body or somatic cell division is called mitosis, which generates the EXACT same cellMitosis: cell division

which produces cells containing SAME number of chromosomes as parent’s cell

○Start: One 2n skin cell

○End: Two 2n skin cells

Gamete or sex cell division is called meiosis, which generates cell with HALF number of chromosomes as originalStart: One 2n cellEnd: Four n sperm or

egg cells

n

n

n n n

nn

nVideo of Mitosis vs. Meiosis

2n

MITOSIS MEIOSISGrowth & repair Making gametes

In body/somatic cells In sex cells/gametes

One 2n cell Two 2n cells One 2n cell Four n cells

One diploid two diploids One diploid four haploids

Exact copy of parent Half copy of parent

2n 2n

2n 2n n nnn

Two repeating stages with precursor stage:

Steps of meiosis

Meiosis I

Meiosis II

Interphase

Interphase – DNA replication (2n 4n)

1. Prophase I – homologous chromosomes visible; crossing over occurs to add genetic variety

2. Metaphase I – homologs move to equator

3. Anaphase I – homologs move to opposite poles

4. Telophase I – 2n sets move to poles; cytokinesis

5. Prophase II – new spindle forms around chromosomes

6. Metaphase II – chromosomes move to equator

7. Anaphase II – centromeres divide; n chromatids to poles

8. Telophase II – n sets move to poles; cytokinesis

ME

IOS

IS I

= 4

n

2n

ME

IOS

IS I

I –

2n

n

Interphase Doubling of

chromosomes (2n 4n) Cell looks same Nucleus is darker

because of chromosomal duplication

A

BC “tetrad”

a

bc

a

bc

A

BC

A A

BBCC

aa

bbcc

Prophase I Nuclear envelope disappears Spindle fibers form from centrioles Chromosomes in homologous pairs Crossing-over occurs (genetic

diversity!)

Homologous pair

A A

BBCC

aa

bbcc

A

BBCC

a

bbcc

aA

Crossing over

Same gene,different variety

Nuclear envelope

centrioles

spindle fibers

Metaphase I Homologs move to

equator How they line up varies

(genetic diversity!) = law of independent assortment

Spindle fibers attach to centromeres

Anaphase I Spindle fibers contract Homologs pulled to poles

What happens when anaphase goes wrong?If tetrads are not

pulled apart evenly, chromosomal abnormalities result

Trisomy (extra chromosome

Monosomy (missing)

Anaphase Mistakes

Telophase I Chromosomes go to poles Cytokinesis occurs; cells split in two 4n 2n (halfway to goal!)

Prophase II New spindle fibers form 2n (2 sets of 23 = 46)

Metaphase II Same as metaphase I

Homologs move to equator Spindle fibers attach to centromeres

Anaphase II Sister chromatids pulled apart at centromere Chromatids move to opposite poles

What happens when anaphase goes wrong?If chromatids are not pulled

apart evenly, chromosomal abnormalities result

Trisomy (extra chromosome

Deletion/Monosomy (missing)

Anaphase Mistakes – A Second Chance

Telophase II Cytokinesis takes place, splitting two cells

into four (sperm or eggs) 2n n

Meiosis Overview

Watch Meiosis Video

Overview 2n

4n

2n 2n

n n n n

Precursor cell

INTERPHASE

MEIOSIS I

MEIOSIS II

diploid

diploid

haploid

polyploid