stages of meiosis

OVERVIEW OF MEIOSIS- One round of ____ Two rounds of ____

DNA replication, cell division

OVERVIEW OF MEIOSIS- Meiosis I

reduction division

OVERVIEW OF MEIOSIS- Meiosis II

separation of sister chromatids

OVERVIEW OF MEIOSIS- Final result:

Four genetically distinct haploid cells.

PROPHASE I

Longest and most complex stage.

PROPHASE I

Chromosomes condense, Homologs pair via synapsis, Tetrads form (4 chromatids).

PROPHASE I

Crossing-over occurs at chiasmata.

Synapsis

precise alignment of homologous chromosomes

SYNAPSIS

Mediated by synaptonemal complex

SYNAPSIS

Allows corresponding genes to align.

SYNAPSIS

Required for accurate segregation.

CROSSING OVER

Physical exchange of DNA between nonsister chromatids

CROSSING OVER

Results in:

• Parental chromatids

• Recombinant chromatids

CROSSING OVER

Increases genetic diversity, Also helps hold homologs together.

Without crossing over

Reduced variability, Increased segregation errors

WHY CROSSING OVER MATTERS

Crossing over ensures proper tension at metaphase I

PROMETAPHASE I

Nuclear envelope breaks down, Spindle fibers form, Kinetochore microtubules attach to homologs.

PROMETAPHASE I

Each homolog attaches to opposite spindle poles

METAPHASE I

Tetrads align at metaphase plate.

METAPHASE I

Orientation of each homologous pair is random

METAPHASE I

independent assortment begins here.

INDEPENDENT ASSORTMENT

Each homologous pair aligns independently of others.

INDEPENDENT ASSORTMENT

If n = 23: 2²³ ≈ 8 million possible combinations per parent.

INDEPENDENT ASSORTMENT

This occurs before fertilization.

ANAPHASE I

Homologous chromosomes separate.

ANAPHASE I

Sister chromatids remain attached.

ANAPHASE I

Chromosome number reduced: 2n → n

ANAPHASE I

DNA still duplicated.

TELOPHASE I

Two haploid cells form.

TELOPHASE I

Each chromosome still consists of two chromatids

TELOPHASE I

Ploidy reduced, DNA still doubled.

INTERKINESIS

Short resting phase, No DNA replication occurs

INTERKINESIS

Cells proceed directly into Meiosis II.

MEIOSIS II OVERVIEW

Mechanistically similar to mitosis.

MEIOSIS II OVERVIEW

Purpose: Separate sister chromatids

MEIOSIS II OVERVIEW

No change in ploidy.

PROPHASE II

Chromosomes condense, Spindle forms, Nuclear envelope breaks down

METAPHASE II

Chromosomes align at metaphase plate.

METAPHASE II

Each chromatid attaches to opposite poles.

ANAPHASE II

Centromeres split, Sister chromatids separate.

ANAPHASE II

Now each chromatid is considered an individual chromosome.

ANAPHASE II

DNA content reduced.

TELOPHASE II

Four haploid cells form.

TELOPHASE II

Each genetically unique.

TELOPHASE II

Each chromosome has one chromatid.

Mitosis:

No synapsis, No crossing over, One division, Produces identical diploid cells

Meiosis:

Synapsis, Crossing over, Two divisions, Produces unique haploid cells

Meiosis II resembles

mitosis

Sister chromatids are held together by protein complexes called

cohesions

In mitosis:

cohesins are cleaved all at once at the end of metaphase

In meiosis:

cohesins along the chromosome arms are cleaved in anaphase I, releasing the homologues

Cohesins at the centromere

are protected until anaphase II

Two-stage release is how one cell gets ___

two different divisions.

___ replication, ___ divisions, ___ haploid cells

one, two, four

Meiosis I separates ___

homologues

meiosis II separates ___

sister chromatids

Prophase I is unique

synapsis, the synaptonemal complex, crossing over, chiasmata

Cohesins are released in two stages

arms in anaphase I, centromeres in anaphase II