+

What occurs during the phases of meiosis?Sections 8.1 2 to 8.18

+Homologous Chromosomes

Somatic Cell Body cells All cells excepts the sperm

and the egg (gametes/sex cells)

Chromosome Number for Humans 46 23 pair

Homologous Chromosomes 2 chromosomes that are

the same length, shape, centromere position and code for the same genetic information

1 from each parent

+Fig. 13-3b

TECHNIQUE

Pair of homologousreplicated chromosomes

Centromere

Sisterchromatids

Metaphasechromosome

5 µm

+Autosome vs. Sex Chromosome

Autosomes Chromosomes that are

the same in males and females

Pairs 1 through 22 on a karyotype

Sex Chromosomes Chromosomes that are

different in males and females

Chromosomes that determine sex

Pair 23 on a karyotype Female – XX Male XY

+Diploid vs. Haploid

Diploid Cells that have 2 sets of

genetic information 2 sets of homologous

chromosomes Designated by “2n” In humans 2n= 46 Normal body cells

Haploid Cells that have 1 set of

genetic information Only one of each homolog Designated by “n” In humans n=23 Gametes

+Gametes vs. Zygote

Gametes Sex Cells Sperm or Egg Haploid

Zygote Fertilized egg Diploid

+Meiosis

• Gamete formation

• Like mitosis, meiosis is preceded by the replication of chromosomes

• Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

• The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis – also called Reduction Division (MI = reduction / MII = division)

• Each daughter cell has only half as many chromosomes as the parent cell – starts with “2n” cell ends with four “n” cells

+Prophase I

Nuclear membrane breaks down

Nucleolus disappears

Chromatin condenses into chromosomes

Synapsis occurs Homologous

chromosomes come together (forming a tetrad)

Crossing over can occur

+Metaphase I

Homologous pairs line up at the metaphase plate

Spindle fibers attach to the kinetechores

+Anaphase I

Homologous chromosomes are pulled apart

Note: centromeres DO NOT split

+Telophase I and Cytokinesis

Double chromosomes are at the poles

Nuclear membrane reforms

Nucleolus reappears

Cytoplasm Separates

Note: at this point there are 2 genetically unique haploid cells - REDUCTION

+Prophase II

Nuclear membrane breaks down

Nucleolus disappears

Chromatin condenses to chromosomes

Note: no synapsis

Note: haploid

+Metaphase II

Chromosomes move to the metaphase plate

Note: single file like mitosis except these cells are haploid

+Anaphase II

Sister chromatids pulled apart

Note: centromere splits just like anaphase of mitosis only with haploid cells

+Telophase II and Cytokinesis

Chromosomes are at the poles

Nuclear membrane reforms

Cytoplasm divides

Note: produces 4 genetically unique haploid cells

+Gamete formation in Animals

+Gamete formation in Plants

+

How does meiosis differ from mitosis?

+Fig. 13-9a

MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Chromosomereplication

Homologouschromosomepair

Chromosomereplication

2n = 6

Parent cell

Prophase

Replicated chromosome

Metaphase Metaphase I

Anaphase ITelophase IHaploid n = 3

Daughter cells ofmeiosis I

MEIOSIS II

Daughter cells of meiosis II

nnnn

2n2n

Daughter cellsof mitosis

AnaphaseTelophase

+Fig. 13-9b

SUMMARY

MeiosisMitosisProperty

DNAreplication

Number ofdivisions

Occurs during interphase beforemitosis begins

One, including prophase, metaphase,anaphase, and telophase

Synapsis ofhomologouschromosomes

Does not occur

Number ofdaughter cellsand geneticcomposition

Two, each diploid (2n) and geneticallyidentical to the parent cell

Role in theanimal body

Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, andtelophase

Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as many chromosomesas the parent cell; genetically different from the parentcell and from each other

Produces gametes; reduces number of chromosomes by halfand introduces genetic variability among the gametes

Mitosis vs. Meiosis

+Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

• In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

+Fig. 13-11-3

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Metaphase II

Daughtercells

Combination 1Combination 2 Combination 3Combination 4

+Crossing Over

• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent

• In crossing over, homologous portions of two nonsister chromatids trade places

• Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

+Fig. 13-12-5

Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

TEM

+Chiasma

The microscopically visible site where crossing over has occurred between chromatids of homologous chromosomes during prophase I