Meiosis ppt

Meiosis

Importance in genetic diversity

Definitions

• Meiosis is a type of cell division which reduces the chromosome number of the parent cell by half to forms four haploid gametes

• Involves two successive divisions: meiosis I and meiosis 2

Meiosis I

• Prophase I: – Chromosomes become visible and condense

(become thicker and shorter) into rod- like structures

– Pairing between homologous chromosomes start– Replication of chromosomes happen; each

chromosome appears to be composed of sister chromatids

– Crossing over happens: the exchange of portions between homologous chromosomes.

Meiosis I

• Prophase I Nuclear membrane

Chromosomes

Cytoplasm

Cell membrane

Meiosis I

• Prophase I

Homologous chromosomes

Meiosis I• Homologous chromosomes: similar in size and shape and contain the

same kinds of genes; remember your alleles? Notice the presence of different alleles carried by the parental gametes.

Male gamete Female gamete

gene

chromosome

Meiosis I• Homologous chromosomes: similar in size and shape and contain the

same kinds of genes; remember your alleles? Notice the presence of different alleles carried by the parental gametes.

zygote

Homologous chromosomes

Genotype:AaRrBb

Meiosis I

• Prophase I Sister chromatids

Light colored: maternal

Dark colored: paternal

Meiosis I

• Prophase I Spindle fibers: responsible for pulling thechromosomes apart

Crossing over: exchange of parts;configuration: tetrad

Meiosis I

• Metaphase I: – Homologous chromosomes migrate to the middle

plane of the cell– spindle fibers attach to the centromeres (area

where the chromatids of a chromosome are attached)

Meiosis I• Anaphase I:

– Spindle fibers pull the homologous chromosomes towards opposite ends of the cell.

Spindle fiber pulling a chromosome

Meiosis I• Telophase I:

– First set of two daughter cells

Sister chromatids

p (petite) arm

q arm

Meiosis II

• Prophase II:– Nothing special; reorganization and preparation

for the next meiotic division

• Metaphase II– Replicated chromosomes move towards the

middle plane; spindle fibers attach to the centromeres

Meiosis II

• Metaphase II

Meiosis II• Anaphase II

– Separation of sister chromatids; note: sister chromatids are separated not homologous chromosomes.

– Migration towards the opposite sides

Used to be sister chromatids

Meiosis II • Telophase II

– Second set of 2 haploid daughter cells– Total gametes produced: 4

Light colored: maternal

Dark colored: paternal

Independent assortment• Members of the different pairs of homologous chromosomes separate

randomly from each other. Paternal chromosomes do not stay only on the right side and maternal only on the left.

1 123456789

10

23

1

3456789

10

Independent assortment• Independent assortment produces variations in the combination of

chromosomes in the gametes.

1 123456789

10

23

1

3456789

10

Example

If crossing over did not happen

Contains dominant allele S (smooth skin form)

Contains recessive allele s (wrinkled skin form)

Example

Recessive allele s

ZygoteSkin form ss (wrinkled skin)

If crossing over did not happen:Zygote

Skin form Ss (smooth skin)

Human chromosomes

Fruit fly (Drosophila melanogaster) chromosomes