Bio Meiosis

Binary Fission

Binary Fission

"division in half"

Binary Fission

asexual reproduction of a prokaryotes and a few single-celled eukaryotes

origin of replication

where binary fission starts

1. Chromosome replication at origin. One copy of the origin moves toward the other end of the cell through an actin-like protein

2. Replication continues while origins are at opposite ends. Cell elongates

3. Replication finished while the cell is pinched in by a tubulin-like protein and a new cell wall is deposited

4. Two daughter cells result

Steps of Binary Fission

Meiosis

a type of cell division in sexually reproducing organisms that reduces the number of chromosomes in gametes (the sex cells, or egg and sperm)

oogenesis

female gamete reproduction

spermiogenesis

male gamete reproduction

spermatogenesis

final stage of spermiogenesis for motile sperm

seminiferous tubules of the testicles after puberty

Where does Meiosis happen in males?

the ovary, producing one large egg cell ; other cells produced in meiosis are polar bodies

Where does Meiosis happen in females?

- Centrosome movement

- Spindle formation

- nuclear envelope breakdown.

- Chromosomes condense

- Synapsis occur

- crossing over occurs

Later in prophase I, microtubules from one pole or the other attach to the kinetochores, one at the centromere of each homolog.

Microtubules move the homologous pairs toward the metaphase plate

Prophase 1

crossing over

The DNA molecules of non-sister chromatids are broken (by proteins) and are rejoined to each other.

synapsis

Pairing of homologous chromosomes

chiasmata

a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands

crossing over

exchange of DNA between paired homologous chromosomes (one from each parent)

crossing over

results in new combinations of alleles in the gametes

•Homologous pairs are now at the metaphase plate

•Each pair has lined up independently of other

pairs.

•Both chromatids of one homolog are attached to kinetochore microtubules

Metaphase 1

Law of Independent Assortment

the alleles of two (or more) different genes get sorted into gametes independently of one another

alleles

variation of the same sequence of nucleotides at the same place on a long DNA molecule

• Breakdown of cohesion

• homologs move toward

• opposite poles

• Sister chromatid cohesion persists at the centromere, causing the two chromatids of each chromosome to move as a unit toward the same pole.

Anaphase 1

• each half of the cell has a complete haploid set of duplicated chromosomes

• Cytokinesis (division of the cytoplasm) usually occurs simultaneously with telophase I, forming two haploid daughter cells.

Telophase 1

• A spindle apparatus forms.

• In late prophase II chromosomes, each

still composed of two chromatids associated at the centromere, are moved by microtubules toward the metaphase II plate.

Prophase 2

• Sister chromatids align at the metaphase plate

• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical.

Metaphase 2

• Breakdown cohesin allows the chromatids to separate and move toward opposite poles.

• Each chromatid has now become an individual chromosome.

Anaphase 2

•meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated) chromosomes.

•genetically distinct

Telophase 2

1. Synapsis and crossing over

2. Alignment of homologous pairs at the metaphase plate.

3. Separation of homologs.

Meiosis vs Mitosis

Synaptonemal Complex

a proteinaceous structure that is transiently formed during meiosis to promote homologous recombination between maternal and paternal chromosomes