Stages of Meiosis

The Phases of Meiosis

Interphase •the cell replicates its chromosomes.•After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

Interphase

The Phases of Meiosis

Prophase I•The chromosomes coil up and a spindle forms.•As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

Prophase I

The Phases of Meiosis

Prophase I•The chromatids in a tetrad pair tightly•In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

Prophase I

The Phases of Meiosis

Metaphase I•During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber.•The spindle fibers pull the tetrads (homologous pairs) into the middle

Metaphase I

The Phases of Meiosis

Prophase I + Metaphase 1•Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.•Genetic recombination results from crossing over during prophase I of meiosis –This increases variation further

The Phases of Meiosis

Anaphase I•begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell.•This critical step ensures that each new cell will receive only one chromosome from each homologous pair.*Random assortment*

Anaphase I

The Phases of Meiosis

Telophase I•Nucleus reforms, chromosomes may or may not unwind•The spindle is broken down and the cytoplasm divides to yield two new cells•Cells are now HAPLOID- half the amount of genetic information

Telophase I

Meiosis 1 overview• 1st division of meiosis

4 chromosomes diploid 2n 2 chromosomes

haploid 1n

Copy DNA during interphase

Line Up in tetrads

Divide 1

prophase 1

metaphase 1

telophase 1

The Phases of Meiosis

Meiosis II•The second division in meiosis is simply a mitotic division of the products of meiosis I.•During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.•Nucleus dissolves

Prophase II

The Phases of Meiosis

Metaphase II•The chromosomes, still made up of sister chromatids, are pulled to the center of each cell and line up randomly at the equator.

Metaphase II

The Phases of Meiosis

•Anaphase II• the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles of each cell.

Anaphase II

The Phases of Meiosis

Telophase II• nuclei reform, the spindles break down, and the cytoplasm divides.•At the end of meiosis II, four haploid cells have been formed from one diploid cell.•These haploid cells will become gametes, transmitting the genes they contain to offspring.

Telophase II

• 2nd division of meiosis– looks like mitosis

Meiosis 2 overview

2 chromosomes haploid 1n

gametes

Line Up 2

telophase 1

metaphase 2

telophase 2

The Phases of Meiosis

•At the end of meiosis II, four haploid cells have been formed from one diploid cell.•These haploid cells will become gametes, transmitting the genes they contain to offspring.

Gamete Formation• Formation of Gametes (egg and sperm- sexual

reproductive cells)– Meiosis in males occurs in the testes and produces 4

sperm cells. The process is called spermatogenesis.– Meiosis in females occurs in the ovaries and produces 1

egg and 3 polar bodies, which disintegrate. The process is called oogenesis.

• Meiosis allows for crossover and genetic recombination (gene mixing), which creates the variation between parents and offspring.

Meiosis = reduction division• Meiosis– special cell division in sexually

reproducing organisms– reduce number of chromosomes• 2n 1n • diploid haploid – half

–makes gametes • sperm, eggs

Meiosis & mitosis

• Meiosis to make gametes– sperm & egg

• Mitosis to make copies of cells– growth– repair– development

Comparison of Mitosis & Meiosis

• MITOSIS MEIOSIS