In which of the following phases of the meiosis I does the crossing over occur?

Skill:

•  Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells

    
Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)

• Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids
• A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage

Meiosis I

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated 

• P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
• M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
• A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
• T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

Meiosis II

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I) 

• P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
• M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
• A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
• T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells 

The final outcome of meiosis is the production of four haploid daughter cells

• These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)

Meiosis Overview

Animation of the Stages of Meiosis

Most of the events that function to differentiate meiosis from mitosis occur in Prophase I

• Homologous chromosomes form bivalents (or tetrads) and crossing over occurs between non-sister chromatids

Prophase I is divided into 5 distinctive sub-stages:

• Leptotene – The chromosomes begin to condense and are attached to the nuclear membrane via their telomeres
• Zygotene – Synapsis begins with a synaptonemal complex forming between homologous chromosomes
• Pachytene – Crossing over of genetic material occurs between non-sister chromatids
• Diplotene – Synapsis ends with disappearance of synaptonemal complex; homologous pairs remain attached at chiasmata
• Diakinesis – Chromosomes become fully condensed and nuclear membrane disintegrates prior to metaphase I

Stages of Prophase I

Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. During reproduction, when the sperm and egg unite to form a single cell, the number of chromosomes is restored in the offspring.

Meiosis begins with a parent cell that is diploid, meaning it has two copies of each chromosome. The parent cell undergoes one round of DNA replication followed by two separate cycles of nuclear division. The process results in four daughter cells that are haploid, which means they contain half the number of chromosomes of the diploid parent cell.

Meiosis has both similarities to and differences from mitosis, which is a cell division process in which a parent cell produces two identical daughter cells. Meiosis begins following one round of DNA replication in cells in the male or female sex organs. The process is split into meiosis I and meiosis II, and both meiotic divisions have multiple phases. Meiosis I is a type of cell division unique to germ cells, while meiosis II is similar to mitosis.

Meiosis I, the first meiotic division, begins with prophase I. During prophase I, the complex of DNA and protein known as chromatin condenses to form chromosomes. The pairs of replicated chromosomes are known as sister chromatids, and they remain joined at a central point called the centromere. A large structure called the meiotic spindle also forms from long proteins called microtubules on each side, or pole, of the cell. Between prophase I and metaphase I, the pairs of homologous chromosome form tetrads. Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes. In metaphase I, the homologous pairs of chromosomes align on either side of the equatorial plate. Then, in anaphase I, the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell. During telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell.

Meiosis II is a mitotic division of each of the haploid cells produced in meiosis I. During prophase II, the chromosomes condense, and a new set of spindle fibers forms. The chromosomes begin moving toward the equator of the cell. During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells. Then in anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell. Finally, during telophase II, the chromosomes are enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of the two cells. At the conclusion of meiosis, there are four haploid daughter cells that go on to develop into either sperm or egg cells.

During which phase of meiosis I does crossing over occur?

Does crossing over occur in meiosis 1?

When crossing over happens in meiosis I what occurs?

During which phase of meiosis I does crossing over occur quizlet?