Prophase I


Prophase I of meiosis I involves five stages during which alleles cross over and recombine to form non-identical haploid chromatids. Meiosis I begins with prophase I, followed by prophase II, anaphase I, anaphase II, metaphase I, and metaphase II.

Prophase I Glossary of Terms

The terms associated with Prophase I of meiosis I are essential to understanding this process. The process of meiosis is only found in eukaryotic cells. In the nucleus of every eukaryote cell, DNA strands contain genetic material. In cell division, these DNA strands are wound into chromatin, which changes shape to form chromosomes.

From a diploid germline stem cell, four haploid daughter cells are produced.

Homologous (same) chromosomes are found in diploid cells. By mitosis, diploid cells can produce exact copies of themselves, or by meiosis, they can produce daughter cells that have half of their genetic material.

The two homologous chromosomes (or homologs) of diploid cells come from the father and the mother, respectively. Mathematically, this is known as 2n, or two sets of homologous chromosomes. It is common for scientific texts to extend this to 2n = 46. It refers to the 23 sets of chromosomes in the human body.

We have the complete genetic information of both parents in our nuclear DNA because our cells have received information from both parents. Each diploid cell has 23 pairs of chromosomes. A diploid is a double.

The centromere connects the two chromatids at the middle of each chromosome. There is only one chromatid per cell.

A haplotype is a gamete or sex cell, such as the spermatozoa or ova of a male or female. Haploid cells contain only half the genetic information of their parent cells. An egg and sperm fertilize each other, resulting in a diploid cell that contains both parents’ DNA. One half of a haploid is one half of a haploid.

Additionally, chromosomes are temporary formations. In the absence of cell division, DNA is packed into the nucleus and held together by binding proteins in a much less organized way than chromatin fibers. As a result, the X-shape of chromosomes is only visible at certain stages of cell division.

The Five Stages of Prophase I (Meiosis)

Understanding the terminology makes it easier to understand the complicated process of meiosis. There are five stages in meiosis I, as already mentioned.

Stage 1: Leptotene

As chromosomes appear under electron microscopy at Prophase I of meiosis I, they appear like ‘a string of beads’, where the beads are called nucleosomes. Several strands of DNA may be nearly a centimeter long if fully stretched out – much too long for a cell nucleolus. Therefore, it is packaged using special proteins.  DNA is coiled around core histones like a sewing thread spool. As DNA wraps twice around the histone core, it forms a structure known as a nucleosome. The unwound DNA gives the appearance of a string of beads, with the wound nucleosomes giving the appearance of the beads.

The chromatids are extremely close to each other, giving the illusion of a single chromosome. At the leptotene stage, DNA double strand breaks occur, preparing for recombination. A recombination occurs when the DNA of one chromatid is broken up and mixed with another chromatid so that more alleles are produced in the offspring. 

‘Crossing over’ is the result of recombination. Due to the fact that the first stage of leptotene is itself a very short process, the next stage is often called the leptotene-zygotene transition.

Stage 2: Zygotene

During meiosis, a pair of homologous chromosomes is formed by connecting two four-chromatid tetrads. Synapses are formed when neurons attach as a pair. Chromosome pairs are attached at a central point by ladder-like filaments. The synaptonemal complex is made up of these filaments. 

When the pair is connected, it becomes a tetrad or bivalent. Recombination can occur over the synaptonemal complex once it has formed, but in some organisms it is not required.

Stage 3: Pachytene

In order to increase gene variation, a small amount of genetic material can be swapped over from the parental DNA sequences through crossing over and recombination. Chromatid sisters (the two strands of a chromosome) begin to separate at this point, although they remain attached as a pair. As a result, they appear much more distinctive under an electron microscope. 

An allele can be exchanged between two non-sister chromatids by means of a chiasma (plural: chiasmata). Sibling chromatids cannot form chiasmata unless they are separated.

Stage 4: Diplotene

When the synaptonemal complex begins to break down, as it does during the diplotene stage, the chromosome pairs begin to move apart. However, they are unable to move far away from each other as they remain attached by the chiasmata. The repelling characteristic of the two chromosomes creates a preliminary shift towards the opposite poles of the as yet incomplete meiosis I spindle apparatus, which will be completed during the prometaphase 1 immediately following Prophase I.

Stage 5: Diakinesis

The chiasmata connections appear at the ends of the chromatid arms of chromosomes during diakinesis. Terminalization is the process of arriving at a destination. The chromosomes are very condensed and still connected by chiasmata; they cannot move any further towards the poles of the as yet incomplete spindle.

Other structural changes occur during meiosis I to prepare for the next phase. As the nucleus dissolves, the nuclear envelope dissolves as well. During mitotic cell division, remnants of spindles formed during mitosis are allowed to migrate along with centrioles (centrosome-forming microtubules). Cell cytoplasm contains microtubules that are the primary building blocks for spindles.


What is Prophase I?

Prophase I is the first stage of meiosis, which is the process of cell division that produces four genetically unique daughter cells. During Prophase I, homologous chromosomes pair up and exchange genetic material in a process called crossing over.

What happens during the pairing of homologous chromosomes in Prophase I?

During Prophase I, homologous chromosomes come together and form a structure called a bivalent or tetrad. This allows for the exchange of genetic material between the chromosomes through a process called crossing over, which can result in new combinations of genes.

How long does Prophase I last?

The duration of Prophase I can vary depending on the organism and the specific cell type. In humans, Prophase I can last for several years in females, while in males it typically takes a few days to complete.

What is the significance of crossing over during Prophase I?

Crossing over is significant because it increases genetic diversity by creating new combinations of genes on the chromosomes. This allows for greater variation within a population, which can be beneficial for evolution and adaptation to changing environments.

What are the sub-stages of Prophase I?

Prophase I can be divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. During each of these sub-stages, different events occur, such as the pairing of homologous chromosomes in zygotene and the separation of bivalents in diakinesis.

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