Prophase () is the first stage of

cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...

in both

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

and

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

. Beginning after

interphase Interphase is the active portion of the cell cycle that includes the G1, S, and G2 phases, where the cell grows, replicates its DNA, and prepares for mitosis, respectively. Interphase was formerly called the "resting phase," but the cell i ...

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

reticulum and the disappearance of the

nucleolus The nucleolus (; : nucleoli ) is the largest structure in the cell nucleus, nucleus of eukaryote, eukaryotic cell (biology), cells. It is best known as the site of ribosome biogenesis. The nucleolus also participates in the formation of signa ...

Staining and microscopy

Microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...

can be used to visualize condensed

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

s as they move through

and

. Various DNA stains are used to treat cells such that condensing

s can be visualized as the move through prophase. The giemsa G-banding technique is commonly used to identify

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

ian

s, but utilizing the technology on

plant cell Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids ...

s was originally difficult due to the high degree of chromosome compaction in plant cells. G-banding was fully realized for plant chromosomes in 1990. During both meiotic and mitotic prophase, giemsa staining can be applied to cells to elicit G-banding in

s. Silver staining, a more modern technology, in conjunction with giemsa staining can be used to image the

synaptonemal complex The synaptonemal complex (SC) is a protein structure that forms between homologous chromosomes (two pairs of sister chromatids) during meiosis and is thought to mediate synapsis and recombination during prophase I during meiosis in eukaryotes ...

throughout the various stages of meiotic prophase. To perform G-banding,

s must be fixed, and thus it is not possible to perform on living cells. Fluorescent stains such as DAPI can be used in both live

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

and animal cells. These stains do not band

s, but instead allow for DNA probing of specific regions and

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

s. Use of fluorescent microscopy has vastly improved spatial resolution.

Mitotic prophase

Prophase is the first stage of

in animal cells, and the second stage of

s. At the start of prophase there are two identical copies of each

in the cell due to replication in

. These copies are referred to as sister chromatids and are attached by

element called the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fiber ...

. The main events of prophase are: the condensation of

s, the movement of the

centrosome In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progre ...

s, the formation of the mitotic spindle, and the beginning of

nucleoli The nucleolus (; : nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis. The nucleolus also participates in the formation of signal recognition particles and plays a ro ...

break down.

Condensation of chromosomes

that was replicated in

is condensed from DNA strands with lengths reaching 0.7 μm down to 0.2-0.3 μm. This process employs the condensin complex. Condensed chromosomes consist of two sister chromatids joined at the

Movement of centrosomes

During prophase in animal cells,

s move far enough apart to be resolved using a

light microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible spectrum, visible light and a system of lens (optics), lenses to generate magnified images of small objects. Optical microscopes ...

Microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

activity in each

is increased due to recruitment of γ-tubulin. Replicated

s from

move apart towards opposite poles of the cell, powered by centrosome associated motor proteins. Interdigitated interpolar

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

s from each

interact with each other, helping to move the

s to opposite poles.

Formation of the mitotic spindle

s involved in the

scaffolding break down as the replicated

s separate. The movement of

s to opposite poles is accompanied in animal cells by the organization of individual radial

arrays (asters) by each centriole. Interpolar

s from both

s interact, joining the sets of

s and forming the basic structure of the mitotic spindle. Plant cells do not have centrosomes and the

s can nucleate

assembly into the mitotic apparatus. In

s gather at opposite poles and begin to form the

spindle apparatus In cell biology, the spindle apparatus is the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter Cell (biology), cells. It is referred to as the mitotic spindle during mitos ...

at locations called foci. The mitotic spindle is of great importance in the process of

and will eventually segregate the sister chromatids in metaphase.

Beginning of nucleoli breakdown

The

begin to break down in prophase, resulting in the discontinuation of ribosome production. This indicates a redirection of cellular energy from general cellular metabolism to cellular division. The nuclear envelope stays intact during this process.

Meiotic prophase

Meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

involves two rounds of

chromosome segregation Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segreg ...

and thus undergoes prophase twice, resulting in prophase I and prophase II. Prophase I is the most complex phase in all of meiosis because

homologous chromosome Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...

s must pair and exchange

genetic information A nucleic acid sequence is a succession of Nucleobase, bases within the nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. This succession is denoted by a series of a set of five different letters that indicate the orde ...

. Prophase II is very similar to mitotic prophase.

Prophase I

Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in mitotic prophase, several crucial events occur within these phases such as pairing of

s and the reciprocal exchange of genetic material between these

s. Prophase I occurs at different speeds dependent on

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

and sex. Many species arrest

in diplotene of prophase I until

ovulation Ovulation is an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and ...

. In humans, decades can pass as oocytes remain arrested in prophase I only to quickly complete meiosis I prior to

Leptotene

In the first stage of prophase I, leptotene (from the Greek for "delicate"),

s begin to condense. Each chromosome is in a

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

state and consists of two sister chromatids; however, the

of the sister chromatids is not yet condensed enough to be resolvable in

microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...

. Homologous regions within

pairs begin to associate with each other.

Zygotene

In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived

s have found their homologous partner. The homologous pairs then undergo synapsis,a process by which the

(a proteinaceous structure) aligns corresponding regions of

on maternally and paternally derived non-sister chromatids of

pairs. The paired homologous chromosome bound by the

are referred to as bivalents or tetrads. Sex (X and Y) chromosomes do not fully synapse because only a small region of the chromosomes are homologous. The

moves from a central to a peripheral position in the nucleus.

Pachytene

The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis.

Chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

has condensed enough that

s can now be resolved in

. Structures called recombination nodules form on the

of bivalents. These recombination nodules facilitate genetic exchange between the non-sister chromatids of the

in an event known as crossing-over or genetic recombination. Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome.

Diplotene

In the fourth phase of prophase I, diplotene (from the Greek for "twofold"), crossing-over is completed.

Homologous chromosome Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...

s retain a full set of genetic information; however, the

s are now of mixed maternal and paternal descent. Visible junctions called chiasmata hold the

s together at locations where recombination occurred as the

dissolves. It is at this stage where meiotic arrest occurs in many

Diakinesis

In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four sister chromatids can be seen in bivalents with

. The rest of the phase resemble the early stages of mitotic prometaphase, as the meiotic prophase ends with the

beginning to form, and the

nuclear membrane The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer polar membrane, membranes that in eukaryotic cells surround the Cell nucleus, nucleus, which encloses the genome, genetic material. The nuclear envelope con ...

beginning to break down.

Prophase II

Prophase II of

is very similar to prophase of

. The most noticeable difference is that prophase II occurs with a

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

number of

s as opposed to the

number in mitotic prophase. In both

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

and

s chromosomes may de-condense during telophase I requiring them to re-condense in prophase II. If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the

model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...

Arabidopsis.

Prophase I arrest

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of

. In humans, as an example, oocytes are formed between three and four months of

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

within the fetus and are therefore present at birth. During this prophase I arrested stage ( dictyate), which may last for decades, four copies of the

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

are present in the oocytes. The adaptive significance of prophase I arrest is still not fully understood. However, it has been proposed that the arrest of oocytes at the four genome copy stage may provide the informational redundancy needed to repair damage in the DNA of the germline. The repair process used appears to be

homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...

al repair Prophase arrested oocytes have a high capability for efficient repair of DNA damages. DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of

fertility Fertility in colloquial terms refers the ability to have offspring. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to reproduce, which is termed fecundity. The fertility rate ...

Differences in plant and animal cell prophase

The most notable difference between prophase in

s and animal cells occurs because plant cells lack

centriole In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic cells, but are not present in conifers ( Pinophyta), flowering plants ( angiosperms) and most fungi, an ...

s. The organization of the

is associated instead with foci at opposite poles of the cell or is mediated by chromosomes. Another notable difference is preprophase, an additional step in plant

that results in formation of the preprophase band, a structure composed of

s. In mitotic prophase I of plants, this band disappears.

Cell checkpoints

Prophase I in

is the most complex iteration of prophase that occurs in both

s and animal cells. To ensure pairing of

s and recombination of genetic material occurs properly, there are cellular checkpoints in place. The meiotic checkpoint network is a DNA damage response system that controls double strand break repair,

structure, and the movement and pairing of

s. The system consists of multiple pathways (including the meiotic recombination checkpoint) that prevent the cell from entering metaphase I with errors due to recombination.

References

External links

* {{Cell cycle Genetics Cell cycle Cellular processes de:Mitose#Prophase