Prophase () is the first stage of

cell division Cell division is the process by which a parent 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 eukaryotes, there ...

in both

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...

and

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

. Beginning after interphase, DNA 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 ...

reticulum and the disappearance of the

nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of sign ...

Staining and microscopy

Microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot 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 micr ...

can be used to visualize condensed

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

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 Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

ian

s, but utilizing the technology on

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

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 Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately res ...

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 giesma staining can be used to image the synaptonemal complex throughout the various stages of

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 predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

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 (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s. Use of

fluorescent microscopy A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microsc ...

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 interphase. These copies are referred to as

sister chromatids A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the dup ...

and are attached by DNA 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 fibers ...

. The main events of prophase are: the condensation of

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

nucleoli The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of s ...

break down.

Condensation of chromosomes

DNA that was replicated in interphase 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

joined at the

Movement of centrosomes

During prophase in animal cells, centrosomes 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 light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsc ...

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 nm and have an inner diameter between 1 ...

activity in each centrosome is increased due to recruitment of γ-tubulin. Replicated centrosomes from interphase 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 nm and have an inner diameter between 1 ...

s from each centrosome interact with each other, helping to move the centrosomes to opposite poles.

Formation of the mitotic spindle

s involved in the interphase scaffolding break down as the replicated centrosomes separate. The movement of centrosomes to opposite poles is accompanied in animal cells by the organization of individual radial

arrays (asters) by each centromere. Interpolar

s from both centrosomes 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 cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a p ...

. In

s gather at opposite poles and begin to form the spindle apparatus at locations called foci. The mitotic spindle is of great importance in the process of

and will eventually segregate the

metaphase Metaphase ( and ) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase). These chromosomes, carrying genetic information, a ...

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

stays intact during this process.

Meiotic prophase

Meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...

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 segrega ...

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 A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...

s must pair and exchange

genetic information A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are u ...

. 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 In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

and sex. Many species arrest

in diplotene of prophase I until ovulation. In humans, decades can pass as

oocyte An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...

s remain arrested in prophase I only to quickly complete meiosis I prior to ovulation.

Leptotene

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

s begin to condense. Each chromosome is in a

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respective ...

state and consists of two

; however, the

of the

is not yet condensed enough to be resolvable in microscopy. 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 synaptonemal complex (a proteinaceous structure) aligns corresponding regions of

on maternally and paternally derived non-sister

chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chr ...

s of

pairs. The paired homologous chromosome bound by the synaptonemal complex 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 Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

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 ...

has condensed enough that

s can now be resolved in

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot 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 micr ...

. Structures called recombination nodules form on the synaptonemal complex of bivalents. These recombination nodules facilitate genetic exchange between the non-sister chromatids of the synaptonemal complex 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 A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...

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 synaptonemal complex 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

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 spindle apparatus beginning to form, and the nuclear membrane 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

number of

s as opposed to the

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. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...

number in mitotic prophase. In both

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

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 (often shortened to model) 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 workin ...

Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (''Arabidopsis thaliana''), one of the model organ ...

Prophase I arrest

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these

s 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 pr ...

within the fetus and are therefor present at birth. During this prophase I arrested stage (

dictyate The dictyate or dictyoteneMedical Physiology, Boron & Boulpaep, , Elsevier Saunders 2005. Updated edition. 1300 pages. is a prolonged resting phase in oogenesis. It occurs in the stage of meiotic prophase I in ootidogenesis. It starts late in fetal ...

), which may last for decades, four copies of the

genome In the fields of molecular biology and genetics, 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 ...

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 ooctyes at the four genome copy stage may provide the informational redundancy needed to repair damage in the DNA of the

germline In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...

. 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 cellular organisms but may ...

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 is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Ferti ...

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, and are ...

s. The organization of the spindle apparatus 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 The preprophase band is a microtubule array found in plant cells that are about to undergo cell division and enter the preprophase stage of the plant cell cycle. Besides the phragmosome, it is the first microscopically visible sign that a plant ...

, 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 DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...

response system that controls

double strand break DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA dama ...

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 Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...

with errors due to recombination.

References

External links

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