Leptotene stage

Last updated

The leptotene stage, also known as leptonema, is the first of five substages of prophase I during meiosis, the specialized cell division that reduces the chromosome number by half to produce haploid gametes in sexually reproducing organisms.

Contents

Terminology

The term "leptonema" derives from Greek words meaning "thin threads". [1] :27 A cell destined to become a gamete enters the leptotene stage after its chromosomes are duplicated during interphase.

Chromosome condensation

During the leptotene stage, the duplicated chromosomes - each consisting of two sister chromatids - condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm (nucleus contents). The chromosomes become visible as thin threadlike structures known as leptonema under a light microscope. [1] :27 [2] :353

Each chromosome consists of two identical sister chromatids held together by cohesin proteins along the entire length, connected at the centromere region. As the chromosomes condense, the nuclear envelope starts to fragment, and the nucleolus disperses as the cell prepares for division.

Chromosome attachment and bouquet formation

During this stage, the chromosomes attach themselves by their ends (telomeres) to the inner membrane of the nuclear envelope. At the transition to the zygotene stage, the telomeres usually aggregate at a sector of the nuclear envelope, thereby forming a "meiotic bouquet" arrangement. [3]

Synapsis initiation

A key event is the initiation of synapsis between homologous chromosomes, which carry the same genetic information but may have different allelic variations. The homologous chromosomes begin pairing and association along their lengths, facilitated by lateral (axial) elements of the synaptonemal complex protein structure that forms between the homologs. [4]

The synaptonemal complex consists of two lateral elements associated with each homolog, and a central region where they are held together. As synapsis begins, the chromosomes adopt a more extended configuration to expose the DNA for pairing.

Genetic recombination

The leptotene stage also sees the initiation of genetic recombination between homologs. This involves programmed DNA double-strand breaks and their repair to generate crossovers between non-sister chromatids of homologous pairs.

Proteins like SPO11 generate the breaks, while strand exchange proteins like RAD51 and DMC1 facilitate repair and exchange of genetic material. Recombination continues through zygotene as synapsis completes, generating new genetic diversity. [5]

DNA damage response

Studies in male mouse meiosis have shown that after DNA damaging treatments like gamma irradiation, two types of repair responses occur depending on the meiotic stage: [5]

Transition

Leptotene is followed by the zygotene stage, where synapsis between homologous chromosomes progresses further and the chromosomes continue condensing. [6]

See also

Synizesis (biology)

Related Research Articles

<span class="mw-page-title-main">Meiosis</span> Cell division producing haploid gametes

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 copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome again.

<span class="mw-page-title-main">Chromosomal crossover</span> Cellular process

Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

<span class="mw-page-title-main">Prophase</span> First phase of cell division in both mitosis and meiosis

Prophase is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.

<span class="mw-page-title-main">Genetic recombination</span> Production of offspring with combinations of traits that differ from those found in either parent

Genetic recombination is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes ; & (2) intrachromosomal recombination, occurring through crossing over.

<span class="mw-page-title-main">Homologous chromosome</span> Chromosomes that pair in fertilization

A pair of homologous chromosomes, or homologs, is 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 along each chromosome that enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance, which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area.

<span class="mw-page-title-main">Heteroduplex</span>

A heteroduplex is a double-stranded (duplex) molecule of nucleic acid originated through the genetic recombination of single complementary strands derived from different sources, such as from different homologous chromosomes or even from different organisms.

<span class="mw-page-title-main">Synaptonemal complex</span> Protein structure

The synaptonemal complex (SC) is a protein structure that forms between homologous chromosomes during meiosis and is thought to mediate synapsis and recombination during prophase I during meiosis in eukaryotes. It is currently thought that the SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities.

<span class="mw-page-title-main">Homologous recombination</span> Genetic recombination between identical or highly similar strands of genetic material

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.

<span class="mw-page-title-main">Synapsis</span> Biological phenomenon in meiosis

Synapsis or Syzygy is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-DNA complex called the synaptonemal complex. During synapsis, autosomes are held together by the synaptonemal complex along their whole length, whereas for sex chromosomes, this only takes place at one end of each chromosome.

Zygotene is the second stage of prophase I during meiosis, the specialized cell division that reduces the chromosome number by half to produce haploid gametes. It follows the Leptotene stage.

The pachytene stage, also known as pachynema, is the third stage of prophase I during meiosis, the specialized cell division that reduces chromosome number by half to produce haploid gametes. It follows the zygotene stage.

<span class="mw-page-title-main">Bivalent (genetics)</span>

A bivalent is one pair of chromosomes in a tetrad. A tetrad is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. In most organisms, each replicated chromosome elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I.

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 segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.

<span class="mw-page-title-main">DMC1 (gene)</span> Protein-coding gene in the species Homo sapiens

Meiotic recombination protein DMC1/LIM15 homolog is a protein that in humans is encoded by the DMC1 gene.

<span class="mw-page-title-main">Homology directed repair</span> Mechanism of DNA repair in cells

Homology-directed repair (HDR) is a mechanism in cells to repair double-strand DNA lesions. The most common form of HDR is homologous recombination. The HDR mechanism can only be used by the cell when there is a homologous piece of DNA present in the nucleus, mostly in G2 and S phase of the cell cycle. Other examples of homology-directed repair include single-strand annealing and breakage-induced replication. When the homologous DNA is absent, another process called non-homologous end joining (NHEJ) takes place instead.

<span class="mw-page-title-main">Chiasma (genetics)</span> Point of contact among homologous chromosomes

In genetics, a chiasma is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis. In meiosis, absence of a chiasma generally results in improper chromosomal segregation and aneuploidy.

<span class="mw-page-title-main">Meiotic recombination checkpoint</span>

The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.

<span class="mw-page-title-main">Synthesis-dependent strand annealing</span>

Synthesis-dependent strand annealing (SDSA) is a major mechanism of homology-directed repair of DNA double-strand breaks (DSBs). Although many of the features of SDSA were first suggested in 1976, the double-Holliday junction model proposed in 1983 was favored by many researchers. In 1994, studies of double-strand gap repair in Drosophila were found to be incompatible with the double-Holliday junction model, leading researchers to propose a model they called synthesis-dependent strand annealing. Subsequent studies of meiotic recombination in S. cerevisiae found that non-crossover products appear earlier than double-Holliday junctions or crossover products, challenging the previous notion that both crossover and non-crossover products are produced by double-Holliday junctions and leading the authors to propose that non-crossover products are generated through SDSA.

<span class="mw-page-title-main">STAG3 (gene)</span> Protein-coding gene in the species Homo sapiens

Stromal antigen 3 is a protein that in humans is encoded by the STAG3 gene. STAG3 protein is a component of a cohesin complex that regulates the separation of sister chromatids specifically during meiosis. STAG3 appears to be paramount in sister-chromatid cohesion throughout the meiotic process in human oocytes and spermatocytes.

Abby F. Dernburg is a professor of Cell and Developmental Biology at the University of California, Berkeley, an Investigator of the Howard Hughes Medical Institute, and a Faculty Senior Scientist at Lawrence Berkeley National Laboratory.

References

  1. 1 2 Snustad, DP; Simmons, MJ (December 2008). Principles of Genetics (5th ed.). Wiley. ISBN   978-0-470-38825-9.
  2. Krebs, JE; Goldstein, ES; Kilpatrick, ST (November 2009). Lewin's Genes X (10th ed.). Jones & Barlett Learning. ISBN   978-0-7637-6632-0.
  3. Scherthan, H. (2001). "A bouquet makes ends meet". Nature Reviews Molecular Cell Biology. 2 (8): 621–627. doi:10.1038/35085086. PMID   11483995. S2CID   32376282.
  4. Llano, E.; Pendás, A. M. (2023). "Synaptonemal Complex in Human Biology and Disease". Cells. 12 (13): 1718. doi: 10.3390/cells12131718 . PMC   10341275 . PMID   37443752.
  5. 1 2 Enguita-Marruedo A, Martín-Ruiz M, García E, Gil-Fernández A, Parra MT, Viera A, Rufas JS, Page J (January 2019). "Transition from a meiotic to a somatic-like DNA damage response during the pachytene stage in mouse meiosis". PLOS Genet. 15 (1): e1007439. doi: 10.1371/journal.pgen.1007439 . PMC   6358097 . PMID   30668564.
  6. "Leptotene Stage - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2024-02-15.